Research On The Analysis Of Power System Harmonics

With the development of modern industry, nonlinear loads of power system are greatly increased. The electrical characteristics of these nonlinear loads, including nonlinearity, impulsion and imbalance, result in voltage aberrance. Voltage aberrance makes power quality severely worsen, endangers power system running safely and economically and affects some users working normally. The content of harmonic in power system is the important parameter in power quality. So, harmonic measurement is important significance in power system.The harmonic wavelet widely applied in mechanical vibration area, is adopted in this paper to deal with the harmonic and inter-harmonic problem in power system, in view of its ability of detail frequency section. Firstly, in order to reduce the impact of Gibbs phenomenon in spectrum, a step of adding window process is employed. And then, the harmonic signal is decomposed by harmonic wavelet transform (HWT) and the instantaneous amplitude and phase of harmonic components are identified; furthermore, in order to increase accurate, the amplitude and phase of each harmonic component are calculated by least squares fitting method.To analyze harmonic accurately is significant to the stability of power system. HHT(Hilbert-Huang Transform) is adopted to analyze harmonic in order to overcome the shortcoming of FFT method and wavelet analysis method. The power quality signals are decomposed by empirical mode decomposition(EMD) method, and then a series of intrinsic mode function(IMF) are obtained. Owing to different IMFs correspond to different harmonic, all harmonic'amplitudes, frequencies and phases can be achieved via Hilbert transform(HT) plus Least Square fitting, to realize accurate analysis of power system harmonic. During the procession of EMD, Improved Extremum field Mean Mode Decomposition(IEMMD) and the piecewise Sinusoidal Interpolating are employed to reduce the end effect. In this way, only one Sine Interpolating is required rather than two. HHT not only can get accurate harmonic component and time characteristic, but also have very strong adaptability and practicability to analyze real-time temporary signal.