CA1233199A - Method and apparatus for high-voltage d. c. transmission with a bypass circuit for malfunctions - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method and apparatus are described for operating a high voltage d.c. (HVDC) transmission line system connecting two a.c. transmission line systems during normal operation and during malfunction in either the rectifier station or the inverter station of the HVDC system. When a malfunction exists in one station of a HVDC transmission line (e.g.,rectifier station A), then the regular thyristor firings are disabled and a bypass circuit, preferably one or several bridge paths of a converter, are fired. As soon as a corresponding change in the current or voltage occurs at the d.c. voltage connections of the other station (B), this station assumes rectifier operation during which the HVDC transmission line is utilized as a reactive load to stabilize the other system. Once the malfunction has ceased to exist, operation is first resumed in station A by extinguishing the bypass thyristors and subsequently resumed also in station B. This procedure permits stable normal operation in both stations, or bypass operation without using remote control signals during which the HVDC transmission line can be rapidly controlled to stabilize the systems.

Description

~;~3~

METHOD AND APPARATUS FOR HIGH-VOLTAGE ~OC.
TRANSMISSION WITH A BYPASS CIRCUIT FOR MALFUNCTIONS

Cross-References to Related Applications This application is related to the following co-pending applications:
Canadian application "Method and Apparatus for Resumptio of Normal Operation of a High Voltage D.C. Transmission Line" by Helmut Neupauer, Serial No. 475,296 filed Feb. 27, 1985.
Canadian application "Method and Apparatus to Operate a High Voltage D.C. Transmission System With Automatic Control of the Converters" by Helmut Neupauer, Serial No. 475,294 filed Feb. 27, 1985.

Background This invention relates to a method to operate a high-voltage d.c. (HVDC) transmission line between two a.c. networks during normal and malfunctioning conditions and an apparatus for said purpose.
In this apparatus a first converter is connected to a first a.c. network NA, and normally operating as a rectifier, 20~ feeds d.c. current into the HVDC transmission line at the first station A, wh~:le a second converter, mounted at a second station B, and feeding into a second a~c. network NB controls the voltage of the HVDC transmission line. Any modification of the HVDC trans-mission line voltage de~ermined by tne second station effects a change of the current fed into the irst station after a lag time ,:
determined ~y the transmission time ("travel time") of the HVDC

transmission line, as long as the control angle of the first ,~

~3~9 converter is not reset. Conversely, any change of the d.c.
current supplied from station A effects a change of the commutation time of the thyristors of the second converter, whose control angle has to be monitored and, : :

:: :
::
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-la-if necessary, shlftQd accordingly in order to ~aintaln the inverter step li~lt.
~ or that reason, the two stations are s~enerally co~nected by remote control lines and interact (e.g., by a so-called ~marginal current~ signal) ~o form the control angle. The con~rollers installed ln the station are set for relatively slow actlon in order to allow tlme for con~roller lnteractlon and for settling time.
If a voltage short circult occurs in station A, then ~hortly thereafter the d.c. current ln the ~VDC transmlssion line lapses or ~he second conver~er i5 shut ~own ln order to avoid further power supply lnto the short circult (rectifier malfunc~lon). The d.c. connec~ions of the firs~ converter can be short-clrculted via a sholt-circuit swltch (~bypass circuit~) ln order to detour any existlng d.c. current by the converter and maintain the current in the line at a reduced voltage.
This short-clrcuit switch can be designed elther as a : relatively slow-actin~ mechaniGal switch or as a thyr~sto : appropriat~ljy rated for relatively low current~. When : lnstalled, ~ t~ ~ ~or ls not suited ~o carry current supplled by station~B in case ~ald station has not been shut down, but nstead is swltched over from lnverter operatlon and is operated as a rectifler durlng the malfunction. ~uch a by~ass ~:~ 25 current would mean a reactlve current for the a.c. NB sy~em, : because ~he ~VDC transmlsslon llne becomes a long cu~rent charged inductor and could involve hlgh levels (e.g., 60 : percent) of the nominal current.
~ While the converter thyrlstocs available ln any even~ for : 30 nor~al operatlon of the flr~t converter ~ould carry such a current, they could only be fired usin~ exeenslve firlng devices 1f due to the rectifier malfunctlon the EVDC
transmlssion line had gone dead and lf also the malfunctlonlng NA ~ystem could not provide firlny energy.

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~;~331~9 For that EQ~son ~ald Dbypa~s opQratlon~ ln which convert~r A is brldged after a rectlfier malfunction and convert~r B 1~
operated as a rectlfier has not, here~ofore, been considered a reasonable approach si~ply for cost reaslons.
Furthermore, the control mechanlsm~ of station B are set for relatively slow action for the reasons e:Kplained above, and sald byeass operation would only result ln a re~ctive load for the NB syste~ whlch would be dlffi~ult to co.ntrol.
In case of a malfunction in 5ta~ tton B si~llar condl~lons would ~revail. During every voltage short circui~ ~he HVDC
trans~isslon llne current wo~ld flow into thls short circult, ; and the inverter would fall. It 15 shut down, and th~ HVDC
transmission llne discharges via the lnver~er thyristors untll the current ~ . A ~hort-ciIcuit switch also installed at that point can be closed until voltage re~umes, and dur1tng this malfunctlon rectlfler A ls sthut down. If the I ~ converter thyrl~tors the~selves were used as short-circuit swltches t~bypass circult~), then additional measures for correct selectlon of the firlng pulse would be.requlred to fire the thyristors ln accordance wlth the system cycle when voltage esurQes~ .
A:further co~pllcating factor ls ~hat the~remote signal ~: l:lne~have transm1~slon and processlng tl~e factors of their own so that the s~ttgnals required to operate thP functl~onlng:
station rQ~arding the~respective ~atu~ of the other sta~ion are only~availa~le aft;er ~ubstantial delays.
~ For that rea on heretofore normal operatlon of ~he HVDC
;~ transmission line~has been~mostly de~igned ln such a manner that, for~exa~le, a failure of the inver~er or other : malfun~tions were avoided, if at all pos ible; however, when a alfunction occurred, the HVDC:transmls~lon line would be held without current by shuttlng down both converters.
The purpose of thls i~vention ls thu~ to deflne a ~ode Qf operat~lon of the NVDC trans~isslon line whereby the HVDC

~33~99 transmission line can be utilized even in the event of a malfunction in one of the stations.
Summary of the Invention In this invention the malfunctioning station is bridged by a special circuit to handle the short during a malfunction designed to withhold the load by a reactive current reflecting the requirements o~ the a.c. ~oltage system in the functioning station, and the functioning station is used as a rectifier to supply the current. The HVDC transmission line can therefore be used to provide a rapid reactive load~ to dampen subsynchronous resonances, or for other balancing procedures, particularly for a maximum stabilization of the functioning a.c. system. This can be achieved, in particular, since in the event of a malfunction the control mechanisms of the functioning station do not have to be coordinated with the bridged other station with respect to their control speed.
In accordance with a broad aspect of the invention there ls provided~a method for operatLng a ~igh Voltage Dlrect Current (HVDC~ transmission line sys~tem connected between two alternating current systems, Network ~ ~N~ and Network (NB), during emèrgency operation resulting from a malfunction having a irst station, connected~to the network ~ which during normal operation operates as a system-synchronous first converter rectifying the incoming alternating current and impressing a normal d.c. current through the HVDC transmission line r and a second station, connected to the network B, determining the normal voltage of the HVDC transmission line, further forming a ~,~

.

3~L95~

leading release signal subsequent to the end of the malfunction;
wherein the second station during normal operatlon operates as a system~synchronous second converter inverting the incoming HVDC
and impressing a normal a.c. current into Network s, and the method adapted to form a short circuit as a bypass circuit around ei.ther a malfunctioning first station or a malfunctioning second station comprising the steps of:
forming a leading fault indication signal at the onset of the malfunction in the malfunctioning station;
firing by means of said leading fault indication signal (i.e.j inducing into conductance~ forming the bypass circuit from bypass thyristors which are designed to withstand as a normal .
operating condition a load current corresponding to the level of reactive current encountered during the malfunction bypass : operation;
forming subsequently in the functioning station a d.erived fault indication signal;
stimulating a rectifier operation of a.c. current into : ~:
~ :: bypass d.c. current in the operating station by said derived ~. :
fault indication signal;
:.
~ impressing said bypass d.c. current through the H~DC
, transmission line and the bypass circuit;
interrupting the bypass circuit causing said bypass ~ : thyristors to become non-conducting by the leading release ;~ signal;
initiating a transition of the previously maIfunctioning . station to system-synchronous normal operation by the leading - 4a -,, , ~;~33~L9~

release signal;
forming a derived .release signal in the previously functioning station;
terminating said rectifier operation by the derived release signal; and said derived release signal further initiating a transition of the previously functioning station from said rectifier operation to system-synchronous normal operation.
In accordance with another broad aspect of the invention there is provided an apparatus for HVDC transmission, including:
a first station connecting to a first a.c. system to draw electrical power therefrom;
a first converter being part o~ said fixst station and being connected to said first a.c. System; and operating as a system- ynchronous rec~ifier during normal operation, a high voltage d.c.~transmission line being connected at one end to said first converter;
a second converter being connected to the other end of 20~ said hlgh voltage d.c. transmisslon line and operating as a system-synchronous inverter dur~ing normal operation;
a second station having said second converter a part thereof and having connections thereto;
a second a.c. voltage syStem ~eing connected to said second station;
a first reference voltage generator located in and connected to station A ~or generating a system-synchronous 4b -,~". ,~

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reference voll-.age ~ith respect to a.c. system A;
a second reference ~oltage generator located in and connected to station B for generating a system-synchronous reference voltage with respect to a.c. svstem B;
a f.irst controller forming a control angle, ~A~
connected to station A;
a second controller forming a control angle, ~B~
connected to station B;
a first drive uni.t, STA, connecting to said first controller for forming a first set of system-synchronous firing commands;
- : a second drive unit, STB, connecting to said second controller for forming a ~econd set of system-synchronous firlng commands;
a~first monitoring:means monitoring the values of the : electrical quantities~of station A and forming t~herefrom a first fault indication~signal when a malfunction is indicated and~a flrs~ release signal when return to normal operatlon is indlcated;
20~ a~second monitoring means monitoring the values of the ~ electrlcal quantities of statlon B and forming therefrom a :~: second fault indication signal when a malfunction is indicated :
and~a second release signal when return to normal operation is ndicated;
a fixst clamping cirauit connected to said first drive ~: :
~ : unit and said first monitoring.means inhibiting the transmission -- ~ c --,, ~, ,, ~33~

of said first fi~ing command~ whenever sai.d first monitoring means forms said first fault indication signal, and transmit~ing said first fixing commands whenever said first monitoring means ~orms said ~ixst relea~e signal; ~nd a second clamping circuit connected to said second drive unit and said second monitoring means inhibiting the transmission of said second firing commands whenever said : second monitoring means forms said second fault indication signal, and transmitting said second firing commands whenever said second :: 10 monitoring means forms said second release signal, comprising:
; a first generating means as a part of the first monitoring means for generating a first leading fault ind~.cation :~ signal when monitoring of the first a.c. system electrical quantitles indicate~the occurrence of a malfunction relative to the first station;
a second~generating means as a part of the second monitoring means ~or generatin~ a second leading fault indicator slgnal when monitoring of the second a.c~ system electrical :quantities indicate the occurrence of a malfunction relative 20 ~ ~to the second station;

. ~ ~
a third generating means as a ~art of the first : monitoring means for generating a first leading .release signal when monitoring~of:the first a.c. system electrical quantities indicates discontinuance of a malfunction relative to the first station;:
a fourth generating means as a part of the second monitoring means ~or generating A second leading release signal - 4d -'~,',;.~,~
~, 33~

when monitoxing the second a.c. s~ste~ electx,ical ~uantities indicates the discontinuance o~ a malfunction relative to the second station;
a fl~th generating means as a part o~ the first monitoring means for generating a flxst derived fault indication signal when monitoring the d.c. voltage electrical quantities indicates the occurrence of a mal~unction in a location other than the first station;
a sixth generating means as a part of the second monitoring means for generating a second dexived fault indication signal when monitoring the d.c. voltage electrical quantities indicates the occurrence of a malfunction in a location other than the second station;
a seventh generating means as~a part o~ the first monitoring means fox~generating a first deri~ed release signal when monitorlng the d.c. voltage electrical~quantlties indicates a discontinuance o~ malfunction outside of the first station;~ ~
an elghth generating means as a part o the second monitoring means~or generating a second dexived release signal when:monltoring the d.c. voltage electrical quantities indicates a dlscontinuance of malfunction outside of the second station;
a first memory as:part of the first clamping circuit of the~firs~station storlng a combination of conVerter thyristors selected as bypass:thyristors in the first station;

~ .
a second memory as part of the second clampin~ circuit of the second station storing a comblnation of converter th~xistors ~::
- 4e -,~

3~9~

selected as bypass thy~isto~s in the second statio~;
a first and second switchlng means proximately located and connected to said firsk and second memory respectively, and further connected respectively to converter thyristors in the first and second converters, inhibiting the system-synchronous firing co~mands to said converter thyristors o~ the respective station when a ~eading fault indication signal occurs therein, and generating bypass firing commands to selected bypass thyristors of said respective station;

: 10 a first and second selector circuit connected to and proximately located to the first and second drive unit respectively, having a third and fourth memory respectively, contained thereinr whereby after the firing of bypass th~ristors : in the respective station, a combination of start thyristors . is stored for resumption of normal operation of the respective ::~: station;
a rirst and second logic circuit connected to said ;~ first and second selector circuit respectively, eliminating the inhibiting of system-synchronous firing commands after said respective leading release signal has occuxred, and subsequently : a firing command for one of the thvr~stors of said respective starting thyristor combination occurs; and ~:~ a first and second pre-programmed circuit, connected , ~
to the first and second control angle controllers respectively, nteracting with~the respeckive;control angle in such a manner ~ that after the staxt of the respective derived fault indication `~ signal, the conver~er in the functional station is operated as -- 4f -~"
~j, 3~

a rectifier in byp~ss opexation, until the occurrence of the respective derived release signal at which time bypass operation is terminated and normal operation resumed.
Brief Desc_ ptlon o~ the Drawings While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention will be better understood from the following description of the preferred embodiment taken in conjunction with the accompanying drawings in which:
Fig. 1 The design of a HVDC transmission line system.
Fig. 2 The relationship between firing angle ~, extinction angle r, the inductive d.c. voltage drop, the d.c.

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~.~33195~
voltage and the a.c. v~ltage when using a converter synchronou~
with the sys~e~.
Fig. 3 The schematlc design of a HVDC trans~lsslon llne short coupllng with automatic control of the control angle for normal functioning operation.
Flg. 4 The configuration of the HVDC transm~ssion system ln accordance ~ith Flgure 3.
Fl~. S A modified deslgn of a HVDC transmission syste~
wlth automatlc control.
Pig. 6 Another sche~atic design of station B with automatic con~rol.
Pig. 7 A design ~odifled from Fig. 3.
Fig. 8 and 9: ~odified designs of station B ln contrast to Fig. 6.
Fig. lO Another schematlc design of statlon A wlth automatlc control.
Fig. ll A design of a monltoring device for station A or B, Fig, 12 or 28: S~gnal pattern and design of a type of llm1t value warnlng devlce for the ~onltoring unlt in : accordance wlth Fig. ll.
Flg. 13 The use of a network model in a statlon with a HVDC remote trans~lssion llne.
Fig. 14 The design of the network ~odel of Fig. 13.
~ig. 15 The detailed design of a ~tation uslng station B
: as an example.
Fig. 16 Slgnal patterns for operation of statlon B ln accordance wlth Fig. l when resum~ng the system-synchronous : operatlon after a shutdown of th2 HVDC ~rans~lssion llne.
;: 3~ Fl~. 17 The clrcuitry oP a clamplng circuit or the arrangement as per Flg. l.
Flg. 18 and l9. The pattern of currents and VoltagQ3 as well as current transit ti~QS for the conver~er thyristors when ~: operatlng accordlng to ~ig. 16.
;: ~

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Flg. 20 Slgnal pa~terns fol operating station B in accordance with Flg. 1 in case of a temporary malfunctlon ~transitlon from normal operation and into nor~al operation).
Fig. 21 The clrcultry of a selector switch ln the design according to Flg. 15.
~lg. 22 The ~at~ern of a returnlngl a.c. voltage following a sy~te~ ~alfunction and the ~ystem-synchronous firing pulse3 of the station as per ~ig. 15.
Fig. 23 The paStern of control angles, voltag~s and : 10 currents ln a preferred e~ample.
Flg, ~4 and 25. The pattern of voltages, currents and sig~als when opera~lng a HVDC transmisslon line wi~h a ; de-energized length durlng a ~alfunctlon of station B or station A.
Fig. 26 and 27. The same patterns in the event of e~ergency bypass operation during a ~alfunctlon of statlon A or B.
Detailed Descrlptlon ~igure 1 shows a HNDC transmisslon line connected to two a.c. systems NA aDd NB via two ~tations A and B, each ; contalning a converter. If uslng thls arrangement d.c.
current is to be trans~itted fro~ the flrst station A to the seco~d statlon B,:then the current flowing through the HVDC
~: 25 transmlssion line will be preset by havlng the first converter located ln stat~on A operatlng as a rectifler ln order to draw from the firs~ syste~ NA (voltage U~) a current which as ~he output d~co current idA of the converter of statlon ~ is lmpressed lnto the HVDC transmission line. The second conver~er (s~atlon B) oparates ln this ~ode as the inverter ln ~: order to supply the in~ut d.c. current idB receiYed vla its d.c. connections lnto the second system NB, with the inverter ~: control angle used for the cur~ent feed controlling the input d.c. voltage UdB of station B and thus the voltage level of the HVDC tran~miss~on llne.

