WO2014146241A1 - Bipolar universal converter station and mtdc system thereof - Google Patents

Bipolar universal converter station and mtdc system thereof Download PDF

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Publication number
WO2014146241A1
WO2014146241A1 PCT/CN2013/072842 CN2013072842W WO2014146241A1 WO 2014146241 A1 WO2014146241 A1 WO 2014146241A1 CN 2013072842 W CN2013072842 W CN 2013072842W WO 2014146241 A1 WO2014146241 A1 WO 2014146241A1
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WO
WIPO (PCT)
Prior art keywords
return
bipolar
converter station
pole
universal converter
Prior art date
Application number
PCT/CN2013/072842
Other languages
French (fr)
Inventor
Dawei YAO
Xiaobo Yang
Chunming YUAN
Original Assignee
Abb Technology Ltd.
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Application filed by Abb Technology Ltd. filed Critical Abb Technology Ltd.
Priority to CN201380038417.0A priority Critical patent/CN104508933B/en
Priority to PCT/CN2013/072842 priority patent/WO2014146241A1/en
Publication of WO2014146241A1 publication Critical patent/WO2014146241A1/en

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Classifications

    • 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
    • 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 invention relates to the field of MTDC(Multi-terminal Direct Current) technology, and more particularly to a bipolar universal converter station and MTDC system thereof.
  • the HV converter station cannot realize current direction reversion, cannot realize current return mode transition between remote return and local return, either.
  • the present invention provides a bipolar universal converter station and MTDC system thereof. According to an aspect of the present invention, it provides abipoiar universal converter station.
  • the bipolar universal converter station can beconfigured to be used as a rectifier or aninverter, to be connected to or disconnected from the DC system smoothly.and/or to be parallel or series connected to a MTDC system; and/or the current directionand/or redesignagedirection of said bipolar universal converter station can be changed flexibly;and/or the grounding current return mode can be transited between metallic return mode and earth return mode.
  • thebipoiar universal converter station comprises: at least one AC/DC converter for each pole.
  • the bipolar universal converter station further comprises: a first switching arrangement ⁇ 11 , 12) for each pole configured to realize the parallel or series connection of the bipolar universal converter station to DC system.
  • bipolar universal converter station further comprises: a second switching arrangement (23, 24) for each AC/DC converter configured to change the current and/or voltage direction of the bipolar universal converter stationwith the coordination with converter isolators (2 , 22).
  • the bipolar universal converter station further comprises: a metallic return line 51 for each pole configured to realize the transition between earth return and metallic return; and the transition between iocai return and remote return.
  • the first switching arrangement comprises two switches, in which one 12 of the two switches is configured to realize the series connection of the AC/DC converter to the DC system; the other 11 is configured to realize the parallel connection of the AC/DC converter to DC system.
  • the second switching arrangement comprises two switches, in which closing of both two switches (23, 24) and opening of both converter isolators (21 , 22) is configured to form a second power flow pathor opening of both said two switches and closing of both converter isolators is configured to form a second power flow path.
  • one terminal of the metallic return line is connected to the pole line 92; the other terminal of the metallic return line is connected to converter side terminal of NBS.
  • connection mode is series or parallel connection; and thegrounding current return mode is metallic return or earth return.
  • the present invention provides a method for changing the connection mode of the bipolar universal converter station mentionedabove.
  • the method comprises: providing the first switching arrangement in each pole between the pole line and neutral bus of the bipolar universal converter station; blocking the converters in each pole; switching the two switches of the first switching arrangement between open status and closed status; and de-blocking the converters in each pole.
  • the present invention provides a method for changing the current and/or voltage direction of the bipolar universal converter station above mentioned.
  • the method comprises: providing the second switching arrangement in each pole; blocking and isolating the converters in each pole; switching the two switches of the second switching arrangement and converter isolators between open status and closed status to form a different power flow path; and de-blocking the converters in each pole.
  • the present invention provides a method for transiting between earth return and metallic return of the bipolar universal converter station above mentioned.
