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WO2021196563A1 - 一种电阻型子模块混合mmc及其直流故障处理策略 - Google Patents

一种电阻型子模块混合mmc及其直流故障处理策略 Download PDF

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Publication number
WO2021196563A1
WO2021196563A1 PCT/CN2020/121131 CN2020121131W WO2021196563A1 WO 2021196563 A1 WO2021196563 A1 WO 2021196563A1 CN 2020121131 W CN2020121131 W CN 2020121131W WO 2021196563 A1 WO2021196563 A1 WO 2021196563A1
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Prior art keywords
mmc
sub
current
module
bridge
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English (en)
French (fr)
Inventor
张哲任
徐政
徐雨哲
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Zhejiang University ZJU
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Zhejiang University ZJU
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Priority to US17/399,740 priority Critical patent/US12040606B2/en
Publication of WO2021196563A1 publication Critical patent/WO2021196563A1/zh
Anticipated expiration legal-status Critical
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    • 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
    • H02M7/02—Conversion of AC power input into DC power output without possibility of reversal
    • H02M7/04—Conversion of AC power input into DC power output without possibility of reversal by static converters
    • H02M7/12—Conversion of AC power input into DC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/21—Conversion of AC power input into DC power output without 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
    • H02M7/217—Conversion of AC power input into DC power output without 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
    • H02M7/219—Conversion of AC power input into DC power output without 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 in a bridge configuration
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/10—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers
    • H02H7/12—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers
    • H02H7/1216—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for AC-AC converters
    • 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
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H1/00—Details of emergency protective circuit arrangements
    • H02H1/0007—Details of emergency protective circuit arrangements concerning the detecting means
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/10—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers
    • H02H7/12—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers
    • H02H7/122—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for inverters, i.e. DC/AC converters
    • H02H7/1227—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for inverters, i.e. DC/AC converters responsive to abnormalities in the output circuit, e.g. short circuit
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/10—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers
    • H02H7/12—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers
    • H02H7/125—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for rectifiers
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
    • H02H7/268—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for DC systems
    • 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
    • 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 technical field of power electronics, in particular to a resistance-type sub-module hybrid MMC and its DC fault processing strategy.
  • HVDC high voltage direct current
  • MMC modular multilevel converter
  • 1It adopts modular design, the requirements for dynamic voltage equalization and triggering consistency of switching devices are lower, and operation and maintenance are more convenient; 2The scalability is good, theoretically, the voltage can be increased by increasing the number of cascaded sub-modules Grade; 3Low loss, which is close to the level of traditional DC transmission (approximately equal to 1%); 4The output waveform has low harmonic content and no filter device is required.
  • MMC has become the most promising converter topology for HVDC transmission occasions.
  • MMC has demonstrated great potential for its application in multi-terminal DC transmission systems.
  • key issues such as the rapid removal of DC side faults have not been well resolved, which greatly limits the application of MMC technology in overhead lines.
  • MMC-HVDC project by opening the AC circuit breaker to deal with the DC side fault method, there are currently two main improvement measures with high technical maturity: (1) The commutation based on the half-bridge sub-module is still adopted. At the same time, a high-voltage DC circuit breaker is installed at both ends of the DC circuit, and the faulty circuit can be quickly opened by the high-voltage DC circuit breaker.
  • ABB released a high-voltage DC circuit breaker with a rated voltage of 320kV, which can break a maximum DC current of 9kA within 5ms.
  • SIEMENS, ALSTOM and State Grid Research Institute are also developing their own high-voltage DC circuit breakers; (2) The use of converters with DC fault self-clearing capabilities can quickly deal with DC line faults.
  • the first method adopts a modular multilevel converter (MMC) plus DC circuit breaker scheme based on half-bridge sub-modules.
  • MMC modular multilevel converter
  • This network configuration method is suitable for DC power grids with any number of terminals;
  • the second network configuration method uses The MMC with DC fault self-clearing capability, such as the MMC based on the full bridge sub-module, but without DC circuit breaker, is suitable for small-scale DC power grids with less than 10 terminals.
  • the half-bridge sub-module MMC plus DC circuit breaker is used to construct the network, the converter station is usually required to continue to operate during the DC line failure and cannot be blocked. The fault line is quickly removed by the DC circuit breaker.
  • the fault handling principle is similar to that of the AC power grid.
  • the relevant inverters in the network are blocked during the DC line failure, and the fault current reaches zero and stabilizes at a value of about 10ms after the blocking, and then the fault line is isolated by the isolating switch, and then the relevant converter Unlock and resume power transmission.
  • the time from the start of the failure to the restoration of power transmission is generally about 20ms, and the impact on the AC power grid is usually within a tolerable range.
  • the DC circuit breaker becomes a key element of the DC grid;
  • the current high-voltage DC circuit breaker construction scheme mainly focuses on three types, which are based on conventional switches.