~33~
Generally, efforts are ~ade to operate the converter wlth low levels of harl~onics, for whlch reason the converters are des~gned as 12- and more-pulse converte.rs containing numerous d.c. current slde serles-connected component con~rters, for e~ample, connected to the HVDC transmlssion line uslng converter reactanee coils LA or LB and/or fllter circults (CFA, LFA and CF~, L~B, respectively) and connected to the e~pective a.c. voltag~ sy3tem NA or NB using convlerter transformers with differing clrcults characterized by thelr transformatlon ratio u (u = 1). Partlcularly for short coupllngs serving to llnk two closely ad~olnlng systems and often containlng only a high-voltage smoo~hlng reactor, the use of the filter ele~ents LP~, CFA and LFB, C~B re~pectively can often ~e omitted. The current appearlng followlng ~he filterlng elements (~HVVC
~rans~ission line current~) is deslgnated with ldLA (and the correspondlng ~HVDC trans~i~sion llne voltage~ wlth UdLA, while the guantities before ~he fllter elements are designated by idA and UdA-The actual values requlred for control purposes are ~: 20 generally obtai~ed as near as possible to the HVDC transmlsslon line conneceion polnt of the respectlve statlon~ i.e., posslbly ~: ~ behlnd the $11ter unlts; in other ca~es, e.g. to ~onltor the HVDC ~rans~slon line under operating condltions), lt ls lrrelevant where the (not deplc~ed) ~easure~ent units regulred :~ 25 to obtaln the ac~ual values are:insta~lled. The component ~ current converters lA', lA'' are serles-connected on the d.c.
;~ current side to connect poles 2,3 of the HVDC transm~ss~on line and each contain one out~ut phase R, S, T of the thyristol groups correspondlng to their transformers des~gnated ~y ~i~ or ~-~, if respectlvely thelr anodes or cathodes are connected to the transfor~er~. Thus, for example, thyrlstor group ~1 is located in the d~rectlon of current flow between the transfor~er and pole 2. A drlve uni~ supplies the ~o~pvnent current converter lA' vlth the flring co~mand seque~ce S'~

~;-"

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which cons~sts of the individual flring commands R~a....
T ~ and is either dlsabled by a cla~ping ci.rcult (switch symbol BS'~ or amplified to for~ a firing pulse sequen~e a~ ind~vidual i~pulses R a..,.~ connected to the thyristors designated with the same sy~bols.
The drive unit ST'~ contalns a reference voltage syste~
UIA5yn from a reference volt~ge generator RG'~ conne~ted to the a.c. voltage lnput of ~he component cu~,rent converter, which for~s the firing commands S~ ~y co~paring UIA5yn wlth a control quantity (e.y., a control voltage UsTA or a control angle a~ ~syste~-~ynchronous operation is formed with control angle aA~)-The control quantlty (for exa~ple, control angle aA~ lssupplled by a control device 4A and accordlng to ~he state-of-the-art is generally shared by all component fcequency converters of the statlon. The component current con~erter lA'' (thyristor groups R+~....T ~ and its control devices (RGA~, STA~, BS~) are deslgned ln the same way as the co~ponent current converter lA'; slm~lar quantities are designated ; 20 accordingly. ~ in most cases it is obvious to those skilled in the art how to control the existing component frequency converters by using the control quantlty of statlon A, the overall current converter ls often designated as lA in ~he ~ following descrip~l~n and the dlfferentiatlon of the quantitles ~ : 25 asslgnsd to the :respective component current converters suppressed.
Station B ls designed analogously as far as posslble; the component frequency converters lB' and lB~, for exa~ple, are often treated as one slngle converter lB. As many structural col~ponents a~d design features are iden~ical for both stat~ons, the differentlatlon using the letters A and ~ is o~itted in ~hese cases.
In the converter lA operating as a rectiier, the control angle is preset near the wide-open setting (~A'0) and .
- U -, . .
~ ,~

~ ~ ~3 ~ 9 ~

initially controls the output d.c. voltage UdA. The output current ldA is then deter~lned by the voltaye drop UdLA -UdA at the filter choke coil L~ accordlng to the followlng equation: ldA = l/LA l(Ud~ ~ UdL~) dt (1) Thus, lf a control guan~l~y l*d~ ls fed to the con~rol and regulatlon unit 4A as a set value for a current control su~plying ~he control ~ngle aA~ then the collapse of voltage UB or U~B as, for e~a~ple, ~rought about by an lnverter failure in station B or any other change in the operatlon of station B after the ~VDC trans~1sslon line ~ravel tlme, results in a change ln UdLA resulting in a current change and excitatlon of the ~urrent controller.
For current control of statio~ A thus UdLA or with the lag time determined by the HVDC transmisslon line travel tlme : 15 UdB operates as the fault indlcating quantity.
The situatlon ls slmllar i~ the active power to be transmitted is used as the con~rol quantity of statlon A. In : thls case an active power:controller supplles, for example, the set value i~dA ln accordance with ~he active ~ower no~inal ~l~ 20 value P~A coodinated with t~e energy balance of system NA.
: Also, ~he d.c. ~on~erter 4B of station B determines by its control angle aB the output d.c. voltage UdB. A5 the d.c.
curent ldB supplied a~ actlve and reac~lve current lnto the : . syste~ NB is impresssd by statioh A, the control and regulation unit 4B can control the reac~ive output into the NB
system in accordance with a reactive power set value Q*B
which can be used as the control quantity to stablize the syste~ voltage. Thereby stat~on B deter~ines ~hic~ voltage :~ level will be established in the HVDC transmlsslon line. The ~:~ 30 curren~ idB and the current flow in syste~ NB develop freely; it corresponds, except for sllght reslsta~ce of losses to the conductor, to the lmplessed current ldA defined prlor to the line travel ti~e.
: :
:: _ g _ ~.Z~3~99 As particularly for a h~gh active current co~ponent of the transmission a control angle normally near the lnverter wide-open control sett~ng (aB near 180 degrees) is the goal, the tl~e lapse during commutatlon fro~ the firiny of ~he succeeding thyristor (firlng angle a~) untl]. complete deactlvation of the precedlng thyristor (i.e., up to the ~e~tlnction anglea YB) 18 relatively long and increases as r the current idB,-rl~es. During the commutatlon ti~e the ~ voltage UdB collapses by a ~o-called ~inductive d.c. voltage 10 ¦ dropU.
¦ The inductive d.c. voltage drop brought about by the d.c.
¦ current i~B (or the impressed current l~A) thus functlons J as the ~ault lndlca~ing quantity for converter d.c. converter 4B.
Thls is e~rtlcularly lmportant s~nce the extlnc~ion angle must not exceed a maximum value (~lnverter step llmit~) which ¦ depends upon the release tlme of the converter thyrlstors 50 that no in~erter failure with short-clrcuiting of voltage ¦ Ud ~ rises. An increase of the i~pressed current idA
~o ~produces -after the HVDC transmiss~on line travQl time- an lncrease of the commutation time in s~ation B and an 1~crease of the lnductive d.c. voltage drop which ha~ to be dealt with by an advance of the ~iring tl~e point of station B (r~duction ~: ~ of aB) lf the ~aximum extinction an~le or a preset extlnctlon angle Y~ used as the control variable of station B is to be : adhered to.
Due to these ~utual control fault lndicating quantities the con~rols of the converters coupled to each ot~er using the H~DC transmisslon line have to be coordlnated in thelr operation. Generally, Infor~ation regarding the operating status o~ one converter (e.g., a marginal current signal or a malfunctlon ~ignal derived fro~ the re~pective contrQl quantity) i5 tran~mitted to the other statlon uslng re~ote control lines. Due ~o the travel tl~e of the HVDC trans~ission , .

~Z33~

line as well as the processlng time for this informa~ion : trans~lssion, stable operation of the HVDC transmlss~on llre can only be attained if the controllers of both statlons are set relatlvQly slowly (on/o~f control times o~ 200 ms, for example).
The HVDC ~ransmission llne can thus s~abllize the relevant a.c. voltage systems glven rapid ~alfunc~ions to a limited extent only. Moreover, Lapld startup of the ~NDC trans~ission line, in ~articular, given a transitory malfunction in a statlo~ and thus a temporary fallure of the HVDC transmlsslon line, for exa~ple, is not posslble. Thua, for example, provislon of an adequate safety margln fro~ the inverter step llmi~ is intended primarlly to avoid an lnverter fallure at the expense of a reductlon in the active ~ower tran~mission.
The oeeratlng procedure described below reduces these problems. It permits, given adequate psotectlon against inverter failure, hlghly dynamic control and rapid startup . after malfunctions.
:, Thus, inltlally the formation and transmlssion of the lnformation speclfied is ~im~llfied and by suitable measures, na~ely automa~ic control of the con~rol angle of one station ; using the fault lndlcating quantity or else a ~odel ~alue to be considered, the startina control times are substantlally hortened. ~ short coupl~ng ln :wh1ch, due to th~close ~roxlmlty of both s~tlons, the lnformatlon regardlng the operatlny status of the o~her station ls available without~ extended transmission tlmes, a d.c.
~ voltage detector for the ~VDC transmission llne vQltage is no : : longer required.:
In a HVDC;line long-dl~tance trans~isslon ~he lnformation required ln o~e sta~ion regarding the o~her station is e actically formed by the operatlng quantities only (partlcularly actual and s~t values) of ~he one statlon~
Thereby remote control llnes are omltted and the lnfor~at1on ls -- 1} --avallable at the earliest ~ossible polnt ln time, namely as soon as the status change of the other station is notlced ln the former station. The automatic control apparatus thereby inltlates very rapid closed loop control circultst ~lth startup ti~es of less than 50 ms (20 ms, for exa~lple) becoming feasible.
If the opera~ing uncertalnty, due to the model fault indicator quantity used, occassionally leads to lnverter failure or to another converter malfunction, the economlc consequences of such a ~alfunctlon can be held to a minimum since the automatlc control utllized per~i~s a rapld restart of the HVDC trans~isslon line followlng a ~alfunction.. Thus the operating procedure can be coordinated erimarily for opt1mum utilization of the HVDC transmlsslon llne. In particular, the HVDC transmlsslon line can also be deslgned to stabillze : 15 dynamlc processes (e.g., balancing processes in ~he sys~ems) : primarily ln order to stabilize the system voltage. Partlcular deslgn features further permit thls utillzation of the HVDC
trans~lssion line even when due to a malfunctlon of a rectifier or 1nverter the active current transmisslon ls interrupted.

~ The I~ductlve D.C. Voltaqe DroP As the Fault Indlcation ~uantitY
: : The method ln accordance wlth thls inventlon ls based upon an Investigation of the~effec~ that d.c. qua~tlties of a convert~r have on lts o~eration.
Fig. 2 depicts in broken l~ne the voltage pat~ern of the ~; individual phases of an a.c. system (system voltage U~
Assuming that the current ln the converter is commuted immediately and co~letely when the thrylstor is flred, the voltage:u d~(t) dependent upon the firing angle ~ arises at the d.c. c~nnections whlch is deslgnated as the instantaneous value of the ~ideal unsmoothed no-load dlrect voltage7.
The ldeal-type assumption that the current ls immediately commuted (so ~hat the com~utation tlme span is zero) ls present .
.~ .
.

31~

only under no-load condi~ions ln whlch the d.c. current itself becomes zero. Glven wide-open control setting, the average value of the ideal no-load dlrect voltage under the~e ideal conditlons is the voltage Udl whlch is proportional ~o the mo~entary syste~ voltage a~plltude U~ or the effective system voltage in accordance with a defina~le ratio which is a pure nu~erical value for the respective co~verter type:

Udl = const . U~ (2) : 1~
l.e., UdiA const UA~ UdiB = -con~t UB

The change in the value for station B takes lnto account the reversed current orientation of the thyristors of converter lB in contrast tv ~he nu~erical orlentatlon of voItage UdB.
Udi responds to the voltage vs ti~e wavefor~ of the rectlfied syste~ voltage.
The respective ldeal unsmoothed no-lvad direct voltage ud~(t) and u~,dl~t) correlate to the two firing angles ~
and ~. T ~ e vs time waYeforms ~hown crosshatched are at~
glven as ~smoothed ldeal no-load direct voltages~ by:
Udl~ = Udi cos ~, Udi~ = Udi cog ~
. Turning now to actual conditlons, it ls assumed that the : converter wlth firing angle ~ ls fired and that the current ld actually flowing~via the ~hyristor~ in the d.c. voltage supply requlres a~certaln commutatlon time (for instance, : ~corresponding to an ~overlapping angle~ u during which both ~ 30 thyristors are current-conducting) in order to pas~ out of the previously flred~thyristor. The end of the commutatlon tlme l~
~ ~peclfled by ~he angle ~, i.e., the extlnction angle y = l~O-~
;~ ~o that the followlng applies:
;

~ ~ -- 13 --~ ' ~33~9 180 ~ u (4) whereby the positloning of the ex~inction angle y, i.e, the value of the overlapplng angle u, depenlds upon th~
magnitude of the commutating current id. Thle unsmoothed d.c.
voltage uda(t) actually arislng at firing angle a ls also deplcted in Fig. ~. It ls eractically the average value 1/2 .
(udia(t) ~ udl~(t~) which the ~voltage-com~utation vs. ti~e slot~ depicted crosshatched on the right divldes in half. Thus, for the actual average value Ud~ of the d.c. voltage U = 1/2 (Udi~ + Udl~) = Udia dla Udi~) (5) l.e., ~he actual d.c. voltage deviates from the ideal no-load direct voltage Udi~ = Udi cosa corresponding to the firlng angle ~ by a voltage differen~lal. This so-called ~induct4Ye d.c. voltage drop~ is proportional to the actual d.c. current id via a converter-specific parameter ~x:

/2 (Udla ~ Udl~) = dx ~d = 1~2 Udi (cos ~
~o~ ~ (6) I
s~ that the followlng relationshlps prevall: Ud = const ~1 U~ cos~ - dx . id~ l.e., glven sultable standardized measure~ent values UA and U~ for the a.c. voltage ¦ amplitudes in both stations, the iollowing equatlons apply for the H~DC transmisslon~voltages Ud~ and UdB:
UdA = UA c~aA ~ dXA ~ ~d~
; UdB = -U~ cS ~B - dXB . ldB
*) ¦ As due to the existing inductances the d.c. current practlcally does not change durlng a commutation, eguatlons 4 and 6 ~er~t a precalculation for each flring ti~e polnt ba~ed on ~he flr~ng angle and the easurement values for the a.c.

* ~t~should be not~lthat ~ is~a~negative quantity du~ to the de~ined PQ1arj.tY ~f idB and Ud~

~ ~33~9~

voltage and the d.c. current wlth regard to the values that the overlap angle~ the extinctlon angle an~ the d.c. voltage will assume during firing.

Nor~al O~eration with Automatlc Control The firs~ application of these relationships is depicted ln Flg. 3 for a short-coupling arrangement, l.e., the HVDC
transmission ltne conslsts in this case only of one lnductance L = LA + LB, arranged without flltering clrcuits between ~he two converters 1 a~d 2. Fig. 4 depicts the structure of thls arrange~ent, whereby inductance L o~ the HVDC transmission line analo~ously to equatio~ 1 is deplcted by an integrator (in~egration constant ~) with the input value UdA - UdB
and the out~ut value ld - idA - ldB.
The action of converter 1~ of drlve unit STA and of ~he reference voltage generator RGA depicted as the current-l~pressing assembly SRA ls deter~ined by the fact that the set firing angle ~A given with incomplete synchronlzatlon of the reference:voltage g~nerator to the actual phase of the a.c.
voltage N~ deter~ines the vol~age UdlA/UA = cos (~A +
) except for an a~gle error ~ (co~ ge~erator 410 in Fi~. 4). By reno~allzing ~multiplier 4113 a~d taking into account the inductlve voltage dro~ XA . ld of this converter ~proportlonal elemen~ ~13, subtraction element 412), one eventuallY obtalns UdA = Udla ld dx~
The lag tl~e of the curren~ impres~lng is symbolized by :~ lag time circult 414, while a dynamic circuit 415 shows the smoothing of the control a~gle in the drive unlt or ln a : generally adv~ntageous lntegral drlve unit smoo~hing mechanism.
This demonstrates that the inductlve voltag~ drop of converter lA can be compensated ln accordance with the principle of auto~atic control by the addltion of a corr~spondlng model control vol~age dXh . id to the e ,~,~

., vP~ 84 P 3393 ~;~33J~9~

control quantity for UA (in ~hls case the slgnal UA' cos aA~ supplied by a current regula~o~ 41A). In many ca~es, e.g., if a UA normalized arcos-function network 40a i5 connected following ~he controller 41A to linearize the control characteristic funct10n, the nor~alization of the automat1c control voltage (model fault lndicatlon qua~tity) corresponding to the normalization of the control quantity is not even ~ ~ecessary so that computlng circuit 43A only requires a : proportional circuit 431~ in order to for~ from the availahle current-measurement value idA (in this case id~ - id ~
idB, l.e., a quantity available at the locatlon of station ~, the mod~l fault lndication quantity ldA . d~A .
If one connects ln additlon or as an alternative ~o the auto~atlc control devlce (addltional ele~ent 42A) the voltage UdB, then also the effect from the converter lB on the ~: current control will be compensated. Station A thus compen~ates, practlcally wlthout any delay, ~umps ln the faul~
indlcatlo~ quantity, comlng from statlon B and the controller : can be ~ore rapidly ad~usted slnce thereby the delaying effect of the HVDC transmisslon lnductivity is obvlated.
. Colrespondingly, the controller can he optlmized strictly ln er~s of tbe time behavlor of the current i~pres~ion (ac~ual value and Be~ value g neratlon as well as s~oothing and delay of the conYerter) without consldering the time constants to be ~-~; 25 allocated to the transmlssIon conductor and the operatlon of .~ station B.
The ~DC current ~ransducer baslcally required for this : o~erati~on can, however, be omitted in accor:dance wlth the~
invention if the actual valu~e UdB for the automa ~ ~c~ontrol ~ 30 is repl~ced by a model fault lndlcation guant;lty UdB`computed by a compu~ing circult 44 from the ~easured value id~ i.e., the HYDC trans~ls610n current:or the lnput d.c. eurrent of the converter lB in accordance w~th equation (7):
.- :
. ~

~233~9 UdB = -UB cos ~B - dXB . ldB
from the actual value id~ the system voltage UB, and the firlng angle (control angle)(~B). In accordance ~ith the clrcultry symbols lF~g. 3) a proportional lin~ ~42 and ~ultlplier 441 and a summat~on point 443, for example, serve that purpose.
: In this context, however, the tlme behavior of both converters wlth their drlve unlts and the HVDC transmlsslon line itself ~ust be taken into account. Thls is handled by a dynam~c element 440 whlch ls preferrably designed as s~veral smoothlng links connected ln series and therefore permits, ln ~ar~lcular, adequate consideration to be glven to the resultlng la~ tl~es.
In summary, the converter lA is operated wlth a control angle aA forming part of a control voltage UsTA = UA .
cos aAX ~ UdAv automatic control voltage UdA can ~ake into account ~he lnductlve d.c. voltage drop ~XAv = ldA .
: dXA, but partlcularly conta~ns the followlng fault ~; 20 lndlcation volta~e UdLA (for short--circuit coupling: UdB
or el~e:U~B) as shown ln equation (1).
In station B one must ~lso take into account that the inductive d.G. voltage drop also functions as a fault : indication quantlty on the voltage Ud~ determ~ned by the control angle aB, ln ad~ition to the fault indication voltage caused by a ynchronizatlon error ~aB. This fault lndlcation : varlable has to be ell~inated by the control quantlty con~roller.
Statlon B shown ln Fig. 4 contains the same structural components for block SRB with a drive unlt smooth~ng component :~ 415B (if necessary designed as an integral smooth~ng clrcuit~
; and a auto~atlc control devlce comprlsing a summing polnt 42B
and, depending upon the ~ature of the aXB slgnal and the ~XB~ automatic control slghal~ a ltnearizing circuit 40B.
, ~. , .

~233~

A5 Flg. 8 shows, initially a slgnal which determlnes a set value of the HVDC transmlssion voltage UdB~ = -U~ . cos ~B~ uslng a preset angle aB*, can be taken from a control devlce which computes th1s voltage set value fro~ a control ~uantity or other operating values of station B (controlled operatlon). In particular, however, aBX (or C05 aB~) if a linearizlng clrcuit 40B is also connected) can be obtained as the output slgnal of a control quantlty ~ontroller.
The ~orresponding model fault indication quantlty is then connected to the automatic control device (sum~ing point 59, FigO 8) as a~ lnductive d.s. voltage drop dXB . ldB (or as the corrsponding ~automatlc control angle~) and onto thls preset angle in such a way that ~he control angle ~B ls determlned by aB = ~B +~v' B dB
15 dXB . idB/ t UB). In the circuit compo~ent SRB, the control command for the inverter thyristors is generated by a reference angle ~(t) of the reference voltage generated by the reference voltage generator RGB whlch ls defined by the condition :: 20 (U dB ~ d~B id~~UB) = cos ( ~o~ ~(t) .~
or B ~ ~Bv ~ ~(t) , o Thereby a ~ump in the fault indlcation quantity can be largely and vlrtually immediately compensated even lf the control quantlty controller:41A generating the faul~ quantlty : 15 ad~usted very rapidly (tlme consta~t less than 50 ms, particularly approxlmately 10 ms or less). Here a~ well the controller can be optimized strictly in terms of the tlme response of lt~ own statlon vithout ~aklng lnto account the - lR -~Z3~
tlme constants to be attrlbuted to ~he HVDC transmission llne and the other s~ation.
AlthQugh this does permlt stable statlonary operatlon wlth rapld control functions, it does no~ assure the ~alntenance of a corre~pondlng minimal ex~inct~on angle (~c;afety angle~) correspondlng to the i~verter step limit.
However, to avold lnverter fallure, the co~mutation always has to bs completed at a certain (~lnimu~) e~tlnction angle Y~. Therefore the thyristors must always be fi~ed at a phase posltion ~(t) of the inverter defined by:

~(t) = 180 - y~ -u In the ideal lnstance in whlch the thyri~tor firlng takes place without swltching delay and with ideal synchronlzation at the phase posltlon ~t) = aB~ wlth the voltage drop being exactly obtained by the model value dXB . idB/(UB), the control angle aB than must be llmited to ~aB)maX = 180 Y*- u = arcos~ - cos y~ dx~ . id~/ UB).
~ In operatlng conditions in which said firin~ angle : ~ liml:tation prevails, the firing condltio~ ~t) = 180 - y~ - u, 180 - y* - u - ~(t~ = arcos(- cos y~`~-(2d ~ dB)~ (t) = O
or i;,.~, -cos y~ ~2dX~ . id ~ (UB) accordingly applies in the drive unlt wlth y* or else cos r*
being a value for the extinctio~ angle which ~ pre~et. This pre~ettlng of the ma~i~um control angle aBmaX can be handled ~ by a li~itlng deYice 58 (Flg. 8) at the lnput channel for the control angle aB of ~he drive unit, wlth aB beln~ preset by a control functlon and, a~ wa~ ~ust explained, capable of belng ~et by automatlc control. The lioita~ion thereby also -- 19 _ ~ . .

