Effect of Frequency on Discharge Characteristics of Inter-layer Materials in HV-HF Transformers

Abstract

High-voltage high-frequency (HV-HF) transformers are usually operated at a non-sinusoidal voltage with a frequency of 1∼20 kHz, and its working frequency has a significant impact on discharge behaviors of inter-layer insulation materials. In order to investigate the influence of voltage frequency on discharge characteristics, an experimental platform with a high frequency power source is set up. Two kinds of inter-layer insulation materials, all of which have excellent thermostability, including the NHN insulation paper which can be resisted high temperature for class $\boldsymbol{H}$ and the modified DMD paper which can be resisted high temperature for class $\boldsymbol{F}$ , are used as test samples. Discharge characteristics tests, including breakdown and surface discharge, are conducted on thermostable inter-layer insulation materials by adjusting applied voltage from 0 kV to 20 kV, and frequency from 1 kHz to 20 kHz, under bipolar high frequency square wave excitation power supply. In terms of high frequency breakdown characteristics, the two-parameter Weibull statistic method is used to calculate the probability of insulation failure, and the relationship between breakdown voltage and frequency is analyzed. The surface discharge characteristics under high frequency is another important research content. The flashover voltage-frequency ( $\boldsymbol{V}-\boldsymbol{f}$ ) discharge characteristic is analyzed based on the Secondary Electron Emission Avalanche (SEEA) model. The experiment results show that, the scale parameter $\boldsymbol{\alpha}$ which is also called the characteristic value of breakdown voltage based on the Weibull distribution decreases with the increase of voltage frequency. Thermal effect is the main reason for the drop of breakdown voltages. In addition, frequency has almost no effect on the surface discharge inception while the flashover voltage is significantly reduced with the increasing frequency. Through the surface insulation tests on DMD paper, it is found that the high-frequency effect has greatly reduced the insulation lifetime.