交直流复合电压下油中局部放电的起始过程

Partial Discharge Initiating Process of Transformer Oil Under Combined AC and DC Voltage

换流变阀侧绕组同时承受交流、直流、交直流叠加、极性反转等多种特殊电压形式,其许用场强却未得到同等关注。因此,采用针-板、板-板典型电极模型和脉冲电流法及宽频带电流传感器检测法,研究了变压器油在交流、直流、交直流复合电压条件下的局部放电起始过程;总结了该起始电压随电压类型变化的规律曲线,为换流变许用场强的提出奠定了基础。试验结果表明:针-板电极模型下局放起始电压基本只与交流、直流、复合电压的幅值有关,而与电压类型、复合电压分量所占比例无明显关系;板-板电极模型中,复合电压下交流局部放电起始电压受直流分量的影响很小。通过高速摄像仪记录复合电压下局部放电引发击穿的全过程并估算出了放电增长速率约为1.8km/s;利用扫描电镜观测放电通道表面形貌特征,进而采用表面能谱分析法检测其主要元素含量由高至低依次为C、O、Cu、Si。最后通过复合电压下油中局部放电产生随机分布气泡的仿真模型算出,气泡使局部放电场强严重畸变,不均匀系数f达1.6。

As a critical parameter for the main insulation design of power transformer, available electric field strength is the important index of the technology and management level. The valve winding of converter transformer withstands several types of voltage, including AC, DC, combined AC-DC voltages. However, the analogous problem for converter transformer has not received enought attention. Therefore, based on broadband sensors and pulse electric methods, the initiating process of partial discharge of transformer oil under combined AC-DC voltage was studied by using needle-plate and plane-plane electrodes. Partial discharge inception voltages ~PDIV） of two electrodes under the different voltages were summarized. The results show that PDIV is not related to voltage types under needle-plate electrodes but only to voltage amplitude. The results also show that AC PDIV has little relation with DC pre-stressed voltage. The process of the dielectric breakdown caused by partial discharge was recorded with an initial velocity of about 1.8 km/s. The surface topography of plasma channel was observed by scanning electron microscope （SEM）. And the amounts of chemical elements C, O, Cu and Si are in sequence from high to low by energy dispersive spectrum （EDS）. At last, an ANSYS structural model of single bubble in oil was established. The simulation results show that electric fields are more easily distorted by bubbles, and non-uniformity coefficient f is about 1.6.