~L233~

functlons as a auto~atlc con~rol wltb a automatic control 5 Bv (~A)ma~ co~puted from r~.
If equatlon 7 applies strictly and a set a~le Y* i~
calculated and ll~ited to ~ln fro~ ~he voltage set value dB Y UB co~ y U d~ + dX~ B~ l:hen the angle ~ for~ed in co~puting stage 43B' (Fig. 8) equal~ a~v =
arcos (-cos y~ - 2dXB . ldB/U~) or, if no llnear1zatlon is performed, ~he co responding voltage UdBv = -U~ . cos y~
- 2dXB . ldB ~rovldes a llmit value for the angle a~B or a correponding control vol~age whlch can be preset in any desired fashlon (i.e., by a rea~tlve current controller or another ~control quantity controller-).
Said control quantlty controller can then si~ply caus~ the control an~le ~ and thus the voltage UdB ln contra~t to the aauto~atlc control angle~ ~Bv to be retracted while wlth glven U~ and ldB the values U~dB and Y~in will not be exceeded. If the angle ~ can be calculated from the control quantity set value as takes place in computlng element 47 of Flgures 6 or 7, then ~he angl~ aBV = 1~0 - y~ - u can be ~: 20 directly con~e~ted to the drlve unlt ln a controlled ; arrangement wlthout any further ad~us~ent.
In the drlve unit the followlng always applles: aBmaX =
~lB0 - y~ - u . wherein u ls the ~omputed overlap angle from y~ and ~he - 25 lnductive d.c. voltage drop (model fault lndication quantity).
: The ~odel value 2d~B ~d/(UB) can thereby be deter~ined from ~omentary valu~s for ld and UB for each phase posltlon of the system. This means that for each momentary phasa positlon of the syste~ the respectlve lnductive voltage drop and thus the overlap angle will be precalculated.
By monitorlng the speclfied firlng condltions, therefore, a thyrl~tor firing will be lnltlated lf ln accordan~e wlth the : precalculated overlap angle the stlll re~alning tl~e period ~331~

until the set time point of a thyristor extlnc~on ~predeter~ined by r~) suffices to complete the co~muSatlon.
In actual converter o~eratlon a conl:rol angle controlling slgnal a~v calculated fro~ a set extinction angle y~ ln accordance ~ith 180 - ~* = aBV ~ u causes at the reference angle ~(t) a thyristor fir~ng which, howeYer, ~akes place at the actual phase positlon ~(t) = ~t) + ~ of the a.c. voltage syste~ and, due to a posslble i~precisios~ ~u of the co~puted overlap an~le leads to the ~ctual value 180- ~ = ~(t~
Thus, there ls a ~et/actual value dlfferentlal Ay = aBV ~
~(t) - u ~ ~or that reason the differential ~ ls preferrabl~
: ellmlnated uslng an extinction angle control1er 41B ~Pig. 8) to whose output ~ignal ~XB the precalculated ~automatic control angle~ aBv is added BO that for the drive unit we have the following conditlon:

QxB + a~v = ~(t~ with ~v = arcos (-cos y~ - 2dXB
~ ' idB/( :: : 20 A controller can, however, be used for ano~her control : quantity ~hereby the set ext~nction angle yt to calculate ~Bv will be de~er~ined from the control quantlty set value (co~pare lte~ 47 in ~lgures 5 or 6) and the control output signal ~XB
wlll correct the extlnction angle to maln~aln the control quantity set value.
~ere as well, the overlapping angle or the inductive d.c.
: voltage drop belonging to y~ will ~e precalculated from momentary values for ld~ and U~ at each phase position ~(t).
Thereby one attains a controlled presettlng of the extinctlon a~gle whlch is u~ed in Flg. a to lim1t the either controlled or regulated preset angle ~B. In the preferred deslgn exa~ple of;~igures 3 and 4 the ~ontrolled preset -automatlc control angle ~Bv is connected directly to the control quantlty controller 41B output as the model fault indlcatlon quantity ~XBy whose output quantity ~B serves to control the control quantlty preset extinction angle in terms of cos y~ - cos y = 0. The automatic con~rol device 42B, 40~ ~s thus in this case connected ~o the auto~atic control voltage QX~v = ~ cos y~ - :; 2i~B ~ d~JUB to generate :~ aB= 180 ~ - u ~ ~X~. This automatlc con1:lol voltage serves thus to take into account the inductive d.c. voltage drop and is co~puted by the 43B co~pu~ation circuit which can be handled, for e~ample~ by ~he co~putation components with the computation clrcuits sho~n in ~he circuitry ~y~bols.
Said taking lnto account of t~e relationships here indicated can, of course, also be handled in accordance wlth deduced relatlonships in modified computation circuits which those skilled in the ar~ can develop as required.
Thus, in the case of a short-circuit coueling one develops an arrangement coordlnated for lapid eliminatlon of operating :;: status changes of both conver~ers whlch requ1res only simple ~ : 20 mechanlsms; particularly ~he high-voltage co~onent only , ~ regulres a current transducer as a detec~lng unit.
:-~ In Fig. 5 these ~elatlonshlps are a~plied ~o the sys~em ln which the converters are conn~cted vla converter reac~ance ~: coils LA an~d L~ and filter circuits LFA, CEA as well as LF~, : 25 CFB to the HVDC transmission line (short-circult coupling or re~ote~ransmlsslon llne).
This arrangement dispenses with any compQnsation of the :~ slight control angle shi~t brought about by the inductive d.c.
voltage drop of the converter lA. Furthermore, to take into account the fault eroceedlng from statlon B, the HVD~
transmis~ion input voltage UdLA is used In place o~ the model : fault lndlcatlon quantity UdB a~ the automatic control `^J
voltage VdAv 1~ statlon A.

~;~33~L~9 In normal operatlon station A requlres no remote actlon signals whlch lnfluence its operation sl~ilarly to ~nformation trans~itted over re~ote contrvl lines regarding ~he s~atus of sta~ion B. Rather, in statlon A all the requlred actual or set value quantlties requlred for normal operatlon ~re avallable.
The same apel~es to station B. The auto~atic control of both stations thereby takes into account the mutual cou~llng of both statlons ln su~h a ~anner that stable opera~ion with short control cycles ls attained.
I~ the d~sign according to Fig. 5, the linearizing circuit 40B is integrated lnto the computing circul~ 43B slnce sn extinctlon an~le controller is used as the control quant~ty : controller 41B, whose output slgnal axB already define~ an angle. The cos Y~ requlred ln the com~utlng clrcult 43B is then for~ed using a func~ion generator 40~'.
The deslgn also foresees that glven sudden lrregular : changes in the respective control quantlties ln both statlons, the respec~ive change rate of the control quantity set values can ~e li~ited by run-up function generator 45A or 45B. The other components already familiar from earlier flgures are identifled wi~h the old reference symbols.
: In statlon B optionally either the extinctlon angle ~, the reactlve power Q, or another suitable con~rol quantity can ~e ~: used to ~alntain constant voltage~ to dampen balanc~ng : 25 processes or for o~her dyna~lc controls. This ls lndIcated by the selector swltch 46, whereby the input of the automatlc control device 42B can be switched between ~he extlnction angle controller 41B and other control quantlty controllers 41B', 41B~. For this swltchover the deslgn foresees that an auxillary computlng clrcuit 47 will supply the relevant ext~nctlon an~le zet value y~ or else the cos (y~) obtainable fro~ the functlon generator 40B based upon the control quantlty set values utilized ln the particular case.

~L2~3~99 The converter para~eters d~A or dXB, respectively, required to generate the model fault indicatlon quantity whlch corresponds to the lnductive voltage drop of the conve~ter, can be automatically reset in ~he relevant co~puting circui~s since always during each thyr1stor extinctlon the actual inductlve voltage drop is determined from ~he measured flring and extlnctlon angle and compared wi~h the computed value in accordance with the following relationship:

UB (cos ~ - cos y) = 2dXB . idB.

In accordance with this comparison the parameter 2dXB can then be ad~usted as indicated by the parameter feedback devlce 48 in Figure 6.
In addltlon, Fig. 6 symbolically shows that the drive unit STB generates the control signals Sa by monltoring the above-specified flring condition.
:~: The precalculatlon of ~he ovsrlap angle which at the momentary phase position ~(~t~ of the system always belongs to he set extinctlon angle y~ as well as the momentary values of B:and UB can~ for example, pro~eed wlthin a mlllisecond PE~ cy ln a mlcrocomputer 43B as shown the case of a short coupling (induc~lvity L3 ln ~ig. 7. Here as well, use is:made of the odel fault indlcation quantlty UdB as the automatic control ~5 ~ Yoltage UdAv of station A, whereby the compu~ing circult 44 can be further substantially slmpllfied if one starts not from the control angla aB itself, but from the:preset extinction angle y* or related guantlties generated ln the microcomputer itself. In parti~ular, for example, in place of UdB, the B Y ~B . idB can be connected by a dynamic clrcuit which lmitates the transmission behavlor of converters and the HVDC transmlssion line to the auto~a~ic control devlee 42A. In addition, Fig. 7 shows ~hat p~eferrably the effectlve output P of the ~VDC transmissions ll~e is used ~23~

as ~he control quantity of stat~on ~. Frc~ the control dlfferential p~-p a superlmposed act~ve power con~roller 51 generates the set value id of the current regulator 41A.
This set value i~d can also ~referrably be reg~lated by a fault indication quantity ld at the addltion clrcuit 52 whereby id = p~/UdB is supplied by the d~vlder 53.
~ lg. B deplcts a design for sta~ion B in which the run-up function generator 66B initlally supplles a control quantlty which (if necessary via a drlve unit smooting circuit 415B) presets the control angle aB f the converter. The quantity a~B can be preset fro~ a non-depicted cont~ol quantity ~ controller (e.g., an active power controller or a controller : for the HVDC transmisslon voltage UdB or can be presst ln accordance wlth the already discussed controlled o~eratlon by computational means. A automatic con~rol dev~ce 59 ls shown : whlch compensates the inductive d.c. voltage drop influencing on the set angle a*B whlch is supplled by a computlng unit 43B~. The mlcrocomputer 43Ba prevlously shown ln Flg. 7 also determlnes the cos1ne of a au~omatlc cantrol angle ~v which takes into account both the voltage differe~ce brought about by the inductive d.c. voltage drop between the ideal no-load dlrect volt~ge relative to y~ and the voltage UB as ~ell aB
the angle shift b~ought a~out by the inductlve d.c. vol~age ; : drop between the angle aB and ~he ideal firing angle :~ 25 belonging to voltage UB.
The 11nearizatlon circui~ 40B' provides a maxlmum extinctlon an~le amaX for a llmlting circuit 58 calculated f~om 180 - (~* - U ~ oxB) , whereby OXB 1~ prepared at the outpu~ signal of an extlnctlon angle controller 67B.
As long as the li~ltlng clrcuit 58 is not ~n operation, ~he converter ls regulated by the preselected value of ~B.
If, however, the co~pen~ated a~gle ~*B attains the pres~t llmlt angle ~a~ the extlnctlon angle control in~ervenes and the converter 1~ operated with the controlled extlnction - ~5 -.

VPA 8~ P 3393 ~3~9 angle. If the e~tinction angle controller 41B is deactivated (for e~ample, hy a short-clrcul~ switch 47B), tklen only the auto~atlc con~rol angle aBV determines the llmit a~gle ~a~. By the action of the angle limitation 58, ~tatlon A ls then operated wlth a regulated extinctloll angle.
As a rule, the incorporatlon of the inductive d.c. voltage drop describ~d assures that the inverter s~ep llmlt is observed as long as i~B and UB do not change all too drastlcally durlng a co~u~atlon process so that the ~reco~puted trans~lssio~ angle u colncides adequately with ~he actual overla~ angle. This eerm~ts operatlon with maximu~ firlng : : angle (~axi~um active power trans~lssion or min~um reactlve ; power). A greater safety ~argln to the lnverter step ll~lt is thus not requlred as long as lt ~5 a~sured that even ln an extre~e ca6e in which sudden changes of ldB or UB can lead to a fallure of the lnverte~, the converter swltches over to malfunction operatlon and after the termlnatlon of a ~alfunc~lon can rapldly resume ~ormal o~eration.
In the preferred design of the control and regulatlng device 4B in Flg. 9, a ~onltor~ng and progra~ing ~wltch 62B 15 included which contalns a ~emory cllcult 63B (for example, a : : programmed ~lcroprocessor) and a ~onitorin~ dQvice 64B whlch will be descrlbed later.
Device 62~ actlvates two selector switches 60B an~ 61B
whose positlon as sho~n ln ~ig. 9 corre~po~ds to nor~al ~: opera~ion. In this normal operation the automatic control : voltage (automatlc control angle ~Bv) generated by the microcomputer 43~' ~s connec~ed via sw~tch posit~on Pl to ~he automatic con~rol de~ice 42B and added to the contEol quantlty ~XB of firing angle aB vhlch ~s generated by a control ~: quantlty con~roller, partlcularly a~ extinctlo~ angle controller 41B. A control devlce 6~B not shown ln detall can, by repositioniny ~witch 60B~ be connected to the auto~atic ~ ~ con~rol devlce so that, for example i~ eaergen~y operation, the ; - 2 6 `, ~;~ 33~

control angle wlll be generated in accordance with the requlrement~ of sys~e~ NB (for e~a~ple, in accordance wl~h a reactive output set value Q*B) requlred to alppro~imately ~aintain a cons~ant voltage.
In swltch ~o~itlon P2 another model va:lue is supplied to the inpu~ of the automa~ic control deYlce 4'2B ln ~lac~ of the model value for ~he model fault lndicat~on quantity aBV.
Said model value is supplied by a run-up fu~c~lon generator 66B. The end value of thl~ run-up function ~enerator is deter~i~ed by values preset in terms of the normal transmission o ~he HVDC trans~lssion line in trouble-free operation. For example, in accordance wi~h a norma~ value U*B for the HVDC transmission llne voltage computed by a divlder 67B as the quotlent of the ~ active power P~ and the nomlnal HV~C transmlsslon llne current i~dB for normal o~eratlon.
Swi~ch pos1tion P3 is reserved for a case in which the control angle ~B has to ba te~porarily shifted due to the surging hookup of an addl~lonal ~t value (I.e., to ~hut down the HVDC transmission llne or to pa~s ~ver into nor~al operatlon~. ~
Station A is very ~i~ilar in:design wlth identical compo~en~s having the same ~oding numbers along with the letter A (Fig~ 10).
A~ide from a ~o~slbly so~ewhat different opera~lon of the lin~açlzing circuit 40A, 5tation A is designed so that the selective switch 60A switches the curIent set value l~dA
pro~ected for norm~l operation for the current regulator 41A to a current set value which ls sup~lied by a superimposed control device 68A ln accordance wlth the requirements of system NA ln ca~e of a malfunction. Moreover, ln positlon Pl of selec~l~e switch 61A the voltage UdL~, i.e., ~he d.c. voltage obtai~ed when using filtering circui~s between the filter circults a~d the HVDC tran~isslon line co~nectlon of statlon A, i~

- ~7 -~33~g~

connected to the automatic co~trol device 42A as the model ~aul~ indicatlon quantity UdAv ln nor~al operation.
In both stationsO moreover, the startup generators ~6A, 66B as well a~ the control variable controll~rs 41A and 41B can be deactivated by the control and programmlng clrcuits. In addition, th~ design foresees that ln the event of a ~alfunction the devices 62A and 62~ tr~n~mlt relevant ~alfunctlon signals to shu~ down the converter, e.g., by triggerlng the cla~ping circul~s B~ $rom Fig. 1.
~onitorin~ ~alfunctio~s In the nor~al o~eration described there are no complications due to delay times and processlng tlmes of remote control ~ignals which often necessltate a slow ad~ustment of the HVDC transmlsslon line operatlon.
Nor~al operatlon of the statlons is, however, posslbly only lf both the NA and N~ systems and both converters are intact. Should one sys~em or converter fall, the operation has to shift lnto emergency ~ode. For that reason both stations requlre a ~onltoring device which ~onitors ths co~rect operatlon in bo~h stations.
Said ~onlto~ing devics 15 depi~ted in Fig. 11 and wlll initially be expla~ned ln i~s operatlon 1~ converter A. Said moni~oring a~d ~he al~ernatlng transition between normal ~: 25 operation and emergency opera~lon given malfunctlons should also, as far as poss~ble, be opera~ive without re~ote control ~ignals. Rather, based upon a further funda~ental concep~ of this i~vention, the r~quired information regarding interrupted : or resu~ed normal operation in the other statlon should be recognlzed fro~ the effects of sald o~eratlon ln t~e former tatlon. It would therefore be advan~ageous to generate clearly recogni2ab~e slgnals that the trans~tlon ln ~he other station ~roceed in a eertain fashion. ~hlle these ~easures for the transltlon from and lnto normal operatlon will be described ~L~3315~

later, we wlll first explain the design of the monitoring arrange~ent based on Figures 11 through 14.
In these flgures the design for the monitoring arrangement o one statloD only i5 shown. The other s~ation contalns an identlcal arrangement and the signals Fde generated in such an arrange~ent are, lf necessary, to be supplemented by the identifying letter of the statlon in which they are yenerated g ' de~' ~dfA ~~~ in station ~.
The advantage of thls monltoring device is that lt does not require remote control signal lines. The start or the end of a malfunct1on in the other statlon is recognized on the d.c.
current ~ide lnpu~ of the ~irst station at the earliest possible mo~ent. If for safety reasons slgnal llnes connect the two statlons, then the signals transaltted th~reby do not normally dlrectly lnitiate measures to ~hut down or start up the flrst station. Rather, this mode of operation ls in the last analysis initlated by the HVDC transmission line currents (e.g., ln station A by measured values l~A and/or l~LA) and the HVDC transmlsslon line voltages ~e.g., Ud~ or UdLA) and the loglcal comblnatlon of the measure~ent variables (~articularly ~he logical comblnation of the ~easured values for amplitude and, if necessary, phase locatlon of the first station's a.c. voltage ~yste~).
The monitorlng devlce is moreover also in the posit~on to notify the sound or ~alfunc~ioning sta~us o~ lts own station and to store the relevant no~lfication.
The system voltage amplltude U~ (in case of s~ation A ~hus voltage UA) ls monltored using a limlt value warnlng devlc~
: 711 to determine whether the preset ~lnlmum value required for eroper operation has been exceeded; the output signal G711 =
thus shows tha~ the a.c. voltage system is intac~).
~ he ~onitoring signal G711 can be u~ed, ~n ~artlcular.
to set the output slgnal Q o~ a malfun¢tion me~ory 700 to the Q
= 1 slg~al corresponding to proper normal operation. The 33~99 memor~ ls designed, for example, as a 1ipflop whose settlny input is set by an OR gate 701 using a releasing slgnal Ff which releases normal operation. Sal~ release ~lgnal is lnitlally generated when endlng the ~al:Eunc~ion of its own system by havlng the resumption of the l~it Yalue Ugrenz impact a tl~e clrcuit 713 via a delay clrcult 712 (ti~e constant T~y~ which thereupon generate~ a pulse (~internal release pul~ea Ffe) f a preset length ~Eollo~lng the preset delay deflned by the delay time T~yn~ 'rhe delay time T~yn takes lnto account that upon resump~ion of the flrst system voltage a certaln tlme ls requlred until lts ow~ reference voltage generator can generate the ~ystem-synchronous reference voltage regylred for normal operation of its own convert2r from the re~urnlng system voltage.
If, however, durlng a system ~alfunctlon, the system's own voltage drops below the limlt value, then from G7~ 0~ a pulse F~e (~lnternal malfunction ~arning pulse~) ls generated uslng asse~bly 74 whlch as lts key element contains a fur~her ti~e clrcult 741, ~hlch ~assing ~hrough ~n OR gate 702 actlvates the reset input of the ~al:function memory 700 wlth the ~orrespondlng malfunction signal Fd = 1. The OR ~ate 701 and 702 ~an also ecelve other ~ultable ~alfunction or release notiflcatlons u~ing further lnpu~s wh~reby other crlt~cal quantltl~s of lts own a.c. voltage sy~tem (e.g., imper~is~lble changes of frequency or phase) are monltored. The li~lt value warnlng devlce 711 and the other monltoring circults to detect : and monitor lts own a.c. voltage can be accommodated thereby artl~ularly in the reference voltage generator utll^izing the slgnals generated therein anyway. For e~ample, the proper operation of its own converter can thereby be monitored or, using a start command, the sys~em can be brought on llne from a standstlll.
The signal status Q = 1 ~et by the release slg~al ~
indicates no~ only the undl~turbed s~atus of its own stat~on, ~33~99 but also of the entire system. Correspondingly, the signal status Q = 1 lndicates a malfunctlon and is set by the ~malfunctlon signal~ (Idefective signal~) ~d~
For that purpose ~he output of components 71 and 74 which monltor their own statlon are loglcally combined with an ; ~external rel~ase pulse~ Ff~ or an ~external malfunctlon warnlng ~ul~e~ Fdf using the OR gates 701 and 702 to the ~release sl~nal^ ~f or to the ~malfunctlon signal~ Pd.
These impulses Fff and Fdf are generated by the component 73 or 72 integral to their own station and notify by monltoring ~he electrlcal varia~les at the d.c. voltage connections of their own station that the other ~external~ station has respectlvely gone lnto normal operation or a malfunctlon-reflectlng emergency o~eration. Insofar as lts own sy~tem i5 operating properly, the pul~e~ Fff and Fdf thus inltiate the suitable operation of their own station depending : on the functlonlng or malfunctloning status of ~he other statlon.
An external ~alfunction, for exa~ple, always exists lf ln 20 the external station a short circuit of ~he syste~ or of the conver~er occurs and t~herefore the d.c. current passes throu~h ~hls ~hort clrcuit as fault ~urrent. 5aid fault current ls notlcable ln its own station afte~ a delay reflectlng the travel time of the HVDC transmission line in that the d.c.
cu~rrent id deviates substantlally from the current set value defined for normal opera~ion; the deviation id* - 1d exceeds critical values. Thls also occurs if given long-term malfunctions the llne is dead~and therefore th~ current at the d.c. voltage side connections of the station ln question no : 30 longer reacts to the control.
For tha~ rea~on, in co~ponent 72 detecting the external malfunctlon the design incorporates a ~ubt~ac~ion clrcult 721 to generate the difference i*d ~ ~d ln lts own sta~ion as well as a connected rectlfier 722. A connected bandpass filter ~ 31 -~3~

