WO2023201922A1 - 混合级联直流输电系统的受端交流故障穿越控制方法 - Google Patents

混合级联直流输电系统的受端交流故障穿越控制方法 Download PDF

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WO2023201922A1
WO2023201922A1 PCT/CN2022/107777 CN2022107777W WO2023201922A1 WO 2023201922 A1 WO2023201922 A1 WO 2023201922A1 CN 2022107777 W CN2022107777 W CN 2022107777W WO 2023201922 A1 WO2023201922 A1 WO 2023201922A1
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receiving end
lcc
mmc
fault
voltage
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French (fr)
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徐政
张楠
张哲任
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浙江大学
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    • 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
    • H02J3/001—Methods to deal with contingencies, e.g. abnormalities, faults or failures
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • 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
    • 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 belongs to the technical field of power systems, and specifically relates to a receiving end AC fault ride-through control method of a hybrid cascaded DC transmission system.
  • LCC-HVDC is very mature and has the advantages of low investment cost and rich practical experience.
  • LCC-HVDC also has the disadvantage that the inverter station is prone to commutation failure, requires the AC system to provide sufficient reactive power support, and cannot provide sufficient reactive power support to the weak AC system. Issues such as power delivery.
  • MMC-HVDC not only does not have commutation failure and reactive power compensation problems, but also can independently adjust active power and reactive power at the same time; however, compared with LCC-HVDC, MMC-HVDC suffers from high equipment cost, large losses and excessive The disadvantage of weak load capacity.
  • the LCC on the inverter side may fail to commutate, and the power transmission of the MMC may also be blocked, which will bring overcurrent and overvoltage challenges to the system; once the inverter The side LCC has commutation failure, and its DC voltage drops to zero. The significant DC voltage difference between the sending and receiving ends will cause the DC current to rise sharply. At the same time, due to the slow control response speed of the rectifier side LCC, the receiving end will bear huge surplus power, forcing the sub-module in the MMC to overcharge, resulting in over-voltage of the sub-module capacitor.
  • the drop in AC voltage at the receiving end will weaken the power transmission capability of the MMC, and the unbalanced power between the sending and receiving ends will be greater, exacerbating the overvoltage level; overcurrent and overvoltage caused by AC system failures at the receiving end will affect the equipment. insulation and service life, and may even cause equipment damage, inverter lock-up and other derivative failures.
  • Controllable self-restoring energy dissipation device suitable for hybrid cascade UHV DC transmission system [J]. Chinese Journal of Electrical Engineering, 2021, 41(02): 514-524.] A controllable self-restoring energy dissipation device is proposed. The device is installed in parallel on the ⁇ 400kV DC bus of the MMC DC port and is used to dissipate the system under AC faults at the receiving end. Transient surplus power.
  • auxiliary energy-consuming devices require high investment costs, and long-term large amounts of energy absorption will accelerate the aging of the arrester.
  • the present invention provides a receiving-end AC fault ride-through control method of a hybrid cascaded DC transmission system to overcome the current problem of insufficient AC fault ride-through capability at the receiving end of the hybrid cascaded DC transmission system and realize overcurrent and overvoltage. of inhibition.
  • a method of AC fault ride-through control at the receiving end of a hybrid cascade DC transmission system adopts LCC, and the inverter side adopts an LCC-MMC hybrid cascade structure, that is, multiple parallel MMCs and LCCs are connected in series. Composition; when a serious AC fault occurs at the receiving end and causes the commutation failure of the inverter side LCC, the rectifier side LCC determines the occurrence of the receiving end AC fault based on the change in the electrical quantity of its own DC port, and then quickly reduces the rectifier side DC by increasing the trigger angle.
  • the MMC that adopts constant active power control on the inverter side modifies its outer loop active power command value to transmit as much active power as possible, thereby suppressing overcurrent and overvoltage and achieving AC fault ride-through at the receiving end.
  • the LCC on the rectifier side adopts constant DC current control
  • the LCC on the inverter side adopts constant DC voltage control
  • the multiple MMCs on the inverter side adopt master-slave control, that is, one of the MMCs adopts constant DC voltage and constant DC voltage control. Reactive power control, the remaining MMC adopts constant active power and constant reactive power control;
  • the outer loop active power command correction value is calculated and the correction value is added to the original active power command value of these MMCs for control;
  • the rectifier side After the rectifier side receives the AC fault removal signal of the receiving end power grid through inter-station communication, it puts in the DC line protection device and changes the trigger angle command value of the rectifier side LCC from Reduce to the steady state value so that the system can smoothly return to the steady state.
  • step (2) when the DC current on the rectifier side is greater than 1.1 p.u. and the DC voltage on the rectifier side is between 0.5 p.u. and 0.9 p.u., it is determined that the system has an AC fault on the receiving end power grid.
