基于脉冲电流法的低频与工频电压下 油中电晕放电特性对比研究

对于电容量较大设备的局部放电试验,采用低频电压代替工频电压可有效降低对试验电源的容量需求。低频电压下试验结果的有效性需要考核,为探究电压频率对绝缘油中电晕放电特性的影响,基于脉冲电流法开展了工频50 Hz到低频5 Hz下油中尖板电极的放电试验。研究结果表明,油中电晕放电的一些基本特性在低频电压下未改变,即正半周放电量明显大于负半周,放电分布在电压峰值附近,呈尖峰状。差异性主要是低频下尖峰特点趋于模糊;随着频率的下降,起始电压稍有上升;正、负半周放电停止相位提前,最大放电量、平均放电量与脉冲重复率整体呈下降趋势;各频率下放电相位分布图谱(PRPD)与统计图谱(qmean-φ、qmax-φ、n-φ与n-q)整体形状一致,但分布范围不同;统计图谱的部分分布特征参数有明显变化。试验结果可作为低频电压下局部放电模式识别的参考。与传统只用50 Hz电压进行试验对比,放电统计量随频率的变化趋势也提供了有关绝缘缺陷的更多信息。     

晶型对等规聚丙烯电导电流和空间电荷特性的影响

聚丙烯材料性能改善过程中,材料内部会出现以α晶或β晶为主的2种晶型。为研究晶型对等规聚丙烯(iPP)电导电流和空间电荷特性的影响,选用α和β成核剂制备了具有不同晶型的iPP–pure(对照组)、iPP–α和iPP–β试样,开展了25℃下电导电流、空间电荷和直流击穿特性的研究。结果表明:iPP–β正负直流击穿强度最高,而iPP–α最低;3组iPP试样的电导电流大小为iPP–αiPP–pureiPP–β,空间电荷限制电流的转折电压大小为iPP–αiPP–βiPP–pure;-20 MV/m电场强度极化30 min后,3组iPP试样在极化过程中均出现正负空间电荷积聚;去极化过程中iPP–pure中空间电荷积聚量最大,而iPP–α中最少。分析认为:α晶和β晶的晶胞形态使得iPP–α中存在大量浅陷阱、iPP–β中平均陷阱深度较深;载流子在iPP–α中容易迁移并产生俄歇效应,而在iPP–β中容易在试样表面附近被捕获。     

交直流复合电压下油纸绝缘典型缺陷局部放电发展阶段评估

随着特高压直流输电工程的持续推进,直流局部放电试验技术对换流变压器绝缘状态评价的实用性问题一直备受关注。为形成有效的状态评价依据,提高换流变压器绝缘状态考核的准确性,本文研究了模拟典型放电类型的4种绝缘缺陷模型(油纸复合绝缘针板电极、变压器油隙针板电极、油隙平行板电极与沿面放电电极)在交直流复合电压下的局部放电特性。研究结果表明,各种缺陷模型在局部放电的不同发展阶段均有可作为判断依据的明显差异特征。本文从三方面给出了局部放电类型和放电发展前期、中期与后期的评估依据:(1)基于放电图谱Q-Δtpr e、Qmean-Δtpre、n-Q、n-Δtpre图谱特点的评估依据;(2)基于放电图谱偏斜度Sk、峭度Ku、不对称度Asy与相关度Cc等18项指纹特征参数的评估依据;(3)基于产气特性的评估依据。     

Characterization and comprehension of corona partial discharge in air under power frequency to very low frequency voltage

For the partial discharge test of electrical equipment with large capacitance, the use of lowfrequency voltage instead of power frequency voltage can effectively reduce the capacity requirements of test power supply. However, the validity of PD test under low frequency voltage needs to be evaluated. In order to investigate the influence of voltage frequency on corona discharge in the air, the discharge test of the tip-plate electrode under the frequency from 50 to 0.1 Hz is carried out based on the impulse current method. The results show that some of the main features of corona under low frequency do not change. The magnitude of discharge in a positive half cycle is obviously larger than that in a negative cycle. The magnitude of discharge and interval in positive cycle are random, while that in negative cycle are regular. With the decrease of frequency, the inception voltage increases. The variation trend of maximum and average magnitude and repetition rate of the discharge in positive and negative half cycle with the variation of voltage frequency and magnitude is demonstrated, with discussion and interpretation from the aspects of space charge transportation, effective discharge time and transition of discharge modes. There is an obvious difference in the phase resolved pattern of partial discharge and characteristic parameters of discharge patterns between power and low frequency. The experimental results can be the reference for mode identification of partial discharge under low frequency tests. The trend of the measured parameters with the variation of frequency provides more information about the insulation defect than traditional measurements under a single frequency (usually 50 Hz). Also it helps to understand the mechanism of corona discharge with an explanation of the characteristics under different frequencies.     

Electrical treeing behaviors in silicone rubber under an impulse voltage considering high temperature

In this paper,work was conducted to reveal electrical tree behaviors(initiation and propagation)of silicone rubber(SIR) under an impulse voltage with high temperature.Impulse frequencies ranging from 10 Hz to 1 k Hz were applied and the temperature was controlled between 30 °C and 90 °C.Experimental results show that tree initiation voltage decreases with increasing pulse frequency,and the descending amplitude is different in different frequency bands.As the pulse frequency increases,more frequent partial discharges occur in the channel,increasing the tree growth rate and the final shape intensity.As for temperature,the initiation voltage decreases and the tree shape becomes denser as the temperature gets higher.Based on differential scanning calorimetry results,we believe that partial segment relaxation of SIR at high temperature leads to a decrease in the initiation voltage.However,the tree growth rate decreases with increasing temperature.Carbonization deposition in the channel under high temperature was observed under microscope and proven by Raman analysis.Different tree growth models considering tree channel characteristics are proposed.It is believed that increasing the conductivity in the tree channel restrains the partial discharge,holding back the tree growth at high temperature.