  • the method comprises: for transition from earth return to metallic return, enabling the metallic return line in the pole with fault, closing NBS in the pole with fault to form a metallic return path and opening MRTB to cut off the earth return path; and for transition from metallic return to earth return, closing MRTB to form a earth return path, opening the NBS in the pole recovering from fault to cut off the metallic return path and disable the metallic return line.
  • it provides a MTDC system.
  • the MTDC system comprises at least one bipolar universal converter station above mentioned.
  • the MTDC system can be operated with different system return modes and the transition between modes can be realized without system shutdown.
  • the system return mode is local return mode or remote return mode in system level.
  • connection mode the connection mode
  • converter mode the connection mode
  • current return mode it is unnecessary for the system to be shutdown or restarted during transferring the connection mode, converter mode and/or current return mode.
  • the present invention provides amethod for transiting between local return mode and remote return mode of MTDC system formed by the MTDC system above mentioned.
  • the method comprises: blocking the converters in each pole of the remote station; enabling the metallic return lines in each pole of the remote station to form a remote return path; connecting neutral bus of local station to form a local return path; opening NBS in remote station to realize the transition from remote return to local return of MTDC system ;and opening NBS in local station to realize the transition from local return to remote return of MTDC system.
  • Embodiments of the present invention provide a bipolar universal converter station and MTDC system thereof, which realizes rectifier or inverter function connected to a MTDC system in parallel or series, and changesthe current and/or voltage direction of thebipolar universal converter station compared with the designeddirection of the MTDC system.
  • Fig.1 illustrates a configuration of a bipolar universal converter station according to an embodiment of the present invention
  • Fig .2 illustrates a configuration of a converter in prior art
  • Fig.3 illustrates a flow chart of a method for changing the connection mode of the bipolar universal converter station according to an embodiment of the present invention
  • Fig.4a illustrates a single line diagram of a 4-terminal series MTDC system with proposed bipolar universal converter stations
  • Fig.4b illustrates a single line diagram of the system under remote return operation after bypassing of the terminal Ri and ; and Fig.4c illustrates a single line diagram of the system under local return operation after return mode transition in system level.
  • Fig.1 illustrates a configuration of a bipolar universal converter station according to an embodiment of the present invention.
  • a bipolar universal converter station can be configured as a rectifier or an inverter connected to an MTDC system in parallel or series;the bipolar universal converter station can be connected to or disconnected from the DC system smoothly;and/or the currentand/or voltage direction of thebipolar universal converter station can be changed flexibly to meet the requirements of MTDC system (or DC grid) operation; and/or the grounding current return mode can be transited between metallic return mode and earth return mode.
  • thebipolar universal converter station comprises:at least one AC/DC converter in each pole (8a, 8b); a first switching arrangement comprised of metallic conduction lines (13a, 13b) and related switches ⁇ 11a, 11b and 12a, 12b); a second switching arrangement comprising switches (23a, 24a and 23b, 24b)in parallel with the converters(8a, 8b); converter isolators (21a, 22a and 21 b, 22b); two NBSs (Neutral Bus Switch, 4a and 4b); metallic return lines (51a, 51 b) and related switches (5a, 5b); anMRTB (Metallic Return Transit Breaker, 6) and an earth electrode 7;and joints of station and system (91a, 91 b and 92a, 92b).
  • a first switching arrangement comprised of metallic conduction lines (13a, 13b) and related switches ⁇ 11a, 11b and 12a, 12b
  • a second switching arrangement comprising switches (23a, 24a and 23b, 24b)in parallel
  • the proposed solution is configured to transit the connectionmode, or to disconnect the bipolar universal converter station from DC system.
  • one switch 12 of the two switches (11 , 12) is configured to realize the series connection of the AC/DC converter to DC system; the other switch 11 is configured to realize the parallel connection of the AC/DC converter to the DC system.
