Assessing Harmonic Impedance By Synchronously Layered Distortion Waves

Since the beginning of the 70s, harmonic pollution has significantly influenced the operation of power system due to the widespread use of non-linear loads. It is determinant to correctly assess harmonic emission levels from different customers for monitoring and inhibiting the harmonic pollution. And to determine whether harmonic currents can cause significant voltage distortions, it is necessary to know the network harmonic impedances. Primarily, the available methods for measuring or calculating harmonic impedances can be divided into two classes, one is invasive and the other is non-invasive. The former is based on injecting (inter)harmonic currents into or switching some branch at the point where the (inter)harmonic impedance is to be measured. Theses method may cause harmful influences on the operation of power system. The latter is to use pre-existing harmonic sources or measured data to assess harmonic impedance.In this paper, a method assessing harmonic impedance by synchronously layered distortion waves is proposed, which is based on the "fluctuations method"-a simple and effective means. However the method is based on the Fourier transform, which can just precisely analyze the signals whose cycle is integer times as broad as that of fundamental wave. With sampling windows widening, the variations of distorted signals in a fundamental period are weakened. Moreover, the higher harmonic order is, the lower distinguishability, and, as a result, the assessment accuracy becomes bad. Considered its prominent localization both in time-domain and in frequency-domain, wavelet is used to the field of assessing harmonic impedance for the first time. We choose Chaari wavelet because of its special band frequency characteristic, which can stratify signals synchronously and locate the distortion time precisely in assessing harmonic impedance.