高压直流控制器交互影响可能导致后续换相失败恶化，为此，分析了故障后各时段换流器控制交互影响机理，设计了降低后续换相失败风险的策略。依据逆变侧控制器动作逻辑，将系统故障发生至恢复稳态划分为4个阶段，基于各阶段的动态轨迹分析，得到逆变侧控制器对换相失败恢复过程的影响规律，结果表明逆变侧定关断角控制与定电流控制的二次交互过程，即电流偏差控制阶段极易引发后续换相失败。提出了一种改进型电流偏差控制策略，通过补偿控制器二次交互期间电流偏差，提升关断角状态识别的准确度，从而抑制因控制器交互不当引发的后续换相失败。最后，在PSCAD/EMTDC仿真平台中以CIGRE标准算例，对不同工况下所提改进控制策略进行测试，结果表明所分析故障后动态过程准确，所提出的后续换相失败抑制策略效果显著。

 The interaction of HVDC controllers may lead to the deterioration of subsequent commutation failure. Therefore, this study analyzes the mechanism of converter control interaction at each stage after a fault and designs a strategy to reduce the risk of subsequent commutation failure. According to the action logic of the inverter-side controller, the process of system recovery from fault occurrence to a steady state is divided into four stages. Based on the dynamic trajectory analysis of each stage, the influence of the inverter-side controller on the commutation failure recovery process is determined. The results showed that the secondary interaction process between the constant extinction angle control and constant DC current control on the inverter side, namely, the current deviation control stage, could easily cause subsequent commutation failure. An improved current deviation control strategy is proposed to improve the accuracy of the extinction angle state recognition by compensating for the current deviation during the secondary interaction of the controllers to suppress the subsequent commutation failure caused by improper controller interaction. Finally, based on the CIGRE HVDC benchmark model, an improved control strategy is tested under different operating conditions using the PSCAD/EMTDC simulation platform. The results showed that the analysis of the fault recovery process was accurate, and that the proposed subsequent commutation failure suppression strategy was effective.

多馈入直流输电系统受端换流站电气距离近,直流系统与交流系统之间、直流系统与直流系统之间相互作用也更为复杂。当交流系统较弱时,故障恢复期间提供的无功支撑不足,容易引起多回直流系统发生后续换相失败。配置无功补偿装置,优化控制环节参数等措施均有利于改善直流系统的故障恢复特性。基于此,提出一种抑制多馈入直流系统后续换相失败的低压限流单元参数优化策略,利用静止同步补偿器(static synchronous compensator,STATCOM)输出的无功大小来衡量直流系统恢复程度,并将STATCOM输出的无功功率和直流电压相结合作为输入信号,随故障发展过程自适应调整低压限流单元输入输出特性,降低系统发生后续换相失败的概率。在PSCAD/EMTDC中搭建含STATCOM的双馈入直流输电系统模型,仿真结果验证了所提策略的有效性。

Due to the short electrical distance of the inverter stations in multi-infeed HVDC system, the interaction between DC and AC and between DC and DC are more complex. When the AC system is weak, insufficient reactive power support is provided during the fault recovery period, which is easy to cause subsequent commutation failure of multi-infeed HVDC system. Configuration of reactive power compensation devices, optimization of control parameters and other measures are conducive to improve the fault recovery characteristics of HVDC system. This paper proposes a parameter optimization strategy of VDCOL to suppress subsequent commutation failure of multi-infeed HVDC system. The reactive power output of STATCOM is used to measure the recovery degree of HVDC system. Combined with DC voltage as input signal, the input and output characteristics of VDCOL can be adaptively adjusted according to the fault development process, so as to reduce the probability of subsequent commutation failure. A dual-infeed HVDC system model with STATCOM is built in PSCAD/EMTDC, the simulation results verify the effectiveness of the proposed strategy.

我国华东、华南等电网已形成多直流馈入受端电网格局，由于直流间强关联耦合作用，交流扰动将引发多直流同时换相失败、电压失稳等问题。该文首先分析了双馈入直流系统逆变站动态无功变化轨迹，揭示了通过优化低压限流环节（voltage dependent current order limiter，VDCOL）参数，实现降低总体无功功率需求的机理。根据换流母线抗扰动能力强弱对直流恢复特性的影响，提出了基于多馈入短路比（multi-infeed short circuit ratio，MSCR）的VDCOL参数优化方案，华东多直流馈入系统仿真结果表明，所提出的VDCOL参数优化方案降低了逆变站总体无功功率需求，提高了受端电网电压稳定性。

