直流电压下纤维小桥对油纸复合绝缘局部放电特性的影响

Effect of Cellulose Bridge on Characteristics of Partial Discharge in Oil-paper Insulation Under DC Voltage

为研究直流电压下油纸绝缘纤维小桥的产生原因及其对局部放电（简称局放）过程的影响,搭建了直流油纸绝缘实验平台,采用水平及垂直针板模型,对包括小桥形貌、局放起始电压、局放重复率、局放放电量在内的多个因素进行了检测分析。结果表明：在直流电场作用下,纸板表面纤维分子链断裂产生游离态纤维杂质,进而形成小桥。由于受力情况不同,小桥呈现堆状、非贯通状及贯通状三种形态。受纤维小桥影响,油纸绝缘针板模型局放起始电压降低,当油隙距离处于12.5～15mm之间时降低程度最大,当小桥呈堆状时降低现象最明显;局放脉冲次数增加,放电在时间上出现了＂伸缩＂效应;局放单次放电量减少,且密集放电阶段末期常出现较高强度的放电脉冲,随后在稀疏放电阶段脉冲强度逐渐减小。分析表明,上述现象是因纤维小桥畸变了油隙内电场分布和电场恢复过程而形成的。

In order to investigate the mechanism of cellulose bridges＇ generation in oil-paper insulation and their effects on characteristics of partial discharge under DC voltage, a high voltage direct current（HVDC） experimental platform was set up. Based on the horizontal and vertical needle-plane model, characteristics including bridge shape, partial discharge inception voltage（PDIV）, partial discharge repetition rate, and partial discharge apparent charges were all recorded and analyzed. The obtained results show that, under the influence of electric field, cellulose chains at the surface of pressboards will be broken and detach into oil to form bridges. According to stress conditions, bridges present in 3 forms, namely, pile-like, nonthrough-like, and through-like. Due to the effect of cellulose bridges, PDIV of oil-paper insulation decreases compared with no-bridge condition. The degree of this inclination reaches the maximum at an oil distance between 12.5 mm and 15 mm, and among three kinds of bridges, pile-like bridge introduces the most severe influence. PD repetition rate of oil-paper insulation increases, and partial discharge process exhibits ＂expansion phenomena＂ on time, partial discharge（PD） charges of oil-paper insulation decrease. Strong PD happens at the end of intensive PD period, after which PD amplitude drops among the sparse PD period. Further analysis indicate that cellulose bridges distort the distribution and recovering process of electric field in oil, which consequently cause the above-mentioned phenomenon.