  • a technically feasible high-voltage DC circuit breaker has been developed, its high cost and huge volume make it difficult to be widely used in the power grid like AC circuit breakers.
  • the purpose of the present invention is to provide a resistance-type sub-module hybrid MMC and its DC fault processing strategy.
  • the proportion of full-bridge sub-modules required by the hybrid MMC is only 30% of all half-bridge sub-modules, and it can be used within 50 ms.
  • the internal isolation of DC faults greatly reduces the cost of constructing overhead line high-voltage flexible DC transmission systems, and has very strong reference significance and use value in engineering.
  • the present invention provides a resistance type sub-module hybrid MMC, which is a three-phase six-leg structure, each phase contains two upper and lower bridge arms.
  • 1 half-bridge sub-module, N 2 resistive sub-modules and a bridge arm reactor are connected in series; for the lower bridge arm, it is composed of a bridge arm reactor and N 2 resistive sub-modules from the high-voltage end to the low-voltage end in sequence
  • N 1 half-bridge sub-modules are connected in series, and N 1 and N 2 are both natural numbers greater than 1;
  • intermediate node A phase upper and lower arms are connected to AC circuit breaker BR 1, BR other end of the AC circuit breaker 1; intermediate node B-phase upper and lower arms are connected to AC circuit breaker BR 2, BR 2 and the other end of the AC circuit breaker Grounding; AC circuit breaker BR 3 is connected to the intermediate node of the upper and lower bridge arms of phase C, and the other end of AC circuit breaker BR 3 is grounded.
  • the number N 2 of resistive sub-modules in each bridge arm is less than or equal to 30% N 1 .
  • the high voltage end of the DC side of the MMC is connected to the DC line through a smoothing reactor and a fast mechanical switch in sequence.
  • the sub-module comprises a resistive type of IGBT anti-parallel diode T 1 ⁇ T 2 and two with a resistor R 0, wherein T emitter of IGBT 1 is connected to the electrode as the tube T and the collector of the IGBT 2 high voltage terminals of the module, the collector of IGBT tube T resistors R 1 0 is connected to one end, the other end of the resistor R & lt IGBT 0 T connected to the gate emitter tube 2 as a low pressure side and submodules.
  • the present invention also discloses a DC failure processing strategy, which is applied to the above-mentioned resistance type sub-module hybrid MMC, and the DC failure processing strategy includes the following steps:
  • the converter station at the other end receives a DC fault handling instruction or detects that the current of its MMC bridge arm exceeds the threshold, and immediately blocks all half-bridge sub-modules in the MMC and conducts all resistance-type sub-modules, and closes the AC circuit breaker BR 1 , BR 2 and BR 3 ;
  • step (2) and step (3) when the following relationship is established, it is determined that the current of the MMC bridge arm exceeds the threshold
  • I pa is the A phase upper arm current of MMC
  • I na is the A phase lower arm current of MMC
  • I pb is the B phase upper arm current of MMC
  • I nb is the B phase lower arm current of MMC
  • I pc is the C-phase upper arm current of the MMC
  • I nc is the C-phase lower arm current of the MMC
  • I rate is the rated current of the IGBT in the sub-module.
  • step (5) requires a reclosing operation after the DC fault processing is completed, and the specific implementation manner is as follows:
  • a closing signal is sent to the fast mechanical switches at both ends of the fault line, and at the same time, an instruction to unlock the half-bridge sub-module and bypass the resistance-type sub-module is sent to the MMC at both ends to restore the system To steady state operation;
  • a close signal is sent to the fast mechanical switches at both ends of the fault line, and at the same time, an instruction to unlock the half-bridge sub-module and bypass the resistance-type sub-module is sent to the MMC at both ends; If the current in the MMC bridge arm of a converter station at one end of the converter station exceeds the threshold is detected again after a certain period of time, the DC fault processing will be performed again according to steps (2) to (5).
  • the bridge arm reactor and the DC line together form an LC oscillation attenuation loop, so that t the half-bridge DC fault currents in the submodules and the blocking resistance-type input submodule time after 3 drops rapidly to a fast mechanical switch capable of cleaving a range of currents, so that the final complete breaking fast mechanical switch.
  • the total time of t 3 + t 4 does not exceed 60 ms.
  • the present invention discloses the following technical effects:
  • the resistance-type sub-module hybrid MMC proposed in the present invention requires only 30% of the resistance-type sub-module ratio of all half-bridge sub-modules. Compared with the sub-module hybrid MMC, it can reduce the number of power electronic devices required, and greatly The construction cost of the converter station is reduced.
  • the resistive sub-module hybrid MMC proposed in the present invention, the resistive sub-module is in a bypass state under normal working conditions, and compared with the existing hybrid sub-module MMC, the operation loss can be greatly reduced.
  • Figure 1 is a schematic diagram of the topological structure of the resistance-type sub-module hybrid MMC of the present invention
  • FIG. 2 is a schematic diagram of the structure of the test system used in the specific implementation of the present invention.
