Study Of Parameter Measurement Methods On Power System Harmonics And Interharmonics Under Dense Frequency Conditions

With the development of electric power system, a large number of nonlinear devices such as inverter and rectifier equipment, electric arc furnace have been implemented,which makes the power grid harmonic pollution aggravating, resulting in failure of sensitive power electronic devices, transformer temperature rising, and many other problems. As an important part of power quality analysis, power system harmonic and interharmonic analysis can provide important scientific basis for harmonic pollution control, energy metering, communication, high impedance fault detection, harmonic power flow calculation, etc. In addition,the accurate measurement of harmonics and interharmonics can also be a vital theoretical precondition for the harmonic source location,the harmonic emission level estimation, harmonic responsibility contribution, etc. This paper is focused on power system harmonic and interharmonic analysis under the condition of dense frequency. The main purpose of the paper is to promote the dense signal analysis including to improve the analysis precision of dense harmonics and the frequency resolution of harmonics with interharmonics, to realize the detection and measurement of the signal with intensive interharmonics, to reduce the interferences on dense interharmonics as well as to achieve parameter measurement of dense frequency signals using a short-time analysis window. Besides, the corresponding algorithms are proposed.Firstly, when steady waves of electric power system are sampled asynchronously, the analysis of dense harmonics using discrete Fourier transform will generate severe spectrum leakage and fence effect. Thus, a new kind of hybrid convolution windows produced by convolving rectangular windows and cosine windows was proposed. The main lobe width and side lobe attenuation of defined L-order hybrid convolution window were analyzed.By comparison with the performance of classical windows, the new windows have faster side lobe attenuation, which can greatly reduce the effects of spectral leakage. The proposed new windows were applied to harmonic analysis, and harmonic interpolation algorithm based on L-order hybrid convolution windows was derived. The results show that the new windows have the excellent property of restraining spectral leakage, and the mutual interference between the various harmonic components can be reduced effectively.The advantage of this method is also more obvious even under noisy conditions, and the new algorithm can be used for high-precision measurement of power system dense harmonics.Secondly, in order to restrain the side lobe interferences and improve the frequency resolution when the harmonic signal contains interharmonics, the harmonic and interharmonic analysis method based on the odd point correction was proposed. The characteristics of Dolph-Chebyshev windows such as equal fluctuation of the side lobe and the minimum side lobe peak with the given main lobe width were used to improve the harmonic analysis ability. The proposed method can suppress the side lobe from other components through phase rotation and the odd point interpolation. The classical harmonic signal and power harmonic signal were analyzed, and the results show that the proposed method has higher precision on harmonic and interharmonic measurement under the same level of frequency resolution, as well as a certain anti-noise ability.Then, power system steady signals with dense interharmonics sampled asynchronously with limited analysis window will generate severe spectrum interferences using discrete Fourier transform. The actual frequency components cannot be accurately identified and measured on account of the limited frequency resolution. By establishing a dense interharmonic simplification spectrum interference model, a multiple interharmonics spectrum separation and parameter measurement method was proposed. Then each main frequency component can be separated with the known spectral information using the spectrum separation algorithm. When come to calculate relatively smaller interharmonics,the spectrum of fundamental wave can be stripped from the original one after calculating its parameters, so as to highlight other smaller components in frequency domain. The spectrum separation process is to separate each frequency around every spectral peak which can be observed, and the true components will be acquired through spectral properties. The performance of the proposed method was validated on the simulation signal and semi-physical signal. The results show that the proposed method can not only accurately identify the dense frequency components, but also obtain the parameters with high precision.After that, dense interharmonics with relatively small amplitude will generate severe mutual spectrum interferences by the spectrum analysis of a signal with dense frequencies.Furthermore, the side lobe leakage of fundamental and harmonic components will make the interharmonic components more complicated. To address this issue, a new dense interharmonic analysis method based on a new quasi-synchronous sampling process was proposed. First, the exact fundamental frequency will be acquired through the spectrum separation and measurement algorithm. Then, a new quasi-synchronous sampling method can be achieved based on Newton interpolation algorithm and the calculated fundamental frequency. The complexity of interharmonics will be reduced by using the quasi-synchronous sampling process, which can greatly inhibits the fundamental and harmonic interferences. By this way, the implementation of the spectrum separation and measurement algorithm can be broadened. The proposed method has been validated by both the simulation signal and a real case signal. The results show that the effect from fundamental wave and harmonics can be restrained and the dense interharmonics can be identified and measured with relatively high precision.Finally, overlapping region in frequency domain among fundamental wave,harmonics and interharmonics will be enlarged when the fundamental signal with harmonics and adjacent interharmonics is analyzed by short-time Fourier transform (STFT)under asynchronous sampling condition. The aggravating mutual effect can result in serious measurement errors. Therefore, the phenomenon of spectrum interference under short-time analysis window was analyzed. Then, a new power system signal parameter estimation method based on short-time spectrum separation and measurement (SSSM)algorithm was proposed. Firstly, short-time multiple frequency signal model was established based on short-time Fourier transform, and the short-time spectrum separation algorithm was deduced subsequently. Then, each parameter of each corresponding frequency component can be calculated. The method has been verified by calculating fundamental parameters via a variety of cases, such as voltage fluctuation and spectrum aliasing, the results show that the proposed method can accurately extract the fundamental component with high temporal resolution and parameter measurement accuracy, and has the ability of anti-noise to some extent. In addition, the method can also be applied to identification and measurement of harmonics or interharmonics.