EP3766170A1 - Ac/dc vsc connected to three or more dc lines - Google Patents
Ac/dc vsc connected to three or more dc linesInfo
- Publication number
- EP3766170A1 EP3766170A1 EP18712839.2A EP18712839A EP3766170A1 EP 3766170 A1 EP3766170 A1 EP 3766170A1 EP 18712839 A EP18712839 A EP 18712839A EP 3766170 A1 EP3766170 A1 EP 3766170A1
- Authority
- EP
- European Patent Office
- Prior art keywords
- terminal
- arm
- voltage
- converter
- voltage source
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Pending
Links
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Classifications
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
- H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
- H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/79—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/797—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
- H02M7/4835—Converters with outputs that each can have more than two voltages levels comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/0083—Converters characterised by their input or output configuration
- H02M1/009—Converters characterised by their input or output configuration having two or more independently controlled outputs
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]
Definitions
- the present invention generally relates to voltage source converters. More particularly the present invention relates to a voltage source converter for conversion between alternating current, AC, and direct current, DC.
- Multiterminal High Voltage Direct Current HVDC
- MTDC Magnetic Multiple Access
- VSCs Voltage Source Converters
- WO 20 13/ 071962 discloses a converter that is connected to two different DC systems. However, it is uncertain if the converter is intended to be used in a DC system or not. There is also no discussion about improving load flow controllability in such a system.
- the invention is provided for addressing this problem of improving the load flow control capability in a DC system.
- the present invention is directed towards obtaining a converter for converting between alternating current and direct current and which converter is capable of improving the load flow control capability in a DC system.
- This object is according to a first aspect of the present invention achieved through a voltage source converter for conversion between alternating current, AC, and direct current, DC, and being connected between at least three direct current, DC, terminals and comprising three parallel AC phase modules each comprising an corresponding AC terminal, each AC phase module further comprising:
- phase leg connected between the first and second DC terminals and the second upper converter arm stretching out from a first connection point on the first phase leg located between the AC terminal and the second DC terminal to a third DC terminal, where the phase leg comprises
- submodules for supplying a first DC voltage between the first and second DC terminal for interfacing with a first DC line and an AC voltage on the AC terminal for connection to an AC network
- the second upper converter arm comprises circuitry for, together with the submodules between the first connection point and the first DC terminal, providing a second DC voltage between the first and third DC terminals for interfacing with a second DC line, wherein the first DC terminal is adapted to be connected to ground.
- the object is according to a second aspect achieved through a direct current power transmission system comprising a voltage source converter according to the first aspect.
- the present invention has a number of advantages. It improves the DC load-flow controllability, which allows an optimization of the load flow distribution to be made, leading to higher transmission capability and lower losses. It can also be used to boost the DC- voltage, which may be useful for example for a long DC line with a significant voltage drop along the line. It also provides a greater flexibility in that it allows the possibility to interconnect DC grids with different voltages without the provision of additional DC/ DC converters.
- fig. 1 schematically shows a first type of multiterminal DC grid comprising a number of interconnected voltage source converters
- fig. 2 schematically shows an expansion of the first multiterminal DC grid
- fig. 3 schematically shows two converters interconnected by a
- fig. 4 schematically shows a first version of a voltage source converter having a number of arms provided for a number of phases, where for one phase there is a lower arm, common valve arm and two branched arms
- fig. 5 schematically shows a second version of voltage source converter having a lower arm, common valve arm and three branched valve arms for a phase
- fig. 6 schematically shows a model of the common valve arm and two branched arms of the converter shown in fig. 4,
- fig. 7 schematically shows a plotting of a boosting factor as a function of active AC input power for a converter modeled according to the model
- fig. 8 schematically shows the boosting factor as a function of the branched arm voltage factor for a converter modeled according to the model
- fig. 9 schematically shows a variation of the voltage source converter having a lower arm, common valve arm and two branched arms for a phase, where one of the branched arms is a branched valve arm and the other is an AC component handling arm, and
- fig. 10 schematically shows the arms in a fourth version of voltage source converter associated with one phase, without a common valve arm, but comprising a lower arm, two branched arms and a disconnector.
- Fig. 1 shows a first type of multiterminal direct current system 10.
- a first converter 12 converting between alternating current (AC) and direct current (DC)
- a second converter 14 converting between AC and DC
- a third converter 16 converting between AC and DC
- VSCs voltage source converters
- MMCs modular multilevel converters
- the first converter 12 is connected to the second converter 14 via a first DC line 18.
- the first converter 12 is also connected to the third converter 16 via a second DC line 20.
- Fig. 2 schematically shows the system of fig. 1, after expansion with two further converters.
