WO2017031991A1 - 一种具有直流故障穿越能力的串联混合型双极直流输电系统 - Google Patents

一种具有直流故障穿越能力的串联混合型双极直流输电系统 Download PDF

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Publication number
WO2017031991A1
WO2017031991A1 PCT/CN2016/078928 CN2016078928W WO2017031991A1 WO 2017031991 A1 WO2017031991 A1 WO 2017031991A1 CN 2016078928 W CN2016078928 W CN 2016078928W WO 2017031991 A1 WO2017031991 A1 WO 2017031991A1
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Prior art keywords
direct current
converter
transmission system
station
series hybrid
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PCT/CN2016/078928
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English (en)
French (fr)
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徐政
王世佳
肖晃庆
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浙江大学
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Priority to US15/522,954 priority Critical patent/US10084387B2/en
Publication of WO2017031991A1 publication Critical patent/WO2017031991A1/zh

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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
    • H02M5/00—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases
    • H02M5/40—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc
    • H02M5/42—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters
    • H02M5/44—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac
    • H02M5/453—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M5/458—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M5/4585—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only having a rectifier with controlled elements
    • 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
    • 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
    • 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
    • H02M7/75—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 thyratron or thyristor type requiring extinguishing means
    • H02M7/757—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 thyratron or thyristor type requiring extinguishing means using semiconductor devices only
    • H02M7/7575—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 thyratron or thyristor type requiring extinguishing means using semiconductor devices only for high voltage direct transmission link
    • 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
    • H02J2003/365—Reducing harmonics or oscillations in HVDC
    • 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
    • H02M1/0095—Hybrid converter topologies, e.g. NPC mixed with flying capacitor, thyristor converter mixed with MMC or charge pump mixed with buck
    • 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
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • H02M1/325—Means for protecting converters other than automatic disconnection with means for allowing continuous operation despite a fault, i.e. fault tolerant converters
    • 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 belongs to the technical field of power system transmission, and particularly relates to a series hybrid bipolar direct current transmission system with DC fault ride through capability.
  • the traditional HVDC transmission system based on thyristor converter has the following three main defects: (1) The inverter station is easy to commutate; because the main device thyristor of the thyristor converter is a semi-controlled device, its commutation The mode adopts grid commutation. Therefore, fluctuations or faults in the AC grid voltage may cause commutation failure of the thyristor converter of the inverter station, which will cause a huge impact on the AC system and seriously affect the safe and stable operation of the system. (2) The dependence on the AC system is strong; the traditional HVDC transmission system based on the thyristor converter cannot transmit power to the weak AC system and the passive network. (3) The converter station has a large area; therefore, there is a big problem in the location of the DC drop point. In summary, the traditional high-voltage direct current transmission system based on thyristor converter has large defects, which restricts its development to some extent.
  • MMC-HVDC Modular Multilevel Converter
  • MMC-HVDC Modular Multilevel Converter based direct current transmission system
  • the basic unit of the topology is a half bridge submodule.
  • MMC-HVDC has many advantages, such as decoupling control of active power and reactive power, low switching frequency, low operating loss, low harmonic coefficient of output voltage waveform, low cost of filtering system, and expandability. Strong, compact footprint, small footprint and no commutation failure.
  • MMC-HVDC is highly competitive in new energy grid-connected, ultra-large-scale urban transmission and distribution and passive network power supply.
  • MMC-HVDC has the following disadvantages: (1) The DC side fault cannot be effectively processed, and the system reliability is low.
  • the anti-parallel diode of the anti-parallel control device forms an energy feed loop between the energy feed point and the fault point, which causes an instantaneous overcurrent, so the AC breaker must be tripped to shut it off.
  • the mechanical response time of the AC circuit breaker also requires 2 to 3 cycles, and the short-circuit overcurrent has increased to a larger value during the 2 to 3 cycles. Therefore, when the device is selected, the rated parameters of the device have to be increased, and auxiliary measures such as a high-speed bypass switch are configured, which greatly increases the construction cost of the converter station.
  • MMC-HVDC often needs to use cable lines with low failure rate and high cost as its DC transmission line, and cannot use overhead lines with high temporary failure rate and low cost, such as flashover, which makes it unusable.
  • grounding branch There are two main grounding methods: one is to install a three-phase star-connected reactive branch on the AC side of the module multilevel converter to provide a reference potential for the converter station, but the choice of reactance parameters is more difficult and will It affects the operating range of the reactive power of the converter itself; the second is to use the DC side large resistance clamp grounding method, but the selection of the large resistance parameter is difficult. When the resistance is too small, the loss during steady state operation will increase.
  • the present invention provides a series hybrid bipolar direct current transmission system with DC fault ride-through capability, which combines the advantages of a thyristor-based converter and an MMC-based direct current transmission topology.