  • the current and/or voltage direction of the bipolar universal converter station can be changed. Specifically, closing of both two switches (23, 24) is configured to form a first power flow path; or opening of both the two switches is configured to form a second power flow path. With the coordination of the first switching arrangement, the second switching arrangement and AC/DC converters, current and/or voltage direction of the bipolar universal converter station can be changed smoothly.
  • the proposed bipolar universal converter station makes the NBS (Neutral Bus Switch) always in a lower voltage side, realizes the transition between earth return and metallic return and/or bypasses a single converter, realizes the transition between remote return and local return and/or bypasses the whole bipolar universal converter station. More details will be described as following.
  • the connection mode of proposed bipolar universal converter station is series or parallel connection.
  • Fig.2 illustrates a configuration of a converter inprior art.
  • aconverter 8 can be configured as a rectifier or an inverter connected to the bipolar universal converter station.
  • the converter comprises a bypass switch (1 ), two converter valves (2, 2') and related AC transformers (3, 3').
  • the method 300 is executed on the basis of the bipolar universal converter station mentioned above.
  • the method comprises: Step 301 , providing the first switching arrangement (11 , 12) in each pole between the pole line and neutral bus.
  • Step 302 blocking the converter 8 in each pole.
  • Step 303 switching the two switches (11 , 12) of the first switching arrangement between open status and closed status.
  • Step 304 de-blocking the converter 8 in each pole.
  • connection mode is series modeas ahigher voltage rectifierterminai, and the connection mode will betransitedto parallel mode.
  • the switches ⁇ 11a, 11 b) are open; the switches (12a, 12b)are closed.
  • the sequencefor changing the connection mode to parallel mode comprises: in each pole, bypassing the converter8; closingthe switch 11 ;blocking the converter 8; opening the switch 12; closingthe switches 4 and 6; finally restarting the converters.
  • converter isolators 21 and 22 are closed and the current direction is from 91a to 92a in positive poie and from 92b to 91 b in negative pole;
  • the current direction is from earth electrode 7 to 92a for positive pole and from 92b to 7 for negative pole;
  • openingconverter isolators 21 and 22 3) openingconverter isolators 21 and 22; 4) closingisolators 23 and 24; and
  • restarting the converter 8 and the current direction is from 92a to 7 for positive pole and from 7 to 92b for negative pole.
  • the grounding current return mode of proposed bipolar universal converter station is metallic return or earth return.
  • metallic return mode it is defined that grounding current is returned through the metallic line of the pole; while for earth return mode is defined that grounding current is returned through the earth electrode. Transition between earth return mode and metallic return mode of the proposed bipolar universal converter stationis introduced as following, it is assumed that converter in negative pole is lost.
  • the MTDC system can be operated with different system return modes and the transition between modes can be realized without system shutdown.
  • As to local return mode it is defined that system current is returned through neutrai bus of remained high voltage level station(s), while for remote return mode, it is defined that system current is returned through metallic return line of blockedlower voltagelevel station(s) and transmission lines between lower voltage level station(s) and higher voltage level station(s).
  • the operation occurs when lower voltage level converter station of MTDC system is blocked.
  • Fig.4a illustrates a single line diagram of a 4-terminal series MTDC system with proposed bipolar universal converter stations.
  • the brief sequence of transiting remote return to local return comprises:
  • the brief sequence of transiting local return to remote return comprises:

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Abstract

A bipolar universal converter stationand MTDC system thereof are disclosed. The bipolar universal converter station can be configured as a rectifier or an inverter, to be connected to or disconnected from the DC system smoothly, and to be parallel or series connected to an MTDC system; and the current directionand/or voltagedirection of thebipolar universal converter station can be changed flexibly compared with the designed direction of the bipolar universal converter station. Considering the MTDC system composed of the proposed bipolar universal converter station, the grounding current return mode of the disclosed bipolar universal converter station can be transmitted between metallicreturn mode and earthreturn mode. The system current return mode of disclosed MTDC system can be transmitted between remote return mode and local return mode.

