Research On Partial Discharge Characteristics And Damage Mechanisms Of High-Frequency Transformer Insulation Under Coupled Electrical-Thermal Stresses

As a novel type of highly controllable and intelligent substation equipment, high-frequency transformer has broad prospects in the field of future power transmission, especially in terms of flexible DC applications. However, due to the harsh and complex working conditions of high-frequency transformer, the polyimide(PI) insulation is prone to discharge, overheating and other defects under long-term coupled electrical-thermal stress, which will result in the insulation failure and bring about a serious threat to its reliability. But so far, it still remains unclear regarding the discharge physics and microscopic deterioration mechanisms caused by strong electrical-thermal stress. Consequently, it is of great significance to study the partial discharge(PD) characteristics and microcosmic pyrolysis mechanisms of PI insulation according to the actual operating conditions of high-frequency transformer.A comprehensive thermal-electrical test rig is established to simulate the frequency-dependent electrical and thermal stress. The leakage current is measured by the broadband pulsed-current sensor. And PD data acquisition system, based on Lab View platform, is in control of the oscilloscope to transfer the measured data to the computer for storage simultaneously. Further, signal processing techniques, including wavelet threshold-filtering method and phase-based fenestration method, are adopted to extract the partial discharge signals from the leakage current. Moreover, the average discharge times, the average discharge amplitude and the spectrum of PD density are introduced to demonstrate the characteristics of partial discharge.Based on the established platform mentioned above, experimental studies were carried out concerning the partial discharge characteristics under different frequencies and temperatures. Partial discharge mainly occurred at the fast rising/falling edge of the positive/negative waveform, whereas only a small amount of the discharge occurred in the peak of the waveform; in the 10kHz-40kHz frequency range and at constant temperature, as the frequency increased, the average number of discharges, average discharge amplitude and maximum amplitude were all decreased; in the 30℃-160℃ temperature range and at constant frequency, the first two parameters mentioned above presented an increasing trend with the rise of temperature, and the impact of temperature on PD characteristics at high-frequency was greater than that at low-frequency; the frequency influenced PD parameters mainly by the way of changing the residual voltage in the air gap, but the temperature affected PD characteristics via altering the electron emission and dissipation behaviors of space charge.A ReaxFF-based molecular dynamics simulation was further employed to study the microscopic impact mechanism of thermal stresses caused by high frequency on the pyrolysis process. The results indicated that initial breaking bond was the C-N bond on the imide ring, and the cleavage of C-N bond connecting two benzene rings was the main reason for PI main chain scission; CO2 and CN were the dominant products of PI pyrolysis, and other small molecular products, including H2O, CO and H2, were detected in the ultimate products as well; the cleavage of C-N bond on the imide ring was the common initial step for forming CO2 and CN; the structure (-C(=O)-O-C-) was conducive for generating CO2, whereas the consecutive dissociation of C-N on the imide ring and the de-carbonization reaction from the benzene were integral for forming CN. With the above discussions, the pyrolysis and failure mechanisms of PI under frequency-dependent thermal effects were revealed in details at the atomic level.