Research On Power System Frequency Measuring Method And Its Application

Frequency is the maj or operating parameter of power system and electrical equipment,and frequency estimation is the foundation of operation,supervision, control and protection of power system and electrical equipment.Applying digital signal process theory,micro-computer technology,and power system automation technology,this dissertation researches on theories and methods for more accurate. faster,less cost,and more practicable electric signal measurement.The paper introduces current power system frequency measuring theories and algorithms,the fundamental principle,applied occasion and practical value of various algorithms are discussed.The paper presents a new approach to estimate the frequency of electrical signal disturbed by harmonics and noise,using an adaptive Infinite Impulse Response(IIR)notch filter based on variable step.size Least Mean Square(LMS)algorithm.The principle that notch filter can eliminate specific frequency from input signal is discussed in detail.In this approach,a second.order adaptive IIR notch filter is designed for the fundamental frequency firstly,and then the notch filter coefficients are adapted by a recursive LMS algorithm with time varying step—size,which is adjusted by the square of the time—averaged estimate of autocorrelation of the notch filter present output signal and the past one.Given constant of step-size iter.ative formula,computing example is shown to estimate frequency.Simulation results for state frequency,varying frequency and frequency of electric machine are also presented to demonstrate the effectiveness of the proposed algorithm.The paper introduces the basic theory,form of fourier transform,and selection parameters of DFT.The reasons of the error caused by the traditional electric signal detection methods base on the windowed DFT are thoroughly analyzed.Besides, many windows are introduced.According to the characteristics of power electric signals,an algorithm for measuring electric signals frequency based on spectrum correction is present.It performs DFT on two sampling sequence of electrical signal, and makes use of the phase—angle difference of two DFT analysis results to estimate correcting variable,then measures power system frequency.Simulation results are presented to demonstrate that the algorithm can meet the demands of computing speed and precision,by choosing different length of data window and corresponding windows.In the end,algorithm is applied in virtual instrument to approve the effectiveness and utility for electric power Dequency measurement.