Study On Accumulative Effect Of Repeated Lightning Impulses On Oil Paper Insulation And Its Influence Factors

Transformer is the core apparatus of power conversion and transmission in electrical power system, the operation stability of which concerns the safety operation of the whole power system. With the improvement of insulation level of transmission lines and automatic reclosing technology, the lightning withstand level of transmission lines are enhanced significantly. But meanwhile, some deep-seated problems are gradually revealed, such as degradation or even premature failure of transformer inner insulation(oil paper insulation) caused by repeated lightning impulses. Based on the above research background, and combined with the problems in engineering practice, we studied the accumulative effect of repeated lightning impulses on the oil paper insulation in this work. The mechanism of accumulative effect is also revealed. The main research contents and corresponding conclusions are summarized as follows,(1) Based on the transformer online monitoring system for invading waves in substations, we studied the identification method of inductive lightning overvoltage and direct lightning overvoltage. By noise elimination and filtering, we obtained the transformer invading lightning impulse voltage, which has oscillation and damping characteristics. To facilitate the comparison of breakdown characteristics of the above two waveforms, we studied the equivalence of oscillatory waveform and double exponential waveform based on energy equality. The probability distributions of wave front/tail time of non-standard lightning impulse voltages were determined in this work, and the equivalent wave front/tail time are 20±2.78 ?s and 198±2.54 ?s, respectively.(2) The accumulative breakdown characteristics of oil paper insulation under standard lightning impulse voltage were determined. Translucent gelatineous substance was detected on the surface of test sample, and the color of sample surface is quite different under electrode compared with the fresh test sample. Based on the Atomic Force Microscope technology(AFM), we studied the influence of repeated lightning impulses on the surface morphology of oil paper insulation. The test results indicate that with the accumulation of repeated lightning impulses, the surface roughness increases dramatically. Based on the Frequency Domain Spectroscopy technology(FDS), we analyzed the influence of repeated lightning impulses on the dielectric parameter. The test results indicate that with the accumulation of repeated lightning impulses, the relative permittivity(real part and imaginary part), dielectric loss factor and volume conductivity increase correspondingly.(3) We determined the V-N characteristics(relationship between the amplitude of applied lightning impulse voltage and the applied times) of oil paper insulation. The influence of test voltage and interval time of applied lightning impulses on the accumulative breakdown characteristics were also studied in this work. We found that with the increase in the test voltage or with the decrease in the interval time, accumulative effect withstand ability of oil paper insulation declines dramatically. Based on the Pulsed Electro-Acoustic Method(PEA), the migration, accumulation, dissipation of space charge in the oil paper under repeated lightning impulses were measured. A large amount of positive charges were detected near the anode. The positive charges accumulated near the oil-paper interface and some of them got through the oil-paper interface and migrated to the cathode. This phenomenon leads to the enhancement of electric field near the cathode, which contributes to the breakdown. The verification test results indicate that the accumulative breakdown of oil paper insulation is caused by electrical degradation. And the space charge accumulation is considered an important factor that contributes to the accumulative breakdown of dielectrics.(4) The V-N characteristics of oil paper insulation under standard double exponential waveform and non-standard double exponential waveform were determined. We analyzed the effect of wave front/tail time on the V-N characteristics, and found that with the decrease in the wave front time or with the increase in the wave tail time, the accumulative effect withstand ability of oil paper insulation declines dramatically. The accumulative breakdown characteristics of oil paper insulation under oscillatory damping impulse voltage were identified in this paper. We found that although the breakdown voltage of oil paper insulation under oscillatory damping impulse voltage and its equivalent impulse voltage is similar, the accumulative breakdown characteristics under these two waveforms are quite different. The accumulative effect withstand ability of oil paper insulation under oscillatory damping impulse voltage is much higher than that under equivalent double exponential impulse voltage.