| The dumping coefficient of interconnection power grid with large capacity,long-distance transmission and heavy load is quite small. When this kind of transmissionsystem is disturbed, relative movements occurs between the generator rotors, which lead tolow-frequency oscillation of the power grid and low-frequency signal superimposed on theoutput voltage. Since a maximum value of tan of XLPE cable appears on the band of0.01to1Hz frequency, using low-frequency signal, which generated by the low-frequencyoscillation of the system, monitoring cable aging can reflect the aging of the cable.Therefore, this paper treats low-frequency signal as research object. Through on-linemonitoring, the frequency, amplitude, duration and other features of low-frequency signalare studied.Based on the generation mechanism of low-frequency oscillation, four-machinetwo-area model of the grid is established, which reaches the features of low-frequencysignal and the difficulties when monitoring it. After that, frequency interference eliminationmethod is studied; various notch circuit simulations are analyzed, and front end circuitwhich used to eliminate frequency interference is chosen. This paper also compares variousdata processing methods. Through analyzing fast Fourier transforming theory, samplingpoints and sampling frequency are determined, and data processing subroutine isprogramed to meet the demands of frequency and amplitude of the low-frequency signal.This paper designs and develops a low-frequency signal on-line monitor, including analogcircuits, digital circuits, electromagnetic interference suppression, the main program andsubprogram.In this paper, functional tests of low-frequency signal on-line monitor are conducted.The test results are basically in line with the design, and the monitor can meet the accuracyrequirements of on-line monitoring. Testing method of low-frequency signal on-linemonitoring is designed, but field test needs to be done in the future. |
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