基于电流相关性的直流线路单极故障保护方案

目前，柔性直流配电网得到快速发展，但其直流线路保护方案还不成熟，特别是基于模块化多电平换流器（modular multi-level converter，MMC）的直流配电系统，单极接地故障时故障电流不明显，给故障识别增加了难度。针对这一问题，考虑直流线路的分布电容效应，提出一种采用Pearson相关系数计算正负极电流相关性的单极接地故障识别方法。区内故障时，线路每端的正极电流与负极电流均呈强正相关特性，而区外故障时，靠近故障端的线路正负极电流正线性相关性较差。以直流电压变化率作为故障识别启动判据，给出具体的单极接地故障保护方案。在PSCAD/EMTDC平台中搭建了光伏直流接入系统仿真模型并进行了验证，结果表明：该方案可快速、可靠地识别区内外故障，并具有一定耐受过渡电阻的能力。

Protection Scheme Based on Current Correlation for DC Lines Against Single-pole Ground Faults

At present, flexible DC distribution technology is developing rapidly, while the DC line protection scheme still needs further investigation. Especially, in the MMC-based DC distribution system, the fault current is not obvious when a single-pole-to-ground fault occurs, which makes it difficult to identify the faulty line. Taking the effect of distributed capacitance into consideration, a method for identifying pole-to-ground fault is proposed, which uses the Pearson correlation coefficient to calculate the correlation between the currents at positive and negative poles. When the internal fault occurs, the currents at positive and negative poles show a strong positive correlation, whereas the positive correlation coefficient of the currents at positive and negative poles near the fault point is much lower under the external fault. And the specific pole-to-ground fault protection scheme is proposed, which uses the change rate of the DC voltage as the starting criterion for fault identification. The simulation model of a PV integration system was established in PSCAD /EMTDC. Simulation results show that the proposed protection scheme can identify the internal and external faults quickly and reliably, and has some ability to tolerate transition resistance.