- a fourth converter 24 converting between AC and DC and a fifth converter 26 also converting between AC and DC, where the fourth converter 24 is connected to the second converter 14 via a fourth power line 28 , while the fifth converter 26 is connected to the third converter 16 via a fifth power line 30.
- the fourth and fifth converters 24 and 26 are finally interconnected via a sixth power line 32.
- Fig. 3 schematically shows a second type of“point-to-point” system where the first, second and third converters are connected in series on the DC side.
- the second converter 14 is here connected to the first converter 12 via the first power line 18 , which is connected to the third converter 16 via the second power line 20. There is thus no DC connection between the second and third converters.
- the first converter 12 may either tap or inject power and may also perform voltage boosting.
- Fig. 4 schematically shows a first type of converter that may be used in the different previously disclosed converter stations, for instance as the first converter 12.
- the converter is an MMC type converter with integrated DC/ DC conversion functionality.
- the converter 12 is a modular multilevel converter (MMC) that converts between Direct Current (DC) and Alternating Current (AC) and may with advantage be connected as a converter in the grid 10 or as a tapping/boosting converter in a point-to-point system.
- the converter has three DC terminals DC1, DC2 and D3 for enabling the provision of at least two different DC voltages and a number of AC terminals ACA1, ACB1,
- the converter comprises three parallel AC phase modules, each
- first phase module comprising a lower converter arm la comprising a lower arm chain link CLAL connected between the first DC terminal DC1 and the AC terminal ACA1 of the first phase.
- the AC terminal ACA1 is further connected to an upper common arm uaO comprising a common chain link CLAC, where the upper common arm uaO with upper common chain link CLAC is in turn connected to a second DC terminal DC2 via a first branching arm ual comprising a first branching chain link CLAB1.
- the upper common arm uaO is in this embodiment furthermore connected to a third DC terminal DC3 via a second branching arm ua2 comprising a second branching chain link CLAB2.
- the common arm uaO and first branching arm ual in this case make up a first upper converter arm that is connected between the first AC terminal ACA1 and the second DC terminals DC2.
- the lower and first upper converter arms la, uaO and ual together form a phase leg connected between the first and second DC terminals DC1 and DC2, where the chainlinks of this phase leg provides a first DC voltage between the first and second DC terminals DC1 and DC2 for interfacing with a DC line, for instance the first DC line.
- the second branching arm ua2 in turn forms a second upper converter arm that stretches out from a first connection point CP on the first phase leg to the third DC terminal DC3.
- connection point CP is located between the AC terminal ACA1 and the second DC terminal DC2, where the connection point CP in this embodiment is a connection point between the upper common arm uaO and the first branching arm ual, which is at a junction between the common chain link CLAC and the first branching chain link CLAB1.
- the common arm uaO thereby stretches between the AC terminal ACA1 and the connection point CP, while the first branching arm ual stretches between the connection point CP and the second DC terminal DC2.
- the chain links comprise submodules and therefore, it is also clear that the first
- connection point CP is provided between two submodules of the first upper arm.
- the second upper converter arm comprises circuitry for, together with the chainlinks CLAL and CLAC between the first connection point and the first DC terminal DC1, provide a second DC voltage between the first and third DC terminals DC1 and DC3 for interfacing with another DC line, such as the second power line 20.
- the circuitry is made of the second branching chain link CLAB2 for forming the AC voltage and the second DC voltage.
- the converter realization in respect of the second and third phases is the same.
- the second converter module thus comprises a lower chain link CLBL connected between the first DC terminal DC1 and a second AC terminal ACB1 of the second phase.
- the second AC terminal ACB1 is also connected to an upper common chain link CLBC, where the upper common chain link CLBC is in turn connected to the second DC terminal DC2 via a first branching chain link CLBB1 and to the third DC terminal DC3 via a second branching chain link CLBB2.
- the third converter module comprises a lower chain link CLCL connected between the first DC terminal DC1 and a third AC terminal ACC1 of the third phase.
- the third AC terminal ACC1 is also connected to an upper common chain link CLCC, where the upper common chain link CLCC is in turn connected to the second DC terminal DC2 via a first branching chain link CLCB1 and to the third DC terminal DC3 via a second branching chain link CLCB2.
- control unit 34 set to control the different chain links provided for the different phases, which control involves the forming of an AC voltage on a corresponding AC terminal for connection to an AC network. At times it may also be involved in handling various faults.
- each chain link may comprise a number of series-connected or cascaded submodules, where a submodule may be realized as a half-bridge submodule, a full-bridge submodule or as a hybrid between the two.
- a half-bridge submodule comprises two switches connected in parallel with an energy storage element, for instance realized as a capacitor. One submodule terminal is then provided at the junction between the two switches and the other submodule terminal is provided at a junction between one of the switches and the energy storage element.
- the half-bridge submodule is configured to either provide a zero voltage or a unipolar voltage corresponding to the voltage across the submodule capacitor.
