US3710232A

US3710232A - Logic-controlled thyristor system for performing tap-changing operations

Info

Publication number: US3710232A
Application number: US00219706A
Authority: US
Inventors: M Matzl
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-01-29
Filing date: 1972-01-21
Publication date: 1973-01-09
Anticipated expiration: 1990-01-09
Also published as: NO132566C; NO132566B; SE376998B; DE2104075B2; FR2124921A5; DE2104075A1; AT315970B

Abstract

A system for performing tap-changing operations on tapped regulating transformers by thyristor networks and relatively movable contact means. The trigger pulse generators for the thyristors of the networks are controlled by sensors whose output is fed into a logic circuitry. The sensors include a pair of sensors responsive to the position of current-carrying contacts, a pair of sensors responsive to the position of disconnect contacts and a pair of sensors responsive to the voltage across a thyristor network.

Description

United States Patent [1 1 Matzl [54] LOGIC-CONTROLLED THYRISTOR SYSTEM FOR PERFORMING TAP- CHANGING OPERATIONS [75] Inventor: Manfred Man], 693 Eberbach/ Neckar, Dr.-Weiss-Strasse 23, Germany 221 Filed: Jan. 21,1672

[21] .Appl. No.: 219,706

[30] Foreign Application Priority Data Jan. 29, 1971 Germany ..P 21 04 075.5 [52] US. Cl ..323/43.5 S [51] Int, Cl. ..G05f [58] Field of Search .t. ..323/43.5, 43.5 S

[56] References Cited UNITED STATES PATENTS 3,437,913 4/1969 Matzl ..323/43.5 S

[451 Jan. 9, 1973 3,466,530 9/1969 Matzl ..323/43.5 S

Pri'mary Examiner-William H.'Beha, Jr.

7 Attorney-Erwin Salzer 57 ABSTRACT A system for performing tap-changing operations on tapped regulating-transformers by thyristor networks and relatively movable contact means. The trigger pulse generators for the thyristors ,of the networks are controlled by sensors whose output is fed into a logic circuitry. The sensors include a pair of sensors responsive to the position of current-carrying contacts, a pair of sensors responsive to the position of disconnect contacts and a pair of sensors responsive to the voltage across a thyristor network.

5 Claims, 4 Drawing Figures PAIENIEDJAn 9 I973 SHEET 1 [IF 2 PATENTEU JAN 9 I973 SHEET 2 OF 2 M m #m A w. 5 A a N1 M 1 w w 47 5. 4 62 w (a? a BACKGROUND OF THE INVENTION The starting point of the present invention are socalled Jansen-type tap-changing switching systems for regulating transformers. A conventional Jansen-type tap-changing switching system includes, in addition to a tapped transformer winding, a selector switch and a transfer'switch, and the latter is provided with a plurality of tap-changing resistors, or change-over resistors. Modern versions of J ansen-type tap-changing switching systems are disclosed in U.S. Pat. No. 3,396,254 to A. Bleibtreu, Aug. 6, 1968 for ARRANGEMENT FOR AVOIDING EDDY CURRENT LOSSES IN TRANSFER SWITCH AND SELECTOR SWITCH UNITS WITH INTERPOSED GEAR DRIVE and in U.S. Pat. No. 3,493,698 to U. G. E. Schweitzer Feb. 3, 1970 for TAP-CHANGING TRANSFORMER IN- 'CLUDING A SELECTOR SWITCH AND A TRANSFER SWlTCl-I. Reference may be had to these patents for a more complete disclosure of Jansen-type tap-changing switching systems.

In more recent times there has been a trend toward substituting tap-changing transfer switches including thyristors for the more conventional transfer switches with their relatively bulky tap-changing resistors, or change-over resistors. Typical examples of tap-changing switching systems including thyristors are disclosed in U.S. Pat. No. 3,502,961 to M. Matzl, Mar. 24, 1970 for TARCI-IANGING TIIYRISTOR CIRCUITRY FOR REGULATING TRANSFORMERS; U.S. Pat. No. 3,534,246 to M. Matzl, Oct. 13, 1970 for TAP- CHANGING SYSTEM INCLUDING Tl-IYRISTORS FOR EFFECTING TAP-CHANGES IN THREE- PHASE TRANSFORMERS and U.S. Pat. No. 3,579,092 to M. Matzl, May 18, 1971 for TAP- CI-IANGING REGULATING TRANSFORMER WITH soup STATE CIRCUITRY. The present invention is an immediate outgrowth of the systems disclosed in detail in the three above patents to M. Matzl to which reference may be had for certain details which will not 'be described below to avoid unnecessary repetitions.