  • Fig. 3 is a schematic diagram of the current waveform flowing through the fast mechanical switch during the fault handling process of the present invention
  • Fig. 4 is a schematic diagram of the DC voltage waveform of MMC1 during the fault handling process of the present invention.
  • Fig. 5 is a schematic diagram of the DC current waveform of the MMC1 during the fault handling process of the present invention.
  • Fig. 6 is a schematic diagram of the active power waveform of the MMC1 during the fault handling process of the present invention.
  • the purpose of the present invention is to provide a resistance-type sub-module hybrid MMC and its DC fault processing strategy.
  • the proportion of full-bridge sub-modules required by the hybrid MMC is only 30% of all half-bridge sub-modules, and can be isolated within 50 ms DC failures greatly reduce the cost of building overhead line high-voltage flexible DC transmission systems, and have very strong reference significance and use value in engineering.
  • FIG. 1 is a schematic diagram of the topological structure of the resistance-type sub-module hybrid MMC of the present invention.
  • the resistance-type sub-module hybrid MMC of the present invention has a three-phase six-arm structure, and each phase includes two upper and lower bridge arms.
  • Each bridge arm is composed of N sub-modules and a bridge arm reactor in series.
  • the outlet of the DC side of the converter is connected to the DC line through a smoothing reactor L dc and a fast mechanical switch K.
  • N 1 series half-bridge sub-modules In all upper arms of MMC, in the order from the high voltage end to the low voltage end, N 1 series half-bridge sub-modules, N 2 series resistance sub-modules and bridge arm reactors are arranged respectively; among all the lower arms of MMC, follow the order from From the high-voltage end to the low-voltage end, respectively arrange the bridge arm reactors, N 2 series resistance sub-modules and N 1 series half-bridge sub-modules.
  • connection point of the upper and lower bridge arms of phase A is grounded through the AC circuit breaker BR 1
  • the connection point of the upper and lower bridge arms of the B phase is connected through the AC circuit breaker BR 2
  • the connection point of the upper and lower bridge arms of the C phase is connected through the AC circuit breaker BR 3 is grounded.
  • the number N 2 of resistive sub-modules in each bridge arm is less than or equal to 30% N 1 .
  • the structure of the resistive sub-module is similar to that of the conventional half-bridge sub-module, that is, the high-voltage and low-voltage ends of the sub-module have two parallel branches. Parallel diodes and sub-module resistors are connected in series.
  • BR 1 , BR 2, and BR 3 remain open, and K remains closed; at the same time, monitor the bridge arm current of each bridge arm, which is the phase A upper arm current I pa and A respectively Phase lower arm current I na , B phase upper arm current I pb , B phase lower arm current I nb , C phase upper arm current I pc and C phase lower arm current I nc .
  • a converter station After a DC fault occurs, a converter station first detects that the current of each bridge arm satisfies the following relationship after the time t 1 has passed, and immediately blocks all half-bridge sub-modules in the MMC and conducts all resistance-type sub-modules, and closes the AC Switch BR 1 , BR 2 and BR 3 , and at the same time issue a blocking command to other converter stations.
  • I rate is the rated DC current of the IGBT in the sub-module used.
  • the equivalent resistance of the resistance-type sub-module and the smoothing reactor, the bridge arm reactor and the DC line form an RL attenuation loop to make the DC fault current locking the half-bridge type resistance sub-modules and sub-modules to rapid mechanical switch rapidly drops off current can be within a range of time t 3 after the access, so that the final K rapid mechanical switch capable of breaking failure.
  • BR 1 , BR 2 and BR 3 After t 4 time (the longest half cycle, namely 10ms), BR 1 , BR 2 and BR 3 return to the disconnected state, so far the DC fault processing is completed; the fault processing period is t 3 + t 4 time period Inside, by closing BR 1 , BR 2 and BR 3 to actively create a three-phase interphase short circuit on the valve side of the converter station to prevent the AC system current from flowing into the DC system and speed up the process of fault current oscillation to 0, where t 3 +t 4 total The duration does not exceed 60ms.
  • a close signal is sent to the fast mechanical switch K, and an instruction to unlock the half-bridge sub-modules and bypass the resistance-type sub-modules is issued to the MMC on both sides, and finally the system returns to a steady state Operating status.
  • a close signal is sent to the fast mechanical switch K, and an instruction to unlock the half-bridge sub-module and bypass the resistance-type sub-module is issued to the MMC on both sides; the fault is a permanent fault
  • t 6 time it will be detected that the bridge arm current value of a converter station exceeds the threshold again, and the fault will be dealt with again according to the above steps.
  • the first-side converter station MMC1 controls the DC voltage
  • the second-side converter station MMC2 controls the transmission power
  • MMC1 transmits 400MW active power to MMC2; at 1.5s, a transient occurs at the midpoint of the DC line Grounding short-circuit fault, the fault duration is 0.1s.
