基于换流器分段线性化模型的交直流碰线保护适用性分析

Adaptability Analysis of AC/DC Line-touching Protection Based on Piecewised and Linearized Model of Converter

交直流混联电网的迅速发展使得交流线路与直流线路发生碰线故障的几率增大。针对交直流碰线故障后直流线路主保护执行移相动作情况下,不同交直流碰线保护判据的适应性进行评估。详细分析了交直流碰线故障下换流器的导通特性,建立了基于换流器导通状态的交直流网络等值模型。由于换流器基频等值阻抗的非线性使得直接计算基频电气量困难,基于换流器基频等值阻抗的分段线性化,推导了工频周期中每个短时间窗内保护安装处的基频电气量,进而利用傅里叶级数算法对工频周期内的等值基频电气量进行求解。最后,通过对比分析整流器移相前后保护安装处的基频电气量,研究了移相后现有的不同交直流碰线保护判据的适应性,分析表明基于基频电流量的交直流碰线保护判据在整流侧存在适应性降低的问题。论文搭建了PSCAD/EMTDC交直流碰线故障仿真模型并进行了大量仿真实验,仿真结果验证了理论分析的准确性。

The rapid development of AC/DC hybrid power grids has increased the probability of the touching faults between AC and DC transmission lines. The adaptability of different AC/DC line-touching protection criteria are evaluated when the main protection of the DC line performs phase-shifting action after the AC/DC line-touching fault. By analyzing the conduction characteristics of the converter under the AC/DC line-touching fault in detail, an equivalent model of AC/DC network of fundamental frequency is established in the conduction state of the converter. Due to the nonlinearity of the converter’s equivalent impedance of fundamental frequency, it is difficult to calculate the fundamental frequency electrical quantity directly. By piecewising and linearizing the converter’s equivalent impedance of fundamental frequency, the electrical quantity of fundamental frequency in the each short time window of the fundamental frequency periods is derived. Then the Fourier Series method is used to calculate the equivalent fundamental frequency electrical quantity in the fundamental frequency period. Finally, by comparing and analyzing the fundamental frequency electrical quantity calculated before and after the phase shift, the adaptability of the existing AC/DC line-touching protection with different criteria after the phase shift is discussed. The analysis shows that the AC-DC line-touching protection with fundamental frequency current as criterion has the problem of reduced adaptability on the rectifier side. An AC/DC line-touching fault simulation model is built on the PSCAD/EMTDC platform and a lot of experiments are carried out. The simulation results verify the accuracy of the theoretical analysis.