The systems disclosed in the three above patents to M. Matzl include thyristor circuits and mechanical switching devices having relatively movable contacts. The latter must be operated in a predetermined sequence and timing. One object of this invention is to provide systems as disclosed in the above patents to M. Matzl with logic circuitry to automate the operation thereof.

Conventional transfer switches including tap-changing resistors, or change-over resistors, as disclosed in the aforementioned U.S. Pat. Nos. 3,396,254 and 3,493,698 have a timing of contact separation and a timing of engagement of the relatively movable contacts other than that required in a thyristor tap-changing system such as disclosed in the three above patents to M. Matzl. It is desirable to eliminate in conventional designs of transfer switches the tap-changing or change-over resistors thereof, to associate such transfer switches with switch-over thyristor networks as shown in U.S. Pat. Nos. 3,502,691; 3,534,246 and 3,579,092 to M. Matzl, and to provide means which enable such a drastic change of the entire system without changing the timing of contact engagement and the timing of contact separation of the constituent contacts of a more or less conventional Jansen-type transfer switch. It is another object of this invention to provide means which allow a transfer switch originally designed to be operatively related to tap-changing resistors, or change-over resistors, to be operatively related to tapchanging thyristor networks such as shown in the above Matzl references.

SUMMARY OF THE INVENTION Tap-changing switching systems or tap-changing transfer switches embodying this invention include a pair of thyristor networks, a pair of disconnect switch means each connected in series with one of said pair of thyristor networks and a pair of current-carrying contact means each arranged to be shunted across one of said pair of thyristor networks and one of said pair of disconnect switch means. The system further includes a pair of voltage sensors each sensing the voltage prevailing across one of said thyristor networks. The system further includes two pairs of position sensors one sensing the position of one of said pair of current-carrying contact means and the other the position of one of said pair of disconnect switch means. The output of said pair of voltage sensors and the output of said two pairs of position sensors is fed into a logic system the output of which controls a pair of trigger pulse generators for triggering the constituent thyristors of the aforementioned pair of thyristor networks.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of a tap-changing system including thyristor circuits to which the invention is intended to be applied; I

FIG. 2 is a diagrammatic representation of the sequence and timing of contact operations in a system as shown in FIG. 1 but including a transfer switch initially designed to be associated with ohmic change-over resistors rather than thyristor networks;

FIG. 3 is a circuit diagram of one logic circuitry embodying the present invention; and

FIG. 4 is a circuit diagram of another logic circuitry embodying the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Referring now to FIG. 1, numerals 1 and 2 have been applied to indicate contacts of a selector switch engaging taps U U of a tapped transformer winding Tr. The system includes two thyristor networks St, and Stg. The former network St, is made up of a pair of inversely parallel connected thyristors 7,8 and the latter network St is made up of a pair of inversely parallel connected thyristors 9,10. Network St, is connected in series with disconnect switch 5 and network St is connected in series with disconnect switch 6. Current-carrying switch 3 is arranged to be shunted across network St, and disconnect 5, while current-carrying switch 4 is arranged to be shunted across network St, and disconnect switch 6. Reference character Y has been applied to indicate an outgoing load current-carrying line.

Assuming it is intended to change from tap U; to tap U This is initiated by closing both disconnects or disconnect contacts 5,6. Thereupon the flow of load current is transferred or commutated from thyristors 7,8 to thyristors 9,10. Finally the current-carrying switch 4 is closed and disconnects 5,6 are opened, thus causing the load current to flow from tap U, by way of current-carrying switch 4 directly to outgoing line Y. Current-carrying switch 3 is opened shortly after closing of disconnects 5,6.

The timing of contact separation and contact engagement in conventional Jansen-type transfer switches is not quite that which has been indicated above. The timing of contact operation in conventional Jansen-type transfer switches involving switching resistors has been shown in FIG. 2 where duration of contact operation is plotted against time. It is apparent from FIG. 2 that in conventional Jansen-type transfer switches the times of operation of all contacts including current-carrying contacts 3,4 and of disconnect contacts 5,6 overlap.

