An Improved Phase Difference Correction Method For Frequency Offset And Frequency Fluctuation

The estimation of electrical parameters in the power system is the guarantee of the stable and safe operation of the power grid.The electrical parameter measurement method based on the discrete Fourier transform(DFT)has been widely used because of its high efficiency and easy implementation in engineering.However,in recent years,with the non-linear load,especially the large amount of renewable energies such as wind power and photovoltaic power,the harmonic pollution of the power system is gradually increased.The load imbalance and system failure cause the fundamental frequency to shift greatly in a short time,Extreme conditions may also be a wide range of fluctuations.In view of the above situation,the DFT measurement method applied to the steady-state electrical parameter detection can not meet the requirements and may even cause problems such as measurement failure.In this paper,we propose two improved algorithms based on the traditional phase difference in order to solve the above problems,ie,fundamental frequency offset and power harmonics estimation in both cases.When the fundamental frequency of the power grid is greatly shifted,the improved algorithm,based on the traditional phase difference method,analyzes the frequency spectrum of the windowed signal and performs polynomial transformation on the spectrum expression to speed up the attenuation of the side lobe and further reduce the spectrum leakage and the spectral lines And then by estimating the frequency offset range to determine the number of sampling points for the signal under test,the sampling sequence of different interception points before and after the two phase difference method,the second phase difference method interception points by the first phase difference method The estimated fundamental frequency is determined to reduce the spectral leakage caused by unsynchronized sampling.When the fundamental frequency of power system fluctuates widely,the improved algorithm has four improvement measures compared with the traditional phase difference method.Firstly,through the polynomial transformation of the windowed spectral expression,the speed of side lobe decay is accelerated and the spectrum leakage is reduced And the interference between the spectral lines;the second by calculating the rate of change of frequency(ROCOF)on the traditional phase difference correction formula was further amended;the third in the calculation of ROCOF to minimize the sampling window length,increased ROCOF Real-time and computational accuracy;fourthly,the problem of spectrum leakage caused by non-synchronous sampling caused by fluctuations of fundamental frequency is alleviated by the fourth phase difference method.The simulation and experimental analysis of frequency offset and fluctuation of the fundamental wave of the power grid are carried out respectively.The simulation results show that the accuracy of the estimation of the electrical parameters of the improved algorithm is greatly improved compared with that of the traditional phase difference method,and is suitable for the power harmonics Wave estimation,and real-time high,the length of the sampling window to meet IEC standards.