  • U r is the AC voltage amplitude on the rectifier side
  • X r is the commutation reactance of the rectifier side LCC
  • U' dcr is the theoretical value of the DC voltage of the rectifier side LCC
  • I * dc is the DC current command value of the rectifier side LCC.
  • the theoretical value U' dcr of the rectifier side LCC DC voltage is set to 0.5pu.
  • U sm,MMCi is the grid-side phase voltage amplitude of the i-th MMC on the inverter side
  • i vd,MMCi is the d-axis component of the valve-side current amplitude of the i-th MMC on the inverter side.
  • the LCC at the sending end and the LCC at the receiving end are used.
  • the rectifier side LCC determines the receiving end based on the change in the electrical quantity of its own DC port.
  • the DC voltage on the rectifier side is quickly reduced by increasing the trigger angle, thereby achieving the effect of quickly suppressing overcurrent.
  • the DC transmission power on the rectifier side is also reduced accordingly.
  • the present invention allows the MMC that adopts constant active power control on the inverter side to transmit as much active power as possible by modifying its outer loop active power command value, thereby reducing the surplus power of the receiving end system and suppressing the MMC sub-module capacitor overvoltage. , thus realizing the ride-through of AC faults at the receiving end of the LCC-MMC hybrid cascaded DC transmission system.
  • the present invention has the following beneficial technical effects:
  • the sending end does not need to rely on inter-station communication, and the response speed is faster.
  • the method of the present invention proposes suppression measures for overcurrent and overvoltage under grid faults at the receiving end of the hybrid cascade DC transmission system of LCC-MMC at the sending end and LCC at the receiving end from the control level, realizing fault ride-through and reducing arrester energy absorption. Provide guarantee for safe and stable operation of DC system.
  • Figure 1 is a schematic diagram of the topology of a hybrid cascade DC transmission system.
  • Figure 2 is a schematic diagram of the improved control principle of LCC on the rectifier side of the system of the present invention.
  • Figure 3 is a schematic diagram of the outer loop active power correction control principle of the inverter side MMC of the system of the present invention.
  • Figure 4 is a schematic waveform diagram of the effective value of the AC voltage of each receiving end grid under the three-phase short circuit fault of the MMC 1 receiving end AC system in the embodiment.
  • Figure 5 is a schematic diagram of the waveform of the valve side current of the inverter side LCC converter transformer under a three-phase short circuit fault in the MMC 1 receiving end AC system in the embodiment.
  • Figure 6 is a schematic diagram of the waveform of the rectifier side DC current under a three-phase short circuit fault in the MMC 1 receiving end AC system in the embodiment.
  • Figure 7 is a schematic waveform diagram of the DC voltage on the rectifier side under a three-phase short circuit fault in the AC system at the receiving end of MMC 1 in the embodiment.
  • Figure 8 is a schematic waveform diagram of the triggering angle of the LCC on the rectifier side under a three-phase short circuit fault in the AC system at the receiving end of MMC 1 in the embodiment.
  • Figure 9 is a schematic waveform diagram of the active power of MMC 3 under a three-phase short circuit fault in the AC system at the receiving end of MMC 1 in the embodiment.
  • Figure 10 is a schematic waveform diagram of the capacitor voltage of the MMC 1 sub-module under a three-phase short circuit fault in the MMC 1 receiving end AC system in the embodiment.
  • the receiving end AC fault ride-through control method of the hybrid cascade DC transmission system of the present invention includes the following steps:
  • the rectifier side LCC adopts constant DC current control
  • the inverter side LCC adopts constant DC voltage control
  • the n MMCs on the inverter side adopt master-slave control, that is, MMC 1 adopts constant DC voltage and constant reactive power.
  • the DC side is equivalent to a short circuit (that is, the DC voltage is zero), the DC voltage on the inverter side drops rapidly, and the DC voltage between the rectifier side and the inverter side The voltage difference causes an increase in DC current.
  • the DC current on the rectifier side is greater than 1.1p.u.
  • the DC voltage on the rectifier side is between 0.5p.u. and 0.9p.u., which is used to determine that the system has an AC fault on the receiving end of the power grid. At this time, exit the DC line protection to avoid malfunction of the DC line protection.
  • the rectifier side LCC quickly increases the firing angle command value to Thereby reducing the rectifier side DC voltage and DC transmission power.
  • U r is the AC voltage amplitude on the rectifier side
  • ⁇ r is the firing angle of the rectifier side LCC
  • X r is the commutation reactance of the rectifier side LCC
  • I dc is the DC current of the rectifier side LCC.
