基于半桥MMC特征信号注入的柔性直流线路频变参数辨识

柔性直流电网故障电流上升速度快与电力电子器件过流能力弱形成突出矛盾，线路保护需要在数毫秒级完成故障判别，输电线路精确参数的获取对于提升继电保护的性能至关重要。然而直流系统中缺乏稳定基频，导致输电线路相关参数难以获取、保护实现较为困难。针对柔性直流线路频变参数难以获取的问题，提出基于半桥模块化多电平换流器(half bridge modular multilevel converter, HB-MMC)特征信号注入的柔性直流线路频变参数辨识方法。首先通过换流器控制在线路中注入特定频率信号，然后利用快速傅里叶分解提取不同频率的信号并计算指定频率下的线路参数，最后依据不同线路参数的频变特性拟合出对应的幅频特性曲线。仿真表明，所提参数辨识方法可以准确拟合保护所需直流线路频变参数，参数辨识频段内相对误差小于1.5%。

Flexible DC line frequency-dependent parameter identification based on characteristic signal injection with HB-MMC

The rapid rise of fault current in a flexible DC grid has a prominent contradiction with the weak overcurrent capability of power electronics elements. The DC line protection needs to complete the fault identification in a few milliseconds. Accurate parameter identification of transmission lines is very important in improving the performance of relay protection. However, the lack of a stable fundamental frequency in the DC system makes it difficult to obtain the relevant parameters of the transmission line and to realize the protection principle. There is difficulty in obtaining the frequency-dependent parameters of flexible DC lines. Thus a flexible DC line parameter identification method based on characteristic signal injection with a half-bridge modular multilevel converter (HB-MMC) is presented. By changing the control strategy of the converter, a characteristic signal is injected into the transmission line. Then the signal of different frequencies is extracted by fast Fourier transform and the line parameters at the specified frequencies are calculated. Finally, the corresponding amplitude-frequency characteristic curves are fitted according to the frequency-dependent characteristics of different line parameters. The simulation results show that the proposed parameter identification method can accurately fit the DC transmission line frequency-dependent parameters, and the relative error of the whole parameter identification is less than 1.5%.