Precise Phase Measurement And Implement Based On PMU

With the wide application of Wide-Area Measurement System (WAMS) in power system, the research of new technology about monitoring power networks is developing greatly. It is the basic function of Phasor Measurements Unit (PMU) to measure precisely the phase of network by the synchronized data from Global Positioning System (GPS), and it is the base of other functions of WAMS.First, PMU will sample the initial signal, and calculate the phase with time tag. Discrete Fourier Transforms (DFT) is the basic method to calculate phase. It will result in the error of calculating phase when the real frequency excursion. If the sampling frequency is regulated as the frequency of network, it can sample at a full period and the error will be reduced. But because of the requirement to synchronization of WAMS, the method is limited.There is two styles of phase , rotating and const phase . The angle of const phase is the difference of real angle and the one with rated frequency. Under the two style, DFT and recursion DFT have different formula.The phase is relative to time. The time of the phase from DFT is relative to the reference point of the data window. Besides, the selection of reference point affects the precision of phase calculating by DFT. When the point is the middle of data window, the error is the least. After reseaching the result of calculating phase by DFT, a new phase revising method is raised. It can be known from the simulated calculteing, that the mothed can calculate the phase precisely when its ample is const and frequency varys from its rated value under statbility of power system.Under the condition of low frequency oscillation, the revising method of phase based on DFT is not suitable any longer. The theoretic signal of oscillation can be regarded as the principal wave whose ample changes as sine curve. It is illustrated by the calculating that the result of phase calculating by DFT can be as the data to anylise low frequency oscillation. After summurising the mothed of calculating oscillation by wavelet transform, this thesis calculated the main frequency of theoretic occilation signal.Based on the principle of calculating phase by DFT, a PMU is designed and implemented under the Windows system, using the fixed sampling frequency. After analyzing the performance index of PMU, this dissertation discusses the Windows system's character and its shortage in the real time performance. Virtual Device Driver (VxD) and multithread are used in the software developing, which have enhanced the real time performance greatly.