  • MMC1 detects that the bridge arm current exceeds the threshold 3kA, immediately blocks all half-bridge sub-modules and turns on all resistive sub-modules, closes the AC switches BR 1 , BR 2 and BR 3 , and then goes to the line
  • the mechanical switches K on both sides send out a breaking signal and at the same time send out a blocking command to MMC2.

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  • Power Engineering (AREA)
  • Rectifiers (AREA)
  • Inverter Devices (AREA)

Abstract

本发明公开一种电阻型子模块混合MMC及其直流故障处理策略，该混合型MMC通过在直流故障处理过程中在换流器交流侧人为制造三相对地短路，阻止故障电流从交流侧进入直流系统，相比起子模块混合型MMC，能够减少所需的电力电子器件数量；同时，该混合型MMC的直流故障处理速度较快，故障处理过程中人为制造的三相对地短路故障持续时间不超过60ms，不会对交流系统产生较大影响。本发明MMC所需电阻型子模块比例仅为半桥子模块的30％，大大降低了建设架空线高压柔性直流输电系统的成本，在工程中具有非常强的参考意义与使用价值。

Description

一种电阻型子模块混合MMC及其直流故障处理策略
本申请要求于2020年4月1日提交中国专利局、申请号为202010252785.7、发明名称为“一种电阻型子模块混合MMC及其直流故障处理策略”的中国专利申请的优先权，其全部内容通过引用结合在本申请中。
技术领域
本发明涉及电力电子技术领域，特别是涉及一种电阻型子模块混合MMC及其直流故障处理策略。
背景技术
随着电力电子技术的蓬勃发展，基于模块化多电平换流器(modular multilevel converter，MMC)的高压直流输电(high voltage direct current，HVDC)技术正受到越来越多的关注。MMC具有以下几个优势：①采用模块化设计，开关器件动态均压以及触发一致性要求较低，运行维护较为方便；②可扩展性好，理论上通过增加级联子模块数量就可以提高电压等级；③损耗低，已经接近传统直流输电的水平(约等于1％)；④输出波形谐波含量低，不需要装设滤波装置。目前MMC已成为高压直流输电场合最具发展前景的换流器拓扑。
MMC凭借其相对于两电平和三电平换流器的技术优势，展现出了应用于多端直流输电系统的巨大潜力。然而到目前为止，直流侧故障的快速清除等关键问题依旧没有得到很好的解决，这极大限制了MMC技术在架空线场合的应用。针对目前MMC-HVDC工程中通过跳开交流断路器来处理直流侧故障方法的不足，目前主要存在两种技术成熟度较高的改进措施：(1)仍然采用基于半桥型子模块的换流器，同时在直流线路两端装设高压直流断路器，通过高压直流断路器来快速开断故障线路。ABB公司在2013年底发布了一款额定电压为320kV高压直流断路器，可以在5ms之内开断最大值为9kA的直流电流。目前SIEMENS公司、ALSTOM公司和国网智研院等单位也都在开发自己的高压直流断路器；(2)使用具有直流故障自清除能力的换流器可以快速处理直流线路故障。
第一种方式采用基于半桥子模块的模块化多电平换流器(MMC)加直 流断路器方案，这种构网方式适用于端数任意多的直流电网；第二种构网方式采用具有直流故障自清除能力的MMC，例如采用基于全桥子模块的MMC，但无需直流断路器，这种构网方式适用于端数小于10的小规模直流电网。采用半桥子模块MMC加直流断路器的构网方式时，直流线路故障期间通常要求换流站继续运行，不能闭锁，故障线路由直流断路器快速切除，其故障处理原则与交流电网类似。采用无直流断路器的构网方式时，直流线路故障期间网内相关换流器闭锁，闭锁后10ms左右故障电流到零并稳定于零值，再通过隔离开关隔离故障线路，然后相关换流器解锁重新恢复送电，从故障开始到恢复送电的时间一般在20ms左右，通常对交流电网的冲击在可以承受的范围之内。