  • the system has high output waveform quality, can independently control active power and reactive power, and has certain supporting effects on the AC grid voltage on the rectifier station side and the inverter station side;
  • Flat DC transmission system which can rely on the action of the converter itself to clear DC faults, has DC fault ride-through capability, and has good economy.
  • a series hybrid bipolar direct current transmission system with DC fault ride-through capability comprising: a rectifier station and an inverter station, which are connected by a direct current transmission line; wherein:
  • the rectifier station is configured to convert three-phase alternating current on the transmitting end alternating current grid into direct current power and then to the inverter station through the direct current transmission line; the inverter station is configured to convert the direct current power into three-phase alternating current Delivered to the receiving end AC grid;
  • Both the rectifier station and the inverter station adopt a bipolar system, that is, both of the positive and negative two-pole converter units are connected in series, and the series node is grounded; the converter unit is composed of a thyristor converter and a modular converter. One end of the thyristor converter is connected to the direct current transmission line, the other end of the thyristor converter is connected to one end of the modular inverter, and the other end of the modular inverter is grounded.
  • the ratio of the rated DC voltage of the thyristor converter to the modular converter is not less than 0.8 to ensure reliable removal of the DC line fault by the system.
  • the modular converter is composed of one MMC or a plurality of MMCs in parallel; if the modular converter is composed of a plurality of MMCs in parallel, the modular converter is configured with current balancing control.
  • a passive filter is connected to the three-phase alternating current bus of the transmitting end AC power grid or the receiving AC power grid; the passive filter is used for filtering the AC side current harmonic generated by the thyristor converter, and compensating The reactive power consumed by the thyristor converter.
  • both the positive and negative ends of the rectifier station or the inverter station are connected to the DC transmission line through a smoothing reactor; the smoothing reactor can suppress the ripple in the DC current to prevent the DC transmission line from being generated.
  • the steep wave front shock wave enters the converter station, causing the device to be damaged by overvoltage, avoiding current interruption, and limiting the DC current rising speed in the DC fault state.
  • the thyristor converter has a DC filter connected in parallel to filter DC side harmonics and reduce DC voltage fluctuations.
  • the thyristor converter adopts a twelve-pulse bridge structure, and each bridge arm is composed of a plurality of thyristors connected in series; the twelve-pulse bridge type thyristor converter can reduce the harmonic current generated by itself.
  • the thyristor converter passes through a three-winding transformer with a wiring mode of Y 0 /Y/ ⁇ or a two-winding transformer and a transmitting end with two wiring modes of Y 0 / ⁇ and Y 0 /Y, respectively.
  • AC grid connection capable of voltage level transformation and isolation of zero sequence components between the converter and the AC system, and such transformers can provide phase angles for the upper and lower six-pulse converter bridges of the thyristor inverter Three-phase alternating current with a difference of 30°.
  • the MMC is connected to the receiving AC power grid by a two-winding transformer with a wiring mode of ⁇ /Y 0 or Y/Y 0 ; the MMC adopts a three-phase six-bridge arm structure, and each of the bridge arms is composed of several The converter modules are cascaded; the three-phase AC output has a small harmonic content, and there is almost no need to configure a filter, and at the same time, the switching frequency of the device is greatly reduced in structural characteristics, thereby reducing losses.
  • the converter module uses a half bridge sub-module (HBSM); it requires a minimum number of semiconductor devices and a minimum steady state operation loss.
  • HBSM half bridge sub-module
  • the half bridge module is composed of two switch tubes T1 T T2 and a capacitor C; wherein one end of the switch tube T1 is connected to one end of the switch tube T2 and constitutes a positive pole of the half bridge submodule, and the other end of the switch tube T1 Connected to one end of the capacitor C, the other end of the switch tube T2 is connected to the other end of the capacitor C and constitutes the cathode of the half bridge module; the control terminals of the switch tubes T1 and T2 receive the switch signal provided by the external device.
  • the switching tubes T1 and T2 are both insulated gate bipolar transistors (IGBTs).
  • the series hybrid bipolar direct current transmission system has three operating modes, namely a bipolar current balancing operation mode, a single maximum reflow mode, and a unipolar metal return line mode.
  • the phase shift control is forcibly applied to the thyristor converter in the faulty pole commutating unit of the rectifier station, and the phase shift angle is 135° to 145°.
  • applying a blocking control to the modular inverter in the faulty pole commutating unit of the rectifier station so that the series hybrid bipolar direct current transmission system outputs a negative voltage on the faulty commutating unit of the rectifier station, thereby eliminating the fault.
  • the fault current provided by the pole commutation unit; and the inverter station fault commutating unit will be naturally cleared due to the single-conductivity of the thyristor converter.
  • the modular multilevel converter of the positive and negative rectifier stations, the thyristor converter of the positive and negative inverter stations, and the modular multilevel converter are all controlled by constant voltage.