  • the size of U dcr depends on ⁇ r and I dc ; compared with adjusting the DC current command value, the response speed of adjusting the firing angle command value is faster, and an excessively low current command value will cause power loss during the fault. Increase and extend failure recovery time. Therefore, the DC current command value is set to a fixed value, and the rectifier side DC voltage is reduced by increasing the firing angle command value. It should be noted that in theory, the DC voltage of the LCC on the inverter side drops to 0 at this time, and the DC voltage of the entire inverter side drops to 0.5pu.
  • I * dc is the DC current command value of the rectifier side LCC
  • U' dcr is the theoretical value of the rectifier side LCC DC voltage.
  • U sm,MMCi is the grid-side phase voltage amplitude of MMC i
  • i vd,MMCi is the d-axis component of the valve-side current amplitude of MMC i .
  • U dci,MMC is the DC voltage of MMC
  • I dci is the DC current on the inverter side.
  • the rectifier side After the rectifier side receives the AC fault removal signal of the receiving end power grid through inter-station communication, it puts in DC line protection and changes the LCC trigger angle command value of the rectifier side from Linearly reduces to its steady-state value, slowing down the recovery speed of the DC voltage, allowing the hybrid cascaded DC transmission system to smoothly return to the steady state.
  • the sending end can respond only after the delay of inter-station communication. Although communication and reducing the trigger angle prolong the recovery process to a certain extent, it improves the fault recovery performance of the entire system and enables it to Smoother transition to steady state.
  • this embodiment takes a hybrid cascade DC transmission system of a certain sending end LCC and receiving end LCC-MMC as an example, that is, the rectifier side uses LCC, the inverter side high-pressure valve group uses LCC, and the low-pressure valve in series with it
  • the group consists of three half-bridge MMCs connected in parallel.
  • the LCC and three MMCs on the inverter side are respectively connected to different receiving end AC systems, and there is electrical coupling between them.
  • the rectifier-side LCC determines the occurrence of an AC fault at the receiving end based on the changes in the electrical quantities of its own DC port, and quickly reduces the rectifier-side DC voltage and DC by increasing the triggering angle.
  • MMC 2 and MMC 3 which use active power control on the inverter side, transmit as much active power as possible by modifying their outer loop active power command values, thereby suppressing overcurrent and overvoltage, and achieving AC fault ride-through at the receiving end;
  • the specific control process is as follows:
  • the rectifier side LCC adopts constant DC current control
  • the inverter side LCC adopts constant DC voltage control
  • the three MMCs on the inverter side adopt master-slave control, that is, MMC 1 adopts constant DC voltage and constant reactive power.
  • MMC 1 adopts constant DC voltage and constant reactive power.
  • MMC 2 and MMC 3 adopt constant active power and constant reactive power control.
  • the fault detection module determines that a fault has occurred in the hybrid cascade DC transmission system at the receiving end of the power grid. In the event of an AC fault, the mode switches to 1; at this time, the DC line protection is exited to avoid malfunction of the DC line protection.
  • the rectifier side LCC quickly increases the firing angle command value to As shown in Figure 2, thereby reducing the rectifier side DC voltage and DC transmission power.
  • the rectifier side After the rectifier side receives the AC fault removal signal of the receiving end power grid through inter-station communication, it puts in DC line protection and changes the trigger angle command value of LCC from decreases linearly to its steady-state value As shown in Figure 2, the hybrid cascaded DC transmission system can smoothly return to a steady state.
  • control strategy of the present invention is verified below by simulating a three-phase metallic short-circuit fault in the AC power grid of the receiving end MMC 1 .
  • the output active power of the LCC is 0, and the active power output of the three MMCs is also blocked to varying degrees due to varying degrees of AC voltage drops; the unbalanced power between the sending and receiving ends causes the receiving end MMC to bear a large surplus power, forcing the sub-module capacitor voltage to charge, and even generate over-voltage.
  • the response curves of the DC current and DC voltage on the rectifier side are shown in Figure 6 and Figure 7 respectively.
  • Figure 8 shows the firing angle response curve of the LCC on the rectifier side.
  • the rectifier side LCC rapidly increases the firing angle command value to At this time, the peak value of the rectifier side DC current is only 1.13pu, which is much lower than the 1.31pu without the control of the proposed invention. Compared with the time when the rectifier side DC current is higher than 1.1pu without the control of this implementation method, the peak value is only 1.13pu. 22ms, and the time when the rectifier side DC current is higher than 1.1pu is only 8ms under the control of this implementation method.
  • MMC 3 can generate more active power through the correction of the outer loop active power command value, so the surplus power on the MMC is reduced.
  • the peak value of the sub-module capacitor voltage is reduced to 1.25pu. , far lower than 1.61pu without the control of the proposed invention, effectively suppressing the MMC sub-module capacitance overvoltage, and realizing the ride-through of AC faults at the receiving end of the hybrid cascade DC transmission system of LCC-MMC at the sending end and LCC at the receiving end. .