- a full-bridge submodule comprises two strings of series connected switches connected in parallel with the energy storage element, where one submodule terminal is provided at the midpoint of one of the strings, while the other submodule terminal is provided at the midpoint of the other string.
- the full-bridge submodule has a a zero and bipolar voltage contribution capability corresponding to the voltage across the capacitor.
- the converters shown so far are all asymmetrical monopole converters where the first DC terminals is connected to ground potential and the second, third and fourth DC terminals are connected to the same or different positive poles of a DC system such as the DC grid.
- the AC terminals have an AC waveshape with a DC offset, which DC offset may be removed using a transformer.
- the converter may be a bipole converter, in which the previously described converter structure is complemented by a mirrored converter structure connected between the first DC terminal and a fifth, sixth and seventh DC terminal for connection to corresponding negative poles.
- the converter may be a symmetric monopole converter.
- the lower arm would be connected to two or more branching arms in the same way as the common arm.
- Fig. 5 shows a variation of the converter that is suitable when connection is made to three DC lines, such as the second converter 14 in fig. 2.
- the converter is the same as in fig. 4 except for the fact that there is a further branching arm for each phase connected to a further DC terminal. There is therefore in this case a fourth DC terminal DC4 for connection to the third DC line.
- each upper common chain link CLAC, CLBC and CLCC is additionally connected to the fourth DC terminal DC4 via a corresponding third branching chain link CLAB3, CLBB3 and CLCB3.
- a converter may be connected to a
- multiterminal DC grid in which case it may be connected to two or more DC lines. If these connections are made using the same DC terminal then there may be a problem of load flow controllability being reduced.
- Each phase leg of the above shown converter realizations comprises a lower arm l a , a common upper arm uaO , and a number of split or branching arms ual,ua2,...,uaN as illustrated in Fig. 5.
- x is a distribution factor for the alternating current between the split arms. Accordingly, the values of x n must satisfy Similar to the lower arm, the active power resulting from the AC and DC components must also cancel out in the upper arm, that is,
- the distribution factor x n of a branched arm thereby depends on the active power of the branched arm, the relationship y between the voltage contribution of the common arm and the voltage contribution Vo of the lower arm and the active power of the AC terminal. It can more
- the distribution factor x n which may depend on the per phase active power being separate from zero, may be provided as the active power of the branched arm divided by the active power of the AC terminal and by a variable comprising the relationship y.
- the variable may more particularly be 1/ k - 1- y, where k is ratio of the AC current distribution between the lower and upper arms.
- the DC voltages Vo and yVo can be chosen such that It can thereby be seen that DC voltage contributions for the lower arm Vo and the common upper arm y*Vo may be chosen so that a maximum absolute value of an AC current distribution factor between the branched arms is kept below or equal to a maximum distribution factor value.
- the invention is in no way limited to voltage differences of ⁇ 10 %. Higher differences such as connecting a 400 kV DC- grid to a 320 kV DC-grid is also possible. For different cases the voltage difference and range in which the voltage can be varied can have a different impact on the required dimensioning of the arms.
- the vdci, Vdc2, and vdc3 are 440 , 360 , and 400 kV, respectively, which may be the voltages between the second and first DC terminal, the voltage between the third and first DC terminal and the voltage between the fourth and first DC terminal.
- the direct current from each line flowing into the converter per phase may as an example be 0.8 kA, 0.7 kA, and -0.5 kA, respectively.
- Pd2 2udcid2 per-phase output power at terminal DC2
- a voltage difference p2 to a nominal DC voltage at the second DC terminal is set based on the power Pdl at the third DC terminal DC3 and on at least one scaling of the power P at the AC terminal, where equation 21 in fact sets out two such scalings.
- the voltage difference p2 to the nominal DC voltage ud at the second DC terminal DC2 is set as a first scaled input power divided by a difference between a second scaled input power and the power Pdl at the third DC terminal DC3, where the first scaling factor comprises the fraction q of the AC voltage applied across the first branched arm ual times the part of the AC current ( l-x) running through the first branched arm ual and the second scaling factor is formed as one minus the first scaling factor.
- Equation (21) Equation (21) will now be evaluated for two cases, which correspond to two realistic applications of the converter.
- a converter equipped with branched arms is placed as a tapping station near the center of a long DC line, such as the converter 12 in fig. 3.
- the line from the sending-end converter 14 is connected to terminal DC3, so Pdl ⁇ 0.
- the tapping converter could be located either at a generation center (P > 0) or at a load center (P ⁇ 0).
- the objective for the boosting functionality is to be performed either at a generation center (P > 0) or at a load center (P ⁇ 0).
- tapping at a load center gives a steeper rise of the boosting factor as I PI increases as compared to tapping at a generation center (P > 0 ).
