主变空载合闸引起的电缆护层过电压分析

变压器空载合闸过程中由于电磁振荡产生过电压,若系统中存在高压电缆,电缆内部过电压耦合到电缆护层造成护层上电压较高,护层过电压与电缆布置方式和护层接地方式有关。文中针对白莲河抽水蓄能电站主变空载合闸时,电缆端部有异常放电声,并可见放电火花的现象,对主变空载合闸时系统产生的过电压进行测量,并对放电现象进行观测;同时采用ATP/EMTP软件计算空载合闸过电压耦合到电缆护层上的过电压大小。测量结果表明:空载合闸时主变侧过电压值并不大,进而计算得到放电处护层过电压最大值为21.49 kV,频率在600 kHz以内,大于过电压保护器动作电压,高频电流注入地网造成电缆支架处地电位升击穿其和护层绝缘间空气间隙。

Analysis of Cable Sheath Overvoltage Due to Main Transformer No-load Closing

In the process of transformer no-load closing,overvoltage appeared due to electromagnetic oscillation,if there are high voltage cables in the system,internal overvoltage can couple to the cable sheath which results in sheath’s high voltage,and this voltage is related to cable arrangement and sheath grounding method.This article aim at the abnormal discharge phenomena in cable ends of Bailianhe Pumped Storage Station when transformer closes in no-load situation,transformer no-load closing overvoltage is measured,discharge phenomena is observed,and ATP/EMTP software is adopted to calculate cable sheath overvoltage coupled by no-load closing overvoltage.Measurement results show that the no-load closing overvoltage of main transformer is not high,then calculation results show that cable sheath overvoltage in the discharge place is 21.49 kV,with the maximum frequency 600 kHz,which is greater than the arrester operation voltage,this high frequency current flowing into the ground results in ground potential rising at cable bracket,which cause the air gap between the cable sheath insulation and cable bracket breaking down.