Study On Subsynchronous Harmonic Detection Method In Dense New Energy Areas

Subsynchronous harmonics will be generated in power electronic devices widely used in new energy wind power generation system.Subsynchronous harmonics severely affect the power quality of the power grid,and it may lead to shafting torsional vibration of thermal power units,resulting in shafting damage and even endangering the safe and stable operation of the whole power system.Consequently,giving great significance to accurately detecting subsynchronous harmonics components.The commonly used subsynchronous harmonic detection method is fast Fourier transform(FFT),whereas,the spectrum leakage and fence effect are caused by asynchronous sampling and non-integer period truncation in discrete analysis of signals,so it is difficult to detect harmonics with high precision by using spectral information.For this phenomenon,the utilization of windowed interpolation algorithm and all-phase FFT(ap FFT)can improve the accuracy of parameter estimation.Based on the analysis of the spectrum characteristics of classical Nuttall window,1st-4th order Nuttall self-convolution window functions are obtained by several 4 terms 5 order Nuttall window self-convolution operations.Compared with classical window functions,self-convolution window functions have better side-lobe characteristics and can further reduce spectrum leakage.Based on the comprehensive consideration of the main-lobe and side-lobe characteristics,the 4 order Nuttall self-convolution window function is used to truncate the signal.According to the principle of quadruple-spectrum-line interpolation,the rectification formula of the 4th order Nuttall self-convolution window function is proposed.According to the phase invariance of ap FFT,the double-spectral-line correction harmonic analysis algorithm based on Nuttall double window ap FFT is deduced by using the ratio of the main spectral line and side spectral line amplitudes near the theory frequency point.The windowed interpolation FFT algorithm has higher estimation accuracy for amplitude and frequency parameters,and ap FFT can effectively extract phase parameters.A combinatorial optimization algorithm is proposed,which uses the first method to detect amplitude and frequency parameters,and uses the second method to detect phase parameter.The combinatorial optimization algorithm and the interpolation FFT algorithm based on classic window functions are used to analyze power harmonic signals respectively,and the accuracy of measuring harmonic parameters is validated under the influence of noise interference and frequency fluctuation.MATLAB simulation analysis indicates that the algorithm can detect the frequency eigenvalues of subsynchronous harmonics more precisely,and suppress the interference between the spectrums.The accuracy of the amplitude,phase and frequency parameters of the harmonic is increased by orders of magnitude compared with other algorithms.In the case of noise interference and frequency fluctuation,the combined optimization algorithm has less influence on the measurement of harmonic parameters,and still has high measurement accuracy.Finally,the combinatorial optimization algorithm is applied to the subsynchronous harmonic detection in Xinjiang,and compared with the Hanning windowed and Nuttall windowed interpolation FFT algorithm.The simulation results show that the algorithm can effectively and accurately achieve subsynchronous harmonic detection purposes.