  • the thyristor converter of the positive and negative rectifier stations converts the received power command value into a current command value for constant current control; the modular multilevel converter of the positive and negative rectifier stations and the inverter station participates in active power control On the basis of this, it is also possible to independently control the reactive power.
  • the modular multi-level inverters of the fault pole are all locked, the rectifier thyristor converter is forced to phase shift, and the firing angle is increased into the inverter mode to output a negative voltage, thereby
  • the DC voltage on the fault rectifier side is negative, the DC current output from the rectifier station side drops to zero; the inverter station has a single-conductivity due to the thyristor converter, which cuts off the AC system between the inverter station and the fault point.
  • the energy feed does not provide DC-side fault current; thus, the fault current at the fault point drops to zero, the fault arc goes out, and the temporary fault on the DC side is cleared.
  • the system is operated by normal operation mode. Switch to the unipolar mode of operation and the fault pole enters the restart wait mode.
  • the modular multilevel converters of the positive and negative poles are all locked, the thyristor converters of the positive and negative poles of the rectifier station are forcibly phase-shifted, and the firing angle is increased to the inverter mode for output.
  • Negative voltage so that the overall DC voltage on the rectifier station side is negative, the DC current output from the rectifier station side drops to zero; the inverter station has a single-conductivity due to the thyristor converter, which cuts off the inverter side AC system and The energy feed between the fault points does not provide a DC-side fault current; thus, the fault current at the fault point drops to zero, the fault arc is extinguished, and the temporary fault on the DC side is cleared.
  • the two-terminal AC system connected to the DC transmission system no longer has energy exchange, and the DC system enters the restart waiting mode.
  • the present invention combines the advantages of conventional high voltage direct current transmission and modular multilevel converters. Compared with the traditional thyristor converter-based direct current transmission system, the system can independently control active power and reactive power, has certain AC voltage control capability, good output waveform quality, and small filter footprint;
  • the modular multi-level flexible HVDC transmission system can rely on the converter's own action to clear the DC fault without the need to configure the DC breaker, and greatly reduces the cost of the full control device, saving costs and reducing losses.
  • the series hybrid bipolar direct current transmission system can be applied to a long-distance, large-capacity direct current transmission field, and is worthy of promotion.
  • FIG. 1 is a schematic structural view of a direct current power transmission system according to the present invention.
  • Figure 3 is a schematic view of the structure of the MMC.
  • FIG. 4 is a schematic structural view of the HBSM.
  • FIG. 5 is a schematic diagram of a bipolar current balancing operation mode of a direct current transmission system according to the present invention.
  • Fig. 6(a) is a schematic view showing the operation of the system of the present invention when a DC fault occurs in the positive power transmission line.
  • Fig. 6(b) is a schematic view showing the operation of the system of the present invention when a DC fault occurs in the negative power transmission line.
  • Figure 6 (c) is a schematic diagram of the operation of the system of the present invention when a bipolar transmission line experiences a DC fault.
  • Fig. 7(a) is a waveform diagram showing the DC voltage of the rectifier station side of the system of the present invention when a DC fault occurs.
  • Fig. 7(b) is a waveform diagram showing the DC current of the rectifier station side of the system of the present invention when a DC fault occurs.
  • Fig. 7(c) is a waveform diagram showing the DC voltage of the inverter station side of the system of the present invention when a DC fault occurs.
  • Fig. 7(d) is a waveform diagram showing the direct current of the inverter station side of the system of the present invention when a DC fault occurs.
  • the present invention has a series hybrid bipolar direct current transmission system with DC fault ride-through capability, including: a rectifier station and an inverter station, which are connected by a direct current transmission line, wherein:
  • the positive and negative poles of the rectifier and inverter stations are each composed of a thyristor converter (LCC) in series with a modular converter, where the modular converter uses one or more modular multilevel converters (MMC).
  • LCC thyristor converter
  • MMC modular multilevel converters
  • the three-phase busbars of the transmitting and receiving AC power grids are connected with passive filters.
  • the specific type, capacity, number of groups and tuning points are determined according to system engineering conditions. Generally, double tuned filters and shunt capacitors can be used. Cooperate to filter out the characteristic subharmonic current generated by the rectifier station, and if necessary, configure a C-type filter to filter out the lower harmonics.
  • the thyristor converter adopts a twelve-pulse bridge structure; wherein each bridge arm is composed of a plurality of thyristors connected in series; the thyristor converter installed at the rectifier station is configured according to a rectification mode, and is configured DC current control; the thyristor converter installed in the inverter station is configured in accordance with the inverter mode and is controlled by a constant DC voltage.
  • the thyristor converter is connected to the AC grid of the transmitting end through two two-winding transformers with Y 0 / ⁇ and Y 0 /Y respectively.
  • the transformer can exchange the voltage level of the three-phase alternating current of the AC system of the transmitting end to adapt to the required DC voltage level.