  • the rectifier side LCC When the rectifier side LCC receives the AC fault removal signal of the receiving end power grid through inter-station communication, its trigger angle command value changes from decreases linearly to its steady-state value As shown in Figure 8, the hybrid cascaded DC transmission system can smoothly return to the steady state.

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  • Supply And Distribution Of Alternating Current (AREA)
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Abstract

本发明公开了一种混合级联直流输电系统的受端交流故障穿越控制方法，即整流侧LCC以其自身直流端口电气量的变化判断受端交流故障发生，通过增大触发角快速降低整流侧直流电压，从而达到快速抑制过电流的效果；逆变侧采用定有功功率控制的MMC通过修正其外环有功功率指令值，尽可能地多输送有功功率，减少了受端系统的盈余功率，抑制了MMC子模块电容过电压。本发明方法从控制层面提出了混合级联直流输电系统受端电网故障下过电流和过电压的抑制措施，实现了故障穿越，并且具有良好的故障恢复过程；除此之外，本发明方法也能够降低避雷器吸能，为直流系统安全稳定运行提供保障。

Description

混合级联直流输电系统的受端交流故障穿越控制方法
本申请要求于2022年04月21日提交中国专利局、申请号为202210438734.2、发明名称为“混合级联直流输电系统的受端交流故障穿越控制方法”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
本发明属于电力系统技术领域，具体涉及一种混合级联直流输电系统的受端交流故障穿越控制方法。
背景技术
LCC-HVDC已经非常成熟，并具有投资成本低、实践经验丰富等优点，但是LCC-HVDC也具有逆变站容易发生换相失败，需要交流系统提供充足的无功功率支持和无法向弱交流系统送电等问题。相比之下，MMC-HVDC不仅没有换相失败和无功补偿问题，而且能够同时独立调节有功功率和无功功率；但是与LCC-HVDC相比，MMC-HVDC存在设备成本高、损耗大和过负载能力较弱的缺点。
为了实现LCC和MMC的优势互补，混合直流输电技术已经成为新的研究热点，是未来远距离、大容量输电的发展趋势。中国目前正在建设的白河滩-江苏混合级联直流输电工程，整流侧采用LCC，逆变侧高压阀组采用LCC，与之串联的低压阀组由三个半桥型MMC并联组成，LCC和三个MMC的受端交流系统分散接入不同的负荷中心，组成了多端直流输电系统。但是由于华东地区的负荷中心相对较近，它们的受端交流系统不可避免地存在不同程度的电耦合，该工程实现了西部地区长距离、大容 量的水电输送，从而缓解了东部地区电力短缺的压力。
当受端交流系统发生了严重故障时，逆变侧LCC可能会发生换相失败，同时MMC的的功率传输也会受到阻塞，这会给系统带来过电流和过电压的挑战；一旦逆变侧LCC发生了换相失败，它的直流电压下降到零，送受端之间显著的直流电压差将导致直流电流急剧升高。同时，由于整流侧LCC的控制响应速度较慢，受端将承受巨大的盈余功率，迫使MMC中的子模块过充电，从而导致子模块电容的过电压。此外，受端交流电压的下降会削弱MMC的功率输送能力，送受端之间的不平衡功率更大，加剧了过电压水平；受端交流系统故障所引发的过电流和过电压，会影响设备的绝缘和使用寿命，甚至发生设备损坏、健全换流器闭锁等衍生故障。
针对混合级联直流输电系统的受端交流故障穿越，现有研究多集中在利用耗能装置抑制过电压的方法上，例如文献[CHENG F,YAO L,XU J,et al.A comprehensive ac fault ride-through strategy for hvdc link with serial-connected lcc-vsc hybrid inverter[J].CSEE Journal of Power and Energy Systems,2022,8(1):175–187.]中提出了一种基于直流斩波器抑制直流过电压的受端交流故障穿越措施；文献[刘泽洪，王绍武，种芝艺等.适用于混合级联特高压直流输电系统的可控自恢复消能装置[J].中国电机工程学报，2021，41(02)：514-524.]提出了一种可控自恢复消能装置，该装置并联安装在MMC直流端口±400kV的直流母线上，用于受端交流故障下耗散系统的暂态盈余功率。然而，上述辅助耗能装置都需要高昂的投资成本，并且长期大量的能量吸收会加速避雷器的老化。