当采用半桥子模块MMC加直流断路器的方式来构成直流电网时，直流断路器就成为直流电网的关键性元件；目前高压直流断路器构造方案主要集中于3种类型，分别是基于常规开关的传统机械型断路器、基于纯电力电子器件的固态断路器和基于二者结合的混合型断路器。虽然目前已开发出技术上可行的高压直流断路器，但其成本高昂，体积巨大，难以像交流断路器那样在电网中广泛使用。
而当采用具有直流故障自清除能力的MMC来构成直流电网时，以全桥子模块为例，与相同容量和电压等级的半桥子模块MMC相比，全桥MMC使用的电力电子器件个数几乎为其两倍，不仅增加投资成本，而且引入了更多的运行损耗。因此有文献提出了由半桥子模块和全桥子模块共同构成的子模块混合型MMC换流站，不仅拥有直流故障处理的能力，同时相比全桥MMC一定程度上减少了电力电子器件个数和运行损耗；然而为了满足直流故障处理速度的需求，通常要求子模块混合型中的全桥子模块数量占所有子模块的50％以上，而在现有实际工程中甚至达到了75％，这大大减弱了子模块混合型MMC相对于全桥MMC的优势，增加了建造成本和运行损耗。
发明内容
基于此，本发明的目的是提供一种电阻型子模块混合MMC及其直流故障处理策略，该混合型MMC所需全桥子模块比例仅为所有半桥子模块 的30％，且可以在50ms之内隔离直流故障，大大降低了建设架空线高压柔性直流输电系统的成本，在工程中具有非常强的参考意义与使用价值。
为实现上述目的，本发明提供了一种电阻型子模块混合MMC，为三相六桥臂结构，每相包含上下两个桥臂，对于上桥臂，其从高压端至低压端依次由N 1个半桥子模块、N 2个电阻型子模块以及一个桥臂电抗器串联构成；对于下桥臂，其从高压端至低压端依次由一个桥臂电抗器、N 2个电阻型子模块以及N 1个半桥子模块串联构成，N 1和N 2均为大于1的自然数；
A相上下桥臂的中间节点连接有交流断路器BR 1，交流断路器BR 1的另一端接地；B相上下桥臂的中间节点连接有交流断路器BR 2，交流断路器BR 2的另一端接地；C相上下桥臂的中间节点连接有交流断路器BR 3，交流断路器BR 3的另一端接地。
可选地，每个桥臂中电阻型子模块的数量N 2小于等于30％N 1。
可选地，所述MMC直流侧的高压端依次通过平波电抗器以及快速机械开关与直流线路相连接。
可选地，所述电阻型子模块包括两个带反并联二极管的IGBT管T 1～T 2以及一个电阻R 0，其中IGBT管T 1的发射极与IGBT管T 2的集电极相连并作为子模块的高压端，IGBT管T 1的集电极与电阻R 0的一端相连，电阻R 0的另一端与IGBT管T 2的发射极相连并作为子模块的低压端。
本发明还公开了一种直流故障处理策略，所述直流故障处理策略应用于上述电阻型子模块混合MMC，所述直流故障处理策略包括如下步骤：
(1)正常运行过程中，使MMC的交流断路器BR 1、BR 2和BR 3保持开断状态，快速机械开关保持闭合状态，所有电阻型子模块保持旁路状态，同时监测MMC每个桥臂的桥臂电流；
(2)直流故障发生后，若MMC所在换流站首先检测到桥臂电流超过阈值，立刻闭锁MMC中所有半桥子模块并导通所有电阻型子模块，同时闭合交流断路器BR 1、BR 2和BR 3，进而向MMC所连的另一端换流站发出直流故障处理指令；
(3)另一端换流站接收到直流故障处理指令或检测到自身MMC桥臂电流超过阈值，同样立刻闭锁MMC中所有半桥子模块并导通所有电阻 型子模块，并闭合交流断路器BR 1、BR 2和BR 3；
(4)经过一定时长t 3之后，当故障线路两端的快速机械开关流经电流降低到一定大小，两端换流站向各自的快速机械开关发出开断信号，快速机械开关完成开断，实现故障线路的物理隔离，与此同时两端换流站向各自MMC的交流断路器BR 1、BR 2和BR 3发出开断信号；
(5)经过一定时间长t 4后，两端MMC的交流断路器BR 1、BR 2和BR 3恢复到开断状态，至此直流故障处理完成。
可选地，所述步骤(2)和步骤(3)中当以下关系式成立则判定MMC桥臂电流超过阈值；
max(I pa,I na,I pb,I nb,I pc,I nc)＞2I rate
其中：I pa为MMC的A相上桥臂电流，I na为MMC的A相下桥臂电流，I pb为MMC的B相上桥臂电流，I nb为MMC的B相下桥臂电流，I pc为MMC的C相上桥臂电流，I nc为MMC的C相下桥臂电流，I rate为子模块内IGBT的额定电流。
可选地，所述步骤(5)在完成直流故障处理后需进行重合闸操作，具体实现方式如下：
对于暂时性直流故障，经过一定的去游离时间后，向故障线路两端的快速机械开关发出闭合信号，同时向两端MMC发出解锁半桥子模块且旁路电阻型子模块的指令，使系统恢复到稳态运行状态；