Description

BIPOLAR UNIVERSAL CONVERTER STATION AND MTDC
SYSTEM THEREOF
FIELD OF THE INVENTION
The invention relates to the field of MTDC(Multi-terminal Direct Current) technology, and more particularly to a bipolar universal converter station and MTDC system thereof.
BACKGROUND OF THE INVENTION
In anMTDC system or DC grid, there are more than twoconverter stations; moreover, all converter stationsin the system should be able to be operated flexibly without system shutdown. In particular, for a series MTDC system, it would be good if the converter stations can be parallel or series connected to the system, can changethe direction of current and/or voltage, can realize the grounding current return mode transition, can realize the system current mode transition, etc.With these functions, the operation flexibility of series MTDC system can be improved greatly.
Prior art CN102082432Adisclosed aHV converter station for series MTDC system. However, the HV converter station cannot realize current direction reversion, cannot realize current return mode transition between remote return and local return, either.
SUMMARY OF THE INVENTION
The present invention providesa bipolar universal converter station and MTDC system thereof. According to an aspect of the present invention, it provides abipoiar universal converter station. The bipolar universal converter station can beconfigured to be used as a rectifier or aninverter, to be connected to or disconnected from the DC system smoothly.and/or to be parallel or series connected to a MTDC system; and/or the current directionand/or voitagedirection of said bipolar universal converter station can be changed flexibly;and/or the grounding current return mode can be transited between metallic return mode and earth return mode.
According to a preferred embodiment of the present invention, thebipoiar universal converter station comprises: at least one AC/DC converter for each pole. According to a preferred embodiment of the present invention, the bipolar universal converter station further comprises: a first switching arrangement {11 , 12) for each pole configured to realize the parallel or series connection of the bipolar universal converter station to DC system.
According to a preferred embodiment of the present invention, bipolar universal converter station further comprises: a second switching arrangement (23, 24) for each AC/DC converter configured to change the current and/or voltage direction of the bipolar universal converter stationwith the coordination with converter isolators (2 , 22).
According to a preferred embodiment of the present invention, the bipolar universal converter station further comprises: a metallic return line 51 for each pole configured to realize the transition between earth return and metallic return; and the transition between iocai return and remote return.
According to a preferred embodiment of the present invention, the first switching arrangement comprises two switches, in which one 12 of the two switches is configured to realize the series connection of the AC/DC converter to the DC system; the other 11 is configured to realize the parallel connection of the AC/DC converter to DC system.
According to a preferred embodiment of the present invention, the second switching arrangement comprises two switches, in which closing of both two switches (23, 24) and opening of both converter isolators (21 , 22) is configured to form a second power flow pathor opening of both said two switches and closing of both converter isolators is configured to form a second power flow path.
According to a preferred embodiment of the present invention, one terminal of the metallic return line is connected to the pole line 92; the other terminal of the metallic return line is connected to converter side terminal of NBS.
According to a preferred embodiment of the present invention, the connection mode is series or parallel connection; and thegrounding current return mode is metallic return or earth return.
According to another aspect of the present invention, it provides a method for changing the connection mode of the bipolar universal converter station mentionedabove. The method comprises: providing the first switching arrangement in each pole between the pole line and neutral bus of the bipolar universal converter station; blocking the converters in each pole; switching the two switches of the first switching arrangement between open status and closed status; and de-blocking the converters in each pole.
According to another aspect of the present invention, it provides a method for changing the current and/or voltage direction of the bipolar universal converter station above mentioned. The method comprises: providing the second switching arrangement in each pole; blocking and isolating the converters in each pole; switching the two switches of the second switching arrangement and converter isolators between open status and closed status to form a different power flow path; and de-blocking the converters in each pole.