  • the difference between the two transformers is the upper and lower six-pulse converter bridge of the twelve-pulse bridge type thyristor converter.
  • a three-phase alternating current with a phase angle difference of 30 0 is provided.
  • the MMC in the modular inverter adopts a three-phase six-bridge arm structure; wherein each bridge arm is composed of several commutation modules in series; the MMC adopts a constant DC voltage and a constant reactive power control strategy. Control, MMC is connected to the receiving AC grid through a two-winding transformer with wiring mode of ⁇ /Y 0 .
  • the converter module adopts a half bridge submodule (HBSM), as shown in FIG. 4, which is composed of two IGBT tubes T1 to T2 and a capacitor C; wherein the emitter of the IGBT tube T1 is connected to the collector of the IGBT tube T2 and One end of the HBSM is formed, the collector of the IGBT tube T1 is connected to one end of the capacitor C, the emitter of the IGBT tube T2 is connected to the other end of the capacitor C and constitutes the other end of the HBSM; the gate of the IGBT tube T1 to T2 The poles receive the switching signals provided by the external device.
  • HBSM half bridge submodule
  • HBSM's switching strategy uses the recent level modulation method and sub-module capacitor voltage equalization strategy.
  • bipolar current balance control is performed under steady state conditions, and the basic working principle is as shown in FIG. 5.
  • the rectifier circuit side thyristor converter of the positive system and the negative system performs constant DC current control according to the same current command value, and the inverter side thyristor converter performs constant output DC voltage control; the positive system and the negative system rectifier station side
  • the modular multi-level converter on the inverter side performs constant DC voltage control and constant reactive power control, and the current multi-level converter performs current equalization control at the same time.
  • the principle of the self-cleaning fault in the DC fault of the present embodiment is as follows: when a positive (or negative) DC fault occurs on the DC line, as shown in FIG. 6(a) and FIG. 6(b), the modular pole of the fault pole is multi-powered.