sees to it that short-term power deviations, particularly those arlslng during the commutation ti~es o~ its own converter, are suppressed in the same way as constant or ~elativ~ly long-term deviations. Sald lony-term devlations arise, for e%a~ple, when startlng up to normal operatlon before the HVDC trans~isslon llne can transait the co~plete d.c. ~urrent l~d follow1ng a ~alfunction. A ~uddenly appearing lncrease or drop of the d.c.
current extending for ~everal mllllseconds 15 noticed at the output of fllter 723 and registered by a limit value warning device 734 . Since sald malfunctlon also arlsss if the own statlon ltself ls ~alfunct~onlng, the output signal of ~he llmit value warnlng device 724 is only released to an AND gate 725 if ~for example, using the ll~it value warning devlce 711) the nor~al status of lts own station has been reported.
If, on the other hand, the external statlon has resumed normal operation after ellminating an external malfunction, then this becomes noticable ln the station in questlon after a delay due to the travel ~ime of th~ HVDC ~ransmlssion l~ne by ~eans of a sudden change of voltage and/or current at the d.c.
~ 20 current connectlons. Therefore, for eacam~le, the input d.c.
`: voltage U~ or Ud~ ~for example Ud = UdA, UdL - UdLA
for ~ta~lon A~ is differe~tiated uslng a differentlation circuit 730 ln order to gene~ate subsequently an exter~al release pulse in a li~it value warning clrcult 731 as soon as the voltage chang~ has exceed~d a ~ertaln ll~it value. The output slgnal of this limit value warnin~ circuit 731 can, in ~ tur~, be co~nected to an aND gate 732 if assembly 71 reports a : malfunctlon of l~s own sy~te~.
In ~his connectlon i~ can be advan~ag~ous ~o generate the requi-ed external release pulse Ff also by a correspondlng monltorlng o~ the current tele~ent 7339 734, 735).
A li~lt value warning clrcult 731 can be designed ln accordance with Flgu~e 12 to lncrease the rellabill~y for the d~eotlon of resumed operation ln the external sta~ion. The ~233~

changes ln the elec~rical quantities ~e.g., UdB) brought about by a resumption of normal operation by the external station (ln the example of Flg. 12: station A~ at the d.c.
voltage connect~ons of the sta~ion ln quç~stloD (station B) indicate a typlcal curve functlon fro~ which pulse ~ff ls derlved.
Figure 12 assumes that the line has become deactlvated during a ~alfunction and converter lA thus blocked or has been regulated to zero d.c. voltage ~A = 90- In order to recognize in station B that station A started nor~al operatlon , the commence~ent of rectifier operatlon is introduced by a :: ~ulsating shift of control angle ~A at time ~1 At time ~2 a smooth runnlng up of the control angle to an operatlng value near to ~ull-open control proceed~. Thls leads to the deplc~ed pattern of UdA and ldA wlth t2), in particular belng preset so that i~a ~ 0 can be maintained.
~: With a delay reflecting the llne travel tlme, a typlcal voltage pattern UdB appears in sta~ion B whose differential in the statlon B monl~orlng dev~ce ls monitored for ~ossible ~` 20 exceeding of either a positive or a negative limit value. The lt v~lue warning devices 731.1 and 731.2 deslgned for that purpose therefore set a fllpflop 731.3 for a tl~e perlod ~T~
whlch is reset from the start of the malfunction tmalfunction signal FdB. Clrcuit 731.4 monltors the tlme perlod aTH or its significance, for example, by entering the output signal of - the flipflop moni~or in a shift ~egister whlch generates an external release l~pulse Fff If the output slgnal ~D ~hows th~ ty~lcal stored perlod ~TH. 5tation B after a preset lag :~ time then also resumes its ~ormal operation by a smooth runn1ng up of the control angle aB at time t'2.
Si~llarly, fur~her monitoring arrangements and ~ogical ;~ combinations of the respective release slgnals or ~alfunetlon ~lgnals can be designed in order to clearly handle any speclal operatlng cases. The deslgn and installa~ion of ~uch loglc : `

~33~
circuits can be additionally lnstalled at any time by ~hose skilled in the art using circuitry such as depl~ted in Fig. 11 as sovn as it is detect~d ln the course of investigating the possible operatlng statuses and operating ~alfunctlons any conceivable a~blgulties in the generatlon of the external release signal, the external malfunct~on signal, the lnternal release sig~als, and ~he lnternal mal~unction slgnal.
The particularly simple ~onltoring ~devlce shown in Fig. 11 can be supplemented or expanded by further mon~toring elements which permit a ~ore precise recognitlon of the operatlng status of ~he external ~tation, with the lnformation obtained fro~ the HVDC trans~i~sion llne connections regarding the operatlng quanti~ies of the external station also belng usable to control the for~er station ln nor~al or emergency operation.
Thl~ ls demonstrated in Flg. 13 for statlo~ A using a HVDC
transmisclon line whose converter lA and lB are respectlvely : connected to the HVDC transmission line using converter : reactance coils and filterlng circuits in accordance with Fig.
1. The HVDC transmlsslon llne voltage itself is depicted in ~20 the replacement circuit as the sequential connec~ion of serles :~inductances Ll through LN and parallel capacltances CO through CN.
In statlon ~ th~ automatlc control device can be recognized by the summatlon polnt 42A and linearization device 40A, which aside from the outpuS signal of the control variable controller 41~' to which can be connected via a selector swltch 61A' the faul~ indicatlon voltage ~HVDC transmisslon line vol~age UdLA or a model value UdB for the fault lndlc~tlon voltage to functlon as the automatlc control voltage UdAv.
~: 30 The computlng Gircult to gen~rate the model value UdB is lnstalled here ln the monltorlng and programming clrcul~ry 62A' as an addltlonal computlng component 43A~ whlch ln part also has a ~onitorlng func~i~n, as wlll be expialned in Fig. 14. At the same tl~e it supplies also a mod~l value ld~ for th~ HVDC

33~gg transmlssion line current idB of the external station which serves as a replacement actual value for the control varlable controller 41A'. The ~orresponding set value i~dB 1~
supplled by the run-up funct~on genera~or 66A whlch optionally (selector swltch 60A) is connected either to the output signal of a superi~posed controller 68A (e.g., for ampll~ude VA of the a.c. voltage system NA) designed for e~ergency operation, or the output signal of a superlmposed ~ontroller 51 deslgned for normal operation. For controller 51, in partl~ular, the same automatic control can be designed as shown ln Fig. 7.
Computer component 43A~ simulates the ~tructure of the HVDC tran~lssion llne as shown, for example, in the ~anner of Fig. 14. For that purpose the actual value of the converter output d.c. current ldA ls obtained at the 5RA bloc~ and supplied to the HVDC transmission llne simulatlng clrcult. The fllter circuits of both statlons are slmulated by these circuits FA and FB.
Thereby model values iFA, UdLA, an CN dLB
the electrlcal values lfA, UdLA, and UdLB of the HVDC
transmisslon llne are generated using the designa~lons of Flgures 1 and 13. Only the converter reactance ~oil LB has to be sl~ulated a~ the integrator ~rom the fllter clrcult of statlon B whi~h supplies, based on UdLB and model value UdB, ~he ~odel current idB- To genera~e UdB, flrst using the subtractlon point 75 one can subtract the ohmlc voltage drop rH . idA (corresponding to the reslstance rH f the HVDC transmisslon llne) fro~ the model value UdLA. In additlon, however, one has to con~lder that the e~ternal converter controls its HVDC trans~lssion line voltage UdB by drawing current fro~ the HVDC transmission llne. Sald current ldB is, however, determlned by the current idA lmpres~ed before the HVDC transmlssion llne ~ravel tlme.
Therefore the control dlfferential id~ ~ idB can be eli~lnated by a reset controller 76 whose output s~gnal - 3~ -VPA &4 P 3393 ~2~ 9 corrects the UdB and eventually ensures that the model current i~B in nor~al operatlon corresponds to the actual current ~d~ f the foreign station.
A fallure of the foreign converter can be as easily recognlzed by the differential UdL~ - Ud~A or the control dlfferential ldA ~ ldB at the comparlson circult 721 of the ~onitoring device of Flgure 11. In sald ~alfunction the d.c.
voltage UdB namely is short-clrcuited and the HVDC
~ransmission line discharges itself through the short circuit.
Given an lnltlally unchanged con~rol angle ~A in the station ~ ln questlon, the current ldA therefore rises following a lag ; deter~lned by the HVDC transmlssion llne travel time. As the s~ulation ln ~ig. 14 has, howeveri not yet taken said short circult into account, the differentlal ld~ - idB ~eaches high values.
, If ln the ~odel clrcuit of Fig. 14 following sald :. malfunction warning, the voltage UdB is now short-clrcuited by selector swltch 77, then the malfunction situatlon of the short-circuited external station (inverter fallure~ ls also slmulated. In thls conditlon the model clrcuit is in a : positlon to recognize from a renewed sudden ~hange of idA ~
idB when the ~ternal converter re~umes its normal operatlon ?nd the HVDC trannsmission line i~presses a voltage UdB not equal to zero. The sele~tor sw~tch 77 is then reposltioned and once agaln the resumed normal operatlon is sl~ulated.
In a ~odel circult ln Plg. 14 one has therefore the f1rst ~ form of a~sys~em observer~ which deter~ines slmulated values::~ for ~he electrical quantItles cf the external station ~ exclusively fro~ the electrlcal guantitles avallable ln lts own :~ 30 statlon. Remote slgnal lines are also not requlred ln this : arrangement, and the information regardlng the operatlng status of the external sta~ion is avallable following the shor~est possible delay, ~amely the travel time of the HVDC transmlsslon : llne it~elf.

~33:3 99 Naturally this prlnclple, whereby the fore~gn statlon by si~ulatin~ ~he HVDC ~ransmlssion llne and several circultry components of the external station ls monltored in the statlon in question, can be further improved and modlfied lf requlred.
The advantage ln this connectlon ls that wlth i~8 and ~dB
model values are availabe for the electrlcal quaDtities of the other station, both for normal as well as malfunctloning operatlon.
Therefore, as already discussed, the ~odel current ld8~
in particular, can be used as the substitute actual value for ~he current controller 41A' ln ~19. 13, which thereby permits control of the current flowing through the 3hort-circuited external station ln the event of a ~alfunction.
Thls prlnciple of system ~on~torlng also permlts recognltion of the end of the ~alfunctlon ln a malfunctlon situatlon and, dependlng upon the ~ype of malfunctlon, transltion lnto normal operation with hlgh power transmlsslon withln a few ~illlseconds (e.g., lO ms approxlmately).

Tran~ltlon to Normal Ope~ation. Thyristor Selectlo~ and Sys~e~-S~nchronou Fir~nq In the normal operation des~ri~ed above the HVDC
;~: transmlssi~n llne can alBo be u~ed due to its capabillty of ~: rapld control of power transmlssion to da~en balanciny processes, to positlon the systems for reactlve load, and for other t~sks. For that reason lt is advantageous for users of such systems that following failure of the HVDC transmisslon llne normal operation resumes as 300n as ~ossible.
In this context varlous types of ~alfunc~lons must be differentiated.
If in the first station operating as th~ rectlf~er, the NA
system breaks do~n or a converter malfunctlons, then a short cir~uit of the d.c. volta~e Ud~ arises. If ~he H~DC
~: transmission llne has fully dlscharged dur1ng the flr3 ~;~33~9~
station's breakdown via a short circuit line or, for example, even via the converter of the second station~ and if the second station is swltched off to avoid an energy flow reversal, without, howev2r, tAere belng a malfunction e$ther ln the a.c.
voltage system o.r ln the converter in the second statlon (~functioning~ station B), then follow~g the end of the malfunctlon ~he ta~k is ~o run up the currentless HVDC
transmlss~on line from the no~ arecuperated~ statlon A when the system voltage UAreturns.
The same task ls also at hand if after init~al lnstallatlon or after maintenance work the HV trans~lssion llne is placed lnto operation.
The lnventlon also deals with the case in whlch following a rectifler ~alfunctlon the HVDC transmission line has not been fully dlscharged. For example, the rectlfier operation could have been lnterrupted only for a short period or the converter lA could have been br~dged by a bypass ctrcult without converter lB being shut down. In ~his bypass operation the : ~VDC trans~lssion line serves only as a react~ve load for the NB syste~ and remalns charged. The c~rrent flowing through th~
bypass line functions as reactive current for the functlonlng statlon.
Such bypass operation has the advantage that the HVDC
transmisslon line does not have to be run up from a fully dlscharged level, but the current rather has to be ralsed only to the level designed for normal energy trans~ission, thereby reducing the startup time. In addi~lon, even durlng the ~alfunetion, the HVDC trans~lsslon line can be used to stabilize the system in the functtonlng station.
The bypass circuit can thereby be closed over lts own bypass swltch (swltch 30A in Fig. 13) of the statlon A;
preferrably, however, the bypass circult ls routed via thyrlstors lying in serles of converter lA (e.g., thyristors R~', R ', R~ , R of Flg. 1~ In both lnstances, 3.233~

though U = O, the bypass circult results in idA ~ ~ UdLA
UdB ~
In this case, thus, one ~ust take i.nto account that the functloning sta~lon B is current~conducti.ng and tha~ the lncreaslng HVDC transmission current arislng during resumption of normal operatlon ~ust not lead to any inverter fallure in that station.
In station B 2 drop of the a.c. voltage UB results in a sudden rlse of ~he current flowlng into system NB, with th~
current co~mutation exceeding the deci6ive ~axlmum extinctlon angle (protectlve angle) for the lnverter step li~it. The inverter ~herefore fails and short-clrcuits the voltage Ud~.
Here as well the design can foresee shutdown of the converter lh of the functloning station based upon a malfunction warnlng 15 30 that the HVDC transmission line dlscharges via the short-circulted station B.
Here as wellO it ls advantageous to contlnue operatinq statlon A and recharge the HVDC transmlssion line wlth station B being short-clrcuited by a bypass circuit. For this bypass 2Q circuit al50 a bypass switch ~BOB per Figure 13~ or speclfic thyri ~ors preselected by programm~ng and arranged in serles in converter lB can be used, or dlrectly the ~hyristors o~
convQrter lB whlch reflecting thelr phase po~ltion of the a.c.
~ voltage sy~tem NB ~inverter beat~) happen to conduct current :~ 25 : dur~ng the Malfunction and lnitiate the fallure. Due to the already descrlbed advan~a~es of bypass operatlon a ~ypass clrcult ln station B it~elf could be deslred, even lf~
depending upon the type of ~alfunction, no lnverter failure arlses necessarlly or the operators permit a punctual shutdowm of converter 1~.
Partlcularly ln ~he event tha~ the bypass ci~cult o station B ls routed over the thyristors whlch during the failure happen to be curre~t-conductlng, the thyristorx wlth which ~he syste~-synchronlzed normal operation ls to be resumed :

~ 2 ~3 ~ ~

have to be dealt wlth depending upon the system beat on the one hand, l.e., o~ the phase lenqth of the retur~ing a.c. voltage UB, and dependlng upon the thyristors already conducting current ln bypass operation, on ~he other hand.
The required phase pos~t~on for the system-synchronous startup of t~e a.c. voltage re~urning to the recuperated ~tation can, however, lnitially only be lnco~pletely detected by the reference voltage generator because thls a.c. voltage usually ha3 ~uperlmposed on it a combination of back-10 electro~otive-force and harmonics wh1ch only decline gradually. The lnvention per~lts aynchronlzation errors up to 30 degrees and permit6 resumption of normal operation in the ; recuperating statlon as early as a few mllliseconds afterwa~ds in such a faxhlon that the functioning statlon can recognlze thls operatlng condltlon rapldly and for its part respond by resu~ing normal operatlon as well.
~ or resu~ption of nor~al operatlon in accordance with this lnvention lt ls important that the recuperated station sends a ignal over the HVDC transmlssi~n line ~hen ~he malfunctlon 15 :~: 20 corrected that is un~istakably recognizable in the functlonlng : statlon. Preferrably the recup~rated station lmpres~e~ a voltag~ ~ulse into the HVDC tran~mi~sion llne. Slnce, however, the impressing of a volt~ge pulse is particularly difficult after a ~alfunction in the lnverter, we desire to initlally ; 25 describe the lmpresslng of t~e voltage pulse in station B based upon Figures 15 through ~2, whereby ~he requlred mechanlsm in ~ station A wlll be seen to be~erely a slmpllficatlon of the : ~echanlsm already d~scrlbed for statlon B.
In the left-hand eortion of ~ig. 15 the already known configu.ration of regulation and co~trol ~echanism 4B , the ~onitoring me~hanls~ 64B and the memory clrcult 63B ase ~hown.
: As ~his f igure rQfers to a converter lB co~prlsiny conponent ~ current converters ln accordance wlth Pigure 1, the de~ign :~ incorporates two run-up functlon generators ~6B~ and 66B~ ln .:

~ 233~99 the run-up function generator 66B already famlliar from the 4B
uni~ in Flgure 8. These two run-up function generators can be started at different times ~fir~ng pulse release signals QIZ~
and can be connected to the auto~atic co~trol device 42B
following a time lag which can be ~et us:lng a delay circult ~S .
The ~onltoring device 64H is only sy~bolically depicted by the ~alfunction memory 700 and the gates 701 and 725, whereby gate 701 issues the release signal FfB ts the ma}functio~
~e~ory even in the event of a star~ pulse ~start^ (e.g., cbtained fro~ the memory circuit 63B) being inputted.
To explaln the circuit, we will start wlth the case in which the ~VDC tra~smlssion line is started up wlth sueh a ~ ~tart command from the de-energlzed condltlon i~ whlch all :~ 15 thyristors will be clamped by the clamplng clrcults. In the left-hand portlon of Fig. 16 said clamped status is depicted.
~: For To the control angles a'B . = ~B~ = 90 are preset corresponding to the zero output vol~age of the converter. The drive u~it STB' then supplies, for example, a system-synchronized 1rlng comma~d ~ correspondlng to ~B = 90 whi~h along with the s~art ~o~mand is pa6sed to a~ ~ND gate GUl lgure 15), yeneratlng only at tlme Tol an output slgnal FSo. T~e fIring command R ' i6 selected fro~ the flrlng : com~and sequence S' ln acco~dance ~lth the automatic ~ : 25 ~rogrammed operatlon mode specifled for this case because the : ~yste~-synchronlzed firing cycle of converter lA' is to begin with~thyrlstors R~' and S
The slgnal FSo is passed through an OR gate GOl as a synchronous s~artup release slgnal FS on ~he dyna~ put of a dy~a~i~ flipflop IZ. The~e it generates the flrlng ~ul~e release signal QIZ= 1, whereby the run-up func~ion generator is t~lggered lf there 15 no syste~ ~alfunctlon ~Q~ = 0).
Figure 16 also shows the flrlng commands ~ through : S a' generated by the drive unit STB' for the component : - 41 -~33~L99 current converter lB' a5 well as the com~ands R~a~ through S ~ generated by STBA for the component current converter lB~, whereby ~o sl~plify the process it ls assumed that ~B' =
aB~. These flring comma~ds generated by the drive unlts are, however, inhiblted before time Tol and thus shown only by broken llnes.
Thls inhlbitlng takes place ln the cla~ping circuits BS~, BS~ triggered by QIZ through AND gate GU2 as indicated ln . Figure 17. By the colncidence of the star~ slgnal and the firlng command R~ at AND gate GUl flnally QIZ = 1 1~ ~et and the lnhiblting of the firing com~ands removed so that ~ow the flring co~ands can be passed through the correspondlng ampllflers YR~ ~hrough VS , thereby flring the thyrlstors ~+ and S ln the respectlve component current converters.
; 15 These thyris~or firings proceed reliably since the slgnal FSo at the sa~e tl~e generates an l~pulse FZo via a pulse ; : generator IFo whlch via OR gates GO3R~ and ~03S
regulates ~he ~pliflers VR+ and VS~. Thls means that the firing pulse Ri' in contrast to the flrlng command R~' ;~ 20 lssued ~y the drive unit i~ extended by the crosshatched su~face ~arked ln ~lg:. 16, while the crosshatched su~faces for ~: ~ the f iri~g pul~es S ~', R~a and S~~e show the pulses FZo coupled over GO3R~ and GO3S .
If a control angle ~<90 i~ prese~ for each relevant component current converter, then thyrlstors R+ and S
fire and pull the voltage to the HVDC transmlsslon llne poles 2 a~d 3 ~station A: UA > O, statlon B: U~ c o ln accordance wlth th8 prese~ symbols~ whlch thereupon by a change he control angle can be ~un up ~o the operatlng value ~: 30 designed for nor~al operatlon, which in the example o~ ~g. 16 represents lncrea~lns the conl;rol angle to ~B = 150 ln statloQ B
~; Thls even applles if the preset control angl~ OL ln : csntrast to the ~eference voltage of the reference voltage ~3~3~9~

generator due to an angle error ~A < 30 of ~he r~ference voltage generator ln con~rast to the actual phase posltlon of the system corresponds to an acutal flrlng angle of a ~aximum of 120 degrees. Only at ~his limit angle does the coup~e~
syste~ phase voltage UR~ whlch ls to be coupled by R~,S
become negatlve and the coupled firing pulsQs R~,S have no impact.
This does not change ~VQn if at time T~o the thyristor co~bination R+,R and/or S~,S are already current-conductlng, as ls foreseen, for example, for a bypass opera~lon during a system malfunc~ion. Slnce even then R+
~ and S~ will lead to the fact that the current co~mutates on ; the ~hyrlstor combinatlon ~,S~.
If the bypass thyristor comblnatlon also lncludes the thyrlstors T~,T, then the fl~ing pulse leads to a current flowing via R+,T , and the coupled pulse S remains simllarly ineffec~lve as does the next T pulse whlch normally follows ln ~he flring cycle on the R~ ~ulse at time Tol ln the flring pu152 release signal QIZ = 1. Only the : 20 subsequent flrlng pulse in the firlng cycle, l.e., S~ ~111 then brlng about the normal curre~t conductance over S+,T . :The voltage, whlch until then was ~onnected to the : HVDC trans~lssion line by the co~mence~e~t of the sys~em-synchronous nor~al operation, thus presents a voltage pulse which corresponds to a temporary rectifier operation wlth UdA>~O or else UdB<<O.
The circult portion described thus far can therefore not : only be used for ~tartup of the de-energized HVDC transmission line using the tart co~mand, but al80 for the com~encement of system-synchronl~ed converter operation ln other opera~ing conditions. If, for e~ample, ~ollowlng an in~ernal aalfunctlon the dynamic fl~pflop IZ ~s set to the output slgnal QIZ = O
over the re~e~ input due to a corres~onding outpu~ s1g~al : QB = 1 of the malfunctlon me~ory 700, then the converter ls , . , ~ 3~ ~ ~9 inhlbited. If given a longer malfunction the HVDC transmlssion llne voltage has dlssipated, then a relea~e signa~ Ffe can now call up the signal status QB - 0 so that the dr~a~ic flipflop IZ' ~an ~tar~ up agaln wlth a startup command and a consequently der~ved pulse FSo = l.
:~ ~ith an lnternal ~alfunction lt can, however, also be deslgned so that a by~ass circuit closes when the current idB
; in station B dlsspipates, in which, for example, the thyristors ; T~ and T are fired.
: This thyrlstor flrlng takes place lf ~he signal 52B set by the internal ~alfunctlon is coupled to me~ories SBT and SBT+ which in this byp~ss thyrlstor comblna~ion are set to 1, using a bypa3s supple~ental pulse BZ gen~rated by a pulse shaper IF uslng the AND gates GU3T and GU3T+ through a firlng pulse VT and VT~. If now the bypass operation ls ended by a start csmmand, then when the firlng co~mand R+~' appears, the gate GUl again lssues a signal FSo ~hich clears the flring conmand inhlblt by the ~lipflop IZ in the clamplng . clrcult BS of Figure 17 and ~oreoYer clrcuits t~e correspondlng flring pulse~ to ~R+ and VS using the pulse genera~Qr ~;~ 20 IFo.
ig. 18 sho~s the pattern of the thyrlstor currents iR~
: through iT , whereby lnitially during bypass ope~atlon lT~
: : and lT flow untll due to the descrlbed normal flrlng pulse : sequence Rl,T ,S~,R --- lnltiated by FZo, the above-described current commutatlons dlscharge. Using as an ~: example a non-symmetrlcal system ~i~h strong har~onic signals :~ : ~hen the system resumes operation, whose pattern is shown in detail in Fig. 22. Flg. l9 dlsplays the resultlng thyrlstor voltages UT~ and US~ for thyrlstors T~,S+ and the resultlng HVDC transmisslon llne voltages and current con~uctance periods whlch are indicated by arrows. The lmpressed voltage pulse ls ~hown crossha~ched and the reference `~:
:

- 4 ~ -.. . .

~ ~ 33~ 9~

polarity change is negatlve corresponding to the transient rec~lfier operatlon o~ s~ation B.
Should the transient voltage pulse be ~ore pronounced, it can easily be corrected by the control a~gle being shifted to .rectlfler operatlon ~n the respec~lve station, with this shift belng prlor ~o the runnlng up of the control angle of Fig. 16 or being superlmposed on the start of the running up.
~; The same applies to the regulatlon of the second component current conver~er lB~, whereby sim~ly th~ use o~ a lag clrcuit ZS which through QB regulates ~he correspondlng dynamic : flipflop I2 at a certain time lag~ assures that ths startup of the second component current converter takes pla~e followlng a cer~aln tlme lag. The deslgn of the other components marked by a double llne ls identical wlth the device descrlbed for the component current converter lB' 50 that ln the following `: discussion both component current conver~ers will be dealt with ,: and ~xplained largely as one slngle converte Wlth thl~ varlan~ lt ls assu~ed for transitlon from bypass ~:~ operatlon ~o ~ormal operation that the HVDC transmlsslon curr:en~ ld had dissipated before closlng the bypass circuit and tha~ the bypass circult was clo~ed ~y a preprogramned selectlon of thylistors connected ln seri~s. The memories SBT+ and SBT can thereby be 1nclud~d ln the ~e~ory circult.
In the case: of inverter failures, the serles thyr~ctors ::are ~lso current-conducting; ~owever, the thyrl~tors whlch form the short clrcult route depend~ on th~ ins~antaneous phase position of ~he systsm durlng the fallure. In order to rapldly transfer to bypass operatlon, these thyristors can be retained as long as their combina~lon is established and then ln accordance wlth thls co~binatlon those thyrl~tors are selected : : among the converter thyrlstors ~hat are suitable for re6tart of normal operatlon.

,~ :

3~

~emories SBR+ through SBR depicted i~ Fig. 17 serve for that pur~ose. They are also regulat~ed by the firi~g commands 5~ and deter~ine at each phase posl~ion of the system, thus at each moment wlthin the cycle of the control so~mand S, - 5 over whlch converter thyristors at that point an inverter fallure could occur. Thls ~ould at least be the respectlve thyristor group which due to the exceedi~g of the ~a~imum extinction angle, ln contrast to the nor~al co~mutation ~: sequenceO stlll earrles ~urrent and the thyristor groups connected ln series thereto.
Flgure 20 assu~es that the ln~ernal ~alfunction ~arning pulse trlggered by the failure occurs before the dri~e unit issues the next flrlng com~and S+ (tlme Tl,l so that lastly thyrlstor T was regulated wlth thyrlstor R~ conductlng.
Therefore a memory SBR whlch can be reset by S+~ is set to ~ the value 1 using the command Ta and lndlcates that thyristor : R+ is involved in the inverter short circult. Whlle by QIZ =
0 at ~U2 the 'regular~ command S~, generated by the drive ~: unit ls lnhibited, at GU3R the bypass supplemental pul~e, shown ~a~ked by crosshatching ln Flgure 20, ls given to thyri~tor ~ vla ~emory SBR and the co~aand BZ. This th~rlstor R , along ~ith thyristor R~, whi~h is not deactivated, now brlngs about a safe closing of bypa~s route R+, R .
If the failure ~arises after another flrlng command, then emories SBR~ and gates GU3R+--- brlng about the relevant selectlon,~to~ag~ and reyulation of ~he possible bypass thyris~or combination. The deslgn can, however, also foresee, partlcularl~ ln sta~ion A, not to mak~ the bypass ~hyristor selectlon so dependent upon the operation, but routed always to establlsh a prepr~grammed co~binatlon (lf necessary several or all converter thyristor groups in serles) i~ the ; ~ memory circuit and to couple them ln the event of a ~hort ~ circult by regulatlng gates GU3 or G02.
~, ~ 33 ~ g To generate the deslred voltage puls~e when proceedlng accordlng to Flgure 16, it ls, however, n~ecessary to ~elect the required inltial cycle ln accordance with the ereset and operation-dependent bypas~ thyristor co~blnation to commence ~he system-synchronous operation. For bypass ~hyrlstors R~,R the syste~-sy~chronous firlng should now s~ar~ by a commu~atlon of thyrlstor S~ or S whlch i~s the function of the selector clrcuit AS of Figur~ 15.
.Therefore, in accordance with Figure 21, a pulse (e.g., pulse S~,S 15 always generated ln the sequence SZ from the : firing pulses obtained follo~ing ~he i~terlock &02 and lssued regardless of whe~her the system is ln normal or bypass operatian whlch is allocated (OR gate ~04R, G04S, G04T) to two serles thyrlstor with one potentlal thyristor forming a bypass palr. Thls potentlal bypass comblnation is clamped by the AND
gate GU4R etc. until in the event of a malfunctlon uslng the bypass supplement~l pulse BZ the by~ass thyrlstor firlng :~ ~ actually occurs and the bypass co~binatlon ls stored in the ~emories SQR etc. whlch can always be reset at the end of a ~alfunction by the firlng pulse release ~lgnal QIZ.
: While thus the me~ories SBR+ etc. of ~he cla~ping clr:cults always deter~ine ~he possible bypas~ thyr~stors from ~: the firlng command seque~ce of ~e drive unlt and ln case of ~;:malfunctions bring ~bout their firlng, the gates G04 select fro~ the actual firing pulses lssued the possible bypass ~hyrlstors which are only stored in case of a malfunction.
Further OR ga~s GOSS, G05R, GOST unite the f~ring commands (e.g., for bypass co~binatlon ~ , thus the flring co~mand combinatlon S~,S conta1ned ln the firlng sequen~e S~ and required always to deco~mutate the current from one of the selected bypass thyr1stors. These firing commands then, by colncldence with the ~tored comblnation deter~ined by g~tes GU5S, GUSR, ~V5T, le~d ~o a sy~chronized signal FS~ .

, ~33~9 The OR gate GOl then issues as the startup release signal the synchronized signal FS instead of the prevlously dlscussed start signal ~start~ whlch in flipflop IZ generates the firing pulse release slgnal QIX if IZ is release~ af~er the end of the system malfunctlon (QB = 0~.
Thereby the deslred startup o~ the syste~-synchronous normal operation starts by itself after the termlnation of the syste~ ~alfunctlon.
To su~marize, ~lgure 20 thus shows ~he followlng: In normal operation slgnals QR~ through QS always indicate a thyristor which ln case of a co~mutation ~alfunction could be used durlng the next firlng as the bypass thyrlstor and should be flred by the byeas~ supplemental pulse FZo ln order to close the bypass. The system malfunct1on lnduces a drop ln the system voltage UB at time Tl below a preset limlt value which via QIZ = O leads to the inhlbitlng of the normal firing co~mand S~ or Za and to the coupllng of the crosshatched bypass ~; supple~ental pulse to R . Dur~ng the ~al~unctio~, therefore, only the thyristor ~ombinatlon R~,~ ls current-conducting and this thyristor combination 1~ stored in the selector : circuit. After the syste~ voltage resumes (tl~e T2) and the ; : release slgnal occurs ~time T~ etting th~ ~alfunction :~ ~emory to QB = 1, ~emorl~s SQR d SQS, ~T of ~he selector clrcuit choo~e a thyristor to resume normal operation and the selector c~rcuit then generates the startup release signal lf ~: the sys~em-synchronous converter operated by the control angle issues the flring co~and corr~sponding to ~his selected thyrlstor. Tbe normal flrlng cycle is then released by the startup release signal FS co~enclng wlth the firlng of ~his thyrlstor and leadlng to the decom~utation of the current from , one of the bypass thyristors.
::~ The a.c. voltage syste~ starts, as Figure 20 for the phase ~:~ voltage (U~)R a~d as the voltage a~plitude UB ~nd~cates. very lrrQgularly. The ~alfunction memory supplies a slgnal QB = l only after a ce~taln time ~equlred by the re~erence voltage ~LZ33~

genera~or to determine a relatively reliable phase position of the system. A rapldly working reference volta~e generator is described in the Ger~an pat~nt application P 33 46 291.7 and assures that thereby the phased positlon of the syste~ is determined down to less than a 30 degree precision level. A
lag circuit containe~ ln the pulse generator 713 ln Fig. 11 can assure that the sslf-release pulse Ffe ln statlon B and thus the release slgnal and QB = 1 are only generated after the . ~inimal tlme required for synchronization.
Figure 22 depicts the overlaylng of the slne-shaped funda~ental frequency by counter voltage and harmonic oscilla~lons as occurs typi~ally for returnlng connected phase voltages USR, UTS, URT of the a.c. voltage system. S~ shows a hypothetical ~et of firlng pulses assoclated with the zero degree of modulatlon (control a B = 90 ) and the fundamental frequency of the recoverlng system.
T~is se~ S*~ is hypothetical, l.e., cannot be realized ~: since ~he fundamental frequency at time T2 cannot yet be ~` detected. Rather the reference voltage generato~ generates a :~ 20 reference voltage UBByn associate~ with the ~l~llarly hypothetical flrlng pulse sequenc~ S~G,R ~,etc. at the hypothe~l~al ~ontrol angle aB 8 90~ Tbis pulse sequence is ~: aIso only hypothetlcal because ln order to implement the ~ procedure accordlng to this invention, no constant control :~ 25 angle aB = 90 is preset. However, a comparlson of both hypothetical ~ulses S+* and S+G shows that at time T3 a reference voltage UBBy~ can devia~e fro~ the (not detectable) funda~ental frequency by a phase dlfference which can rQach up to 30 degrees. Onlr at ti~e T4 (typically T4 - T3 = B
msec) has the reference voltage generator built up ~o a polnt that the reference voltage U~s~n is practically ~ync~ronous ~: wlth the fundamental frequency of the system (stlll overlaid by :~ har~onlc osclllationsj and the eguation R a * = ~ G
applies~

-- 4g --~L2331~

In order to resu~e normal operatlon as 500n as possible after the sy~tem resumes, the deslgn does not have a provislon to walt t~ll tl~e T4 for the release of the flring pulse.
Rather, lt suffices 1f the synchronizatlon error of the refarence voltage is less than 30 degrees, i.e., ~he flring pulses are released already as early as time T3.
Flgur~ 22 assumes that for the resumption of normal operatlon wl~h the control angle (~B = 90~ (i.e., control . angle at lnitial ~et~lng of O correspondlng to a d.c. voltage set v~lue UdB~ = O the system can begin and run up to a maxl~um value~ a~ wltbin approxi~ately two syste~
sycles which corresponds to the nomlnal value pro~ected for normal operation. Consequently thyristor S~ ls flred at ti~e T3 by the flrlng ~ulse S~ - S~G, whlle at approxlmately : 15 time T4 the actual firing command R ~, in contrast to the hypothetlcal command appertainlng to R G, ~B = 90 ls displaced by 90)/2. At time T5. a~ax ~:~ ls attained.

Transltlon ~o NoLmal Operat-lon; Presettinq the Çon3l~LL~ e In accordance with the previous explanations it is thus possible that after a malfunctlon has end~d, one statlon i~sues a pulse to the HVDC t~ansmlsslon line, partlcularly a voltage puIse, wh~ch can be detected by the other side and lnltiates an lnternal release pulse whereby normal operation ls resu~ed in that s~ation as well. The arrangement shown ln Flg. l5 as :~ hardware, can also be ~artlally realized ln sof~ware, and : incorpora~ed with the me~ory clrcuit 63B. The memory circuit 63B baslcally only has to deter~ine which mode of e~ergency operatlon (e.gO~ ~ypass operation or shutdown of the llnej are :~ ~o be selected for ~he partlcular malfunctlon in ques~lon. The -~ methods d~scrlbed below for the varlous typ~s of e~ryen~y ~; operatlng ~ode~ for resuming nor~al o~eration will ~rovide those skilled ln the art wlth ade~uate lnfor~atlon as to how ., , ~.~33~

the indtvldual slgnals of ~he monitorlng devlce are to be comblned ln the me~ory circuit for each individual case in order to execute the relevant startup progra~ ln co~3unctlon ~ith the design of the statlons pel ~igures 8, 9, 10 and 13.
~ In the followlng discussion, that particular station whose : malfunctlon ~as caused an interruption of normal oeeratlon will always be designated as ~the former station. ~ In case of a rectif ier ~alfunction this would be statlon A and ln case of an inverter ~alfunction it would be statlon B. In thls context, dependlng upon the energy flow d1rectlon ~hosen for normal : operatlon, the de~ign can foresee that the statlon arranged at one end of the HVDC trans~isslon llne once plays the role of sta~ion A and another tl~e the role of station B slnce the task and deslgn of the ~onltorlng devlce and the ~emory circuit both for rectifler as well as for lnverter operation ale largely the ~ same so that both stations are deslgned to be as identical as : ~ possible.
After overcoming the malfunction in the former station, the correspondlng ~onltoring station (deslgnatsd as the former .20 ~tation ~onitoring unit~) generates an lnternal release pulse which in thls former station produ~e~ the release signal (described as the ~l@adlng releass slgnal2) whe.reby the startup 1~ lnltlated.
Thls ~tartup ts then obtained after the HVDC transmission line travel time at the d.~. voltage slde connectlons of the latter (undisturbed) s~ation or of the latter converter, thereby lnitiatin~ ln ~he other monitor~ng device an external release pulse which produces ~he ~derivative~ release slgnal.
The baslc concept for startup is then that when a non-problematic (~recuperatedg) status o~ the prevlou~ly malfunctionlng form2r ~tation is establl~hed, a leadlng release : signal ls generated, lnitlating the resumption of ~y6tem-: syn~hrQnous nor~al operation ln the former ~tation converter.
~ In the lat~er s~atlon, on the other hand, the ef f a~ts of this ., :, ~L~33~1L9~

