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在交流系统故障或者甩负荷等暂态过程中,由于有功功率不平衡,电压源换流器型高压直流(VSC-HVDC)输电系统直流侧会出现过电压或者欠电压现象,危及换流阀、直流电容器和直流电缆等设备的安全。为了限制暂态过程中直流电压波动幅度,提出了一种改进的控制策略,即在外环有功功率控制器中增加了一个依赖于直流电压的电流指令限制单元(VDCCOL),它通过内环电流控制器有功电流指令的变化来反映直流电压的变化。当直流电压异常时,VDCCOL根据预设的特性迅速修正外环功率控制器输出的有功电流指令值,减缓有功功率的不平衡,从而达到限制直流过电压/欠电压的发展、保护设备安全运行和避免频繁切机等目的。仿真研究表明,文中提出的控制策略能有效地抑制故障条件下直流电压波动幅度,提高系统的运行特性。
During the transient process such as AC system failure or load shedding, due to unbalanced active power, over-voltage or under-voltage may occur on the DC side of the VSC-HVDC transmission system, which may endanger the converter valve, DC capacitors and DC cables and other equipment safety. In order to limit the fluctuation of DC voltage during the transient process, an improved control strategy is proposed, in which a DCCC-based current command limiting unit (VDCCOL) is added to the active power controller in the outer loop, Controller active current command changes to reflect the DC voltage changes. When the DC voltage is abnormal, VDCCOL rapidly correct the active current command value output by the outer loop power controller according to the preset characteristic, slow down the imbalance of active power, so as to limit the development of DC overvoltage / undervoltage and protect the safe operation of the equipment. Avoid frequent cutting and other purposes. The simulation results show that the control strategy proposed in this paper can effectively restrain the fluctuation of DC voltage under fault conditions and improve the operating characteristics of the system.