MMC-HVDC系统阀区单相接地故障定位研究

模块化多电平的直流输电系统(MMC-HVDC)换流变压器阀侧中性点采用大电阻接地。任何一换流站阀侧发生单相接地故障后,导致两个换流站的中性点电流幅值和相位均相等。两个换流站都触发中性点过流保护元件动作,无法准确定位故障位置,对后续的柔直检修工作带来极大困扰。针对此问题,深入分析了阀侧单相故障产生的故障电流通道机理,指出一条放电回路为非全相放电通道,结合柔直的平衡性控制特点,该通道即不可控制通道;另一条零序放电回路为故障放电通道。提出了在一侧换流站增加零序补偿策略,该策略在原调制电压上叠加零序电压,导致两换流站中性点电流出现差值而实现故障准确定位。新策略不增加任何测量装置,简单可靠,易于实现。最后通过RTDS试验平台验证所提策略的正确性。

Research on location of single-phase grounding fault in valve side of MMC-HVDC system

High resistance grounding is used in the neutral point of valve-side winding of the converter transformer in a modular multi-level HVDC system.Both the amplitude and phase of neutral point current at the two converter stations are equal when the single-phase grounding fault occurs at the valve-side winding of any converter station.The neutral point over-current protection elements of both converter stations will act.It is then not possible to locate the fault position accurately and brings difficulties for the subsequent maintenance of HVDC.To solve these problems,this paper analyzes the mechanism of the fault current channel caused by a single-phase fault on the valve-side winding,and points out that one discharge circuit is a non-full-phase discharge channel.Combining with the characteristics of balancing control on HVDC,one channel is uncontrolled,and the other through a zero-sequence circuit is a fault discharge channel.A control strategy is proposed to add zero-sequence compensation voltage to the one-side converter station.This strategy superimposes a zero-sequence voltage on the original modulation voltage,resulting in the difference of neutral point current between the two converter stations and thus enabling accurate location of the fault.The new strategy is simple,reliable and easy to implement without adding any measuring devices.Finally,the correctness of the proposed strategy is verified by an RTDS test platform.