柔直电网阀侧单相接地故障过电压产生及影响因素研究

在真双极柔性直流输电系统中,虽然换流变压阀侧交流单相接地故障出现的概率较低,但是一旦发生,直流侧会产生严重的过电压。文中研究了柔性直流输电系统模块化多电平换流器(MMC)阀侧发生单相接地故障时的过电压特性及产生机理。首先,分析闭锁前后MMC桥臂子模块电容的充放电回路,对桥臂过电压和健全极线过电压的产生机理进行研究。随后,基于厦门柔性直流输电系统,对阀侧单相接地故障特性分析的准确性进行验证,并仿真分析闭锁延时对过电压的影响。结果表明,桥臂子模块电压的升高是由于直流线路分布电容的放电作用,健全极线过电压是由于闭锁前子模块电容的放电以及闭锁后交流侧电压的充电作用,闭锁时间越短,健全极线和故障相子模块电容过电压幅值越低。

Analysis of single-phase-to-ground fault overvoltage at the valve-side of HB-MMCs in bipolar HVDC systems

In the true bipolar modular multilevel converter based high voltage direct current (MMC-HVDC) systems, the probability of occurrence of the single-phase-to-ground faults at the valve-side is arguably low, but it may cause serious overvoltage on the direct current (DC) side. The overvoltage characteristics and generation mechanism is studied when a single-phase-to-ground fault occurs on the valve-side of the MMC in HVDC systems. Firstly, the charging and discharging circuits of the sub-module capacitors for each phase are given before and after blocking. The generation mechanism of the overvoltage of the arm and healthy pole line is investigated then. Finally, simulations are performed based on Xiamen two-terminal HVDC grid to verify the accuracy of the mechanism, and the effect of the blocking delay on the overvoltage is simulated and analyzed. The results show that the increase in the voltage of the arm, the healthy pole line and the metallic return line are caused by the charging and discharging of the distributed capacitances in DC lines. Besides, the shorter the blocking time, the lower the overvoltage amplitude.