Study On Application Of DFT Filter Bank To Power System Interharmonics Measurement

Power semiconductor devices are widely used in electric power distribution system. Because of their nonlinear load characteristics, hannonics pollution has worsened. The electric power system exists many non-integer harmonics.The harm brought by interharmonics have attracted people's attention more and more. Current research shows that interharmonics voltage should be restrained to a low enough level. The study of harmonics is a very important topic for electric power quality. Research topics include harmonic source analysis and efforts, measurement technology and elimination strategy.When we use FFT to analysis harmonics and interharmonics, the interharmonics component will result in a phenomenon of frequency spectrum leakage . The FFT has a higher error in the non-integer hannonics analysis of the electric power system.The paper discussed the results of higher error.The tapered-window-length in the non-integer harmonics analysis was given. Some estimation formula for non-integer harmonics parameters was given, which is based on hanning window and sampling time equal to 10 times of period. Hower the FFT may produce aliasing spectral leakage and picket fence effects, thus affecting its direct application to this field.The paper presents a discrete-Fourier-transform (DFT) filter bank for power system interharmonic measurement. It has the ability to measure interharmonics contents in time and frequency domains, and is ideal for long-term of observation harmonics. It can also overcome shortcomings of the windoeed fast-Fourier-transform (FFT) algorithm. The subject of the DFT filter bank is closely releted to multi-rate digital-signal-processing (DSP) technique, narrow band low pass FIR filterprocesses sharp transition band on the frequency domain, hence for implementation of narrow band low pass FIR filter with traditional methods, too many weight coefficients and multiplications are needed, it is unrealistic. The technique of multi-samples rate digital processing, makes transition band wide, overcomes the faults of narrow band.Hence , the method using the multi-rate technique is very efficient. Finally, The similating result shows the new method reach a higheraccuracy.