温度对方波电压下环氧树脂局部放电及击穿特性的影响

Effect of Temperature on Partial Discharge and Breakdown Characteristics of Epoxy Resin Under Square Wave Voltage

电力电子变压器用中频变压器环氧固封绝缘长期承受方波电压应力和变压器温升带来的热应力,是制约电力电子变压器可靠性的关键。为分析温度对方波电压下环氧固封绝缘强度的影响,对室温至110℃温度条件下环氧树脂薄片在1 kHz、50%占空比单极性方波下的局部放电和绝缘击穿特性进行了实验研究,对局部放电起始场强和击穿场强进行了Weibull分布拟合。实验结果表明,80℃以下,击穿场强随温度升高略微下降,局放起始场强略微升高,110℃温度条件下,击穿电压明显下降,局放起始场强降低至5.44 kV/mm。借助聚合物自由体积击穿理论,解释了环氧树脂绝缘强度在玻璃化温度出现转折的原因。对局部放电事件的统计表明,局放在方波上升沿和下降沿对称分布,且小幅值放电频率高、相位分布广,大幅值放电在相位上相对集中,局放最大幅值随温度升高而先降低后增加。通过建立局部放电随机仿真模型,得到了不同温度下与实验相似的局部放电幅值-相位分布规律,发现温度通过改变脱陷电子产率影响局部放电延迟,继而决定局部放电最大幅值和相位分散性。

The epoxy resin(EP)encapsulant insulation for medium-frequency transformer in power electronic trans-formers(PET)is subjected to the long-term square wave voltage stress and thermal stress caused by temperature rise,which is a key restriction to the reliability of PETs.To analyze the effect of temperature on the electrical strength of EP under square wave voltage,the partial discharge(PD)and breakdown(BD)characteristics of EP specimens under a 1 kHz 50%duty cycle unipolar square wave from room temperature to 110℃are studied.The Weibull distribution is applied to fit to the measured PD inception field strength(PDIE)and breakdown field strength(BDE).Experimental results show that,below 80℃,BDE decreases slightly and PDIE increases slightly with increasing temperature;however,at 110℃,BDE significantly decreases and PDIE decreases to 5.44 kV/mm.Combined with free volume breakdown theory of pol-ymers,the variance of BDE at glass transition temperature is explained.Statistical analysis of PD events shows that PDs are symmetrically distributed on the rising and falling edges of the square wave.Small signal PDs are frequently and dis-persively located in phase domain,while large signal PDs are relatively concentrated in phase domain.The maximum PD magnitude decrease first and then increase with increasing temperature.PD amplitude-phase distribution patterns similar to experimental results are obtained through a PD stochastic simulation model.It is found that the temperature affects PD delay by changing the detrappable electron emition rate,further determines the PD magnitude and PD dispersity in phase domain.