对于永久性直流故障，经过一定的去游离时间后，向故障线路两端的快速机械开关发出闭合信号，同时向两端MMC发出解锁半桥子模块且旁路电阻型子模块的指令；由于是永久性直流故障，若经过一定时间再次检测到某一端换流站MMC桥臂电流超过阈值，则重新根据步骤(2)～(5)进行直流故障处理。
可选地，当MMC中的半桥子模块闭锁且电阻型子模块投入后，电阻型子模块中的电阻与平波电抗器、桥臂电抗器和直流线路共同构成一个LC振荡衰减回路，使直流故障电流在半桥子模块闭锁且电阻型子模块投入后的t 3时间内迅速下降到快速机械开关能够断开的电流范围之内，最终使得快速机械开关完成开断。
可选地，在故障处理的t 3+t 4时间段内，通过闭合交流断路器BR 1、 BR 2和BR 3在MMC阀侧主动制造三相相间短路，阻止交流系统电流流入直流系统，加快故障电流衰减的过程，t 3+t 4的总时长不超过60ms。
根据本发明提供的具体实施例，本发明公开了以下技术效果：
1、本发明提出的电阻型子模块混合MMC，所需电阻型子模块比例仅为所有半桥子模块的30％，相比起子模块混合型MMC，能够减少所需的电力电子器件数量，大大降低了换流站的建设成本。
2、本发明提出的电阻型子模块混合MMC，电阻型子模块在正常工作状态下处于旁路状态，相比已有的子模块混合型MMC，可以大大降低运行损耗。
说明书附图
为了更清楚地说明本发明实施例或现有技术中的技术方案，下面将对实施例中所需要使用的附图作简单地介绍，显而易见地，下面描述中的附图仅仅是本发明的一些实施例，对于本领域普通技术人员来讲，在不付出创造性劳动性的前提下，还可以根据这些附图获得其他的附图。
图1为本发明电阻型子模块混合MMC的拓扑结构示意图；
图2为本发明具体实施方式中所采用的测试系统结构示意图；
图3为本发明故障处理过程中流经快速机械开关的电流波形示意图；
图4为本发明故障处理过程中MMC1的直流电压波形示意图；
图5为本发明故障处理过程中MMC1的直流电流波形示意图；
图6为本发明故障处理过程中MMC1的有功功率波形示意图。
具体实施方式
下面将结合本发明实施例中的附图，对本发明实施例中的技术方案进行清楚、完整地描述，显然，所描述的实施例仅仅是本发明一部分实施例，而不是全部的实施例。基于本发明中的实施例，本领域普通技术人员在没有做出创造性劳动前提下所获得的所有其他实施例，都属于本发明保护的范围。
本发明的目的是提供一种电阻型子模块混合MMC及其直流故障处理策略，该混合型MMC所需全桥子模块比例仅为所有半桥子模块的30％，且可以在50ms之内隔离直流故障，大大降低了建设架空线高压柔 性直流输电系统的成本，在工程中具有非常强的参考意义与使用价值。
为使本发明的上述目的、特征和优点能够更加明显易懂，下面结合附图和具体实施方式对本发明作进一步详细的说明。
图1为本发明电阻型子模块混合MMC的拓扑结构示意图，如图1所示，本发明电阻型子模块混合MMC为三相六桥臂结构，每相包含上、下两个桥臂，每个桥臂由N个子模块以及一个桥臂电抗器串联构成，这N个子模块包括了N 1个半桥子模块和N 2个电阻型子模块即N＝N 1+N 2，N、N 1和N 2均为大于1的自然数。换流器直流侧出口处通过一个平波电抗器L dc和一个快速机械开关K与直流线路相连接。MMC所有上桥臂中，按照从高压端到低压端的顺序，分别布置N 1个串联半桥子模块，N 2个串联电阻型子模块和桥臂电抗器；MMC所有下桥臂中，按照从高压端到低压端的顺序，分别布置桥臂电抗器，N 2个串联电阻型子模块和N 1个串联半桥子模块。A相上、下桥臂的连接点通过交流断路器BR 1接地，B相上、下桥臂的连接点通过交流断路器BR 2相连，C相上、下桥臂的连接点通过交流断路器BR 3接地。每个桥臂中电阻型子模块的数量N 2小于等于30％N 1。电阻型子模块的结构与常规的半桥子模块类似，即子模块的高压端和低压端有两个并联支路，其中一个支路由IGBT和反并联二极管构成，另一个并联支路由IGBT、反并联二极管和子模块电阻串联构成。
对于上述混合型MMC，其直流故障处理策略的步骤如下：
(1)正常运行过程中，BR 1、BR 2和BR 3保持断开状态，K保持闭合状态；同时，监测每个桥臂的桥臂电流，分别为A相上桥臂电流I pa、A相下桥臂电流I na、B相上桥臂电流I pb、B相下桥臂电流I nb、C相上桥臂电流I pc和C相下桥臂电流I nc。
(2)直流故障发生后，经过t 1时间后某换流站首先检测到各桥臂电流满足下述关系式，立刻闭锁MMC中所有半桥子模块并导通所有电阻型子模块，闭合交流开关BR 1、BR 2和BR 3，同时向其它换流站发出闭锁命令。