This raises the problem of adapting a switch mechanism having overlapping contact operating times such as shown in FIG. 2 to control a pair of thyristor networks as shown in FIG. 1 in order to effect a tapchanging operation from a first tap U, to a second tap U,, or vice versa, from a second tap U, to a first tap U F I68. 3 and 4 each shows diagrammatically a circuitry which offersa solution to the above problem. The same reference characters have been applied in FIGS. 1, 3 and 4 to indicate like parts.

Thus in FIG. 3 reference character St, has been applied to indicate a first network including a pair of thyristors 7,8 inversely connected in parallel and reference character St, has been applied to indicate a second network including a pair of thyristors 9,10 inversely connected in parallel. The network St, further includes capacitor 40 and resistor 42 which are connected in series and the network St, further includes the capacitor 41 and the resistor 43 which are connected in series. Thyristors 7,8 may be triggered by trigger pulse transformer 11 the primary winding of which may be energized by trigger pulse generator 13. Thyristors'9,l0 may be triggered by trigger pulse transformer 12 the primary winding of which may be energized by trigger pulse generator 14.

In FIG. 3 reference numeral 15 has been applied to indicate a memory device having two inputs S and L and two outputs A and A. Application of a voltage signal at S results in a voltage signal at A, and application of a voltage signal at L results in a voltage signal at inverse output A. Memory device 15 is under the control of, two

position sensors

20,21 whose output indicates the respective position of current-carrying contact means 3,4. The state of memory device 15 stores information regarding the direction of tap change, i.e., whether the load current is supplied from tap U, by the intermediary of current-carrying contacts 3 and the circuitry is ready for a change to tap U,, or whether the opposite condition prevails, i.e., the load current is derived from tap U, by the intermediary of current-carrying contacts 4 and the circuitry is ready for a change to tap U,. If memory device 15 is in its state c there is an output voltage at A indicative of the fact that currentcarrying contacts 3 are closed and current-carrying contacts 4 open and the system is ready to switch outgoing load line Y from tap U, to tap U, On the other hand, if memory device 15 is in'itsstate F there is an output voltage at A rather than at A indicative of the fact that current-carrying contacts 4 are closed and current-carrying contacts 3 open and that the system is ready to switch outgoing load line Y from tap U, to tap U In FIG. 3

reference numerals

22 and 23 have'been applied to indicate a pair of switch position sensors of which the former senses the position of disconnect 'contacts S and the latter senses the position of disconnect contacts 6.

Reference numerals

24 and 25 have been applied to indicate a pair of voltage sensors of which the former senses the voltage prevailing across thyristor network St, and the latter senses the voltage prevailing across thyristor network

St Reference numerals

26,27,30,

31'and

38,39 have been applied in FIG. 3 to indicate AND gates arranged in pairs and reference nu- -

merals

34,35 have been applied in that figure to indicate a pair of OR gates. The output of AND-

gates

38,39 controls the trigger pulse generators l3 and 14, respectively, for triggering the thyristors in network St and network St respectively. The interrelation of the various circuit elements or components of FIG. 3 has clearly been indicated by appropriate lines and, therefore, does not need to be described by words.

The system shown in FIGS. 1 and 3 operates as stated below: Assuming that current-carrying contacts 3 are initially closed and that disconnect contacts 5 and 6 as well as current-carrying contact 4 are initially open. These positions of contacts 3,5,6 and 4 have been shown in FIG. I. The memory device 15 under control of sensor 20 emits a voltage signal at its terminal A which is a criterion for a transfer from tap U, to tap U The OR- gate 34 receives a voltage signal at one of its input terminals and therefore, transmits a voltage signal at to one of the two input terminals of AND-gate 38. The other input terminal of AND-gate 38 rece i ves a voltage signal from the inverse output terminal A of OR-gate 35. Since both input terminals of AND-gate 38 receive a voltage signal, the output signal of AND-gate 38 renders the trigger pulse generator 13 operative, thus causing triggering of thyristors 7 and 8.

