为降低高压直流输电系统发生后续换相失败的风险,分析了换相失败机理,并结合对称与不对称故障检测器提出了一种可根据故障严重程度调整电流偏差控制(current error control,CEC)的改进方案。分析了电流偏差控制的斜率大小对换相失败的抑制效果,发现电流偏差控制中斜坡函数的斜率越大则对输入电流偏差越敏感且换相裕度增量越大,越有利于抑制后续换相失败。最后,在PSCAD/EMTDC 仿真软件中基于 CIGRE HVDC标准测试系统实现了所提控制方案。仿真结果表明,所提的电流偏差控制方法能有效降低高压直流输电系统发生后续换相失败的风险,提高直流输电系统的运行特性。

<p id="p00010">In order to reduce the risk of subsequent commutation failure in HVDC transmission system, the commutation failure mechanism is analyzed, and combined with symmetrical and asymmetric fault detectors, an improved scheme of current deviation control which can be adjusted according to the severity of fault is proposed. The suppression effect of the slope of current deviation control on commutation failure is analyzed. It is found that the larger the slope of slope function in current deviation control is, the more sensitive it is to input current deviation, and the larger the commutation margin increment is, the more conducive it is to suppress subsequent commutation failure. Finally, the proposed control scheme is realized on the basis of CIGRE HVDC Standard Test System in PSCAD / EMTDC simulation software. The simulation results show that the current deviation control method proposed in this paper can effectively reduce the risk of subsequent commutation failure of HVDC system and improve the operation characteristics of HVDC system.</p> <p id="p00015">This work is supported by State Grid Corporation of China Research Program (No.52130420001S).</p>

换相失败是特高压直流输电系统的常见故障之一,常在送、受端交流电网引起剧烈的无功波动。投入旁通对是对直流系统进行保护的重要控制措施之一。当直流输电系统发生故障时,逆变侧保护装置动作后投入旁通对有助于达到快速停运直流输电系统,隔离故障的目的。而现在有关旁通对的研究多集中于其在直流故障中的应用,关于其在换相失败问题中的应用研究较少。通过分析旁通对控制对特高压直流输电系统送、受端电压特性的影响,论证了旁通对控制策略对换相失败后整流侧过电压、逆变侧低电压的改善作用。进而提出了一种换相失败后投旁通对的控制方法,以逆变阀组换相失败及交流电压跌落程度为旁通对控制的启动判据,根据直流运行状态对直流电流进行动态调节,然后根据受端交流系统恢复程度退出旁通对。PSCAD/EMTDC仿真表明,所提旁通对控制器在交流故障导致特高压直流换相失败后,能够起到快速隔离交直流系统、减轻无功电压波动的控制效果。

As an inherent drawback of LCC-UHVDC, commutation failure often causes dramatic reactive power fluctuation in the AC power grid both on the rectifier side and the inverter side. The application of bypass pair is one of the important control strategies of DC system protection. When a fault occurs in the HVDC system, the switch of bypass pair after the protection action of the inverter side will help to achieve the purpose of stopping the HVDC system quickly and isolating the fault. At present, the research of bypass pair is mainly focused on the application under DC fault cases, whereas the application of that in commutation failure is lacking. According to the analysis of the impact of bypass control on the voltage characteristics on the rectifier side and the inverter side, this paper demonstrates the contribution of reasonable bypass control on improving the voltage characteristics of UHVDC transmission line after commutation failure. And a bypass pair control scheme is proposed, which takes the commutation failure in inverter valves and the reduction of AC bus voltage as the initialization criterion to dynamically adjust DC current. Then, the bypass pair will quit as the system recovering. The effectiveness of the proposed bypass pair control scheme on isolating AC and DC systems and reliving the reactive power and the voltage fluctuation during the transient process of AC system fault is verified in PSCAD/EMTDC.

为了在实际工程中减小换相失败的概率,在PSCAD 中建立了晶闸管最小关断角动态模型,通过确定动态最小关断角得到换相失败的实际判据。然后,将晶闸管阀关断角的变化量引入控制系统中,提出了一种动态关断角控制方法。在PSCAD/EMTDC上进行仿真验证,仿真结果表明,换相失败实际判据能有效提高换相失败识别的精准性,关断角动态控制方法能较好地抑制单相及三相短路故障导致的换相失败,有利于直流输电控制保护系统调整关断角控制裕度,具有工程实际意义。

To reduce the probability of commutation failure in practical engineering, the dynamic turn-off angle model of thyristor is established in PSCAD, and the actual criterion of commutation failure is obtained by determining the dynamic minimum turn-off angle. Then, the variation of turn-off angle of the thyristor valve is introduced into the control system, and a dynamic control method for turn-off angle is proposed. The proposed control method was improved on the CIGRE standard test model and realized by PSCAD/EMTDC simulation. The simulation results show that the actual criterion of commutation failure can effectively improve the accuracy of commutation failure identification. The dynamic control method for turn-off angle can mitigate the commutation failure caused by single-phase or three-phase short-circuit faults better. It can be beneficial for the control and protection system of DC transmission to adjust the control margin of turn-off angle of converters, which has practical engineering significance.