change at the d.c. voltage s~de connections are converted into a derlvative release signal which ln that s~ation si~llarly lnitiates the system-synchronous nor~al ~peratlon whlch thus begins after the shortest posslble delay upon recuperatlon of the for~er ~tation. Remote signal lines are not required.
This assu~es on the one hand that the electrlcal processes brought a~out ln the former statlon are clearly recognized in the latter s~atlon and, on the other hanld, that ~he processes in both statlons ~ust be coordlnated wlth each other.
Both ~rerequi~ites are met by having the HVDC transmlsslon line in the for~er statlon be i~pressed lnitlally by a voltage surge ~ith the co~trol angle of the former ~tatlon converter belng run up ln accordance ~ith a preprogrammed run-up function ~ ap~roxl~ately to the value pro~ected for normal operation : 15 w~ereby, a~ ~hown ln Flg. 22, the voltage surge ls elther necessarlly generated by the run-up of the control angle or, lf regulred by the sltuatlon, can be generated by a separate and .~ te~porary pulsatlng shift of the control angle. The control ~: angle is then similarly ru~ up ln the latter ~tation (startlng `~ 20 f om an initlal value of approxi~ately zero correspo~dlng to an ;: angle of approximately 90 degree~ 1n accordance with a preprogra~ed run-up functlon to the value pro~ect~d for normal ~ operatlon.
; Both run-up functions are coordina~ed with each other.
Preferrably thelr run-up time would be approxl~a~ely two a.c.
voltage cycles.
~: Duri~g the malfunction the malfunctioning converter of the ~ form~r statlon ls either totally shut down or shQrt-c~rsuited '~ usln~ the bypass ~oute. Thereby during e~ergency operatlon the : 30 model fault ind1ca~ion volt~ge ln the auto~atic co~trol devlce ln ~he control channel of the former sta~ion drlve unit does ~ not have to be swltched on, On the o~her hand, the relevant ;~ control angle at the end o~ the run-up cy~le has to attain the ~; control angle pro~ected value pro~ected for steady state operatlon at the end of the run-up cycle or at least attaln thls steady state value, if posslble wit~lou~ substantlal 3umps. If therefore a s~ooth run~up is generated by a run-up generator at the input for the preset angle, then thls preset angle w~lch genexally i5 generated by a ~uperimposed con~rol quantity controller (e.g., the current con~roller 41A or the extlnction angle controller 41B) would have to be retracted to the extent that now the automatic control model fault . indlcation quantity determines tAe control angle.
The preferred arrangement is therefore that the run-up generator 66R geDerating the run-up function be locate~ -at least ln sta~lon A (Flgo la)- at the entry to the automatlc control voltage of the automatic control devlce ln order to connect using the selector switch 61A the ~odel fault lndication quantity of the output signal of the run-up generator as a substltute value durlng e~ergency operation and the run-up cycle. Only after atta1ning the run-up function final value does the syste~ switch over to the measured value U~LA (or the correseondlng ~odel value UdB)by switchlng the sel~ctor swltch 61A. On~e s~eady state normal operatlon has been resumed, then also the preset angle will be obtained from the current controller 41A as designed fo~ normal operat~on, but wh~ch can be dea~tivated durlng the malfunct~on.
In statlon 8 the model fault indlca~ion quanti~y serves to set the extlnction angle and to maintain the inverter step limit. If during emergency opexation t~e syste~ dlspenses with the automat:ic control wi~h the corresponding auto~at~c control angle ~y~ no danger arises regarding any possible inverter fa11ure, since even ln case:of bypass operatlon the control angle ls still ~ar from the inverter s~ep llmit. Thls al~o applles, lf after a rectlfler ~alfunctlon, station A ~tarts ~o increase the current. If therefore statlon B resu~es inverter : operation af~er a derivatlve release signal, then the ~reset run-up functlon in that statlon generates a controlled model ~33199 substitute value for the model fault inslication guantlty whlch can also r~place the model fault indicatlon quantity ob~alned fro~ the actual inductive d.c. vo~tage drop and lead to a limitation o~ ~he extinctlon angle. Therefore here as well the run-up generator can be arranged at the automatic control angle input of the automatic control device and be s~ltched by the selector swl~ch between the regulated preset model value supplied by the generator and the model fault lndication quantity whose value is computed from the lnductive d.c.
voltage drop lf or ~hen the run-up value has approxlmately been ob~ained.
The two run-up functio~s thus provlde substltute values fo~ the relevant automatlc control quan~itie~. It thus see~s lnitlally necessary that both run-ue functions coinclde, l.e.
are ldentlcal ln terms of thelr curve and plateau level. Such rigid coordinatlon of both run-up functlons is, however, not requlred. For example, the run-up end value of the run-up generator of statlon A can generally be somewhat hlgher and ea~d to a HVDC ~rans~lssion current ~hlch could, wlth regard ~o the pçeset control an~le in the run-up function of statlan ~, ad ~o an extinction angle beyond the inverter step li~it.
The extinctlon angle control of s~ation B ls, however, in a posltlon to retract the con~rol angls ~B in accordance with the inverter step llait.
25 ~ For the various types of emergency operatlon the following oSsible varian~s resulted:
a) Startu~ from a deactlvated llne.
In th~s case, a~ter th~ malfunctlon arise~, both conver~ers are inhlblted whereby the dlscharge of the line, e.g., during a short circuit of the vol~age U~B, ~ould be handled by the ~hort clrcult ln th~ ~tation ltself or also, for e~ample, given a rec~lfier malfunction in statlon A, ~y a forced te~porary flring angle 6hift in the other functionlng station.

.

~L~33~

li~hen the malfunctionlng sys~em goes back on llne, the correnpondlng lnternal release pulse ln the forrue~ monitor$ng station lnitlates the leadlng release signal. The lesuming nor~al operation in the former station initiates the derived release signal in the latter s~atlon, whlch there, too, initiates the startup in accordance wi~h ~he preprogrammed run-up functlon. Both run-up functlons start at an initial value of approsimately zero ~control angle approxlmately 90 degress).
~s a supplement, the design can foresee that the control angle of the for~er statlon ls determined fro~ the run-up ~unctlon and a te~porary additlonal shift ln the direction of the rectifier wide-open control, wherein this addltional shift can elther be set before or superimposed upon the run-up function.
Given a malfunction of the lnverter operatlon ln station B, thls Deans that the already described ne~ative voltage pulse, ~hlch is l~pressed when resuming nor~al operatlon, ls reinforced. This s~mplifles both the generatlon of the derived release pulse as w@ll a~ the start of rectlfi r operation ln station ~. ~ven after a ~al~unc~lon of rectifier opexation in sta~lon A, such a pulse is ~dvantageous since i~ ~eans that : current ldA and voltage UdA rlse rapidly and charge up the ~: HVDC tra~ission llne so that both the run-up of ~he power : 25 transmission to the value pro~ected for steady state normal o~eration can take place ~ore rapidly and the derlved r*lease slg~al of station B can be qenerated ~ore easily.
:~1 Emer~ency operation wlth bypass circult.
In case of longer malfunctlons the operation can proceed u~ing the bypass circult, whereby ln the ~alfunctlon~ng former statlon, when the internal ~alfunction ~arning pulse arlse~.
the system-synchronous flring co~mands of the conver~er ln the former statlon are inhibited and the bypass clrcult closed. As already explal~ed, ~hls bypa~s circuit c~n best be closed by . .

~ ~3~
~L ~ ~

the firing of selected converter thyrlstors ln serles. In the latter statlon using the external ~al~unc~ion warnlng pulse, the control angle ls shifted to such an extent ~o rectifler operation as requlred to feed bypass currlDnt ln~o the HVDC
trans~lssion line to meet the requirement~; of the latter sy~tem.
The transl~ion fro~ this bypass operation to nornmal operation ta~es place when the system of thi6 former station has recupera~d and genera~e ~he leading release signal whlch initia~es resu~ption of system-synchronous converter operation with the control angle runnlng up ~o the proper operatl~g degree of control. Si~ultaneously the bypass clrcuit ls extingulshed which generally no longer requires any special measures to be taken lf no specific bypass switch or bypass thyrlstor, but rather a series of converter thyristors required for nor~al operatlon, were being utilized.
the latter station the derived release slgnal then in~tiates re~umption of normal operation w~ereby the control ~:~ angIe runs up ~o the control angle steady s~ate value in accordance wl~h the run-up f~ctlo~ from th~ co~trol an~le d~ter~lned by the bypass current.
~ If ~he bypass operation resulted fro~ a ~alfunction of :: : station A, ~hen it ls not necessary ~o underta~e an lnternal ; pulse shaplng dlsplacement of the control angle at the start of the run-up cycle, and the system-synchronous firing at the ~: start of the run-up cycle, 1.e., the introduction of nor~al rectlfler operatlon, poses no problems. Rather, in accordance : with Flg. 16, the rectlfier opera~lon can begln wlth the synchronous operation of specific preprogra~m~d thyristors.
Even ~he selection of the bypass thyrlstors 18 preerrably program~ed whereby the by~ass curren~ can be routed not ~ust : over one bypass clrcuit for~ed by thyristors ln s~rles, but :~ rather over several or even all th2 converter thyri~tors. Thls is posslble and under glven circu~stances advantageous lf, for example, sultable ~easures are ta~en in statlon A to ensure . _ 5~ -~ ~33~L9~

that after a malfu~ction ~he returning sys~em NA wlll not l~nedlately dlscharge agaln via ~he bypass clrcult.
Durlng bypass opera~ion wherein the bypass clrcuit ls closed by thyrlstors ln serie~ of the con~erter of station ~, the selectlon of the bypass ~hyristors for the duratlon of the malfunction 15 stored. In accordance wlth this ~tored thyrlstor co~bination one of the thyrt stors is ~elected by the selector circuit as desGribed ln Flg. 15 for resumption o~ the syste~-synchro~ou~ normal operation. In the cycle of the system-sync~ronous flring co~ands this thyri~tor then corresponds to ~he deco~mutation of the current fro~ one of the by~ass ~hyristor~.

Fluctuation Between Normal OPeratlon and Emerqency Operatlon The nor~al operation descrlbed p~rmits, in accordance wlth the auto~atic con~rol prlnctæle, ra~id control of ~he HVDC
trans~is~lon llne wl~hout, however, being abl~ to exclude a ~alfunctlon altog~ther. The principle of programmed run-up :~ wlthout the u~e of lnitlatlng remote control ~lg~al~ permlts transition ln~o nor~al operatlon both given a deac~lvated HVDC
tran~isslon llne a~ we}l as ln cas~ of bypass ~peration wlthin ~: very short star up ti~es. The bypass operatlon principl~
descrlbed b~low thus per~its, glven a ~alfunc~lon of one of the tWQ xta~ions, the operatlon of the HVDC transmissio~ line even during the mal~unct:lon as the reactive load for the a.c.
voltaye sy~tem of the nor~ally func~loning st~tlon and to be r~gula~ed or controlled ln accordance with ~he regulre~ents of the normally operating syste~.
~ach of these three principlss (automatic con~rol, program~ed run-up, and bypass operation~ can, ln con~unction with other operating p~oc~dur~s of a ~VDC transml~sion llne, be advantagssusly used separately or 1~ con~un~tion wl~h a~other one of ~hese princlples. The ~o~blnation of all three principles descrlbed below perDlts rap~d control of the HVDC

~3~

transmlsslon line, both ln normal as well as ln e~ergency o~eration, ln order thereby, for exa~ple, ~o da~p~n sub-synchronous resonance occurances or other dynaolc bala~clng processes in the sy~tem~, to minimize ~he econo~lc effects of a malfunction, particularly of a voltage drop in one of the a.c.
voltage system~ and always to be able to complete the transltlon bet~een normal operatlon, bypass operation an~
: (occaslonal unavoldable) operation wlth deactlva~on of the . HVDC trans~ission line, with~n the shortest possible tl~e.
The principle of bypass operatlon, lncludlng the transltion lnto bypass operation ln case o~ a ~alfunctlon, ls explalned ln furth~r detall based upon the slgnal patterns in Flgures 26 and 27 and the design models of the two stations shown in Flgures g and lO~ Aslde fro~ ~his bypass operation it 15 could be necesary, depending upon the type of malfunct1on, to deactivate the HVDC trans~lssion line during the ~alfunction;
this type of e~ergency operatlon is sho~n ln Figures 24 and 25. To resu~e nor~al operatlon, it could be desilable, as already explained, both fol a HVDC-tran mtssion over long distance as well as for a ~hort coupling ~ha~, for example, the transltion f-o~ bypass operatlon to nor~al o~eratlon be d~signed so that the ~alfunctloning syste~ prl~arlly carries reactive curre~t when the voltage resumes.
Thls transition can be influenced ~ot only by the d.c, current it~elf which has to be run up, but also from the number of 2vailable co~ponent current converters sl~ultaneously : ~artlcipatlng ln the transition. ~hereby the possibility arlses ; of decoupling the deslred grada~ions in ~he d.c. current rlse from the rise of the system reactive current in di~crete steps so that thereby the requlremen~s ~ade of the syste~ and of the ~ur~ent control can be handled separa~ely to a cer~aln sstent in accorda~ce with ~he nu~ber of separately connected co~ponent current conver~ers.

~....

~3~

Thls can be done ln accordance with Figure 15 by havlng the timer circuit æs, whlch releases t~e generatlon of the flring pulse release signal to disable the clamping ~wltch B5 of the compoDent current converter IBR at the reset input of the dynamic flipflop, delay ~he start of ~he system~synchronous converter operatlon against the co~ponent: current converter lB'. The pul~e release of the lndlvldual component current converters selected and dependent upon the combinatlon of . bypass thyrlstors, thus proceeds in steps (with tlme delays), : 10 whereby the up~wing of the system-r~actlve load also proceeds in step5.
In the followlng discus~ion, a run-up functlon ls selected for ~he lncrease of the control angle in general whlch drives the con~rol angle fro~ a value whlch, lf necessary, devlates : 15 sllghtly from 90 degrees ln a llnear fashion tlll it attalns the steady ~tate condition~ Although lnltially it seems essential that the two r~n-up functlons, whlch as regulated : values substitute for the preset o~erat~ng model value of the model fault indlcation voltage when ~ransitloning into normal operàtion, be practlcally identlcal, such tight coo~dinatlon of both run up functlons 15 not in fact required. It ~an ln fact : be advantageous to preset the two ru~-up functlons to run ln opposlte directions lnsofar as, for exa~ple after a malfunctlon : in s~ation B the control angle o~ the rectlfler can be run up lnitially with a larger and later wlth a decllnl~ng slope~ while the inverter control angle inltially ~ounts slowly and later ; mounts more rapidly.
Thereby the functlonlng station is ~o be recharged by : active curren~ as quickly as posslble so that the HVDC
: 30 transmlsslon llne can be recharged quickly. For the lnl~ially :~ m~lfunctionlng statlon whose returning voltage o~ten displays ~: marked switch-on ~eaks, ~he HVDC transmlssion line lnitially functlons a~ a voltage reduclng reactive load whil~ at the sa~e time attainlng rellable com~utatlon.

~ .~
- 5g -' ~L~33~

a) Example with difPeren~ run-up functlons Figure 23 shows ~his run-up variant uslng as an example a system shortclrcult in NB and a bypass operation durlng the ~alfu~c~ion.
At ti~e klo voltage U~ bre~ks down and the llm~t value warnlng output ~lgnal G711B lndlcates a shortfall of the llmlt voltage U~renz and sets the controlling ~alfunction signal QB = O. This initates a commutation lock ln the clamping devlce of converter lB, ~hereby all regular flrlng ~:~ 10 commands of the drlve unit are suppressed whlle si~ultaneously the bypa~s circuit ls fired by a æupplemental bypa~s pulse;
thls condition is sy~bollz~d by a control angle ~B = 90 in accordance wlth the output d.c. voltage UdB = O.
Uslng the bypass clrcult, the charg~ in the HVDC
transmlsslon llne and the fllter of station B is reversed as can be recognized by a current flo~ idR (which often does ~ot even attain the condltlon idB = 3 whose ~xtent is exaggerated ln the depietion of Fig. 23, and the o~illation of voltage U~L~ at the end of the converter reactance coil LB
faclng away fro~ the converter. The voltage Ud~ whlch during nor~al opera~ion, for exa~ple, was held con~tant at a~
operatlny value U*dBO and was ~hort-clrculted du~ing the : current surge takes on a negativs value, if the current ldB
due to the charge reversal becomes zero and the bypa~s thyriators ext~nguish. The control quantlty control 41B (Fig.
~ : 9) alr~ady made llne-~neffective due to the disabling of :~ ~ommutation and whic~ would only di~play lrregular values during the ~alfunction by a defined ~alue ~xB = O.
Slmilarly, swltch 61B can be swltched to the te~porarlly lnactlve run-up functlon generator 66B at the sam~ tlme or after the appearance of the ~alfunction signal Q~ , O fro~ the circuit 43B' wbich co~putes th~ ~odel fault indlcation guantlty fro~ actual value~ ldB and U~.

.-~ . ...

....

~33~l99 The current idA of station A which during normal operatlon is controlled to the operatlng set value l*dAo (nor~al value) slmilarly shows, followlng ~he transmisslon travel tlme, an increase wherein the difference i*d~ - ldA
attalns a li~it value whlch leads to the derlved malfunction slgnal QA = 0 ~t'lo). Thereby the ~ontrol angle wlll no longer be preset in a normal fashion wherein the corresponding nominal value ~*Ao (e.g., ~Ao- 150~ respectively.
~X~A is generated by current controller 41A (Fig. 10) 13 ad~usting the i~dAo parameter and whereby ~he fault lndlcation voltage UdAv is generated by a ~easurement circult as the automatic control quantity UdAv. Rather by reverslng the switch 60A ~t tlme tlo the system swltches from the set value for the operating current to a new set value supplled by ~ 15 a bypass controller, ~.e., a controller for the voltage U'A.
; A~ in the case under conslderation only a bridglng clrcult of converter thyristor groups ln series was selected and fired as the bypass circuit and as thls brldginq circult has to be capable of bearlng permanent current, its thermal load bearlng capability ls lower ~han for normal system-synchronous ~ : opera~ion. Therefore the bypass current set value supplied by the bypass controller ls throttled back ln contrast to the no~lnal value i*dAo.
The control angle sl7pplled largely by the bypass ~: 25 controller 68A is symbolically assumed to ~e a constant in Fig.
23. Until the end of ~he bypass operatlon lt can be ~ pre-regulated by the ~easured voltage UdLA or even by the :~ substltute UdLA or UdB generated accordlng to Flg. lq;
accordln~ to Flg. 23, at time t'12 the auto~atic cont~ol iS
switched onto the (lnitlally inactive) run-up generator 66A ln : order to prepare for a nor~al start.
The d.c. current idb~paS~ now supplied by the ~: funct~on~ng station A flows -after th~ voltage UdB passes to the zero polllt ttime t~ over the bypass thyrlstors untll ~:

vP~ ~4 P 3393 ~233~

voltage UB returns at time t20 ~osslbly with su~stantlal voltage spike~.
The limlt value warning si~nal G711B supplles at a delay TSyn in ~ccordance with the synchronizing time of the reference voltage generator the leading release signal QB = 1 u~ed to start the component current converter lB' with control angle ~B' - 90~ whereby the component curlent converter lB' i5 shifted at a low lncrement of, for exar~ple, lO degrees ~nto ~onverter opera~ion. Follow~ng a delay T~S supplled by the ti~er clrcuit ZS (Fig. 15~, the component current converter lB~
~ is also released.
: Thereby initially, as already discussed, a negatlve voltage tl~e waveform arlses ln UdB and the sy~tem NB ls charged with reactlve current whlch leads to the de~lred voltage decllne and a reduction of voltage splkes in UB.
; As the co~ponent current converter 15 ~wltched on after a delay, the reactlve load al50 rises ln steps. Preferrably the control angle a'B of tbe component currQnt converter started first is left in the area ~ear gO degrees by havlng the ~V~C
~ransmisslon curr~nt function ~rimarlly as a r active load, and ls run up to the oeerating inver~er degree ~odulation o~ly ~: after all the other co~ponent current c3nverters co~ute fully. Thereby, as Figure 15 indi~ates, a distinct run-up function can be selected for each component current converter.
According to Flgure 23, however, one single run-up functlon ~: generator u~usally sufflces for aB~ whereby the ~lope of a' initially impact~ only on lB' and starts at a low lncrement to :con~inue at a higher increment to the nominal value ~O only after time t20 ~ TZS after the la~t co~ponent current ~onverter is started wlth (a~BO = ~.
The voltage pusles i~pressed by sta~io~ B leads following : ~he tran~mlssion line travel tl~e Tts to a motion of Ud~, ln other words, UdLA and idA. ln other words, ldLA, whlch ls detected in station A and inltiates the derived relea~e slgnal ~, ,.

