Research On Harmonic Detection Methods In Power System Base On HHT

In recent years, with the development of the power electronic technology, variousnonlinear equipments were widely applied in the power grid. The pollution ofharmonic in the distribution net-work lowered the power quality level and caused allsorts of electrical equipment failure. Therefore, to study of appropriate harmonicdetection method, and basing on test results to put forward the harmonic controlmeasures had important practical significance.Fourier transform and wavelet transform were popular method of analysis of timedomain. It was also the most commonly methods being used in harmonic signaldetection. Most of the harmonic detection tools and power quality analyzer were basedon these two theories. But Fourier transform has defect of a single resolution andwavelet transform has the problem of overly depending on wavelet base. When it wasused in detecting the signal analysis for time-varying harmonic, attenuation harmonic,discontinuity harmonic and other non-stationary signal of harmonic would appeardeviation. The effect of instantaneity detection was poor. In this paper, a method ofHilbert-Huang Transform (HHT) to the analysis of harmonic signal detection would beintroduced. HHT is a kind of frequency -domain analysis method which can beadaptive to deal with the nonlinear and non-stationary signal. This method couldovercome the defects of the traditional method to detect the mutation in the harmonicsignal, but there were also many other problems such as curve envelope, modalaliasing.In this paper, in order to solve the problem of HHT method in analysis harmonicsignal, an ideal, and using shape-preserving piecewise cubic Hermite interpolatingpolynomial to instead of three spline interpolation function and related coefficient todistinguish the false IMF component was to put forward in this paper, through theMatlab simulation program to verify the effect. Then, using HHT methods to simulatefour analog signal of harmonic wave(linear combination between harmonics andinter-harmonics, continuous harmonics signal ,attenuation harmonics signal,harmonics signal with noise)，and compared the results with FFT and wavelettransform. The results showed that detecting of mutations, attenuation, continuousharmonic signals; HHT is more accurately than wavelet transform and FFT.To eliminate the influence of accuracy of the base wave components in harmonicanalysis, in this paper, a processing of filtering was first do on the original signal. Then a second order digital filter was designed to curb modal aliasing phenomenon inEMD process, through the method of HHT to simulate original signal. With the leastsquares fitting to extract harmonic parameters. The simulation experiment showed thatcan get a good effect of harmonic detection, and stability was good.This paper collected the actual harmonic current data from the steel factory of thearea in guang-dong province, using the method of HHT to analysis the signal,extracting the every harmonic current component to contrast to the measured value ofthe power quality analyzer (Unipower-U901). The result showed that this methodowned high accuracy and has application value.