According to another aspect of the present invention, it provides a method for transiting between earth return and metallic return of the bipolar universal converter station above mentioned. The method comprises: for transition from earth return to metallic return, enabling the metallic return line in the pole with fault, closing NBS in the pole with fault to form a metallic return path and opening MRTB to cut off the earth return path; and for transition from metallic return to earth return, closing MRTB to form a earth return path, opening the NBS in the pole recovering from fault to cut off the metallic return path and disable the metallic return line. According to another aspect of the present invention, it provides a MTDC system.The MTDC system comprises at least one bipolar universal converter station above mentioned.The MTDC system can be operated with different system return modes and the transition between modes can be realized without system shutdown.
According to a preferred embodiment of the present invention, the system return mode is local return mode or remote return mode in system level.
According to a preferred embodiment of the present invention, it is unnecessary for the system to be shutdown or restarted during transferring the connection mode, converter mode and/or current return mode.
According to another aspect of the present invention, it provides amethod for transiting between local return mode and remote return mode of MTDC system formed by the MTDC system above mentioned. The method comprises: blocking the converters in each pole of the remote station; enabling the metallic return lines in each pole of the remote station to form a remote return path; connecting neutral bus of local station to form a local return path; opening NBS in remote station to realize the transition from remote return to local return of MTDC system ;and opening NBS in local station to realize the transition from local return to remote return of MTDC system. Embodiments of the present invention providea bipolar universal converter station and MTDC system thereof, which realizes rectifier or inverter function connected to a MTDC system in parallel or series, and changesthe current and/or voltage direction of thebipolar universal converter station compared with the designeddirection of the MTDC system.
BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter of the invention will be explained in more details in the following description with reference to preferred exemplary embodiments which are illustrated in the drawings, in which: Fig.1 illustrates a configuration of a bipolar universal converter station according to an embodiment of the present invention;
Fig .2 illustrates a configuration of a converter in prior art;
Fig.3illustrates a flow chart of a method for changing the connection mode of the bipolar universal converter station according to an embodiment of the present invention;
Fig.4a illustrates a single line diagram of a 4-terminal series MTDC system with proposed bipolar universal converter stations;
Fig.4b illustrates a single line diagram of the system under remote return operation after bypassing of the terminal Ri and ; and Fig.4c illustrates a single line diagram of the system under local return operation after return mode transition in system level.
DETAILED DESCRIPTION OF PREFERRED EMBODI ENTS
Exemplary embodiments of the present invention are described in conjunction with the accompanying drawings hereinafter. For the sake of clarity and conciseness, not all the features of actual implementations are described in the specification.
Fig.1 illustrates a configuration of a bipolar universal converter station according to an embodiment of the present invention. As shown in Fig.1 ,a bipolar universal converter station can be configured as a rectifier or an inverter connected to an MTDC system in parallel or series;the bipolar universal converter station can be connected to or disconnected from the DC system smoothly;and/or the currentand/or voltage direction of thebipolar universal converter station can be changed flexibly to meet the requirements of MTDC system (or DC grid) operation; and/or the grounding current return mode can be transited between metallic return mode and earth return mode.
In detail, thebipolar universal converter station comprises:at least one AC/DC converter in each pole (8a, 8b); a first switching arrangement comprised of metallic conduction lines (13a, 13b) and related switches {11a, 11b and 12a, 12b); a second switching arrangement comprising switches (23a, 24a and 23b, 24b)in parallel with the converters(8a, 8b); converter isolators (21a, 22a and 21 b, 22b); two NBSs (Neutral Bus Switch, 4a and 4b); metallic return lines (51a, 51 b) and related switches (5a, 5b); anMRTB (Metallic Return Transit Breaker, 6) and an earth electrode 7;and joints of station and system (91a, 91 b and 92a, 92b).
By means of the first switching arrangement (11 , 12) for each pole, the proposed solution is configured to transit the connectionmode, or to disconnect the bipolar universal converter station from DC system. Specifically, one switch 12 of the two switches (11 , 12) is configured to realize the series connection of the AC/DC converter to DC system; the other switch 11 is configured to realize the parallel connection of the AC/DC converter to the DC system. With the coordination of the first switching arrangement and AC/DC converters, smooth transition between connection modes can be realized
By means of the second switching arrangement (23, 24) for each AC/DC converter, the current and/or voltage direction of the bipolar universal converter station can be changed. Specifically, closing of both two switches (23, 24) is configured to form a first power flow path; or opening of both the two switches is configured to form a second power flow path. With the coordination of the first switching arrangement, the second switching arrangement and AC/DC converters, current and/or voltage direction of the bipolar universal converter station can be changed smoothly.