33~9 ~L

QA = 1 (t'20~. There the control angle ~A is run up to A- , wl~h the lncrement initlally being prefer~ably large and later being thro~tled down. 5ince inl~lally only the fault voltage Ud~, in other words UdLA, impres~ed by the initially sllqht lncrement of a8 ~mpacts on statlon A, idA
ri~es rapldly to the nominal value ld~o, and converter lA is soon in a range ln which the current idA loading the syste~
NA is prl~arily ac~ive current.
During ru~-up switch 60A ls ~eversed to the operating set value i~d~ whereby the bypass controller ls out of actlon.
After the run-up functlon starts ln ~lg. 23 at tl~e t21 the control quantlty controller 41B ls activated and appro~imately at the end of ~he ~un-up function (at the preset tlmes t22, t'22 con~rolled by the lag circuit VZ) the operatlng automatic control slgnal ls always connected so that normal ~ operatlon can resume.
; In normal operation thu~ both stations are released by the release slgnal QA - 1, QB = 1, while switch 61A and SlB are always posltlo~ed by ~he la~ firlng pulse release slgnal QIZ =
~: ~ 20~ l ~o that the control angles a~ and ~ are deter~ln~d ln ~he ~`: auto~a~ic control device by the addltion of the pro~ected ~odel fault lndlca~lon quantlty and the re3pectlve preset a~gle (l.e., the angles deter~lne~ by the quantltles ~xB and ~xA
ln Figur~ 9 and 10) for nor~al operatlon. Slmilarly, rev~rsing ~: 25 ~witches 60A and 60B are held ln a posltion whereby the preset angle aA*~ and respectlvely, YB~ ls d~termined by the control quantlty Gontrol ~ro~ected for normal operatlon. For exa~ple, in accosdance with Figure 7, to generate XA, the set value i~d~:of the current controller 41A in statlon A is sueplied by a superl~po~ed controller (actlve power controller 51, Flgure 7~ in accordance wi~h the active power balance or : other require~ent~ of he undisturbed system N~. In ~tation B, ~XB is supplled ln accordance wlth a ~et value ~
~upplied elther by an extinction angle controller ~41B, ~lgure .: , ~33~9~3i 5) sr a rea~tlve current controller (41B') o~ voltage regulator (41B~), wlt~ ~* derlvable ln the compu~er clrcuit 47 f~o~ the system requlrement for reactlve current ol constant voltage.
Since for malntenance on the HVDC transmlssion line ltself it ~lght be necessary following a malfunc~ion to deactivate the ~VDC transmlssion llne ln ter~s of curre~t and vol~age, l.e., not undertaklng any bypass operation~ wa wlll first look at thls situation.
. b) ~xamples wlth Trans~lssion Line Deactivated In a typlcal sequence as per Flgure 24 we fl~t look at the case in which the former station B at time Tlo faces a short circuit of sy~tem NB, i.e., a breakdown of voltage UB.
In the ~onitoring statlon the li~it value alar~ 711 issues the correspondlng slgnal ~711B. The HVDC trans~isslon llne, the : 15 converter reactance coil LB as well as t~e filtsr elements CFB
and LFB dlscharge via the short circult So that the currents ldB, in other words, idLB lnitially rise, leading to an i~ver~er fallure. At the same time, the malfunction sig~al QB
0 ls set and the firlng ~uls~s o~ converter lB are lnhlbited as indicated ln Flgure ~4 by the control angle (aB = 90~
~: Thereby the voltage UdB d~ops and the HVDC ~ransmlssion llne curren~ s~tingui~hes.
In s~atlon A follo~ng the travel time Tts deter~ined by the HVDC trans~lss~on line ~low tlme there ~s a si~llar : 25 in~rease ln ~he HVDC transmisslon line current idA, ln other ~: words l~LA, which over the external m~lfunction warnlng pul~e ~; eroduces a malfunctlon signal and Ieads to QA = 0 (ti~e ~ tlo~ Thereby the other s~ation A is also shut down (aA =
: ~ 90 ).
Figure 24 assumes that due ~o the B~s1de short cl~cuit the HVDC tr2nsm1s~10n line has been practlcally compl~tely discharged wlth the excep~ion of ~light osclllatlons in the .~: HVDC transmlssion line, ~hlch ls now locked on bo~h ~ldes. In this context it can be of particular advantage if With QA = 0 :

: - 6~ -~ ~ 3 3~g~ , the other converter lA is not lm~ediately locked, but rather the HVDC trans~lssion line current and voltage dis~lpat~ fully down to the value zero over the controller of the ~tation A.
Such a procedure is par~icularly advantageous if, for e~ample, followlng a short-ter~ lnterruptlon the HVDC transmisslon llne i5 to be started up as rapidly as possible from the deactiva~ed position.
In this re~tart, as soon as the voltage U~ has attained a sp~ciflc li~it value ~time point t20~ and the li~it value alarm 711 has activated in the one station followlny a certaln : lag ti~e T~yn ~hlch is necessary to synchronlze the reference voltage generatvr as well as to generate the flring pul~e release slgnal QIZ, the run-up function generator 66B ls actlvated and the control angle aB run up smoothly.
In order to then impress the XVDC transmission llne with a deflned voltage pulse which can releas~ an external release pulse in the other station A, lt is preferrably foreseen to add an additlonal pul~e ~a to the smooth run-up of the ~ontrol ~- angle a~ or to overlap said pulse, by ~eans of which the

2~ conve~rter lB is te~porarily o~erated ln rectlfler operation.
lg. ~4 shows that the HVDC trans~lsBlon li~e curlent ~dB is hus rapldly exclted and the HVDC trans~lsslon llne voltage UdB b:ecomes te~po~arlly n~gatiYe.
: As a consequence, again at time t20 deter~ined by the HVDC transmlsslon Ilne travel ~ime, a negative voltage vs. time wavefor~ in the thus far: locked station A and a slgnal QA = 1 ~:~ result, ~lth which the run-up functlon generator lna~tivated du~ing malfunctlon is activated there a~d the control angle ;~ ~Ais ru~ u~ s~oothly.
In both stations durin~ the ~alfun~tion lnactlvated run-up func~ion generators ~u~ply du~ing the run u~ ini~lally a sub titute value for the model fault guantlty, although one can recogni~e from the pa~tern of ~he control angles a~ a~d B
that the smooth preset value supplled by the run-up function ~A 84 P 3393 ~233~

during the run-up is leplaced by the pulsa~ing model fault quantlty as soon as the reverslng swltches 61A and 61B and also swltches 60A and 60B are reversed an~ the undisrupted normal operatlon is resumed.
~ig. 25 de~icts the case of a malfunction in statlon A.
This assu~e~ that in system NA at the time tlo only one of the phase voltages fails 50 that the voltage a~plitude UA of this sys~em sho~s a pulsating pattern. I~ thls case the llmlt value alar~ 711 also supplies a ~ulsatlng slgnal to one monltoring devlce whlch, hvwever, is only shown by ~he broken llne in Fig. 25 because, for ex~mple, the time constant of pulse shaper 713 ean be set so that a constant signal suppres~lng thes2 oscillatlo~s can be generated. In ~he case already ~entloned, wherein no specific limlt value alarm ls deslgned to form the slgnal G711B, but the pulse generatlon is : rather handled by the refelence voltage generator whlch handles all the fluctuations of the a,c. voltage system anyway, the relevant constant signal for the total extent of the ;~ ~alfunction can be generated there-easIly.
: 20 ~ I~ general, for ~uch a ~alfunction another type of emerg~ency operatlon ls deslyned whlch iB not the ~ub~ect of ~:~ this i~ventlon. The strategy deslgned for a malfunction of the rectifler wlth deactlvlation of ~he HVDC trans~ission line wlll be expla1ned, however, for this case as ~ell.
The inhibiting of the flring pulses of converter lA
~ genera~ed by QA = 0 br~ngs about the extinctlon ef the HVDC
`~ ~ curren~ idA, although admlttedly these is no necessary immediate discharge of ~he HVDC trans~lssion line. Such a : discharge ~an be forced, as already explained in connectlon wlth Fig. 24, by a tempsrary flring a~gle ~hift ln the undlsturbed other 5~at10n B, w~ich, however, ls dlspensed with in the exam~le of ~lg. 25. Rather, ~he ~1rlng pulses o~
conveEter lB ln ~tatlon B are inhiblted followlng the lag tlme ; Tts by the collapslng ~VDC trans~ission llne curren~ idB or ~ - 66 -~;233~39 ~VDC transmlsslon l~ne voltage UdB ~o that there, too, the current ls discharged and the converter shut down. The HVDC
trans~ission llne then no longer conveys current, but stlll conveys voltage.
In such ~ case it is often ~ot necessary following the return of the syste~ NA (ti~e T20 to begln ~he transltlon to normal operatlon with a temporary supplement~l pulse ~a on the control an~le. This applles, in particular, if the HVDC
transmission llne stlll retains a posltive residual voltaye as shown ln Fig. 25, whlch when flrlng the thyristors ln lB, generates a current flow. TAe control angle can then be run up ~oothly from ap~roxl~ately ~A = 90' In t~e situatlons of ~igure 24 and 25 the deslgn foresees ln case of emergency operation durlng a ~alfunction a shutdown . l5 ~deactlvatlon) of the HVDC transmission llne at least to the extent that the ~VDC transmisslon line current ls equal to zero whlch is attained by havin~ the HVDC transmlssion line ~ separated ~rom the a.c. voltage systems by a lock of both : convsrters during the malfunction, i.e., before the resu~ption of nor~al o~eration.
~:: The subseguent transltlon to nor~al operation ls thus also sultable for the in~tial startup of the HVDC trans~ission line followlng ins~allatlon or after a thorough ~a~ntenance.
The ~VDC trans~ission llne voltage at time t20 rapldly changed by the resumptlon of nor~al o~erat~on ls detected at the latter station B at time point t'20, where 1~ leads to the deriv~d release slgnal QB - 1 and si~llarly to the resumption of normal operation.
~: During a malfunction the swltches 60, 61 and 67 of the s~at~ons in Flgures 9 ~nd 10 are ln a positlon at wh~c~ the deactivated controller 41 and the run-up funct~on genera~or 66 ~et the control angle gO degrees ln accordance wlth the lnitlal set~ing step of zero. The control angle i5 run up a~ tlme polnt t2~, in o~her wo~ds, t'20 by activating ~he run-up ' ' ' :

., vP~ 84 P 3393 ~;~33~

function generator, whereby posltlon P3 of switch 61 permits temporary connection of the supple~ental pulse ka durlng the system-synchronous start of the drlve unlt. If t~e controllers 41 are activated simultaneously with the run-up functlon generators 66, then the run-up is superi~posed by the buildup of oscillation3 of the controller, while at tlme points t : and t'21 it can be seen fro~ the wave pattern of the automatic control voltage ~hat now the control angle is belng deter~ined by the automatic control quantities UdL~, in other words, aBV which as the ~easurement value of the faul~
quantity replaces the aodel value generated and regulated by the run-up functlon generator.
c) Examples with By~ass Operatlon If, ln t~e event of a ~alfunc~ion ln the former statlon, glven undisturbed operatlon of ~he latter statlon, a bypass opera~lon ls specified due to the already mentloned advantages, then in the undl~turbed statlon upon commence~en~ of the alfunctlon the bypass thyristors s~lected for the bypass circult are flred by the lnternal ~alfunctlon warning pulse, and the HVDC transmlsslon line connections of thls s~a~lon are ~hort-clrculted. In the latter station the external maIfunctlon warnlng pulse ~df initlates a rec~ifier actlvity ln this ty~e of operatlon whereby any deslred bypass current is ~; su~plled lnto the ffVDC transmlsslon llne. This bypa s current is preferrably derived ln accordance with the requirements of the latter station, i.e., ~ased upon the measured values of the a.c. ~oltage syst~ avallable in that statlon.
After the ~alfunctlon ls over, ~he bypass circuit ls interrupted again in the ~recu~erated~ statlon by means of the lnternal release pulse Ffe and normal operatlon ~ynchronous wlth the ~ystem resumed, resulting ln ~he functionlng other -~ station in an external release pul~e Fff by which the bypass current feed ls discontlnued and nor~al operation synchronous with the system re~umes. Figures 26 and 27 deplct advantageous ' ~ A 84 P 3393 ~33~ ~

types of designs of said ~alfunction operatlon wlth ~y~ass current.
Flgure 26 assumes a malfunction of the rectlfler operatlon (Statlon A).
The collapslng voltage U~ of the system NA collapslng at time tlo 1~ again recorded i~ station A by the internal malfunction warning pulse Fde and release~ ~A - 0. Converter 1 is inhibited. The current idA thus goes off, and the . voltages UdA and UdLA, respectively are caused to osclllate. Sald oscillation ls depicted by a broken line and 1B dependent on the further events ln station B.
After a delay ti~e TtS (time T'lo) there ~ a correspondiny drop in voltage UdBI in other words, UdLB and in current ~dB at the d.c. connections of statlon B.
Therefore, by monltoring the current ldB, for example, an external malfunctlon warnlng pulse Fdf ls generated at sald locatlon, resulting i~ QB = ~
: If ln the event of sald ~alfunction, bypass o~eratio~ ls fore een, then t~e functionlng stat~on B as~umes r~ectifier ~ 20 operat:lon with the external mal~unctlo~ warning pulse, with the ;~:: control angle belng provlded by a ~uperimposed controllBr. If, ~: for lnstaDce~ the bypass o~eration is to serYe to keep the voltage UB constant or to co~trol the react~ve load, then a :~ voltage regulator or a reac~ive power controller ls provided as : 25 a super~mposed regulator for ~he ~ypass operation. The output signal of this bypass controlles 68B (Fl~. 9) for~s value XB,~which colresponds to the set value i~dB for the current to be ~aken fro~:the ~unctioning syste~ and is :
connected wlth ~he control set as control an~le aB in the : 30 bypass operatio~ vla the switch 60B. The co~puting circu1t : 4~B' ls separated in this ln tance, e.g., by the ~wltch 61B
belng switched to ~:he outpu~ o~ the run-up ~unction generator :~ 66B ~which ls inactlvated durlng the ~alfu~ction).

:, .

-- 6g --~233~

In one configuratlon, at QB = 0 at the polnt in ti~e t'10 fir~t the flrlng commands o~ the coDverter are inhibited. It then depends on the phase position of ~he system N~ if and when current ~ extlnguishes. Subsequently, rectifier operation ls co~menced in statlon B which results ln a voltage rever6al in ~he HVDC transmlssion line ln accordance wlth the polarity of the converter thyristors.
The ~e t'll for commencing this rectifler operatlon (posslbly with the pulse ~ beiny give~) is practically freely selectable. The automatlc control anyle ~B~ ls switched over to the run-up functlon generator (inactl~ated during the malfunctlon) 50 that ~he control angle aB 1~ determlned solely by the bypass con~roller which has now been swltched on.
The bypas~ operation now taken up by ~tation B has the effect that an increase ln the voltage UdA occurs at the d.c.
connec~ions in the malfunctionlng station A at time ~ with ;~ the storage clrcuit 63A belng able to recognlze the bypass operation taken up by the functlonlng sta~lon by ~eans of the simultaneous occurence of signal G711A and the internal ~alfunctlon warning pul~e Fds, respectlvely~ and ~he ex~ernal :~ relea~e ~ignal ~ff derived from t~e voltage o~cillatlon. As ldA = 0 at thls ~l~e, lt doe~ not ~tter ~hich converter : thyrls~or~ of sta~ion A connecte~ 1n serles are flred as ~he ;: bypass:circult. Several parallel bypasses can be clo~ed, for exam~le. In the case at hand a selection of ~hose byp~ss : thyristors servlng as ~bypass thyrist4rs- a~ong the thyristors of converter lA regui~ed for normal o~eration is ~re-program~ed and only a slngle bypass ls foreseen.
The flrlng commands for thls bypass clrcuit can be ~ormed upon occurrence of the in~ernal ~alfunctlon warnlng pulse or shortly thereafter; ~iri~g is, however, not effected until the voltage UdA ~ue to the voltage ~pression by the hyp~s ~ rectlfier operation of statlon B~ has changed polarlty. Then a :~ current ld~ fl~ws through the converter lA and the HVDC
-- 7 r~ -~3~ag~
trans~lssion llne, which, howev2r, is separated from the malfunc~loning network NA.
The n~twork NA resumlng operation at tlme t20 reaches at t2l the preset lil~it value at which the internal release pulse ls for~ed. Af~er the tl~e TSyn requ~red for the formation of ~he syfitem-synchronous reference voltage o~ the drlve unit, the malfunction memory QA a 1~ and the converter lA
is run up ~l~h tha control angle ~A in a smooth fashion.
In order to avoid a short-clrcul~ current to flow through converter lB~ whlch in bypass operatlon i~ltially stlll operate~ a~ a rec~lfler, and converter lA, which has already gone to nor~al rectifler opera~ion, the run-up of the control angle ~A can be delayed as against an initially lmpres~ed voltage surge so that station B can ~ake the transltlon to lnverter operation in a timely ~anner after recognitlon of thls voltage surge.
Furthermore, at time t20 the swltch 61A is switched in ~uch a manner that the run-up function generator output si~nal forms the auto~atic control slgnal UdAy~ ~ this run-up ~ 20 qenerator i~ lnactlvated during the ~alfunction and not - released again untll the sy~tem resu~es, the deplcted pattern results for the vol~age V~Av, wl~h a smoo~h i~crease upon occurrence of ~he leading release ~lgnal QA = l.
At a ti~e t2~ allowed by ~he progra~ clrc~lt t~e voltage UdLA l~ again u~ed as automatic control voltage by switching swltch 61A. Therefore dur~ng nor~al operation the control anqle oA is practlcallY given by UdAv = UdLA (or el5e : UdLB) and is modlfled only sllgh~ly by th~ functlon of the current regulator, As ~he cl~mplng circult lnhiblts the flrin~
com~ands of the drlve unit STA derlved from ~A durlng bypass o~eration, lt ls inslgniflcant when in the intarval between tlo and t~o the ~emory circult swltche~ from UdAv =
: UdLA to the automat$c control voltage supplled by the run-up functlon generator. Preferrably the curren~ controller i~

~ 33 ~ ~ ~

lnactive during this tl~e a~d is not ac~ivated until after the return of ~he sy~te~ via the progra~ circuit, prefelrably at the end of the ~mooth run-up or upon occurrence ~t~e t21) of the effects of the nor~al oeeration initiated by the other statlon.
In statlon B the rectlfler operatlon resumed by statlon A
ln a smoo~h ~anner effects at t'20 an lncrease in ~he voltage UdB (or else UdLB) and ln the current i~B~ thus triggering the release ~lgnal ~B = 1 der1ved here, witA which the system is now switched from bypass current con~rol to normal control, l.e., from bypass controller 68B to the : extinction angle controller 41B in Fig. 9 (switch 60B). At the same time the run-up function generator is activated th~ere with sald external release slgnal of sta~ion B, whlch now supplles the automatic control angle aBV for station B lnstead of ele~ent 43B'. As a co~sequence, the control angle ~B
increases again in a ~mooth manner to the angle characteristic of normal inverter operatlon untll a~ tl~e t'22 ~he swltch 61B is again ~wltched ~rom ~he run-up fu~ctlon generator to the computi~g circuit 43B'.
The sa~e principle can be i~ple~e~e~ ~f statlon B has a ~alfunctlon. F~g. 27 a~su~es this case.
In nor~al operat~on the ~onverter lB ln station B is operated as an lnverter wl~h a control anyle a~ near the lnverter step li~lt, whlch ls for~ed by ~he extlnctlon angle controller (or a controller for the reac~lve power or another control quantity) and ls controlled ~ith the automatlc control ; ~
angIe calculated from the inductive voltage drop. At tlme : tlo the voltage UB of the syste~ NB colla~ses wlth the effect ~hat vla a corresponding signal ~711B in the ~onltorlng device of ~tatio~ ~ the normal fillng pulses are inhlblt~d there.
;: Thls short clrcult alBo r~sults ln the collap~e of the d.c. voltage Ud~ and a rapldly lncreaslng dlrect current ~ 33 idB flowlng into the short ci~cul~ so that th~ lnverter l~
becomes unstable. The capacitance of the HVDC transmlsslon llne at the statlon B connection are thus discharged lnto the short clrcuit, resultlng in a reversal of the voltage Ud~B
and finally ln an extinction of the direct current idB.
Meanwhlle the llmlt value warning devlce has al50 effected condltio~ QB = 0 via the ~nternal malfunction warning pulse, by whlch the normal commutatlon of the converter lB is lnhlbited and the firlng of the bypass thyrlstors initiated.
According to the line's dlstributed tlme delay, the d.c.
cu~rent ld~ lncreases ln 8tation A and the d .c. voltages UdA and UdLA drop accordingly. Yla the external malfunctlon warning ~ulse of station A this results in condltion ~A = 0 at which ~he operatlonal current set value l~dA ls swltched to a lower set value which is supplled by a superimposed bypass controller, e.g., the reactive power controller 6~A or a voltage regulator for voltage UA (switch 60A in ~lg. lO).
The converter IA therefore feeds only the react~ve curren~ i~to the 0VDC ~ransmlssion line whlch is required for as continuous : .20 an operation as possible of net~ork NA. Furthermo~e, the signal QA = 0 trlggers the switching of the au~offlatic control voltage UdAv fro~ the ~easurlng value output for UdLA to the output of the run-up functlon genera~or (switch 61A), whlch swltching takes place at a preset later tlme tl2, for exa~ple.
As the statlon A continues to feed current lnto the HVDC
trans~lssion llne, the end of sald ~rans~ission line discharged : through~the sys~em short clrcuit is recharged in statlon B.
Thus the voltage UdA acqulres positlvs values again (~lme tl2) so that upon attaining a preset ~osltlv~ lt value, -30 certain thyrlstors f~re ln s~ation B wh~ch are selected for t~
: formatlon of the bypass circuit and at w~lch corre~pondlng firing voltages arise at tl~e tl2. Thus the bypass circuit ls ncw closed and ~he bypass operatlon initlated during which the ~::HVDC trans~lssio~ lin~ ls operated a~ reactive impedance for ~ystem NA.

