基于N-1规则的多端柔性直流输电系统联合控制策略

在多端柔性直流(voltage source converter based multi-terminal DC,VSC-MTDC)输电系统安全运行时,该系统必须满足N-1法则,即当该系统任何一个换流站由于故障或者检修退出运行时,剩余系统具备功率调节能力,能够恢复功率平衡,保持系统稳定运行,且暂态过电压不会超过设备绝缘裕度。为了维持VSC-MTDC直流电压尽可能地稳定在原有水平,提出了一种考虑到VSC-MTDC中任一换流站退出运行时的联合控制策略。该策略结合了VSC-MTDC系统主从控制与下垂控制的优点,令VSC-MTDC系统中容量最大的换流站为定直流电压控制,其余换流站为直流电压-有功功率下垂控制,并设置定直流电压控制换流站参与功率调节的优先级高于其余换流站,仅当定直流电压控制换流站传输功率达到上限时其余换流站才参与功率的调节。最后,在PSCAD/EMTDC中搭建了VSC-MTDC的仿真模型,对所提出的联合控制策略在N-1故障条件下进行仿真验证。仿真结果表明:所提联合控制策略在换流站退出运行时有效保证了直流电压的稳定以及系统功率的紧急输送,提高了VSC-MTDC的运行稳定性。

Coordinated Control Strategy for VSC-MTDC Systems Based on N-1 Criterion

The operation of voltage sourced converter based on the multi-terminal high voltage direct current ( VSC-MTDC) transmission system is required to meet the minimum N-1 criterion. Specifically, when one converter is out of operation due to fault or maintenance, the surviving converters have the ability to regulate power, so as to restore the balance of power and maintain the transient overvoltage without exceeding the equipment insulation margin. In order to keep the DC voltage stability of VSC-MTDC as much as possible, this paper proposes a coordinated control strategy when any converter station in VSC-MTDC is out of operation. This strategy combines the advantages of master-slave control and droop control of VSC-MTDC system. The converter owing the largest capacity is controlled with a constant DC voltage while other converters are controlled by a DC voltage-active power droop controller. Besides, the converter with constant DC voltage has a higher priority to regulate active power than other converters, and other converters start to regulate active power only when the power of the converter with constant DC voltage reaches the upper limit. Finally, the VSC-MTDC model is developed in PSCAD/EMTDC and the proposed control strategy is verified. The results show that the proposed control strategy can effectively guarantee the stability of DC voltage and the emergency transmission of power when converter station is out of operation, which improves the operation stability of VSC-MTDC.