  • the flat converters are all locked, the thyristor converter of the fault pole rectifier station is forced to phase shift, and the firing angle is increased into the inverter mode to output a negative voltage, so that the DC voltage of the fault pole rectifier station side is negative, and the rectifier station side
  • the output DC current drops to zero;
  • the inverter station has a single-conductivity due to the thyristor converter, which cuts off the energy feed between the AC system on the inverter side and the fault point, and does not provide DC-side fault current;
  • the fault current at the fault point will drop to zero, the fault arc will go out, and the temporary fault on the DC side will be cleared.

Abstract

一种具有直流故障穿越能力的串联混合型双极直流输电系统，包括整流站和逆变站；两者通过直流输电线路相连；整流站和逆变站的正极和负极均由晶闸管换流器与模块化换流器串联组成，其中模块化换流器采用一个或多个模块化多电平换流器并联组成。该系统分为正负两极加强了运行的可靠性，并且故障下能够依靠换流器自身的动作清除直流故障。

Description

一种具有直流故障穿越能力的串联混合型双极直流输电系统 技术领域
本发明属于电力系统输电技术领域，具体涉及一种具有直流故障穿越能力的串联混合型双极直流输电系统。
背景技术
随着电力科学技术的发展，基于晶闸管换流器的传统高压直流输电系统(LCC-HVDC)的造价逐年降低，电力输送的可靠性不断提高，站内损耗渐渐减小。目前，基于晶闸管换流器的传统高压直流输电系统已经被广泛应用于大容量远距离输电、海岛输电以及异步电网背靠背互联等场合。但是，基于晶闸管换流器的传统高压直流输电系统有以下三个主要的缺陷：(1)逆变站易换相失败；由于晶闸管换流器的主要器件晶闸管属于半控型器件，其换相方式采用电网换相，因此交流电网电压的波动或者故障有可能导致逆变站的晶闸管换流器出现换相失败，这将造成交流系统的巨大冲击，严重影响系统安全稳定运行。(2)对交流系统的依赖性强；基于晶闸管换流器的传统高压直流输电系统无法对弱交流系统以及无源网络进行输电。(3)换流站占地面积大；因此直流落点的选址上存在着较大的问题。综上所述，基于晶闸管换流器的传统高压直流输电系统存在着的较大的缺陷，一定程度上制约了它的发展。
基于模块化多电平换流器(Modular Multilevel Converter，MMC)的直流输电系统(MMC-HVDC)是一种较为新颖的柔性直流输电系统，其拓扑结构的基本单元是半桥子模块。相较于LCC-HVDC，MMC-HVDC有着诸多优点，例如能够实现有功功率及无功功率解耦控制、开关频率低、运行损耗低、输出电压波形谐波系数低、滤波系统成本低廉、拓展性强、结构紧凑占地面积小和不存在换相失败故障等。在新能源并网，超大规模城市输配电和无源网络供电等场合，MMC-HVDC有着很强的竞争力。然而，MMC-HVDC存在如下不足：(1)无法有效地处理直流侧的故障，系统可靠性低。当直流侧发生接地故障时，全 控型开关器件反向并联的续流二极管会形成能量馈送点与故障点之间的能量馈送回路，这会造成瞬时的过电流，因此必须跳开交流断路器将其切断。但交流断路器的机械响应时间最快也需要2～3个周波，短路过电流在这2～3个周波的时间内已经增大到较大的数值。因此在选择设备时不得不增大设备的额定参数，并且配置高速的旁路开关等辅助性措施，这大大地增加了换流站的建造成本。由于这个原因，MMC-HVDC常常需要使用故障率低、造价高的电缆线路作为其直流输电线路，而无法使用闪络等暂时性故障率高、造价低的架空线路输电，这导致了其无法应用于长距离直流输电场合。(2)接地支路设计、安装困难。目前采用的接地方式主要有两种：一是在模块多电平换流器的交流侧安装三相星形连接的电抗支路为换流站提供参考电位，但是电抗参数选择较为困难，并且会对换流器本身的无功功率的运行范围造成影响；二是采用直流侧大电阻嵌位接地方式，但是大电阻参数的选取较为困难，当电阻取得过小则稳态运行时的损耗会增大，当电阻取得过大则失去了接地的意义。(3)相对于LCC-HVDC，MMC-HVDC单位容量的投资成本较高，在远距离大容量输电场景下MMC-HVDC经济性相对较差。综上所述，MMC-HVDC存在的不足一定程度上制约其在工业输电领域的应用。
