逆变侧换流变压器网侧接地故障对差动保护影响分析

特高压直流输电系统逆变侧换流变网侧单相接地故障后,网侧电压跌落可能会引发直流系统换相失败,有直流分量流入换流变压器绕组,使变压器铁芯磁通发生偏移,引起换流变饱和,产生故障性涌流,进而影响其安全稳定运行。针对逆变侧换流变网侧单相接地故障产生的故障性涌流问题,通过开关函数理论讨论了换流阀正常换相以及换相失败两种情况下网侧故障电流特征,分析了逆变侧网侧单相接地故障后换流变磁通的变化情况。在此基础上,研究了电流互感器饱和对换流变差动保护的影响。结果表明,换流变网侧区内故障情况下,网侧电流互感器饱和,使得换流变差动电流二次谐波含量超过门槛值,导致差动保护误闭锁。最后通过PSCAD/EMTDC仿真验证了分析的正确性。

Analysis of influence of grounding fault of the network side of an inverter-side converter transformer on differential protection

After a single-phase ground fault of the inverter side converter side of a UHV DC transmission system, the voltage drop on the grid side may cause commutation failure of the DC system, such that the DC component flows into the converter transformer winding, and causes the transformer core flux to shift, causing the converter transformer to become saturated, causing a faulty inrush current, which in turn affects its safe and stable operation. In this paper, the problem of fault inrush current is generated for the single-phase ground fault of the inverter side commutation network. The characteristics of the grid side fault current under the two conditions of normal commutation and commutation failure of the converter valve are discussed using the theory of switching function. The variation of commutation flux after a single-phase ground fault on the side of the side network is studied. On this basis, the influence of current transformer saturation on commutation differential protection is studied. The results show that the grid-side current transformer is saturated under the fault condition in the side of the converter network, so that the second harmonic content of the commutating differential current exceeds the threshold value, resulting in differential protection mis-locking. Finally, the correctness of the analysis is verified by PSCAD/EMTDC simulation. This work is supported by Key Project of Smart Grid Technologies and Equipment of National Key Research and Development Program of China (No. 2016YFB0900600) and Science and Technology Project of State Grid Corporation of China (No. SGNX0000DKJS1900460).