Furthermore, the proposed bipolar universal converter station makes the NBS (Neutral Bus Switch) always in a lower voltage side, realizes the transition between earth return and metallic return and/or bypasses a single converter, realizes the transition between remote return and local return and/or bypasses the whole bipolar universal converter station. More details will be described as following. In the present invention, the connection mode of proposed bipolar universal converter station is series or parallel connection.
Fig.2 illustrates a configuration of a converter inprior art. As shown in Fig.2,aconverter 8 can be configured as a rectifier or an inverter connected to the bipolar universal converter station. In detail, the converter comprises a bypass switch (1 ), two converter valves (2, 2') and related AC transformers (3, 3'). Fig.3iliustrates a flow chart of a method for changing the connection mode of the bipolar universal converter station according to an embodiment of the present invention.
As shown in Fig.3, the method 300 is executed on the basis of the bipolar universal converter station mentioned above. The method comprises: Step 301 , providing the first switching arrangement (11 , 12) in each pole between the pole line and neutral bus.
Step 302, blocking the converter 8 in each pole.
Step 303, switching the two switches (11 , 12) of the first switching arrangement between open status and closed status. Step 304, de-blocking the converter 8 in each pole. in detail, the process will be explained for transiting the connection mode by switching the switches 11 and 12 based on the bipolar universal converter station proposed above. Firstly, it is assumed that connection mode is series modeas ahigher voltage rectifierterminai, and the connection mode will betransitedto parallel mode. For series connection mode of proposed bipolar universal converter station, the switches {11a, 11 b)are open; the switches (12a, 12b)are closed. So generally speaking, the sequencefor changing the connection mode to parallel mode comprises: in each pole, bypassing the converter8; closingthe switch 11 ;blocking the converter 8; opening the switch 12; closingthe switches 4 and 6; finally restarting the converters.
To transit connection mode to series modefor converter station at high voltage level,the following steps will be conducted in sequence comprising: blocking the converter 8; opening the switches 4 and 6; closing the switch 12; closing the bypass switch (including but not limited to a bypass isolator) of the converted; opening the switch 11 ; de-blocking the converter 8. Finally, the transition is realized from parallel mode to series mode. Taking athyristor based LCC converter station as an exampie.changing of its current direction is introduced. It is assumed that the connection mode of the station is series mode.
1 ) before transition, converter isolators 21 and 22 are closed and the current direction is from 91a to 92a in positive poie and from 92b to 91 b in negative pole;
2) bypassing and blocking the converter 8, after the blocking, the switch 11 is closed and 12 is opened;
3) openingconverter isolators 21 and 22, closing converter isolators 23 and24; and
4) closing 12 and de-blocking the converter 8, during the de-biocking, the switch 11 is opened and after the de-blocking, the current direction is from 92a to 91a in positive pole and from 91 b to 92b in the negative pole.
Assuming the connection mode of above converter station is parallel mode, the sequence is as following:
1 ) before transition, the current direction is from earth electrode 7 to 92a for positive pole and from 92b to 7 for negative pole;
2) stopping the converter 8;
3) openingconverter isolators 21 and 22; 4) closingisolators 23 and 24; and
5) restarting the converter 8 and the current direction is from 92a to 7 for positive pole and from 7 to 92b for negative pole.
It's obvious to the person skilled in art that the proposed invention takes advantages from the current and/or voltage direction changing. Some types of converters are current unidirectional and some types are voltage unidirectlonal.This abiiity brought by this invention can improve the flexibility of a MTDC system or DC grid, in which different types of converters are applied.