发明内容
针对现有技术所存在的上述技术问题，本发明提供了一种具有直流故障穿越能力的串联混合型双极直流输电系统，该系统结合了基于晶闸管换流器以及基于MMC的直流输电拓扑的优点；相对于传统高压直流输电系统，该系统输出波形质量高，能够独立控制有功功率和无功功率，对整流站侧和逆变站侧的交流电网电压具有一定支撑作用；相对于模块化多电平直流输电系统，该系统能够依靠换流器自身的动作清除直流故障，具有直流故障穿越能力，且经济性较好。
一种具有直流故障穿越能力的串联混合型双极直流输电系统，包括：整流站和逆变站，两者通过直流输电线路相连；其中：
所述的整流站用于将送端交流电网上的三相交流电转换为直流电后通过直流输电线路输送给逆变站；所述的逆变站用于将该直流电转换为三相交流电并 输送给受端交流电网；
所述的整流站和逆变站均采用双极系统，即均由正负两极换流单元串联组成，其串联节点接地；所述的换流单元由晶闸管换流器和模块化换流器构成，其中晶闸管换流器的一端与直流输电线路连接，晶闸管换流器的另一端与模块化换流器的一端相连，模块化换流器的另一端接地。
进一步地，晶闸管换流器与模块化换流器的额定直流电压之比不低于0.8，以保证系统对直流线路故障的可靠清除。
进一步地，所述的模块化换流器采用一个MMC或由多个MMC并联组成；若所述的模块化换流器由多个MMC并联组成，则该模块化换流器配置电流均衡控制。
进一步地，所述送端交流电网或受端交流电网进站的三相交流母线上连接有无源滤波器；无源滤波器用于滤除晶闸管换流器产生的交流侧电流谐波，并补偿晶闸管换流器消耗的无功功率。
进一步地，所述整流站或逆变站的正负两端均通过平波电抗器与直流输电线路相连接；平波电抗器能够对直流电流中的纹波进行平抑，防止直流输电线路产生的陡波前冲击波进入换流站导致器件遭受过电压而损坏，避免电流断续，同时限制直流故障状态下直流电流上升速度。
进一步地，所述的晶闸管换流器并联有直流滤波器，以滤除直流侧谐波，减少直流电压波动。
进一步地，所述的晶闸管换流器采用十二脉动桥式结构，每个桥臂均由若干个晶闸管串联组成；十二脉动桥式晶闸管换流器可减少自身所产生的谐波电流。
进一步地，所述的晶闸管换流器通过一台接线方式为Y0/Y/△的三绕组变压器或通过两台接线方式分别为Y0/△和Y0/Y的双绕组变压器与送端交流电网连接；能够起到电压等级变换和隔离零序分量在换流器与交流系统之间传递的作用，且这样的变压器能为晶闸管换流器的上下两个六脉动换流桥提供相角差为30°的三相交流电。
进一步地，所述的MMC通过接线方式为△/Y0或Y/Y0的双绕组变压器与受端交流电网连接；所述的MMC采用三相六桥臂结构，每个桥臂均由若干个换 流模块级联组成；其输出的三相交流电具有很小的谐波含量，几乎不需配置滤波器，同时其在结构特性上大大降低了器件的开关频率，进而减少损耗。
进一步地，所述的换流模块采用半桥子模块(HBSM)；其所需半导体器件数量最少，稳态运行损耗最小。
所述的半桥子模块由两个开关管T1～T2和一个电容C构成；其中，开关管T1的一端与开关管T2的一端相连并构成半桥子模块的正极，开关管T1的另一端和电容C的一端相连，开关管T2的另一端与电容C的另一端相连并构成半桥子模块的负极；开关管T1和T2的控制端均接收外部设备提供的开关信号。
所述的开关管T1和T2均采用绝缘栅双极型晶体管(IGBT)。
所述的串联混合型双极直流输电系统具有三种运行方式，即双极电流平衡运行方式、单极大地回流方式和单极金属回线方式。
当所述串联混合型双极直流输电系统的直流输电线路发生接地故障时，则对整流站故障极换流单元中的晶闸管换流器强制施加移相控制且移相角为135°～145°，同时对整流站故障极换流单元中的模块化换流器施加闭锁控制，使所述串联混合型双极直流输电系统在整流站故障极换流单元上输出负电压，从而消除由该故障极换流单元提供的故障电流；而逆变站故障极换流单元由于晶闸管换流器的单向导通性，故障电流将被自然清除。
本发明直流输电系统在正常工作时，正负极整流站的模块化多电平换流器、正负极逆变站的晶闸管换流器和模块化多电平换流器均采用定电压控制，正负极整流站的晶闸管换流器将接收的功率指令值转化为电流指令值进行定电流控制；正负极整流站和逆变站的模块化多电平换流器在参与有功功率控制的基础上，亦能够独立对无功功率进行控制。
在直流线路发生单极直流故障时，故障极的模块化多电平换流器全部闭锁，整流站晶闸管换流器强制移相，增大触发角转入逆变模式以输出负电压，从而使故障极整流站侧的直流电压为负，则整流站侧输出的直流电流下降为零；逆变站由于晶闸管换流器具有单向导通性，截断了逆变站侧交流系统与故障点之间的能量馈路，亦不会提供直流侧故障电流；这样，故障点的故障电流就会下降到零，故障电弧熄灭，直流侧的暂时性故障被清除。此时系统由正常运行模 式切换到单极运行模式，故障极进入重启等待模式。
在直流线路发生双极直流故障时，正负极的模块化多电平换流器全部闭锁，整流站正负极的晶闸管换流器强制移相，增大触发角转入逆变模式以输出负电压，从而使整流站侧的整体直流电压为负，则整流站侧输出的直流电流下降为零；逆变站由于晶闸管换流器具有单向导通性，截断了逆变站侧交流系统与故障点之间的能量馈路，亦不会提供直流侧故障电流；这样，故障点的故障电流就会下降到零，故障电弧熄灭，直流侧的暂时性故障被清除。此时直流输电系统所连接的两端交流系统不再有能量交换，直流系统进入重启等待模式。
