绝缘纸特性和极性反转对油纸绝缘空间电荷效应的影响

Influence of Insulation Paper Characteristics and Polarity Reversal on Space Charge Effect of Oil-paper Insulation

油纸绝缘系统的空间电荷效应将畸变绝缘介质内部的电场,进一步影响其老化和击穿等电气特性。在直流电场、复合电场及极性反转等复杂工况下,换流变压器内绝缘系统的空间电荷效应将更为突出。基于双极性电荷输运模型,通过数值模拟方法探究绝缘材料厚度、老化状态和极性反转等关键因素对空间电荷效应的影响。研究表明,绝缘纸厚度的变化不改变空间电荷密度曲线中正/负电荷区的极性偏移特性,但厚度变小缩短或消除了零电荷区;绝缘纸老化程度的加深,其电荷陷阱及深陷阱数目均会增多,这一方面加强了空间电荷效应的极性偏移,另一方面使得正电荷区呈变小趋势,从而使绝缘纸内电场畸变加剧;极性反转工况下,绝缘纸中陷阱电荷的脱陷或复合均需一定的时间滞后,使得净空间电荷密度曲线的负/正电荷区呈现初值不对称和沿介质厚度方向分布均不对称的特征,致使绝缘纸内部电场畸变愈发严重。研究结果可以为换流变压器油纸绝缘设计与防护提供参考。

The space charge effect of oil-paper insulation distorts the electric field inside the dielectric, and it further affects the aging and breakdown of the dielectric. Under the complex working conditions of electric field, compound electric field and polarity reversal, the space charge effect of the oil-paper insulation system within converter transformers will be more prominent. Based on the bipolar charge transport model, the effects of insulation thickness, aging state and polarity reversal on space charge effect were investigated through numerical simulation method. The results show that the change of the thickness of insulation paper/board doesn’t change the polarity shift of the positive/negative zone of the space charge density curve, but the decrease of the thickness shortens or eliminates the zero charge area. The number of charge traps and deep traps will increase as the aging degree of insulation paper deepens, not only the polarity shift of space charge effect is strengthened, but also the positive charge area becomes smaller, thus the internal electric field distortion of insulation paper is aggravated. Under the condition of polarity reversal, the lag time of trap charge for detrapping or recombination in the insulation paper is needed, which makes the negative/positive charge area of the net space charge density curve present the characteristics of asymmetric initial value and asymmetric distribution along the direction of the thickness of the dielectric, resulting in more serious internal electric field distortion of the insulation paper. The conclusions can provide a reference for the design and protection of oil-paper insulation within converter transformers.