max(I pa,I na,I pb,I nb,I pc,I nc)＞2I rate
其中：I rate为所用子模块内IGBT的额定直流电流。
(3)其它换流站接收到闭锁信号或检测到自身桥臂电流超过限值后，同样对闭锁MMC中所有半桥子模块并导通所有电阻型子模块，同时闭合交流开关BR 1、交流开关BR 2和交流开关BR 3，从第一个换流站闭锁到直流网络内所有换流站完成闭锁经过t 2时间。
(4)所有换流站闭锁后经过t 3时间，故障线路两侧快速机械开关K在流经电流降低到0.2kA之后，对应的换流站向快速机械开关发出开断信号；快速机械开关完成开断，实现故障线路的物理隔离，同时向BR 1、BR 2和BR 3发出开断信号。
当MMC中的半桥子模块闭锁且电阻型子模块接入后，电阻型子模块的等效电阻与平波电抗器、桥臂电抗器和直流线路共同构成一个RL衰减回路，使直流故障电流在半桥子模块闭锁且电阻型子模块接入后的t 3时间内迅速下降到快速机械开关能够断开的电流范围之内，最终使得快速机械开关K能够开断故障。
(5)经过t 4时间(最长为半个周期，即10ms)，BR 1、BR 2和BR 3恢复到开断状态，至此直流故障处理完成；故障处理期间即t 3+t 4时间段内，通过闭合BR 1、BR 2和BR 3在换流站阀侧主动制造三相相间短路，阻止交流系统电流流入直流系统，加快了故障电流振荡至0的过程，其中t 3+t 4总时长不超过60ms。
当故障处理完成后需进行重合闸操作，具体实现方式如下：
对于暂时性直流故障，经过t 5的去游离时间后，向快速机械开关K发出闭合信号，向两侧MMC发出解锁半桥子模块且旁路电阻型子模块的指令，最终系统恢复到稳态运行状态。
对于永久性直流故障，经过t 5的去游离时间后，向快速机械开关K发出闭合信号，向两侧MMC发出解锁半桥子模块且旁路电阻型子模块的指令；由于故障为永久性故障，经过t 6时间将再次检测到某个换流站桥臂电流值超过阈值，重新按照上述步骤对故障进行处理。
在图2所示的两端直流输电测试系统直流故障处理过程具体实施例中，两侧换流站均采用本发明所提出的电阻型子模块混合MMC，具体参数如表1所示：
表1
Figure PCTCN2020121131-appb-000001
仿真场景：稳态运行状态下，第一侧换流站MMC1控制直流电压，第二侧换流站MMC2控制传输功率，MMC1向MMC2传输400MW有功功率；在1.5s时，直流线路中点发生暂时性接地短路故障，故障持续时间0.1s。
(1)经过t 1＝3ms后，MMC1检测到桥臂电流超过阈值3kA，立刻闭锁所有半桥子模块且导通所有电阻型子模块，闭合交流开关BR 1、BR 2和BR 3，向线路两侧机械开关K发出开断信号，同时向MMC2发出闭锁命令。
(2)经过t 2＝1ms后，MMC2检测到桥臂电流超过阈值3kA，立刻闭 锁所有半桥子模块且导通所有电阻型子模块，闭合交流开关BR 1、BR 2和BR 3。
(3)经过t 3＝40ms后，流经线路两侧机械开关K的故障电流均降低到0.2kA，两侧开关完成开断，故障线路被隔离，同时向两侧换流站发出开断BR 1、BR 2和BR 3的信号。
(4)经过t 4＝10ms后，两侧换流站的BR 1、BR 2和BR 3均恢复到开断状态，至此直流故障处理完成。
上述过程总共花费54ms，在1.544s时故障线路被顺利隔离，其中两侧换流站BR 1、BR 2和BR 3均闭合持续时间为51ms，对交流系统产生冲击较小，该过程中流过线路两侧机械开关K的电流波形如图3所示。
故障处理结束后，等待t 5＝300ms的去游离时间，向快速机械开关K发出闭合信号，向两侧MMC发出解锁半桥子模块且旁路电阻型子模块的指令，最终系统恢复到稳态运行状态。从故障发生至恢复到稳态运行状态过程中，MMC1的直流电压波形如图4所示，直流电流波形如图5所示，向MMC2传输有功功率波形如图6所示。
本说明书中各个实施例采用递进的方式描述，每个实施例重点说明的都是与其他实施例的不同之处，各个实施例之间相同相似部分互相参见即可。
本文中应用了具体个例对本发明的原理及实施方式进行了阐述，以上实施例的说明只是用于帮助理解本发明的方法及其核心思想；同时，对于本领域的一般技术人员，依据本发明的思想，在具体实施方式及应用范围上均会有改变之处。综上所述，本说明书内容不应理解为对本发明的限制。

Claims (9)

  1. 一种电阻型子模块混合MMC，为三相六桥臂结构，每相包含上下两个桥臂，其特征在于，对于上桥臂，其从高压端至低压端依次由N 1个半桥子模块、N 2个电阻型子模块以及一个桥臂电抗器串联构成；对于下桥臂，其从高压端至低压端依次由一个桥臂电抗器、N 2个电阻型子模块以及N 1个半桥子模块串联构成，N 1和N 2均为大于1的自然数；