本发明结合了传统高压直流输电和模块化多电平换流器的优点。相对于传统基于晶闸管换流器的直流输电系统，所述系统能够独立控制有功功率和无功功率，具有一定的交流电压控制能力，输出波形质量好，滤波器占地面积较小；相对于基于模块化多电平的柔性高压直流输电系统，能够依靠换流器自身的动作清除直流故障，无需配置直流断路器，且大大减少了造价昂贵的全控器件数量，节约了成本，降低了损耗。综上所述，所述串联混合型双极直流输电系统能够应用于远距离、大容量的直流输电领域，值得大力推广。
附图说明
图1为本发明直流输电系统的结构示意图。
图2为十二脉动桥式晶闸管换流器的结构示意图。
图3为MMC的结构示意图。
图4为HBSM的结构示意图。
图5为本发明直流输电系统在双极电流平衡运行方式的示意图。
图6(a)为正极输电线路发生直流故障时本发明系统的运行示意图。
图6(b)为负极输电线路发生直流故障时本发明系统的运行示意图。
图6(c)为双极输电线路发生直流故障时本发明系统的运行示意图。
图7(a)为发生直流故障时本发明系统整流站侧直流电压的波形示意图。
图7(b)为发生直流故障时本发明系统整流站侧直流电流的波形示意图。
图7(c)为发生直流故障时本发明系统逆变站侧直流电压的波形示意图。
图7(d)为发生直流故障时本发明系统逆变站侧直流电流的波形示意图。
具体实施方式
为了更为具体地描述本发明，下面结合附图及具体实施方式对本发明的技术方案及其相关原理进行详细说明。
如图1所示，本发明具有直流故障穿越能力的串联混合型双极直流输电系统，包括：整流站和逆变站，两者通过直流输电线路相连，其中：
整流站和逆变站的正极和负极均由晶闸管换流器(LCC)与模块化换流器串联组成，其中模块化换流器采用一个或多个模块化多电平换流器(MMC)并联组成；
送端和受端交流电网进站的三相母线上连接有无源滤波器，其具体类型、容量、组数和调谐点等根据系统工程条件来确定，一般可采用双调谐滤波器和并联电容器相配合，以滤除整流站所产生的特征次谐波电流，必要时可配置C型滤波器以滤除低次谐波。
如图2所示，晶闸管换流器采用十二脉动桥式结构；其中，每个桥臂均由若干个晶闸管串联构成；安装在整流站的晶闸管换流器按照整流模式进行配置，并采用定直流电流控制；安装在逆变站的晶闸管换流器按照逆变模式进行配置，并采用定直流电压控制。
晶闸管换流器通过两台接线方式分别为Y0/△和Y0/Y的双绕组变压器与送端交流电网连接。变压器能够对送端交流系统的三相交流电进行电压等级交换，以适应所需的直流电压等级，两变压器接线方式的不同为十二脉动桥式晶闸管换流器的上下两个六脉动换流桥提供相角差为300的三相交流电。
如图3所示，模块化换流器内的MMC采用三相六桥臂结构；其中，每个桥臂均由若干个换流模块串联组成；MMC采用定直流电压和定无功功率控制策略控制，MMC通过一台接线方式为△/Y0的双绕组变压器与受端交流电网连接。
换流模块采用半桥子模块(HBSM)，如图4所示，其由两个IGBT管T1～T2和一个电容C构成；其中，IGBT管T1的发射极与IGBT管T2的集电极相连并构成HBSM的一端，IGBT管T1的集电极和电容C的一端相连，IGBT管T2的发射极与电容C的另一端相连并构成HBSM的另一端；IGBT管T1～T2的门 极均接收外部设备提供的开关信号。
HBSM的投切策略采用最近电平调制法和子模块电容电压均衡策略。
本实施方式在稳态情况下进行双极电流平衡控制，基本工作原理如图5所示。正常工作时正极系统与负极系统的整流站侧晶闸管换流器按照相同电流指令值进行定直流电流控制，逆变站侧晶闸管换流器进行定输出直流电压控制；正极系统与负极系统整流站侧和逆变站侧的模块化多电平换流器均进行定直流电压控制和定无功功率控制，模块化多电平换流器内部同时进行电流均衡控制。在该控制方式下，当正负极功率指令值相同时，正负极直流电压相同；当正极直流功率指令值大于负极功率指令值时，正极直流电压大于负极直流电压，反之则正极直流电压大于负极直流电压。所述系统的正负极因此可以按照相应功率指令值输送有功功率，维持接地极地电流为零，同时在整流站侧和逆变站侧分别控制注入交流系统的无功功率。
本实施方式在直流故障时的自清理故障的原理如下：在直流线路发生正极(或负极)直流故障时，如图6(a)和图6(b)所示，故障极的模块化多电平换流器全部闭锁，故障极整流站的晶闸管换流器强制移相，增大触发角转入逆变模式以输出负电压，从而使故障极整流站侧直流电压为负，则整流站侧输出的直流电流下降为零；逆变站由于晶闸管换流器具有单向导通性，截断了逆变站侧交流系统与故障点之间的能量馈路，亦不会提供直流侧故障电流；这样，故障点的故障电流就会下降到零，故障电弧熄灭，直流侧的暂时性故障被清除。此时系统由正常运行模式切换到单极运行模式，故障极进入重启等待模式。在直流线路发生双极直流故障时，如图6(c)所示，正负极的模块化多电平换流器全部闭锁，整流站正负极的晶闸管换流器强制移相，增大触发角转入逆变模式以输出负电压，从而使整流站侧的整体直流电压为负，则整流站侧输出的直流电流下降为零；逆变站由于晶闸管换流器具有单向导通性，截断了逆变站侧交流系统与故障点之间的能量馈路，亦不会提供直流侧故障电流；这样，故障点的故障电流就会下降到零，故障电弧熄灭，直流侧的暂时性故障被清除。此时直流输电系统所连接的两端交流系统不再有能量交换，直流系统进入重启等待模式。
