无联接变柔直配电网的零序电流抑制控制策略

A Zero-sequence Current Suppression Control Strategy for Transformer-less VSC-MVDC

在基于模块化多电平换流器(MMC)的电压源换流器-柔性直流配电网(VSC-MVDC)换流站中,联接变压器是占地面积和造价仅次于换流阀的一次设备。取消VSC-MVDC换流站的联接变压器,可有效降低换流站占地面积,减小项目投资,还可降低系统损耗,减少维护和服务,提高系统经济性和实用性。但针对消弧线圈接地和电阻接地系统,无联接VSC-MVDC在发生接地故障时,会产生零序电流,并且可穿过直流侧流入对侧交流系统。零序电流若不进行抑制,会导致一次设备的电流应力增加,严重时可影响系统稳定性。此处针对该工况,提出了一种附加的零序电流抑制控制器,可有效抑制故障时零序电流,增强无联接变压器的MMC换流站的故障穿越能力。通过RTDS仿真验证了所述零序电流抑制控制器的有效性。

In a voltage source converter-medium voltage direct current(VSC-MVDC)station based on modular multilevel converter(MMC),the coupling transformer is the second largest and costliest equipment.Elimination of transformer from the VSC-MVDC station may give significant attractive advantages like cost and loss reduction and reduce project investment.At the same time,it can reduce maintenance and service,reduce the space of the converter stationa and improve system economy and practicability.However,for the arc-suppression coil grounded and resistance grounded system,the transformer-less VSC-MVDC transmission system will generate zero-sequence current when a ground fault occurs and can flow into the opposite-side AC system through the DC side.If the zero-sequence current is not suppressed,the current stress of the primary device will increase which may affect the stability of the system in severe cases.An additional zero-sequence current suppression controller is proposed to effectively suppress the zero-sequence current in the fault and enhance the fault ride-through capability of the transformer-less MMC station.The effectiveness of the zero sequence current suppression controller is verified by RTDS simulation.