,::

~L~33~
In the case depicted by Fig. 27 the lnhlbitlng of the normal firing commands supplled by the drlve unit took place at ~he ti~e tlo in the malfunctloning station B with QB = O, whereby the current ldB lnitlally continues to flow through the thyristor~ of converter lB involved ln th~ shutdown of converter lB. Only af~er extinction of these thyristors the HVDC trans~lssion line has been recharged wlth a ~urrent whlch ~ at tlme tl2 leads to a positive response value of UdB, : while ~ta~ion A has started the bypass operation, i~ the voltage applied to the thyrlstors selected for bypass operatlon, which voltage resul~s in ~he firlng of the bypass ~hyristors and thus ln a recurring current idB. In this event the bypass thyrlstors can be s~lected independent of the thyristors involved in the inverter shutdown. However, as ln : 15 statlon B ~he thyrlstors involved in the shutdown can be selected as bypass thyrlstors independent of operation, a frequently undesirable complete extinction of the current idB
~ must not be waited for.
: Partlcularly in the case deplcted in Flg. 27 substantlal negatlve values o~ UdB can result in the time l~terval between tlo and tl2, :whlch the users try to prevent ln many cases when operatlng a HVDC tra~s~ission line.
This can be achieved if the control angle ~ of s~atlon A is not i~mediately reduced to a value near zero correspondlng to the bypass operatlon wh,en the derived ~alfunction signal QA
~` - O, but rather to a control angle whlch ls initially shlfted ;~ ~ in the direction of rectifler wide-open settlng in order to recharge the H~DC transmission line as guickly as possible.
This value can be given ~y the storage circuit S3A via the ~osition P3 of switch 61A according to Flg. 10. If a n¢~work monitor 43A' is available, ho~ever, a~ explalned in Flgur~s 13 and 14, lt ls then possible to keep the current id~ almost constant, by controlllng the substltute actual value id~ f t~e current regulator 41A' calculated by the netwo~k monltor to , ~33~

a set value supplled by the voltage regulator 68A servlng to malntain a constant voltage UA. If for nc~rmal operatlon, for example, a superimposed active power regulator 51 i5 provlded, then through QA = 0 thls superimposed res~ulator is switched to a bypass controller (voltag~e regulator 68A), w~ile in the observer station the inverter shutdown oi. malfunctionlng station B ~s si~ulated by t.he closlng of swltçh 77.
In order ~o convert from the bypass operation back to the . system-synchronous nor~al operatlon upon the re~urn of the network NB (t1~e t20 ln Fi0. 7) and upon occurrence of the leadlng release signal QB = 1, those thyristors are fired ln the already described ~anner which have been pre-programmmed wlth a pre-programmed bypa2~s thyristor co~blnatlon ln order to lnitlate the system-synchronous operation ~hlle lmpresslng a voltage surgeO or those thy~rlstors are fired whlch have been : ~elected by the selector switch dependent on the operation, in accordance ~lth the bypass thyristors f~red dependent of ~: operation.
Flgure 27 depicts ~he voltage UdB occul{ing after the tlme T~yn requlred for synchronization of the reference : voltage generator, wh~ch voltage is a func~ion of the run-up of ~. At t~e tl~e t'20 t:hen the release signal QA = 1 occurs, with a correspondlng lncrease of the angle:~A to the rectifler wide-open setting provided for nor~al operatlon and a :; 25 correspondlng pattern of voltage UdA, with the bypass con~rol now dlscontinued in the functionlng statlon A and normal : sontrol belng swi~ched on via the current controller. At times t22 and $'22 switching from the automatic control value eset as a controlled run--up functlon to the ~easured fault ~: 30 quantity as a automatic colltrol quantlty takes plaçe.
In this deslgn ~he particular advantage is that durlng normal o~eration ~he two s~;ations functlon i~depe~dant of one another, 1. e., that no inf or~ation to be transmitted via remote control lines is required i^rom the respective other statlon for ,:

3L~33~99 the control of the two convelters. Therefore both converters can be guickly controlled by correspondlng auto~atlc control wlthout forci~g a slow control pattern due to the delay time of remote control slynal transmlsslon. Even ln the event of malfunctlo~ the trans~lsslon of ~orresponding malfunction signals is not effec~ed via re~ote control llnes, but via the HVDC transmisslon llne ltse~lf 50 that ln the event of a ~alfunction in the one station the necessary infor~ation on the malfunc~lon is available ill the other statlon as ~ell within the shortest tt~e possible. Th~ resumptlon of the fault-free normal operatlon in the one station is communica~ed in the same way wlthin the shortest po~3sible tl~e so that very short on-cont~ol ti~es result for the resumption of normal operation.
Furthermore, it is posslble by said rapld control to utllize the }~ C transmission line ltseIf to control or regulate the electrical quantlties of tlhe respective syste~s, e.g., for reactive current control o;r during bypass operation to ~aiDtain a fault-ree system constant, or to dampen other proc~sses, ; e.g., balanclng processes in the systems.
As w~ll be evldent from the foregoing descrlptlon, certain aspects of the inventlon are not ll~ited to the particular d~tall~ of ~he examples lllustrated, and lt 1~ therefore i contemplated that other modiflcations or appllcations will occur to those skilled ln the art.

:

' :

':

Claims (38)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for operating a High Voltage Direct Current (HVDC) transmission line system connected between two alternating current systems, Network A (NA) and Networ}s (NB), during emergency operation resulting from a malfunction having a first station, connected to the network A which during normal operation operates as a system-synchronous first converter rectifying the incoming alternating current and impressing a normal d.c. current through the HVDC transmission line, and a second station, connected to the network B, determining the normal voltage of the HVDC
transmission line, further forming a leading release signal subsequent to the end of the malfunction; wherein the second station during normal operation operates as a system-synchronous second converter inverting the incoming HVDC and impressing a normal a.c. current into Network B, and the method adapted to form a short circuit as a bypass circuit around either a malfunctioning first station or a malfunctioning second station comprising the steps of:
forming a leading fault indication signal at the onset of the malfunction in the malfunctioning station;
firing by means of said leading fault indication signal (i.e., inducing into conductance) forming the bypass circuit from bypass thyristors which are designed to withstand as a normal operating condition a load current corresponding to the level of reactive current encountered during the malfunction bypass operation;
forming subsequently in the functioning station a derived fault indication signal;
stimulating a rectifier operation of a.c. current into bypass d.c. current in the operating station by said derived fault indication signal;
impressing said bypass d.c. current through the HVDC
transmission line and the bypass circuit;
- 77a -interrupting the bypass circuit causing said bypass thyristors to become non-conducting by the leading release signal;
initiating a transition of the previously malfunctioning station to system-synchronous normal operation by the leading release signal;
forming a derived release signal in the previously functioning station;
terminating said rectifier operation by the derived release signal; and said derived release signal further initiating a transition of the previously functioning station from said rectifier operation to system-synchronous normal operation.

2. The method according to claim 1 further including the steps of:
presetting said bypass d.c. current in accordance with the requirements of the a.c.
system, which is connected to the functioning station, particularly in accordance with the output signal of a bypass controller for said a.c. system which is connected to a functioning station voltage signal or reactive power signal whereby said bypass controller is located in the station supplying said bypass d.c. current.

3. A method according to claim 1, further including the steps of shifting a control angle, .alpha., during malfunction operation in the converter in the functioning station to rectifier operation by means of said derived fault indication signal; and impressing a voltage, during malfunction operation by said functioning station, across the HVDC transmission line which is positive in the direction of the current flow direction of said bypass thyristors in the malfunctioning station and of sufficient level to fire said bypass thyristors.

4. A method in accordance with claim 3, further including the steps of:
adding to said control angle during malfunction a pulse, .DELTA..alpha. initiating a temporary voltage pulse on the HVDC
transmission line for assumption of malfunction operation, partic-ularly adding said pulse to the control angle of the second station for the rapid recharging of the HVDC transmission line in the event of a malfunction in the first station.

5. A method in accordance with claim 3, further including the steps of:
discharging the HVDC transmission line via series connected thyristors in the second station in the event of a malfunction in the second station, with subsequent firing of said bypass thyristors in the second station after impressing by the first station of a positive voltage sufficient to fire the bypass thyristors on the HVDC transmission line.

6. A method according to claim 3, further including the steps of:
setting of a leading fault indication signal in the second station by the occurring of a malfunction in the second station;
firing immediately, thereafter by means of said leading fault indication signal, said bypass thyristors; and setting a derived fault indication signal in the first station; and presetting a control angle from said derived fault indication signal;

whereby the HVDC transmission line is recharged without the extinction of the HVDC transmission line current.

7. A method according to claim 6, further comprising the steps of:
forming said control angle in such a manner as to provide an input to a system simulation model located in the first station;
forming from said control angel input to said system simulation model, a model quantity, which is proportional to the d.c. current of the second station; and controlling said model quantity, to a value of maximum constancy.

8. A method according to claim 1, in which said bypass thyristors that are being fired are a selected combination of the thyristors used in the malfunctioning converter during normal operation.

9. A method according to claim 8, further comprising the steps of:
presetting a start value of a control angle in the previously malfunctioning station for resumption of system-synchronous normal operation following the end of the malfunction;
forming from said start value of said control angle and from a system-voltage synchronized reference voltage the normal system-synchronous operation firing command sequence; and selecting for a start of said normal system-synchronous operation firing command sequence, a firing command in said sequence corresponding to the decommutation of the current from one of said bypass thyristors in said firing command sequence.

10. A method according to claim 9, in which said combination of bypass thyristors and selection of said start firing commands are preprogrammed.

11. A method according to claim 9, further comprising the steps of:
determining, as a function of which thyristors are conducting current at the occurrence of said leading fault indication signal, said combination of bypass thyristors; and selecting, as a function of the combination of bypass thyristors determined as a function of the occurrence of said leading fault indication signal, the firing commands to start normal operation.

12. A method according to claim 11 determining, from the firing command last given during normal operation from the respec-tive sequence of the system-synchronous firing commands, said bypass thyristor combination;
storing said determined bypass thyristor combination for subsequent firing of thyristors;
inhibiting the firing commands by the occurrence of the leading fault indication signal; and firing said stored bypass thyristor combination thyristors.

13. A method according to claim 12 further comprising the steps of:
setting a memory unit upon firing of the bypass thyristors;
reading out, subsequently, at the occurrence of said leading release signal, information stored in said memory unit releasing the firing command selected thereby for start-up of normal operation.

14. A method according to claim 9 further comprising the steps of:
forming, by monitoring the a.c. voltage system, said leading release signal;
suppressing, initially, said leading release signal following a malfunction of the system until the synchronizing of a reference voltage generator within a maximum permissible synchronization error of 30 degrees.

15. A method according to claim 1, further comprising the step of:
resuming of normal operation in the non-malfunctioning station occurring as a result of impressing on the HVDC transmis-sion line a voltage pulse corresponding to temporary rectifier operation.

16. A method according to claim 1, further comprising the step of:
resuming normal operation of a converter after either a leading release signal or a derived release signal including the running up of the control angle .alpha.* from a value approximately the value of the converter output d.c. voltage of zero (that is .alpha. ? 90°), to a final value approximately corresponding to normal operation, having the start of the run-up function capable of being preceded by a temporary control angle shifting to a recti-fier operation or a respective superimposing.

17. A method according to claim 16, in which said run-up time is approximately two periods of the fundamental a.c. system voltage.

18. A method according to claim 16, in which the running up of the control angles of the first and the second converters is pre-programmed and mutually compatible.

19. A method according to claim 16, further comprising the step of:
presetting a control angle, in either one or both stations by an automatic control apparatus, after the running up of said control angle, by connection to the output signal, of a control quantity controller and to a model value for the fault indication quantity generated by the functioning in normal operation.

20. A method according to claim 16, further comprising the steps of:
presetting of said control angle of one or both stations by an automatic control apparatus, said apparatus which is switchably connected to a bypass controller output signal deter-mining the bypass current, such as, a voltage regulator for the a.c. voltage of the malfunctioning system or of a reactive output control during malfunction condition operation, and during normal operation to the output signal of a control quantity controller;
and which is additionally connected or to a run-up function generator during run-up, and to the model value of the fault indication quantity during normal operation and preferably up to the start of the run-up function.

21. A method according to claim 19, in which a voltage, is connected in the first station as a model value for the case of a HVDC remote transmission line at the end of a converter reac-tance coil following the first converter on the d.c. voltage side.

22. A method according to claim 19, in which a voltage is connected in the first station as a model value to model quantity formed by a computed inductive d.c. voltage drop of the second converter for the case of the HVDC transmission line short coup-ling.

23. A method according to claim 19, further comprising the step of:

forming a model quantity in the first station by a model simulation circuit driven by operational data of the first station.

24. A method according to claim 19, further comprising the steps of:
determining, from the inductive d.c. voltage drop of the second converter, a model quantity, whereby said inductive d.c. voltage drop is computed from a given set extinction angle, .alpha.*, preset from the set value of the control quantity controller and measured values of the d.c. current and the system d.c. vol-tage of the second station, added to the output signal of a con-trol quantity controller, such as a controller for the voltage or the controller for the transmitted reactive power.

25. A method in accordance with claim 16, in which said control angle of station A during the transition from emergency operation to normal operation initially having a larger and later decreasing run-up increment to normal rectifier operation, and said control angle of station B having a run-up to normal inverter operation in the opposite increment sequence from that of station A.

26. A method in accordance with claim 16, further comprising the steps of:
staggering the start up times of component current converters in converters consisting of several component current converters during normal operation, whereby the component current converter starting first is rapidly run up to a control angle preset by the operation and then taken back to the extent that the run-up of the other component current converters is completed.

27. A method in accordance with claim 1, in which said leading fault indication signal and the leading release signal are generated from operating data of the malfunctioning converter, such as from measuring values of the a.c. system voltage, and said derived fault indication signal and said derived release signal are generated from measuring values of the d.c. connections of the non-malfunctioning station as soon as effects of the interrupted or resumed normal operation are detected therein.

28. An apparatus for HVDC transmission, including:
a first station connecting to a first a.c. system to draw electrical power therefrom;

a first converter being part of said first station and being connected to said first a.c. system; and operating as a system-synchronous rectifier during normal operation;
a high voltage d.c. transmission line being connected at one end to said first converter;
a second converter being connected to the other end of said high voltage d.c. transmission line and operating as a system-synchronous inverter during normal operation;
a second station having said second converter a part thereof and having connections thereto;
a second a.c. voltage system being connected to said second station;
a first reference voltage generator located in and connected to station A for generating a system synchronous reference voltage with respect to a.c. system A;
a second reference voltage generator located in and connected to station B for generating a system-synchronous reference voltage with respect to a.c. system B;
a first controller forming a control angle, .alpha.A, connected to station A;
a second controller forming a control angle, .alpha.B, connected to station B;
a first drive unit, STA, connecting to said first controller for forming a first set of system-synchronous firing commands;
a second drive unit, STB, connecting to said second controller for forming a second set of system-synchronous firing commands;
a first monitoring means monitoring the values of the electrical guantities of station A and forming therefrom a first fault indication signal when a malfunction is indicated and a first release signal when return to normal operation is indicated;

a second monitoring means monitoring the values of the electrical quantities of station B and forming therefrom a second fault indication signal when a malfunction is indicated and a second release signal when return to normal operation is indicated;
a first clamping circuit connected to said first drive unit and said first monitoring means inhibiting the transmission of said first firing commands wherever said first monitoring means forms said first fault indication signal, and transmitting said first fixing commands whenever said first monitoring means forms said first release signal; and a second clamping circuit connected to said second drive unit and said second monitoring means inhibiting the transmission of said second firing commands whenever said second monitoring means forms said second fault indication signal, and transmissing said second firing commands whenever said second monitoring means forms said second release signal, comprising:
a first generating means as a part of the first monitoring means for generating a first leading fault indication signal when monitoring of the first a.c. system electrical quantities indicate the occurrence of a malfunction relative to the first station;
a second generating means as a part of the second monitoring means for generating a second leading fault indicator signal when monitoring of the second a.c. system electrical quantities indicate the occurrence of a malfunction relative to the second station;
a third generating means as a part of the first monitoring means for generating a first leading release signal when monitoring of the first a.c. system electrical quantities indicates discontinuance of a malfunction relative to the first station;

a fourth generating means as a part of the second monitoring means for generating a second leading release signal when monitoring the second a.c. system electrical quantities indicates the discontinuance of a malfunction relative to the second station;
a fifth generating means as a part of the first monitoring means for generating a first derived fault indication signal when monitoring the d.c. voltage electrical quantities indicates the occurrence of a malfunction in a location other than the first station;
a sixth generating means as a part of the second monitoring means for generating a second derived fault indication signal when monitoring the d.c. voltage electrical quantities indicates the occurrence of a malfunction in a location other than the second station;
a seventh generating means as a part of the first monitoring means for generating a first derived release signal when monitoring the d.c. voltage electrical quantities indicates a discontinuance of malfunction outside of the first station;
an eighth generating means as a part of the second monitoring means for generating a second derived release signal when monitoring the d.c. voltage electrical quantities indicates a discontinuance of malfunction outside of the second station;
a first memory as part of the first clamping circuit of the first station storing a combination of converter thyristors selected as bypass thyristors in the first station;
a second memory as part of the second clamping circuit of the second station storing a combination of converter thyristors selected as bypass thyristors in the second station;
a first and second switching means proximately located and connected to said frist and second memory respectively, and further connected respectively to converter thyristors in the first and second converters, inhibiting the system synchronous firing commands to said converter thyristors of the respective station when a leading fault indication signal occurs therein, and generating bypass firing commands to selected bypass thyristors of said respective station;
a first and second selector circuit connected to and proximately located to the first and second drive unit respectively, having a third and fourth memory respectively, contained therein, whereby after the firing of bypass thyristors in the respective station, a combination of start thyritors is stored for resumption of normal operation of the respective station;
a first and second logic circuit connected to said first and second selector circuit respectively, eliminating the inhibiting of system-synchronous firing commands after said respective leading release signal has occurred, and subsequently a firing command for one of the thyristors of said respective starting thyristor combination occurs; and a first and second pre-programmed circuit, connected to the first and second control angle controllers respectively, interacting with the respective control angle in such a manner that after the start of the respective derived fault indication signal, the converter is the functional station is operated as a rectifier in bypass operation, until the occurrence of the respective derived release signal at which time bypass operation is terminated and normal operation resumed.

29. An apparatus according to claim 28, further comprising:
a first and second switching means, connecting to the first and second drive unit respectively, whereby during bypass operation of one of the stations said switching means in the other, non-malfunctioning station provides an electrical connection from the drive unit and a bypass controller of the other station.

30. An apparatus according to claim 29, whereby said bypass controller is a voltage regulator or a reactive power controller.

31. An apparatus according to claim 28, further comprising:
a third and fourth switching means, connected to the first and second drive unit respectively, connecting a first and second run-up function generator respectively to the respective drive units when said respective release signal occurs forming a respective firing angle.

32. An apparatus according to claim 28, whereby said clamping circuits, selector circuits, and run-up function generators are used by their respective component current converter of their respective converters to provide resumption of normal operation in a time-staggered fashion and by different run-up functions in the converters.

33. An apparatus according to claim 31, further comprising:
a first and second device for determining a fault indi-cation quantity respectively in the first and second station, whereby after completion of the respective run-up function, said third and fourth switching means switch from connection to respec-tive run-up function generator and to connection to said device for determining a fault indication respectively.

34. An apparatus according to claim 33, whereby:
said first or second fault indication quantity determin-ing device in the first station is connected to monitor the d.c.
voltage, UdLA, from the first station; and the inductive d.c.

voltage drop, dXB . idB of the respective converter in the second station.

35. An apparatus according to claim 33, further comprising:
a first and second automatic control device connected in series with an input to said first and second drive unit respectively, connecting to the respective fault indication quantity and the output signal of the respective control quantity controller as inputs upon the occurrence of a release signal.

36. An apparatus according to claim 28, whereby said memories of said clamping circuits, and said memories of said selector circuits being Read Only Memories, preferably contained in said pre-programming circuit.

37. An apparatus according to claim 28, further comprising:
a first and second memory control means connected to said first and second memory respectively, and to said third and fourth memory respectively, setting said first and second memory at the occurrence of an output signal from the respective driven unit to a respective bypass thyristor combination dependent upon the momentary control status of the respective converter thyristors; and said third and fourth memory to a respective start thyristor combination dependent of the bypass thyristors.

38. An apparatus according to claim 28, whereby:
said first and second monitoring device in order to provide the respective fault indication release signals; and said control quantity controllers in order to provide the respective control angle in the respective station;
having as electrical quantity inputs of actual and set values provided in the respective stations without the use of remote control signals.