为了进一步验证本实施方式的有效性和可行性，通过在电力系统暂态仿真 软件PSCAD/EMTDC中搭建相应模型，具体仿真参数如表1所示：
表1
Figure PCTCN2016078928-appb-000001
单极接地故障是输电线路最容易发生的故障类型，对于本实施方式，在1s时刻与正极直流线路中点引入接地故障，接地电阻为1欧姆。直流故障的处理逻辑为，当故障极整流站侧直流电流超过1.5倍额定值时触发故障极闭锁信号，故障极MMC全部闭锁，整流站LCC触发角强制移相到135°，逆变站触发角降为90°。当检测到直流电流为零后，故障极整流站和逆变站继续保持相应动作0.2s，以恢复弧道绝缘特性。弧道绝缘特性恢复后，将对故障极进行重启。故障极重启逻辑为，解锁整流站和逆变站的MMC，同时整流站触发角自45°起，在0.2s内线性下降至15°，逆变站触发角自120°起，在0.2s内线性上升至140°。当整流站和逆变站触发角分别达到15°和140°后，认为故障极重启完成，故障极转入稳态运行控制。
故障响应特性如图7所示，其中图7(a)为发生直流故障时整流站侧直流电压随时间变化的波形；图7(b)为发生直流故障时整流站侧直流电流随时间变化的波形；由图可见，在发生正极直流故障后，整流站侧LCC通过强制移相使之闭锁，直流电流降为0，而整流站侧负极直流电压电流在小幅波动后运行状态正常。图7(c)为发生直流故障时逆变站侧直流电压随时间变化的波形；图7(d)为发生直流故障时逆变站侧直流电流随时间变化的波形；由图可见，在发生正极直流故障后，逆变站侧正极换流站由于LCC单向导通性，直流电流降为0，而逆变站侧负极直流电压电流在小幅波动后运行状态正常。在故障清除之后，系统能够平滑地恢复至稳定运行值。

Claims (11)

  1. 一种具有直流故障穿越能力的串联混合型双极直流输电系统，包括：整流站和逆变站，两者通过直流输电线路相连；其特征在于：
    所述的整流站用于将送端交流电网上的三相交流电转换为直流电后通过直流输电线路输送给逆变站；所述的逆变站用于将该直流电转换为三相交流电并输送给受端交流电网；
    所述的整流站和逆变站均采用双极系统，即均由正负两极换流单元串联组成，其串联节点接地；所述的换流单元由晶闸管换流器和模块化换流器构成，其中晶闸管换流器的一端与直流输电线路连接，晶闸管换流器的另一端与模块化换流器的一端相连，模块化换流器的另一端接地。
  2. 根据权利要求1所述的串联混合型双极直流输电系统，其特征在于：所述的模块化换流器采用一个MMC或由多个MMC并联组成；若所述的模块化换流器由多个MMC并联组成，则该模块化换流器配置电流均衡控制。
  3. 根据权利要求1所述的串联混合型双极直流输电系统，其特征在于：所述送端交流电网或受端交流电网进站的三相交流母线上连接有无源滤波器。
  4. 根据权利要求1所述的串联混合型双极直流输电系统，其特征在于：所述整流站或逆变站的正负两端均通过平波电抗器与直流输电线路相连接。
  5. 根据权利要求1所述的串联混合型双极直流输电系统，其特征在于：所述的晶闸管换流器并联有直流滤波器。
  6. 根据权利要求1所述的串联混合型双极直流输电系统，其特征在于：所述的晶闸管换流器采用十二脉动桥式结构，每个桥臂均由若干个晶闸管串联组成。
  7. 根据权利要求2所述的串联混合型双极直流输电系统，其特征在于：所述的晶闸管换流器通过一台接线方式为Y0/Y/△的三绕组变压器或通过两台接线方式分别为Y0/△和Y0/Y的双绕组变压器与送端交流电网连接；所述的MMC通过接线方式为△/Y0或Y/Y0的双绕组变压器与受端交流电网连接。
  8. 根据权利要求2所述的串联混合型双极直流输电系统，其特征在于：所述的MMC采用三相六桥臂结构，每个桥臂均由若干个换流模块级联组成；所 述的换流模块采用半桥子模块，所述的半桥子模块由两个开关管T1～T2和一个电容C构成；其中，开关管T1的一端与开关管T2的一端相连并构成半桥子模块的正极，开关管T1的另一端和电容C的一端相连，开关管T2的另一端与电容C的另一端相连并构成半桥子模块的负极；开关管T1和T2的控制端均接收外部设备提供的开关信号。
  9. 根据权利要求1所述的串联混合型双极直流输电系统，其特征在于：所述的串联混合型双极直流输电系统具有三种运行方式，即双极电流平衡运行方式、单极大地回流方式和单极金属回线方式。
  10. 根据权利要求1所述的串联混合型双极直流输电系统，其特征在于：当所述串联混合型双极直流输电系统的直流输电线路发生接地故障时，则对整流站故障极换流单元中的晶闸管换流器强制施加移相控制且移相角为135°～145°，同时对整流站故障极换流单元中的模块化换流器施加闭锁控制，使所述串联混合型双极直流输电系统在整流站故障极换流单元上输出负电压，从而消除由该故障极换流单元提供的故障电流；而逆变站故障极换流单元由于晶闸管换流器的单向导通性，故障电流将被自然清除。
  11. 根据权利要求1所述的串联混合型双极直流输电系统，其特征在于：
    所述的系统中晶闸管换流器与模块化换流器的额定直流电压之比不低于0.8。
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