因此，目前缺乏对于混合级联直流输电系统受端交流系统故障所引发 的过电流抑制策略的研究，文献[NIU C,Yang M,XUE R,et al.Research on inverter side ac fault ride-through strategy for hybrid cascaded multi-terminal hvdc system[C].2020 IEEE 4th Conference on Energy Internet and Energy System Integration(EI2),2020：800–805.]提出了在站间通信正常的情况下，整流侧LCC接收到受端交流系统故障信号后，定直流电流控制切换到响应速度更快的PI控制器，并通过调低电流指令值快速降低直流电流的控制策略。但是，站间通信的速度较慢，并且过低的电流指令值会使故障期间功率损失增多并延长故障恢复时间。
发明内容
鉴于上述，本发明提供了一种混合级联直流输电系统的受端交流故障穿越控制方法，以克服目前混合级联直流输电系统受端严重交流故障穿越能力不足的问题，实现过电流和过电压的抑制。
一种混合级联直流输电系统的受端交流故障穿越控制方法，所述混合级联直流输电系统整流侧采用LCC，逆变侧采用LCC-MMC混合级联结构即多个并联的MMC与LCC串联组成；当受端发生严重交流故障导致逆变侧LCC换相失败时，整流侧LCC以其自身直流端口电气量的变化判断受端交流故障发生，然后通过增大触发角快速减小整流侧直流电压和直流输送功率；同时逆变侧采用定有功功率控制的MMC通过修正其外环有功功率指令值，尽可能地多输送有功功率，从而抑制过电流和过电压，实现受端交流故障穿越。
进一步地，所述受端交流故障穿越控制方法的具体步骤如下：
(1)系统稳态运行时，整流侧LCC采用定直流电流控制，逆变侧 LCC采用定直流电压控制，逆变侧的多个MMC采用主从控制，即其中一个MMC采用定直流电压和定无功功率控制，其余MMC采用定有功功率和定无功功率控制；
(2)在整流侧判断系统是否发生受端电网交流故障，若发生故障，则将系统的直流线路保护装置退出，避免该装置误动作；
(3)将整流侧LCC的触发角指令值迅速增大至
Figure PCTCN2022107777-appb-000001
以降低整流侧直流电压和直流输送功率；
(4)对于采用定有功功率和定无功功率控制的MMC，通过计算外环有功功率指令修正值，将该修正值附加到这些MMC原有功功率指令值上加以控制；
(5)当整流侧通过站间通信接收到受端电网交流故障切除信号后，投入直流线路保护装置，并将整流侧LCC的触发角指令值从
Figure PCTCN2022107777-appb-000002
减小到稳态值，使得系统能够平滑恢复至稳态。
进一步地，所述步骤(2)中当整流侧的直流电流大于1.1p.u.且整流侧的直流电压在0.5p.u.到0.9p.u.之间，则判定系统发生受端电网交流故障。
进一步地，所述触发角指令值
Figure PCTCN2022107777-appb-000003
的计算表达式如下：
Figure PCTCN2022107777-appb-000004
其中：U r为整流侧的交流电压幅值，X r为整流侧LCC的换相电抗，U' dcr为整流侧LCC直流电压的理论值，I * dc为整流侧LCC的直流电流指令值。
优选地，所述整流侧LCC直流电压的理论值U' dcr设定为0.5p.u.。
进一步地，所述外环有功功率指令修正值的计算表达式如下：
Figure PCTCN2022107777-appb-000005
其中：
Figure PCTCN2022107777-appb-000006
为外环有功功率指令修正值，U dci,MMC为MMC的直流电压，I dci为逆变侧的直流电流，P s,MMCi为逆变侧第i个MMC的瞬时输出有功功率，S N为MMC的额定容量，n为逆变侧MMC的数量。
进一步地，所述瞬时输出有功功率P s,MMCi的计算表达式如下：
Figure PCTCN2022107777-appb-000007
其中：U sm,MMCi为逆变侧第i个MMC的网侧相电压幅值，i vd,MMCi为逆变侧第i个MMC阀侧电流幅值的d轴分量。
本发明在送端LCC受端LCC-MMC的混合级联直流输电系统在受端发生严重交流故障导致逆变侧LCC换相失败时，整流侧LCC以其自身直流端口电气量的变化判断受端交流故障发生，通过增大触发角快速降低整流侧直流电压，从而达到快速抑制过电流的效果，同时整流侧的直流输送功率也相应降低了。此外，本发明使逆变侧采用定有功功率控制的MMC通过修正其外环有功功率指令值，尽可能地多输送有功功率，减少了受端系统的盈余功率，抑制了MMC子模块电容过电压，从而实现了LCC-MMC混合级联直流输电系统受端交流故障的穿越。
