Multi-Level Set Single-Cycle Power Grid Frequency Measurement Method And Its Implementation

In the electric power system,the detection of the fundamental frequency is related to the safe and stable operation of the system,which makes it significant to achieve the fast and accurate measurement of the frequency in the power system.However,the precision and real-time detection cannot be met at the same time with the traditional frequency measurement method.Therefore,it is of great significance both in theoretical and practical to study a fast and accurate method for measuring fundamental frequency of power system.In this dissertation,a single-cycle measurement method of power system frequency based on multi-threshold is proposed after the analysis with the shortcomings of the traditional frequency measurement methods.This method filters out the high frequency interference in the signal with the low-pass filter before reconstructing the sampling point near the threshold with the three spline interpolation method and estimating the frequency with the intersection of multiple thresholds and signals.Moreover,the optimal frequency of the power system is obtained by using the optimal combination weighting method.The method,based on the time-domain frequency estimation method,has the characteristics of low computation and high precision,which can realize the accurate measurement of the power system frequency in a single cycle.The main contents of this dissertation include:Firstly,it introduces the traditional frequency measurement method of power system along with its advantages and disadvantages before illustrating the application of each algorithm.Secondly,the frequency estimation algorithm based on single threshold of power system is deduced according to the principle of three spline interpolation and not-a-knot.On the basis of this,the expression of the frequency estimation based on the single-cycle measurement algorithm is after obtaining the optimal weight with the Lagrange multiplier method.Moreover,it can be concluded after comparing the proposed method with the interpolation FFT method based on Hanning window that the amount of data required is much less than the amount of data needed to the interpolation FFT method.Thirdly,it conducts the comparative simulation experiments to illustrate the accuracy of the proposed method with a single-level set algorithm,single threshold method,multi-threshold average weighting method,multi-threshold power system single-cycle frequency measurement algorithm and interpolation FFT algorithm based on Hanning window,and it analyzes the results of fundamental frequency measurement under the fundamental frequency fluctuation and with the consideration of the white noise and harmonic.The simulation results show that the proposed method is lower than the interpolation FFT method based on Hanning window in time complexity and is higher than the of the interpolation FFT method,single-level set algorithm in the accuracy.Moreover,the stability of the proposed method is weaker slightly than the interpolation FFT method but is stronger than the single-level set algorithm,single threshold method,multi-threshold average weighting method.Therefore,its comprehensive performance of the proposed method is most prominent.Finally,it builds the experimental platform according to the requirements of the GB/T 19862-2005,GB/T 15945-2008 and line standard DL/T 1028-2006.Furthermore,it compares and analyzes the frequency results obtained by the single level set algorithm and a single cycle frequency measurement algorithm based on multi-threshold.And these meet the measurement requirements of GB-T15945-2008,in the line with engineering applications.