Investigation Of Time-frequency Hybrid Insulation Diagnosis Method Based On PDC Analysis

In the study of the dielectric spectroscopy on insulation diagnosis,extractions of aging relaxation parameters are the basis of theoretical research. Inaddition, getting the dielectric frequency response and recovery voltage bynumerical computation to the measuring time-domain polarization anddepolarization current (PDC) current, and cascaded methods of linear highvoltage amplifier are also the key test problems.The dissertation firstly analyzed the total current response in step excitation,shows the characterizations of parameters based on multi exponential functionfitting which can be used to solve the branch resistance and capacitance indielectric Debye model. The effects of different relaxation time branches,conductance parameters on time and frequency characteristics of the complexpermittivity and dielectric dissipation factor are simulated in oil-paper insulation,which are consistent with the experimental results in the literatures and provideinsulation diagnosis of dielectric spectroscopy with a theoretical analysis method.The micro current measurement circuit is designed for the PDC analysis. Forsolving the problem of high resistance error, a T-type resistor network is used toobtain50times of high resistance. The design of micro current measurementcircuit is reasonable and effective by the PDC tests of oil-paper insulation.Considering that the recovery voltage in high impedance dielectric can notbe measured, the recovery voltage characteristics are obtained through theoreticalcalculation according to the dielectric polarization and depolarization current(PDC) testing data, and the numerical algorithm transforming polarization anddepolarization current to the recovery voltage is presented. The recursion formulais deduced in this dissertation, and effectiveness of the formula can be testifiedfrom simulation and test results. The dissertation demonstrates the calculating formulae of complexpermittivity and dielectric dissipation factor on the basis of the elementparameters fitting from the PDC data in dielectric Debye model. Because of thelimitation of traditional Harmon approximation and longer time consuming forthe low frequency test in FDS method, the dissertation presents ContinuousFourier Transform and Fast Fourier Transform on the basis of time-domain PDCanalysis. The FDS characteristics are obtained by simulation, which is inagreement with the test results.The dissertation carries out a large number of experimental studies of time-frequency hybrid insulation diagnosis based on PDC analysis. The validity ofconverting the polarization and depolarization current (PDC) into complexcapacitance and loss factor characteristics in frequency domain is verifiedexperimentally with insulation pressboard. A test system platform for theexperiments of transformer oil is built for laboratory accelerated electro-thermalaging experiment to simulate the onsite flowing transformer oil. The effects ofdifferent moisture content in oil, different treatments of oil samples and differentaging factors on the FDS curves are analyzed, which is consistent with thetheoretical analysis. For the dielectric spectroscopy insulation diagnosis of cross-linked polyethylene, experimental studies are carried out to investigate effects ofaging time and aging temperature. The experiments show that the test minimumvalue of dielectric dissipation factor remain nearly constant with the same agingtime and different test temperature for cross-linked polyethylene with differentaging degree due to the heat, oxygen and copper catalyzed accelerated thermalaging effect. The minimum dielectric dissipation factor in frequency domain willrise with the increase of aging degree, and can be used as the evaluation criterionof insulation aging degree in cross-linked polyethylene.High voltage linear amplifier is the key part of dielectric spectroscopyinsulation diagnostic. Because the traditional high voltage power supply can notbe used for a wide frequency range of insulation diagnosis and present highvoltage amplifier is limited to the output voltage ranges, a new type of seriescascaded topology of high voltage linear amplified circuit is presented in thisdissertation. The circuit unit and cascaded topology are given, which solve theproblem of high-voltage power supply by use of differential output of dual operational amplifier, can obtain slew rate of40V/μs for one unit, and beamplified40times linearly by one unit, therefore,40n times of high-voltagelinear amplification can be fulfilled by cascaded n level with the frequency rangeof10-3-104Hz. A two units of series cascaded high voltage linear amplifier isdeveloped, and the amplitude frequency and phase frequency characteristics aremeasured, which verified the rationality and validity of the circuit design.Since the insulation diagnosis compared the dielectric parameters of timedomain or frequency domain with the changes of test curve in traditionaldielectric spectroscopy, it is difficult to determine the insulation assessmentcriteria accurately. Based on the systematic research of the insulation diagnosistheory and experiment of dielectric spectroscopy in the time and frequencydomain, the dissertation presents a multivariable nonlinear least square fittingmethod of dielectric relaxation time distribution characteristic parametersextraction. According to the measuring PDC data, each branch parameters of theDebye model can be fitting with a multi exponential function analysis. Thefrequency characteristics of the dielectric parameters will be calculated accordingto the formula of complex permittivity and dielectric dissipation factor infrequency domain analysis, moreover, the empirical formula of complexpermittivity is given by use of the modified Cole-Cole model. The multivariablefunction nonlinear least squares fitting method is presented to extract thecharacteristic parameters of the dielectric relaxation time distribution. Thevalidity of the method is proved by use of the aging FDS experimental data oftransformer oils. The characteristic parameters of relaxation time distribution areextracted in transformer oils with different accelerated aging time of50-200hours. The results show that the characteristic parameters change significantlywith the increase of aging time, which can be used for condition assessment ofinsulation aging.The dissertation presents the multivariable nonlinear least squares fittingmethod for extracting the characteristic parameters of dielectric relaxation timedistribution, which can provide theoretical basis to determine the quantitativecriterion for dielectric insulation aging. With the fast Fourier transform methodfor converting the time domain measurement of polarization depolarizationcurrent into that of the frequency domain, the lower frequency range of curves can be obtained instead of directly measured by the frequency domain method,which reduces the measurement time to a great extent. The application scope ofthe traditional PDC test can be broadened by the numerical algorithm of RVMtest waveform obtained directly from the PDC test data. In addition， Thedissertation presents the minimum dielectric dissipation factor in frequencydomain as the evaluation criterion of insulation aging degree in cross-linkedpolyethylene and the series cascaded topology of high voltage linear amplifiedcircuit. Related theoretical and experimental studies of dielectric spectroscopydiagnosis in this dissertation are valuable in theory and application.