与现有技术相比，本发明具有以下有益技术效果：
1.采用本发明方法，在送端LCC受端LCC-MMC的混合级联直流输电系统受端交流故障发生后，送端无需依赖站间通信，响应速度更快。
2.本发明方法从控制层面提出了送端LCC受端LCC-MMC的混合级联直流输电系统受端电网故障下过电流和过电压的抑制措施，实现了故障穿越，降低了避雷器吸能，为直流系统安全稳定运行提供保障。
说明书附图
图1为混合级联直流输电系统的拓扑结构示意图。
图2为本发明系统整流侧LCC的改进控制原理示意图。
图3为本发明系统逆变侧MMC的外环有功功率修正控制原理示意图。
图4为实施例中MMC 1受端交流系统三相短路故障下各受端电网交流电压有效值的波形示意图。
图5为实施例中MMC 1受端交流系统三相短路故障下逆变侧LCC换流变压器阀侧电流的波形示意图。
图6为实施例中MMC 1受端交流系统三相短路故障下整流侧直流电流的波形示意图。
图7为实施例中MMC 1受端交流系统三相短路故障下整流侧直流电压的波形示意图。
图8为实施例中MMC 1受端交流系统三相短路故障下整流侧LCC触发角的波形示意图。
图9为实施例中MMC 1受端交流系统三相短路故障下MMC 3有功功率的波形示意图。
图10为实施例中MMC 1受端交流系统三相短路故障下MMC 1子模块电容电压的波形示意图。
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进行详细说明。
本发明混合级联直流输电系统的受端交流故障穿越控制方法，包括如下步骤：
(1)稳态运行时，整流侧LCC采用定直流电流控制，逆变侧LCC采用定直流电压控制，逆变侧n个MMC采用主从控制，即MMC 1采用定直流电压和定无功功率控制，其余的MMC x(x＝2,3,…,n)均采用定有功功率和定无功功率控制。
(2)当整流侧的直流电流大于1.1p.u.同时整流侧的直流电压在0.5p.u.到0.9p.u.之间时，判断为混合级联直流输电系统发生了受端电网交流故障。
当受端电网严重交流故障导致逆变侧LCC发生换相失败时，其直流侧相当于短路(即直流电压为零)，逆变侧直流电压迅速下降，整流侧和逆变侧之间的直流电压差导致了直流电流的升高。在整流侧故障检测模块中，整流侧的直流电流大于1.1p.u.同时整流侧的直流电压在0.5p.u.到0.9p.u.之间，用于判断系统发生了受端电网交流故障。此时，将直流线路保护退出，避免直流线路保护的误动作。
(3)整流侧LCC迅速增大触发角指令值至
Figure PCTCN2022107777-appb-000008
从而降低整流侧直流电压和直流输送功率。
整流侧LCC的直流电压U dcr和直流输送功率P dcr的表达式如下：
Figure PCTCN2022107777-appb-000009
P dcr＝U dcrI dc；
其中：U r为整流侧的交流电压幅值，α r为整流侧LCC的触发角，X r为整流侧LCC的换相电抗，I dc为整流侧LCC的直流电流。
由上式可知，U dcr的大小取决于α r和I dc；与调节直流电流指令值相比，调节触发角指令值的响应速度更快，并且过低的电流指令值会使故障期间功率损失增多并延长故障恢复时间。因此，将直流电流指令值设为定值，通过增大触发角指令值降低整流侧直流电压。需要注意的是，理论上此时逆变侧LCC的直流电压降为0，整个逆变侧的直流电压降到了0.5p.u.，但是受端交流电压跌落越严重，MMC的输出功率受阻，送受端之间不平衡的功率越大，MMC子模块电容的过电压越严重，导致逆变侧直流电压越高，实际上整个逆变侧的直流电压会高于0.5p.u.。因此，为了保证受端能够把吸收的直流功率全部送出去，从而抑制子模块过电压，假设U dcr是常数并设置为理论值0.5p.u.，在这样的假设下，可以计算出
Figure PCTCN2022107777-appb-000010
Figure PCTCN2022107777-appb-000011
其中：I * dc为整流侧LCC的直流电流指令值，U' dcr为整流侧LCC直流电压的理论值。
(4)计算出逆变侧MMC x(x＝2,3,…,n)的外环有功功率指令值修正值
Figure PCTCN2022107777-appb-000012
并将其附加到原有功功率指令值
Figure PCTCN2022107777-appb-000013
上，以确保逆变侧输出更多的有功功率。
在正常运行时，
Figure PCTCN2022107777-appb-000014
其可调节范围为
Figure PCTCN2022107777-appb-000015