    A相上下桥臂的中间节点连接有交流断路器BR 1，交流断路器BR 1的另一端接地；B相上下桥臂的中间节点连接有交流断路器BR 2，交流断路器BR 2的另一端接地；C相上下桥臂的中间节点连接有交流断路器BR 3，交流断路器BR 3的另一端接地。
  2. 根据权利要求1所述的电阻型子模块混合MMC，其特征在于，每个桥臂中电阻型子模块的数量N 2小于等于30％N 1。
  3. 根据权利要求1所述的电阻型子模块混合MMC，其特征在于，所述MMC直流侧的高压端依次通过平波电抗器以及快速机械开关与直流线路相连接。
  4. 根据权利要求1所述的电阻型子模块混合MMC，其特征在于，所述电阻型子模块包括两个带反并联二极管的IGBT管T 1～T 2以及一个电阻R 0，其中IGBT管T 1的发射极与IGBT管T 2的集电极相连并作为子模块的高压端，IGBT管T 1的集电极与电阻R 0的一端相连，电阻R 0的另一端与IGBT管T 2的发射极相连并作为子模块的低压端。
  5. 一种直流故障处理策略，所述直流故障处理策略应用于上述权利要求3所述的电阻型子模块混合MMC，其特征在于，所述直流故障处理策略包括如下步骤：
    (1)正常运行过程中，使MMC的交流断路器BR 1、BR 2和BR 3保持开断状态，快速机械开关保持闭合状态，所有电阻型子模块保持旁路状态，同时监测MMC每个桥臂的桥臂电流；
    (2)直流故障发生后，若MMC所在换流站首先检测到桥臂电流超过阈值，立刻闭锁MMC中所有半桥子模块并导通所有电阻型子模块，同时闭合交流断路器BR 1、BR 2和BR 3，进而向MMC所连的另一端换流站发出直流故障处理指令；
    (3)另一端换流站接收到直流故障处理指令或检测到自身MMC桥 臂电流超过阈值，同样立刻闭锁MMC中所有半桥子模块并导通所有电阻型子模块，并闭合交流断路器BR 1、BR 2和BR 3；
    (4)经过一定时长t 3之后，当故障线路两端的快速机械开关流经电流降低到一定大小，两端换流站向各自的快速机械开关发出开断信号，快速机械开关完成开断，实现故障线路的物理隔离，与此同时两端换流站向各自MMC的交流断路器BR 1、BR 2和BR 3发出开断信号；
    (5)经过一定时间长t 4后，两端MMC的交流断路器BR 1、BR 2和BR 3恢复到开断状态，至此直流故障处理完成。
  6. 根据权利要求5所述的直流故障处理策略，其特征在于：所述步骤(2)和步骤(3)中当以下关系式成立则判定MMC桥臂电流超过阈值；
    max(I pa,I na,I pb,I nb,I pc,I nc)＞2I rate
    其中：I pa为MMC的A相上桥臂电流，I na为MMC的A相下桥臂电流，I pb为MMC的B相上桥臂电流，I nb为MMC的B相下桥臂电流，I pc为MMC的C相上桥臂电流，I nc为MMC的C相下桥臂电流，I rate为子模块内IGBT的额定电流。
  7. 根据权利要求5所述的直流故障处理策略，其特征在于：所述步骤(5)在完成直流故障处理后需进行重合闸操作，具体实现方式如下：
    对于暂时性直流故障，经过一定的去游离时间后，向故障线路两端的快速机械开关发出闭合信号，同时向两端MMC发出解锁半桥子模块且旁路电阻型子模块的指令，使系统恢复到稳态运行状态；
    对于永久性直流故障，经过一定的去游离时间后，向故障线路两端的快速机械开关发出闭合信号，同时向两端MMC发出解锁半桥子模块且旁路电阻型子模块的指令；由于是永久性直流故障，若经过一定时间再次检测到某一端换流站MMC桥臂电流超过阈值，则重新根据步骤(2)～(5)进行直流故障处理。
  8. 根据权利要求5所述的直流故障处理策略，其特征在于：当MMC中的半桥子模块闭锁且电阻型子模块投入后，电阻型子模块中的电阻与平波电抗器、桥臂电抗器和直流线路共同构成一个LC振荡衰减回路，使直流故障电流在半桥子模块闭锁且电阻型子模块投入后的t 3时间内迅速下降到快速机械开关能够断开的电流范围之内，最终使得快速机械开关完成 开断。
  9. 根据权利要求5所述的直流故障处理策略，其特征在于：在故障处理的t 3+t 4时间段内，通过闭合交流断路器BR 1、BR 2和BR 3在MMC阀侧主动制造三相相间短路，阻止交流系统电流流入直流系统，加快故障电流振荡衰减的过程，t 3+t 4的总时长不超过60ms。
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