Research On The Algorithm Of Harmonic Detection

In recent years, with the development of electrical technology, a lot of rectifier, inverter,voltage regulator devices and nonlinear load such as arc furnace, household electric applianceswidespread used, These devices inject harmonics into the power system which causing severeharmonic pollution, reduction power quality and electrical casualty. Harmonics is a great publichazard which has threat the power system safety, high quality and economical operation. Accuratemeasurement harmonic is the premise and foundation of harmonic suppression, equipmentinspection and other harmonic problems.Power system except contain harmonics, there also contain Inter-harmonics andtime-varying harmonic components. In order to estimate frequency, amplitude phase and otherparameters from different types of harmonic components mentioned above, the paper mainlyinvestigated the characteristics of Fourier transform, wavelet transform and estimation of signalparameters via rotational invariance technique (ESPRIT) in harmonic estimation, analyzed theperformance of these algorithm, and compared there advantages and disadvantages in harmonicparameter estimation. A hybrid harmonic parameter estimation method based on ESPRIT andwavelet transform is proposed, which combined the feature of high frequency estimationprecision and suitable for time-varying harmonic detection. The simulation results show thatthe proposed method has higher inter-harmonic parameter detection accuracy than Fouriertransform algorithm, and make up the deficiency of Fourier transform in detecting time-varyingharmonics. Related research summary as follows:(1)The harmonic analysis based on Fourier transform algorithmThe reason of the spectrum leakage and picked-fence effect in harmonic parameter estimationthrough discrete Fourier transform method is analyzed based on the deep research on the basis ofthe Fourier transform. Aiming at the existence of spectrum leakage and picked-fence effect,windowed interpolating Fourier transform methods are studied and the selecting principle ofwindow function is given. Simulation signals with different noise interference which containingdifferent length and different harmonic component are analyzed with the given method.(2)The harmonic analysis based on wavelet transform algorithmAccording to Fourier transform algorithm detection accuracy is affected by noise and can’tanalysis time-varying harmonics, wavelet transform algorithm investigated. This paper deduced theformula to calculate the RMS of each harmonic from the discrete wavelet decompose coefficient, selected the dmey wavelet as the mother wavelet function, and analysis the performance ofdetecting harmonic ratio and time-varying harmonics; Wavelet ridge is introduced into CWT forextracting the harmonics and inter-harmonics according to the maximum value of the ridge, andgiven parameter selection principle for CWT. Using the method of self-tuning scaling to analysisthe signal containing harmonics and inter-harmonics, the simulation result is compared to Fouriertransform analysis result.(3) The harmonic frequency estimation based on ESPRIT algorithmDiscrete Wavelet algorithm can’t estimation the frequency of harmonics and inter-harmonics,and using CWT decompose the signal in the whole scale range which cause amount of calculation.Aiming at these problems, harmonic frequency estimation algorithm based on ESPRIT isintroduced. This paper studied minimum description length (MDL) method to extraction thenumbers of harmonics and inter-harmonics in power signal, and using euler’s theorem convert thetime domain signal to airspace signal. The influence of time-varying harmonic, noise, number ofarray elements and length of signal on frequency estimation precision was analyzed. A hybridharmonics and inter-harmonics method proposed based on ESPRIT and wavelet transformalgorithm, and testing steps formulated.(4)Simulation verificationThe simulation model of time-varying harmonic generator Synchronous generator three-phaseshort circuit and static harmonic generator high voltage DC transmission system are constructed.This paper analysis the characteristic of the harmonics which generated by the model above, and theproposed harmonic parameter estimation method is used to analysis the generated harmonics. Theanalyzed result is consistent with the theoretical calculation result, thus verify the performance andfeasibility of the new method.