S N为MMC的额定容量；当受端电网发生交流故障后，MMC i(i＝1,2,…,n)的瞬时输出有功功率P s,MMCi的表达式如下：
Figure PCTCN2022107777-appb-000016
其中：U sm,MMCi为MMC i的网侧相电压幅值，i vd,MMCi为MMC i阀侧电流幅值的d轴分量。
因此，可以计算得到：
Figure PCTCN2022107777-appb-000017
其中：U dci,MMC为MMC的直流电压，I dci为逆变侧的直流电流。
(5)当整流侧通过站间通信接收到受端电网交流故障切除信号后，投入直流线路保护，并将整流侧LCC触发角指令值从
Figure PCTCN2022107777-appb-000018
线性减小到其稳态值，减缓直流电压的恢复速度，使得混合级联直流输电系统能够平滑恢复至稳态。
当受端交流故障清除后，送端经过站间通信的延时后才能响应，虽然通信和减小触发角在一定程度上延长了恢复过程，但是提高了整个系统的故障恢复性能，使其能够更加平滑地过渡到稳态。
如图1所示，本实施方式以某送端LCC受端LCC-MMC的混合级联直流输电系统为例，即整流侧采用LCC，逆变侧高压阀组采用LCC，与之串联的低压阀组由三个半桥型MMC并联组成，逆变侧的LCC和三个MMC分别接入不同的受端交流系统，并且相互之间存在电气耦合。当受端发生严重交流故障导致逆变侧LCC换相失败时，整流侧LCC以其自身 直流端口电气量的变化判断受端交流故障发生，通过增大触发角快速减小整流侧直流电压和直流输送功率，同时逆变侧采用有功功率控制的MMC 2和MMC 3通过修正其外环有功功率指令值，尽可能地多输送有功功率，从而抑制过电流和过电压，实现受端交流故障穿越；具体控制过程如下：
(1)稳态运行时，整流侧LCC采用定直流电流控制，逆变侧LCC采用定直流电压控制，逆变侧三个MMC采用主从控制，即MMC 1采用定直流电压和定无功功率控制，MMC 2和MMC 3均采用定有功功率和定无功功率控制。
(2)当整流侧的直流电流大于1.1p.u.同时整流侧的直流电压在0.5p.u.到0.9p.u.之间时，如图2所示，故障检测模块判断为混合级联直流输电系统发生了受端电网交流故障，模式切换到1；此时，将直流线路保护退出，避免直流线路保护的误动作。
(3)整流侧LCC迅速增大触发角指令值至
Figure PCTCN2022107777-appb-000019
如图2所示，从而降低整流侧直流电压和直流输送功率。
(4)根据图3计算出逆变侧MMC 2和MMC 3外环有功功率指令值修正值
Figure PCTCN2022107777-appb-000020
后，将其附加到原有功功率指令值
Figure PCTCN2022107777-appb-000021
上。
(5)当整流侧通过站间通信接收到受端电网交流故障切除信号后，投入直流线路保护，并将LCC的触发角指令值从
Figure PCTCN2022107777-appb-000022
线性减小到其稳态值
Figure PCTCN2022107777-appb-000023
如图2所示，使得混合级联直流输电系统能够平滑恢复至稳态。
结合图1，采用本实施方式下的送端LCC受端LCC-MMC的混合级联直流输电系统参数如表1所示：
表1
Figure PCTCN2022107777-appb-000024
以下通过模拟受端MMC 1交流电网发生三相金属性短路故障来验证本发明控制策略的效果。
假定在t＝1s时，受端MMC 1交流电网发生三相金属性短路故障，由图4可以看出，受端LCC、MMC 1、MMC 2和MMC 3各自的交流母线电压有效值跌落程度不同，这是由于它们的交流系统存在不同程度的电耦合。由图5可见，逆变侧LCC发生了换相失败，它的直流电压降低到零，使得整流侧和逆变侧之间的直流电压差很大，混合级联直流输电系统会产生过电流。此外，LCC的输出有功功率为0，三个MMC的有功功率输出也因其不同程度的交流电压降而受到不同程度的阻塞；送受端之间的不平衡功率使得受端MMC承受很大的盈余功率，迫使子模块电容电压充电，甚至产生了过电压。
采用本实施方式后，整流侧的直流电流和直流电压的响应曲线分别由图6和图7所示，图8给出了整流侧LCC的触发角响应曲线。从上述图中可以看出，受端MMC 1交流电网发生三相金属性短路故障后，当故障检测模块检测到整流侧的直流电流大于1.1p.u.，同时整流侧的直流电压在0.5p.u.到0.9p.u.之间时(即t＝t FD)，整流侧LCC迅速增大触发角指令值至
Figure PCTCN2022107777-appb-000025
此时，整流侧直流电流的峰值只有1.13p.u.，远低于不含所提发明控制情况下的1.31p.u.，相比于不采用本实施方式控制情况下整流侧直流电流高于1.1p.u.的时间有22ms，采用本实施方式控制情况下整流侧直流电流高于1.1p.u.的时间只有8ms。
