Study On Failure Characteristics Of Oil-paper Insulation And Method Of Waveform Equivalence Under Different Impulse Waveforms

The standard waveforms for the dielectric withstand impulse voltage test specified in IEC 60060-1-2010 are the double exponential waves of 1.2（±30%）/50（±20%）μs（lightning）and 250（±20%）/2500（±60%）μs（operation）.At present,the double exponential waveform recommended by IEC is adopted in the research,design,and test of transformers.However,the overvoltage waveform invading a transformer is not the double exponential waveform specified by the IEC but the nonstandard waveform due to various factors,such as attenuation of incoming line,catadioptric reflection in the station,wave process and resonance of transformer winding,and operation of arrester.Therefore,many challenges are often encountered in the optimization of insulation coordination and insulation safety assessment of power transformers.Aiming at the above background and the problems existing in the current research,this paper studied the failure characteristics of oil-paper insulation and a method of waveform equivalence under different impulse waveforms.The research works are as follows:1)Based on the characteristics of measured impulse voltage waveform parameters,a comprehensive experimental platform for double exponential impulse voltage and bipolar oscillatory attenuated impulse voltage was built.By changing the impedance parameters of the test circuit（including the wave front resistance R_f,the wave tail resistance R_t,the wave front capacitance C₁,the damping inductance L,the damping resistance R）,we can adjust the wave front time T_f,the wave tail time T_t,the wave front time T₁,the oscillation frequency f,and the attenuation constantα,respectively.This platform can generate double exponential impulse voltage with a waveform parameter range of 1.2μs≤T_f≤400μs and 20μs≤T_t≤5400μs,and can generate bipolar oscillatory attenuated impulse voltage with a waveform parameter range of 0μs≤T₁≤70μs,50 Hz≤f≤100 k Hz,and 0.1≤α≤0.99.2)Based on the failure probability characteristics of oil-paper insulation under different impulse voltage waveforms,the dispersion of the failure probability characteristics of oil-paper insulation was studied.The failure probability characteristics of oil-paper insulation under the double exponential impulse voltage and bipolar oscillatory attenuated impulse voltage with different waveform parameters obey the Logistic distribution.The fitting parameter d which characterizes the dispersion of failure probability characteristics of oil-paper insulation is positively correlated with the50%breakdown voltage(U_50%),that is,the larger the U_50%of the oil-impregnated insulating paper is,the larger the value of the fitting parameter d is,and the greater the dispersion of the failure probability characteristics of oil-paper insulation is.Through the mathematical method of normality test,it is verified that the relative dispersion of the failure probability characteristics of oil-paper insulation under double exponential wave and bipolar oscillatory attenuated wave is consistent.Based on this consistency,the prediction of the failure probability characteristics of oil-paper insulation under different impulse voltages waveforms is realized.3)Based on the breakdown characteristics of oil-paper insulation under different impulse voltage waveforms,an equivalent method of different impulse voltage waveforms was studied.The effects of wave front time and wave tail time on the U_50%of oil-paper insulation under the double exponential impulse voltage was obtained through experiments.The U_50%gradually increases with the increase in wave front time and finally becomes saturated.The U_50%gradually decreases with the increase in wave tail time and finally becomes a constant value.When the wave front time is relatively short,the range of wave tail time that affects the U_50%of oil-paper insulation is short as well.The U_50%of oil-paper insulation under the bipolar oscillatory attenuated impulse voltage increases with the increase in attenuation constant and finally become saturated.The U_50%changes significantly when 0.3<α<0.7.The U_50%shows a“U”shape with the increase in oscillation frequency.Furthermore,the frequency corresponding to the lowest breakdown voltage is approximately f=0.6 k Hz.Based on the breakdown characteristics of oil-paper insulation under the double exponential impulse voltage and the bipolar oscillatory attenuated impulse voltage,we established the U_50%prediction mathematical models.Meanwhile,all parameters of the models were estimated.Based on the U_50%prediction mathematical models under these two kinds of waveforms,the waveform equivalence maps were established.We proposed an evaluation method for waveform equivalence between the two types of waveform parameters（（T_f,T_t）（?）（f,α））.This method considered the amplitude and second-order partial derivative of the impulse voltage.The waveform equivalence sets between any nonstandard impulse voltages can be determined using this method.