基于暂态功率方向的柔性直流配电网线路单极接地保护方法

单极接地故障是柔性直流配电网最为常见的故障。采用小电流接地方式的MMC型柔性直流配电网发生单极接地故障时，短路电流小(主要是分布电容电流)，故障识别困难，柔性直流配电网接地故障检测亟待解决。利用相模变换建立了基于MMC的多端环状柔性直流配电网单极接地故障的零模网络电路，分析了零模网络换流器和直流线路的阻抗特性。在所关注的特征频段内将零模网络简化等效，分析得到故障线路两端的零模电压导数与零模电流的极性均相反，非故障线路两端至少有一端的零模电压导数与零模电流极性相同。利用该故障线路与非故障线路差异特征，设计了一种利用暂态功率的方向纵联保护方法。其中故障线路两端暂态功率方向均为负，非故障线路两端功率方向至少有一端为正。最后在PSCAD/EMTDC平台搭建仿真验证。结果表明，所提保护方法能可靠识别区内区外故障，不依赖线路边界元件，无需数据同步，具有较好的耐受过渡电阻能力。

A flexible DC distribution network line unipolar grounding protection method based on transient power direction

Unipolar grounding faults are the most common faults in flexible DC distribution networks. When a single-pole grounding fault occurs in MMC-type flexible DC distribution networks using small current grounding methods, the short-circuit current is small (mainly distributed capacitance current) and fault identification is difficult. The ground fault detection of flexible DC distribution network is an urgent matter. Phase-to-mode conversion is used to establish a multi-terminal ring-shaped flexible DC distribution network with a single-pole ground fault zero-mode network circuit based on MMC, and the impedance characteristics of the zero-mode network converter and the DC line are analyzed. In the characteristic frequency band of interest, the zero-mode network is simplified and made equivalent, and the relationship between the transient zero-mode current and the zero-mode voltage of each line is obtained. Based on the distribution characteristics of the transient zero-mode current and zero-mode voltage of each line, a directional longitudinal protection method using transient power is designed. The transient power directions at both ends of the fault line are all negative. At least one end of the power direction at both ends of the non-faulty line is positive. Finally, a simulation verification is built on the PSCAD/EMTDC platform. The results show that the protection method is simple and can be implemented reliably. It also has a good ability to withstand transition resistance.This work is supported by the National Natural Science Foundation of China (No. 52077221).