如图9所示，MMC 3通过外环有功功率指令值的修正使得它能够发出更多的有功功率，因此MMC上的盈余功率减少，由图10可见，子模块 电容电压的峰值降低到了1.25p.u.，远低于不含所提发明控制情况下的1.61p.u.，有效地抑制了MMC子模块电容过电压，实现了送端LCC受端LCC-MMC的混合级联直流输电系统受端交流故障的穿越。当整流侧LCC通过站间通信接收到受端电网交流故障切除信号后，其触发角指令值从
Figure PCTCN2022107777-appb-000026
线性减小到其稳态值
Figure PCTCN2022107777-appb-000027
如图8所示，使得混合级联直流输电系统能够平滑恢复至稳态。
上述对实施例的描述是为便于本技术领域的普通技术人员能理解和应用本发明。熟悉本领域技术的人员显然可以容易地对上述实施例做出各种修改，并把在此说明的一般原理应用到其他实施例中而不必经过创造性的劳动。因此，本发明不限于上述实施例，本领域技术人员根据本发明的揭示，对于本发明做出的改进和修改都应该在本发明的保护范围之内。

Claims (7)

  1. 一种混合级联直流输电系统的受端交流故障穿越控制方法，所述混合级联直流输电系统整流侧采用LCC，逆变侧采用LCC-MMC混合级联结构即多个并联的MMC与LCC串联组成；其特征在于：当受端发生严重交流故障导致逆变侧LCC换相失败时，整流侧LCC以其自身直流端口电气量的变化判断受端交流故障发生，然后通过增大触发角快速减小整流侧直流电压和直流输送功率；同时逆变侧采用定有功功率控制的MMC通过修正其外环有功功率指令值，尽可能地多输送有功功率，从而抑制过电流和过电压，实现受端交流故障穿越。
  2. 根据权利要求1所述的受端交流故障穿越控制方法，其特征在于：该控制方法的具体步骤如下：
    (1)系统稳态运行时，整流侧LCC采用定直流电流控制，逆变侧LCC采用定直流电压控制，逆变侧的多个MMC采用主从控制，即其中一个MMC采用定直流电压和定无功功率控制，其余MMC采用定有功功率和定无功功率控制；
    (2)在整流侧判断系统是否发生受端电网交流故障，若发生故障，则将系统的直流线路保护装置退出，避免该装置误动作；
    (3)将整流侧LCC的触发角指令值迅速增大至
    Figure PCTCN2022107777-appb-100001
    以降低整流侧直流电压和直流输送功率；
    (4)对于采用定有功功率和定无功功率控制的MMC，通过计算外环有功功率指令修正值，将该修正值附加到这些MMC原有功功率指令值上加以控制；
    (5)当整流侧通过站间通信接收到受端电网交流故障切除信号后， 投入直流线路保护装置，并将整流侧LCC的触发角指令值从
    Figure PCTCN2022107777-appb-100002
    减小到稳态值，使得系统能够平滑恢复至稳态。
  3. 根据权利要求2所述的受端交流故障穿越控制方法，其特征在于：所述步骤(2)中当整流侧的直流电流大于1.1p.u.且整流侧的直流电压在0.5p.u.到0.9p.u.之间，则判定系统发生受端电网交流故障。
  4. 根据权利要求2所述的受端交流故障穿越控制方法，其特征在于：所述触发角指令值
    Figure PCTCN2022107777-appb-100003
    的计算表达式如下：
    Figure PCTCN2022107777-appb-100004
    其中：U r为整流侧的交流电压幅值，X r为整流侧LCC的换相电抗，U' dcr为整流侧LCC直流电压的理论值，I * dc为整流侧LCC的直流电流指令值。
  5. 根据权利要求4所述的受端交流故障穿越控制方法，其特征在于：所述整流侧LCC直流电压的理论值U' dcr设定为0.5p.u.。
  6. 根据权利要求2所述的受端交流故障穿越控制方法，其特征在于：所述外环有功功率指令修正值的计算表达式如下：
    Figure PCTCN2022107777-appb-100005
    其中：ΔP s *为外环有功功率指令修正值，U dci,MMC为MMC的直流电压，I dci为逆变侧的直流电流，P s,MMCi为逆变侧第i个MMC的瞬时输出有功功率，S N为MMC的额定容量，n为逆变侧MMC的数量。
  7. 根据权利要求6所述的受端交流故障穿越控制方法，其特征在于： 所述瞬时输出有功功率P s,MMCi的计算表达式如下：
    Figure PCTCN2022107777-appb-100006
    其中：U sm,MMCi为逆变侧第i个MMC的网侧相电压幅值，i vd,MMCi为逆变侧第i个MMC阀侧电流幅值的d轴分量。
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