It is immaterial as far as the operation of the system of FIG. 3 is concerned whether disconnect contacts 5 for thyristor network St, are closed following initiation of a tap-changing operation from tap U, to tap U,, or had remained closed during the stationary operation of the system, i.e., during the period of time network St, and disconnect contacts 5 had been shunted by current-carrying contacts 3. The function of disconnecting contacts 5 is to isolate thyristors 7 and 8 while the load current is being carried by current-carrying contacts 3. Disconnect contacts 5 must, of course, be closed when current-carrying contacts 3 open to make it possible for the current to commutate to network S1,. When disconnect contacts 6 close as the tap-changing operation progresses, one of its input signals is removed from AND-gate 30 and there is no voltage signal on any of the inputs of OR-gate 34. Hence there is no voltage signal at the AND-gate 38, as a result of which trigger pulse generator 13 is rendered inoperative and thyristors 7,8 are not triggered any longer. As soon as thyristors 7,8 begin to block the flow of current, the voltage sensor 24 transmits a voltage signal to the AND-gate 27 whose other input terminal is receiving a voltage signal from the terminal A of the tap-changing direction-determining memory device 15. As a result, trigger pulse generator 14 is rendered operative by the intermediary of OR-gate 35 and AND-gate 39, thus causing triggering of thyristors 9 and in network St When disconnect contacts 5 open both input terminals of AND-gate 31 receive a voltage signal and, therefore, another of the input terminals of OR-gate 35 is supplied with a voltage signal. Closing of current-carrying contacts 4 has the same effect as opening of disconnect contacts 5. In the former case sensor 21 transmits a voltage signal to OR-gate 35, and this voltage signal is also transmitted to the input terminal L of the tapchanging direction-determining memory device 15, t hus causing a voltage signal to appear at the terminal A of memory device 15. Now the system is ready for the next tap-changing operation.

In FIG. 4 the same reference characters have been applied as in FIG. 3. Therefore FIG. 4 calls for description only to the extent that it differs from that of FIG. 3.

The system of FIG. 4 is intended to be connected to two thyristor networks identical to the thyristor networks St and St of FIG. 3. This may be achieved by trigger pulse transformers as shown in FIG. 3. FIG. 4 differs from FIG. 3 by addition in the former of a pair of

memory devices

16 and 17 of which the former stores the condition of thyristors 7,8 in network St, and the latter stores the condition of thyristors 9,10 in network

St Memory devices

16 and 17 are preferably interlocked.

In FIG. 4 the same reference characters as in FIG. 3 have been applied to gates correlating the same parameters, i.e., those sensed by sensors to 25.

Reference characters

28,29 in FIG. 4 have been applied to indicate AND-gates and

reference characters

32,33 and 36,37 have been applied to indicate OR- gates.

The interconnection of the components of FIG. 4 is self-explanatory by the lines indicating the connections thereof.

The operation of the system of FIGS. 1 and 4 is as follows:

Assuming that in the initial state of the system currentcarrying contacts 3 are closed, and that disconnect contacts 5 and 6 as well as current-carrying contacts 4 are open so that the load current flows from tap U by way of contacts 1 and 3 directly to the outgoing line Y. Memory device 16 then has an output voltage since current-carrying contacts 3 are closed, and memory device 17 has no output voltage since current-carrying contacts 4 are open. Since current-carrying contacts 3 are closed, there is a voltage signal at the output terminal A of the memory device 15. Upon closing of disconnect contacts 5 and subsequent opening of current-carrying contacts 3 the load current is commutated to thyristors 7,8 of network St since trigger pulse generator 13 is operative. Closing of disconnect contacts 6 causes sensor 23 to transmit a voltage signal to AND-gate 28. The latter transmits a voltage signal to OR-gate 32, and OR-gate 32 transmits a voltage signal to the input terminal L of memory device 16 controlling the state trigger pulse generator 13 is in. Memory device 16 then causes trigger pulse generator 13 to become ineffective so that thyristors 7,8 are not being triggered any longer. Though thyristors 7,8 in network St are not triggered any longer, the flow of the load current through thyristors 7,8 continues until the next zero of the current wave. Following that current zero, thyristors 7,8 block the flow of current. As soon as the blocking voltage of thyristors 7,8 reaches a predetermined level sensor 24 transmits a voltage signal to one of the input terminals of AND-gate 27. The other input terminal of AND-gate 27 is likewise supplied with a voltage signal since there is a voltage at the output terminal A of memory device 15. As a result, trigger pulse generator 14 is rendered operative by the intermediary of memory device 17 and OR-gate 37, thus triggering thyristors 9,10 in network St Thereupon disconnect contacts 5 are opened and current-carrying contacts 4 closed. As a result, the state of memory device 15 storing the direction of the tapchanging operation is reversed. The last step in the tapchanging operation consists in fully opening of the disconnect contacts 6. Henceforth the load current flows from tap U by way of contacts 2 and 4 to the outgoing line Y.

