基于初始电压行波频域衰减速率的MMC-HVDC线路保护方案

MMC-HVDC line protection scheme based on frequency-domain attenuation rate of initial voltage traveling wave

针对目前基于模块化多电平换流器的高压直流(MMC-HVDC)电网的线路保护中存在的问题,提出了一种基于初始电压行波(IVTW)频域内衰减速率的MMC-HVDC线路单端量保护方案。首先,构建四端MMCHVDC电网的等效模型,分别计算直流线路区内外故障下IVTW的频域表达式。由于输电线路两端限流电抗器的边界作用,区内外故障下IVTW随频率增大而衰减的速率具有不同的特点。以不同尺度的平稳小波变换(SWT)提取IVTW在不同频带下的细节系数,并用不同尺度下的小波变换模极大值(WTMM)的绝对值分别表征其在相应频带中的幅值,根据IVTW频域内等效衰减速率的平方构造了识别线路区内外故障的判据。最后,各种故障情况下的仿真结果验证了所提保护方案的有效性。

Aiming at the problems currently existing in transmission line protections of MMC-HVDC(Modular Multilevel Converter based High Voltage Direct Current)grid,a single-terminal MMC-HVDC transmission line protection scheme based on the frequency-domain attenuation rate of the IVTW(Initial Voltage Traveling Wave)is proposed.Firstly,the frequency-domain expressions of IVTW under internal and external faults are respectively calculated based on the equivalent model of four-terminal MMC-HVDC grid.Due to the boundary effect of the current limiting inductors on both terminals of the DC line,the attenuation rates of the IVTW with the increase of frequency under internal and external faults have different characteristics.The detail coefficients of the IVTW in different frequency bands are extracted by stationary wavelet transform with different scales,and the amplitudes of the IVTW in different frequency bands are quantified by the absolute values of the WTMMs(Wavelet Transform Modulus Maximums)under the corresponding scales.The criterion to identify internal and external faults is constructed based on the square of the frequencydomain equivalent attenuation rate of the IVTW.The simulative results under diverse fault conditions verify the effectiveness of the proposed protection scheme.