Study On Power System Interharmonic Detection Algorithm And Motor Interharmonic Source

With extensive applications of nonlinear loads, power electronic devices and fluctuating loads in power system, it causes an increase of interharmonic content in the grid, and the interharmonic of the grid leads to a series of problems, besides the typical problems caused by harmonics such as the load power factor reductions, the variety of energy loss increase, and the excitation of various resonance modes with supply system, it creates some new problems, such as voltage flicker, the passive filter overload, zero offset of the voltage waveform. So it causes power quality deterioration and affects the safe and economic operation of the power grid. Comparing with the traditional harmonics, the interharmonics have a more extensive damage for the power system, so it must be suppressed. The accurate estimation of interharmonics parameters and the deep understanding of their origins are a premise of the suppression of them. The interharmonics have the time variability and randomness characteristics, and their amplitudes are usually very small, and their frequencies can appear any point of the spectrum, so the traditional harmonic analysis methods have a serious deficiency to the interharmonic analysis. According to the interharmonic characteristics and the research status at present, this paper researches on the interharmonic detection algorithm, the voltage flicker envelope extraction method and the motor interharmonic source. The main work is as follows:(1) About interharmonic detection, a new method of interharmonic spectral estimation based on the optimal window Burg algorithm is presented. Firstly, the principle of the interharmonic analysis based on the AR model is elaborated, and the high resolution characteristic of AR model is analyzed. The Burg algorithm of AR model is often used, but the error will appears in the process of AR model parameter recursion. It causes that the AR model parameters don’t consistent with the actual values, leading the shortcoming of the peak bias and spectral line splitting. Aiming at this shortcoming of the Burg algorithm, a windowed method is present to suppress the error. Firstly, the optimal window can be obtained by minimizing the variance of average frequency error, and then the Levinson recursion is operated by windowed reflection coefficient. Since the method can guarantee to obtain a stable AR model, its spectral estimation performance is better than the Marple algorithm of AR model. By comparing the optimal window Burg algorithm with the eigenvalue methods from the points of the view of computational complexity and spectrum estimation performance, we can find that they both have the high frequency resolution and aren’t affected by the asynchronous sampling, but the computational complexity of the optimal window Burg algorithm is far less than the eigenvalue methods. According to the acquired power spectrum by using the optimal window Burg algorithm, the amplitudes and initial phases of every harmonic and interharmonic are computed further by least square method in this paper. (2) About the voltage flicker envelope extraction, a new time-frequency atom-based method to detect voltage flicker is proposed. By means of matching pursuit, the voltage flicker signal after the synchronous detection is divided into a series of time-frequency atoms in which each atom is from redundant time-frequency atom dictionary and the chosen atoms can match to the signal structure. The frequency parameters of the chosen atoms are accepted or rejected to achieve the purpose of low-pass filtering. There is no convolution operation in the proposed method, so the boundary effect can be avoided. The results show that by use of the proposed method, not only undistorted envelope detection can be implemented, but also the boundary effect brought about by the traditional filtering methods, FIR filter, IIR filter and wavelet transform, can be avoided.(3) The oscillating load torque leads to the induction motor stator current oscillations, and the oscillating current contains interharmonics. According to this phenomenon, the interharmonic source model of the induction motor with oscillation mechanical load is established. Compared to previous work, this model takes into account the more parameters of the induction motor, so the model is more detailed and has a higher accuracy. The mathematical model of the induction motor, which is nonlinear when the load torque oscillates, is linearized firstly. On this basis, the expression of the stator interharmonic currents is derived. The matlab simulation results verify the correctness of the proposed expression. Furthermore, the relationship between load torque oscillations and the spectrum of the stator interharmonic currents is analyzed by the proposed expression, and the effect of the various motor steady output powers on the spectrum of the interharmonics is investigated.(4) The interharmonic source model of the induction motor driven by the voltage source inverters (VSI) considering mechanical load oscillation is established. Firstly the motor input voltage spectral characteristics is analyzed, on this basis, the formula to forecast the frequencies of the stator interharmonic currents is presented, and the process that the stator interharmonic currents pass through VSIs is analyzed using the modulation theory and instantaneous power conservation theory, and then, the expression of the interharmonic currents on supply system side is derived, and the frequencies and spectrum characteristics of interharmonic currents on supply system side can be analyzed by this expression, and finally, the effect of the load torque oscillation on the spectrum of the interharmonic currents on supply system side is investigated by simulation.