The circuitry of FIGS. 1,3 and 4 requires that there be an overlap of the time of operation of the disconnect contacts 5,6, i.e., that both remain simultaneously closed for a period of time lasting at least half a period, or half a cycle, of the current wave. If this condition were not met, one of the disconnect contacts 5,7, i.e., the one pair that had been carrying the load current, would be compelled to part under load, or break a load current, because its associated thyristors have not turned as yet to their blocking state. This would be a rather undesirable condition since the disconnect contacts 5,6 are not intended to be used for current interruption and not designed to perform this function.

The systems shown in FIGS. 1-4 are but two of many possible embodiments of the invention. The invention may be expressed more generally in terms of Boolean algebra. Vitally necessary structural elements of the invention are the three pairs of

sensors

20,21;22,23 and 24,25. The state of the various components of the circuitry of FIG. ll may be described by the following symbols:

c tap-change in one direction and Ftap-change in the opposite direction. 0 and Fare indicated by the output of memory device 15.

a and lithe two states which one of the pair of disconnect contacts may assume, a referring to the open contact state and Eto the closed state.

b and Ethe two states which the other pair of disconnect contacts may assume, b referring to the open contact state and Fto the closed contact state.

d and Jthe two states of one of the thyristor networks, d referring to the state wherein there is a blocking voltage and Zi to the state wherein there is no such voltage.

h and Fthe two states which the pairs of current-carrying contacts may assume, h referring to the open contact state and i zto the closed contact state.

The logic circuitry of this invention may then be described by the term In the embodiment of the invention of FIGS. 1 and 4 wherein the

trigger pulse generators

13,14 are under works. This end is achieved by virtue of the fact that the logic circuitry according to this invention compensates for the timing of contact operation of Jansen-type tap-changing transfer switches originally designed to include tap-changing resistors, or switching resistors, which timing is normally unacceptable where changes from one tap to another are intended to be effected by a pair of thyristor networks.

The invention makes it further possible to convert a regulating transformer including a resistor transfer switch by substituting a pair of thyristor networks for the resistors of the transfer switch and adding the above logic to the regulating transformer. In effecting such a change the existing auxiliary contacts of the transfer switchwhich had been previously connected to the resistors thereof are connected to a pair of thyristor networks, as described above.

A system as shown in FIG. 1 requires, in addition to a transfer switch, also a selector switch to make it possible to change between a large number of taps rather than merely two taps of a tapped transformer winding. The selector switch has been deleted in FIG. 1 since its presence or absence has no bearing on the present invention and since selector switches and the way to connect them into circuits is well known in the art and fully disclosed, inter alia, in the above referred-to US. Pat. Nos. 3,396,254 and 3,493,698.

I claim as my invention:

1. In a transfer switch for tap-changing regulating transformers the combination of a. a pair of thyristor networks each including means for connection thereof to one of a pair of taps of a tapped transformer winding, each of said pair of networks including a pair of inversely parallel connected thyristors;

i b. two pairs of disconnect contacts each serially connected to one of said pairs of networks to connect selectively each of said pair of networks to, and to disconnect selectively each of said pair of networks from, an outgoing load current-carrying line;

c. two pairs of current-carrying contacts each arranged to be shunted across one of said pair of networks and one of said two pairs of disconnect contacts;

d. a pair of voltage sensors each for sensing the voltage across one of said pair of networks;

e. a first pair of position sensors each sensing the position of one of said two pairs of disconnect contacts;

f. a second pair of position sensors each sensing the position of one of said two pairs of current-carrying contacts;

g. a pair of trigger pulse generators each triggering the constituent thyristors of one of said pair of networks; and

h. a logic circuitry under the control of said pair of voltage sensors, said first pair of position sensors and said second pair of position sensors for selectively activating and de-activating said pair of trigger pulse generators.

2. A transfer switch as specified in claim 1 wherein said logic circuitry includes a memory device for storing information as to the direction of the tap-changing operation to be effected, said memory device being under the control of said second pair of position sensors.

3. A transfer switch as specified in claim 2 wherein said logic circuitry includes a pair of additional memory devices for storing information as to the direction of the tap-changing operation to be effected, each of said pair of additional memory devices being under the control of said second pair of position sensors and having an output terminal connected to an input terminal of one of said pair of trigger pulse generators.

