Study On Direct Detection Technology Of Space Charge In Polyimide Under Square Wave Electrical Field

Polyimide(PI)film has been widely used in inverter-fed motor insulation system because of its excellent electrical properties.Space charge is an important factor leading to insulation aging.However,the polarity reverse time of square wave stress is ns to μs level,which makes it difficult to directly detect the space charge distribution under square wave stress.Therefore,the influence of core parameters of pulsed electro-acoustic method(PEA)equipment was analyzed under square wave stress.And direct dynamic detection of space charge distribution under square wave stress was realized by automatic equal phase shift principle(AEPS).Square wave voltages with rising edge time adjustable within 100μs to 1000μs were obtained by the optimization of the system parameters,and space charge distribution of unaged and thermal aged PI films was measured under the DC and square wave fields.At the same time,the electrical properties of PI films after different thermal aging were analyzed through scanning electron microscopy(SEM),broadband dielectric spectroscopy and breakdown tests.The formation mechanism of space charge in PI film was preliminarily explained.The high resolution requirement of space charge detection under the square wave field can be realized by using AEPS principle,and the highest phase resolution is 1.79°.By reducing the value of the protective resistor and coupling capacitance,the waveform distortion problem can be effectively solved.The phenomenon of double carrier injection under the DC electric field is observed in unaged PI film,and the deep trap injection exists in high field condition.Those space charge dissipates slowly during short circuit,and the bipolar space charge exists for a long time.The internal space charge mainly comes from the homocharge injection coming from the aluminum electrode under 10kV/mm square wave field.While those space charge can be quickly extracted in the polarity reverse process,and no significant space charge accumulation phenomenon is observed inside the film.SEM and dielectric spectroscopy experimental results show that thermal aging may cause PI molecular chain fracture,thus affecting the surface morphology and leading to dielectric constant increase at high frequency condition.Breakdown test results show that thermal aging does not significantly impair the breakdown resistance.Thermal aging weakens the positive charge injection from upper electrode under the DC electric field,and the speed of space charge dissipation grows up with the aging time during the short circuit condition.Thermal aging has no obvious effect on the space charge distribution under the square wave electric field,and the space charge density of aged PI films is basically consistent with that of unaged one.The rising time in the range from 100 μs to1000 μs does not affect the space charge distribution significantly.The polarity of the space charge changes with the polarity of the lower electrode simultaneously throughout the whole test process.