In the present invention, the grounding current return mode of proposed bipolar universal converter station is metallic return or earth return. As to metallic return mode, it is defined that grounding current is returned through the metallic line of the pole; while for earth return mode is defined that grounding current is returned through the earth electrode. Transition between earth return mode and metallic return mode of the proposed bipolar universal converter stationis introduced as following, it is assumed that converter in negative pole is lost.
The brief sequence of the process for transferring earth return to metallic return mode comprises. In this example, it is assumed that the converter 8b at negative pole has been blocked. 1 ) closing the switches (4b and 5b) to form a metallic return path through metallic return line 51 b;
2) opening the MRTB 6 to cut-off the current in earth return path through the earth electrode 7.
On the contrary, the brief sequence of the process for transferring metallic return to earth return mode comprises:
1 ) closing the MRTB 6 to form an earth return path through the earth electrode 7;
2) opening the NBS 4b to cut-off the current in metallic return path through line 51b. It's obvious to the person skilled in art that the proposed invention takes advantages from the earth return mode and metallic return mode transition. Earth return mode can maintain the unbalanced system operated for a short term, and during the transition from metallic return mode to earth return mode, grounding currentcan be re-directed by using the NBS 4. This means that the Ground Return Transfer Switch (GRTS), which is usually used in the DC yard of a classic HVDC station, can be removed in this invention and its function is replace by the NBS 4. in the other aspect of the present invention, it also provides a MTDC system, wherein the MTDC system comprises at least one of universal converter station mentioned above. The MTDC system can be operated with different system return modes and the transition between modes can be realized without system shutdown.As to local return mode, it is defined that system current is returned through neutrai bus of remained high voltage level station(s), while for remote return mode, it is defined that system current is returned through metallic return line of blockedlower voltagelevel station(s) and transmission lines between lower voltage level station(s) and higher voltage level station(s).For the transition between local return mode and remote return mode of MTDC system, the operation occurs when lower voltage level converter station of MTDC system is blocked.
Fig.4a illustrates a single line diagram of a 4-terminal series MTDC system with proposed bipolar universal converter stations. A4-terminal series MTDC system includingfourconverterstations (Ri, F , li and ), is taken as an example. It is assumed that Ri and hare to be blocked. The brief sequence of transiting remote return to local return comprises:
1 ) bypassing the converter stations of Ri and by using metallic return lines (51 ) with closed 5 and operating the system under remote returnmode as shown in Fig.4b;
2) closingswitches4 and 6 in R2 and l2to form a local return circuit;
3) openingswitches 4 and 6 in Ri and o cut-off the current in remote return circuit; 4) openingthe switch 12 in R2 and \2.
Then the transmission lines between Ri and R2 (and and l2) are isolated from the remained system as shown in Fig.4c.
Correspondingly, taking de-blockingthe rectifier Ri and inverter H as an example, the brief sequence of transiting local return to remote return comprises:
1 ) closingswitches 4, 5 and 6 in Ri and M
2) closingswttch 12 in R2 and l2 to form a remote return circuit;
3) openingswitches 4 and 6 in R2and l2 to redirectground current in local return to remote return; 4) de-b!ocking Ri and l-i. it's obvious for the person skilled in art to know that the MTDC system with such universal converter station is not necessary to be stoppedduring transferring the connection mode, converter mode, and/or return mode.
Although the present invention has been described on the basis of some preferred embodiments, those skilled in the art should appreciate that those embodiments should by no means limit the scope of the present invention. Without departing from the spirit and concept of the present invention, any variations and modifications to the embodiments should be within the apprehension of those with ordinary knowledge and skills in the art, and therefore fall in the scope of the present invention which is defined by the accompanied claims.

Claims

1. A bipolar universal converter station, characterized in that said universal converter station can beconfigured to be used as a rectifier or an inverter, to be connected to or disconnected from the DC system smoothly,and/or to be parallel or series connected to an MTDC system; and/or the current directionand/or voltagedirection of said bipolar universal converter station can be changed flexibly;and/or the grounding current return mode can be transited between metallic return mode and earth return mode.