In East China and South China power grid, it has formed the pattern that many HVDC transmission systems feed into the receiving-end grid. Because of the strong coupling of multi-infeed HVDC systems, AC voltage disturbance will lead to the commutation failure of multiple HVDC at the same time, even voltage instability. At first, this paper analyzes the inverter station dynamic reactive power change track of dual-infeed HVDC test systems, and reveals the mechanism that the optimization of voltage dependent current order limiter （VDCOL） parameters can reduce the total reactive power demand. According to the influence of resisting disturbance capacity of converter bus on DC power recovery characteristics, this paper proposes the optimization scheme of VDCOL parameter based on multi-infeed short circuit ratio （MSCR）. The simulation results of East China multi-infeed HVDC systems show that proposed optimization scheme of VDCOL parameter can reduce the reactive power demand of inverter station and improve the voltage stability of receiving end of power grid.

随着直流输送容量不断增加，直流落点越来越密集，多馈入直流（MIDC）输电系统的换相失败问题日益凸显。为降低多馈入直流输电系统换相失败的概率，首先分析了多回直流同时或相继换相失败的根本原因，然后结合常规低压限流（VDCOL）控制，提出了电压补偿式变斜率VDCOL协调控制策略。该控制策略充分利用直流故障电流的变化特点，综合考虑了MIDC输电系统中各回直流自身特性影响因素，选取合理的补偿系数K，使各回直流能在故障后有序且稳定地恢复，从而大大减少后续换相失败的发生。同时根据故障的严重程度选择性地切入K值以达到提高直流系统恢复性能的目的。最后在PSCAD/EMTDC中搭建算例系统及实际系统模型进行仿真验证，结果表明该控制策略能有效降低多馈入直流输电系统的后续换相失败的概率，使各回直流在故障后能够有序地恢复，同时在保证良好的恢复性能的基础上大大提高了多馈入直流输电系统的可靠性。

With the increasing of DC transmission capacity, HVDC connecting points are becoming more and more crowded, and commutation failure becomes increasingly prominent in MIDC transmission systems. To reduce the probability of commutation failure in MIDC systems, this paper first analyzes the fundamental causes of simultaneous or successive commutation failure, and then combing with the tradition VDCOL control puts forward the coordinated control strategy of variable slope VDCOL with voltage compensation. The control strategy takes advantage of the DC current characteristics while fault occurs, considers the influence factors of the DC systems characteristics in MIDC transmission system, and selects the reasonable compensation coefficient K, so that the MIDC systems can be recovered stably after faults, and the occurrence of subsequent commutation failure will be reduced. At the same time, according to the severity of the fault, the K value is selectively introduced in order to improve the recovery performance of the DC systems. Finally the example system and the actual system are built in PSCAD/EMTDC. Simulation results show that the control strategy can effectively reduce the probability of following commutation failure in MIDC transmission systems, and makes all DC system recovery orderly after fault, meanwhile ensures the good recovery performance which can greatly improve the safety and reliability in MIDC transmission systems.

在以新能源和直流馈入为主的新型受端电网中,常规直流换相失败问题常威胁着系统的安全运行。为此,归纳和提出了三项用于评估直流换相失败问题严重程度的指标,并以江苏电网为例分析了直流换相失败严重程度。然后,针对严重换相失败问题,提出了一种用于加强直流系统恢复能力的动态无功协调控制方法。该方法先根据潮流控制目标确定系统运行边界和柔性直流换流站等设备出力,再基于电压交互因子筛选出与目标直流系统强相关的动态无功源,进而通过动态无功协调控制策略加强目标直流系统换相失败后的恢复能力。最后,在江苏电网实际算例中进行了测试,验证了所提动态无功协调控制方法的有效性。

The safe operation of new receiving-end power grids that rely heavily on renewable energy and DC systems is often threatened by commutation failures within the DC systems. Therefore, three indicators used to evaluate the severity of these commutation failures are summarized and proposed in this study. For illustrative purposes, the severity of the DC commutation failures in the Jiangsu Power Grid are analyzed based on these parameters. Subsequently, a reactive power coordination control method is proposed to enhance the DC recovery ability in response to serious commutation failures. The following is an outline of the phases involved in our study. First, the operating state of the system and the output of some devices, such as MMC-DC converters, were obtained by considering the power flow control objectives. Second, dynamic reactive power sources strongly related to the concerned DC systems were identified based on voltage interaction factors. Third, the dynamic reactive power coordination control method was employed to enhance the recovery ability of the DC systems. Finally, the effectiveness of the proposed coordination control method was verified in a case study of the Jiangsu Power Grid.