- q ⁇ 0 must be used for P ⁇ 0 , so an increased rating of the common-arm cell string may be needed.
- the second application is a converter station where the DC lines have the same power direction, i.e., the signs of Pdl and Pd2 are equal.
- the voltage difference p2 to the nominal DC voltage ud at the second DC terminal DC2 is set as a fraction q of the AC voltage applied across the first branched arm ual divided by a difference between one half and the fraction q. More generally speaking, if a fraction n of the total power P is to be provided by the second branching arm, then the boosting level p2 of the first branching arm may be set as q divided by ( 1 - q - n), where q is the part of the phase voltage taken up by the split arms.
- circuitry may be in the form of a parallel LC circuit tuned to the fundamental frequency. This is shown in fig. 9, where the second branching arms of the three phases are realized as filters FA, FB and FC.
- the circuitry could be realized as a pure inductance. This would result in an AC component in the second branched arm, but it would be 90° phase shifted to uv and thus would not produce active power in the first branched arm, where it would circulate. However, a very large inductance would be needed to obtain a low enough current.
- the AC component can be reduced to a minimum even with a relatively small inductance.
- Tapping/ boosting (Pd l ⁇ 0) with inverter operation (P ⁇ 0 ) requires— as predicted by theory— a modest increase of the peaks of the common-arm insertion indices. Since the branched-arm insertion indices are within the range [0 , 1] , use of half-bridge cells in the branched arms of the second DC terminal is sufficient.
- Tapping/ boosting (Pd l ⁇ 0) with rectifier operation (P > 0 ) reduces the peaks of the common-arm insertion indices, but the branched-arm insertion indices are now periodically negative. This implies that at least some of the cells in the branched arms of the second DC terminal must use full bridges.
- a boosting of p2 2% may be sufficient.
- the branch-arm chain link responsible for the boost can then be reduced to only six submodules rated at 3 kV.
- the first connection point CP is thus at the AC terminal ACA1.
- the first branched arm in this case comprises a first and a second first branched arm chainlink CLAB1A and CLAB1B, while the second branch arm comprises a first and second branched arm chain link CLAB2A and CLAB2B.
- the second and third DC terminals DC2 and DC3 are also interconnected by a mechanical disconnector MD.
- the use of the converter may be the following:
- the converter is used to force the current through the mechanical disconnector MD to zero by diverting it through the submodules in the chain links CLAB1A, CLAB1B, CLAB2A and CLAB2B.
- the mechanical disconnector MD may be opened
- the mechanical disconnector is a part of a DC circuit breaker that in this case is also connected between the two DC terminals DC2 and DC3.
- the configuration in fig. 10 is also interesting without a mechanical disconnector MD.
- a DC fault on one of the lines can be handled by the converter.
- the converter may also remain operational during and after the fault (with one DC voltage level reduced to zero).
- control unit may be realized in the form of discrete components.
- a computer program product carrying this code can be provided as a data carrier such as one or more CD ROM discs or one or more memory sticks carrying the computer program code, which performs the above-described control functionality when being loaded into a processor performing the role of control unit of the voltage source converter.
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- Engineering & Computer Science (AREA)
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Abstract
Description
Claims
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
PCT/EP2018/056507 WO2019174732A1 (en) | 2018-03-15 | 2018-03-15 | Ac/dc vsc connected to three or more dc lines |
Publications (1)
Publication Number | Publication Date |
---|---|
EP3766170A1 true EP3766170A1 (en) | 2021-01-20 |
Family
ID=61763949
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP18712839.2A Pending EP3766170A1 (en) | 2018-03-15 | 2018-03-15 | Ac/dc vsc connected to three or more dc lines |
Country Status (2)
Country | Link |
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EP (1) | EP3766170A1 (en) |
WO (1) | WO2019174732A1 (en) |
Family Cites Families (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US9065328B2 (en) * | 2011-11-16 | 2015-06-23 | Abb Technology Ag | AC/DC multicell power converter for dual terminal HVDC connection |
CN106253728B (en) * | 2016-08-15 | 2019-02-22 | 上海交通大学 | Multi-port modular multi-level converter for Multi-end flexible direct current transmission application |
-
2018
- 2018-03-15 EP EP18712839.2A patent/EP3766170A1/en active Pending
- 2018-03-15 WO PCT/EP2018/056507 patent/WO2019174732A1/en active Application Filing
Also Published As
Publication number | Publication date |
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WO2019174732A1 (en) | 2019-09-19 |
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Inventor name: NEE, HANS-PETER Inventor name: NORRGA, STAFFAN Inventor name: HEINIG, STEFANIE Inventor name: HARNEFORS, LENNART Inventor name: JOHANSSON, NICKLAS Inventor name: ILVES, KALLE |
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