2. The bipolar universal converter station according to claim 1 , characterized in that saidbipolar universal converter station comprises: at least oneAC/DC converter for each pole.
3. The bipolar universal converter station according to claim 2, characterized in that said bipolar universal converter station further comprises: a first switching arrangement (11 , 12) for each pole configured to realize the parallel or series connection ofsaid bipolar universal converter station to DC system.
4. The bipolar universal converter station according to claim 2 or 3, characterized in that said bipolar universal converter station further comprises: a second switching arrangement (23, 24) for each AC/DC converterconfigured to change the current and/or voltage direction of said bipolar universal converter station with the coordination with converter isolators (21 , 22).
5. The bipolar universal converter station according to any one of claims2-4, characterized in that said bipolar universal converter station further comprises: a metallic return line (51 ) for each pole configured to realize the transition between earth return and metallic return, and the transition between local return and remote return.
6. The bipolar universal converter station according to claim 3, wherein the first switching arrangement comprises two switches, in which one (12) of the two switches is configured to realize the series connection of the AC/DC converter to DC system; the other (11 ) is configured to realize the parallel connection of the AC/DC converter to the DC system.
7. The bipolar universal converter station according to claim 4, wherein the second switching arrangement comprises two switches, in whichclosing of both said two switches (23, 24) and opening of both converter isolators (21 , 22) is configured to form a first power flowpath; or opening of both said two switches and closing of both converter isolators is configured to form a second power flowpath.
8. The bipolar universal converter station according to claim 5, one terminal of said metallic return line is connected to the pole line (92); the other terminal of said metallic return line is connected to converter side terminal of NBS.
9. The bipolar universal converter station according to claim 1 , characterized in that said connection mode is series or parallel connection; and said grounding current return mode is metallic return or earth return.
10. A method for changing the connection mode of said bipolar universal converter station according to any above claim comprises:
providing the first switching arrangement in each pole between the pole line and neutral bus of said bipolar universal converter station;
blocking the converters in each pole;
switching the two switches of said first switching arrangement between open status and closed status; and
de-blocking the converters in each pole.
11. A method for changing the current and/or voltage direction of said bipolar universal converter station according to any one of claims 1-9comprises: providing the second switching arrangement in each pole;
blockingand isolating the converters in each pole;
switching the two switches of said second switching arrangement and converter isolators between open status and closed status to form a different power flow path; and
de-biocking the converters in each pole.
12. A method for transiting between grounding current earth return and metallic return of said bipolar universal converter station according to any one of claims 1-9comprises:
for transition from earth return to metallic return, enabling the metallic return line in the pole with fault, closing NBS in the pole with fault to form a metallic return path and opening MRTB to cut off the earth return path; and
for transition from metallic return to earth return, closing MRTB to form a earth return path, opening the NBS in the pole recovering from fault to cut off the metallic return path and disable the metallic return line.
13. AnMTDC system, characterized in that said MTDC system comprising at least one bipolar universal converter station according to c!aims1-9, can be operated with different system return modes and the transition between modes can be realized without whole system shutdown.
14. The MTDC system according to claim 13, characterized in that said system return mode is local return mode or remote returnmode in system level.
15. The MTDC system according to claim 13, characterized in that it is unnecessary for said system to bestoppedor restarted during transferring the connection mode, converter mode and/or current return mode.
16. A method for transiting between local return mode and remote return mode of MTDC system formed by said MTDC system according to any one of claims 13-15 comprises:
blocking the converters in each pole of the remote station;
enabling the metallic return lines in each pole of the remote station to form a remote return path;
connecting neutral bus of local station to form a local return path; opening NBS in remote station to realize the transition from remote return to local return of MTDC system ;and
opening NBS in local station to realize the transition from local return to remote return of MTDC system.
PCT/CN2013/072842 2013-03-19 2013-03-19 Bipolar universal converter station and mtdc system thereof WO2014146241A1 (en)

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