4. A transfer switch for tap-changing regulating transformers including a. a pair of thyristor networks each including means for connection thereof to one of a pair of taps of a tapped transformer winding, each of said pair of networks including a pair of inversely parallel connected thyristors;

b. two pairs of disconnect contacts each serially connnected to one of said pairs of networks to connect selectively each of said pair of networks to, and to disconnect selectively each of said pair of networks from, an outgoing load current-carrying line;

f. a second pair of position sensors each sensing the.

position of one of said two pairs of currentcarrying contacts;

h. a logic circuitry under the control of the output signals of said pair of voltage sensors, said first pair of position sensors and said second pair of position sensors and complying with the Boolean term wherein a signifies the position of one of said two pairs of disconnect contacts, b signified the position of oneof said two pairs of current-carrying contacts, 0 and Fsignify the respective direction in which the tap-changing operation is effected, d signifies the blocking voltage of one of said networks, and h signifies the position of one of said two pairs of current-carrying contacts.

b X ac) for selectively activating and de-activating said pair of trigger voltage generators.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. Dated Jan. 9,

Invenwfl Manfred Matzl It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' On the cover page, left column, insert the following after line 6 Assignee: Maschinenfabrik Reinhausen Gebruder Scheubeck K. G.

I Regensburg, Ger a Signed and sealed this 13th day of NOT/ember 1973 (SEAL) Attest:

EDWARD M. PLETCI-IERJR. RENE D. TEGTMEYER 'Attesting Officer v Acting Commissioner of Patents mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pawn No. 3, 232 T Dated Jan. 9, 1973 Invenwfl Manfred Matz].

I, It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

0n the cover page, left column, insert. the following after line 6 Assignee: Maschinenfabrik Reinhausen Gebruder Schenbeck K. G.

Regensburg, Ger a Signed and sealed this 13th day of November 1973.

(SEAL) 4 Attest:

EDWARD M. FLETCHEILJR. RENE D. TEGTMEYER At testing Officer Acting Commissioner of Patents

Claims

1. In a transfer switch for tap-changing regulating transformers the combination of a. a pair of thyristor networks each including means for connection thereof to one of a pair of taps of a tapped transformer winding, each of said pair of networks including a pair of inversely parallel connected thyristors; b. two pairs of disconnect contacts each serially connected to one of said pairs of networks to connect selectively each of said pair of networks to, and to disconnect selectively each of said pair of networks from, an outgoing load current-carrying line; c. two pairs of current-carrying contacts each arrAnged to be shunted across one of said pair of networks and one of said two pairs of disconnect contacts; d. a pair of voltage sensors each for sensing the voltage across one of said pair of networks; e. a first pair of position sensors each sensing the position of one of said two pairs of disconnect contacts; f. a second pair of position sensors each sensing the position of one of said two pairs of current-carrying contacts; g. a pair of trigger pulse generators each triggering the constituent thyristors of one of said pair of networks; and h. a logic circuitry under the control of said pair of voltage sensors, said first pair of position sensors and said second pair of position sensors for selectively activating and deactivating said pair of trigger pulse generators.

4. A transfer switch for tap-changing regulating transformers including a. a pair of thyristor networks each including means for connection thereof to one of a pair of taps of a tapped transformer winding, each of said pair of networks including a pair of inversely parallel connected thyristors; b. two pairs of disconnect contacts each serially connnected to one of said pairs of networks to connect selectively each of said pair of networks to, and to disconnect selectively each of said pair of networks from, an outgoing load current-carrying line; c. two pairs of current-carrying contacts each arranged to be shunted across one of said pair of networks and one of said two pairs of disconnect contacts; d. a pair of voltage sensors each for sensing the voltage across one of said pair of networks; e. a first pair of position sensors each sensing the position of one of said two pairs of disconnect contacts; f. a second pair of position sensors each sensing the position of one of said two pairs of current-carrying contacts; g. a pair of trigger pulse generators each triggering the constituent thyristors of one of said pair of networks; and h. a logic circuitry under the control of the output signals of said pair of voltage sensors, said first pair of position sensors and said second pair of position sensors and complying with the Boolean term a X b + c(c) X d + h wherein a signifies the position of one of said two pairs of disconnect contacts, b signified the position of one of said two pairs of current-carrying contacts, c and c signify the respective direction in which the tap-changing operation is effected, d signifies the blocking voltage of one of said networks, and h signifies the position of one of said two pairs of current-carrying contacts.

5. A transfer switch as specified in claim 4 wherein said logic circuitry includes means complying with the Boolean terms c(c) X d + h and b X c(c) for selectively activating and de-activating said pair of trigger voltage generators.