Abstract
To address the problems still existing in line protection for flexible DC grids currently, this paper proposes a non-unit frequency-based line protection scheme for flexible DC grids. Firstly, the values of the initial voltage traveling waves (IVTWs) generated by the fault point under internal and external faults are respectively calculated based on the equivalent model of the four-terminal flexible DC grid. Due to the boundary effect of the current limiting inductors on both ends of DC lines, the attenuation rates of the IVTWs 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 at different scales, and the amplitudes of the IVTW in different frequency bands are expressed by wavelet transform modulus maximums (WTMMs). A criterion to identify internal and external faults is constructed based on the square of the frequency-domain equivalent attenuation rate of the IVTW. Lastly, simulation results under diverse fault conditions validate the availability of the proposed protection method.
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