【作者】 刘玉；

【导师】 蓝磊；

【作者基本信息】 武汉大学 ， 高电压与绝缘技术， 2017， 硕士

【摘要】 高压单芯交联聚乙烯电缆由于其性能稳定,安全可靠,被电力系统广泛采用。单芯电缆在正常运行、雷电和操作暂态以及系统故障状态下,其金属护层均会产生感应电压,特别是在系统发生单相接地等故障时,护层感应电压过高可能导致护层外绝缘的损坏,从而造成系统多点接地事故。单芯电缆护层的外绝缘损坏将对电力系统的安全稳定运行构成危害,因此,研究电缆金属护层感应电压的产生及其抑制措施具有重要意义。本文针对GIS合空变时,电缆与GIS接头附近电缆支架处放电现象展开研究。首先,根据放电现象的观测结果确定了电缆放电的位置为电缆与GIS接头附近的支架与电缆外皮之间空气间隙;然后,通过理论分析得到了电缆护层电压产生原因为合空变产生的励磁涌流和高频过电压;最后,针对500kV单芯电缆在变压器空载合闸时的护层电压进行了仿真计算,并提出抑制放电的措施。通过上述研究工作,本文取得的结论如下:(1)在变压器空载合闸暂态计算中,饱和变压器模型计算结果和实际结果偏差较大,BCTRAN模型计算结果接近实际值,但励磁涌流波形有差异;Hybrid模型更好的体现了磁通在铁心柱、铁轭以及气隙的流通路径,能更为准确的仿真变压器空载合闸暂态过程的励磁涌流和过电压。(2)针对500kV抽水蓄能电站主变空载合闸过程进行了计算,合闸角为0°时,励磁涌流最大,达到4380A,为额定电流的5倍,涌流频率大多集中在1000Hz以内,同时含有幅值较小的高频分量。合闸角为90°时,过电压最严重,电缆芯线过电压是557kV,过电压幅值较小,衰减快,电压频率大多集中2kHz内,同时具有由波的折反射产生的幅值较低的50～100kHz高频分量。(3)GIS合空变过电压频率高导致电缆芯线流过高频电容电流,高频电容电流和励磁涌流感应得到的电缆金属护层电压其幅值较大,合闸角从0°～90°变化,A相电缆护层电压为12.70kV～20.50kV,代表了其在各种合闸角下电缆护层可能出现的电压大小。(4)GIS合空变暂态过程产生励磁涌流和高频过电压,若合闸回路中连有电缆,电缆芯线的励磁涌流和过电压产生的高频电容电流通过电磁感应耦合到电缆护层,导致护层采用单端接地方式时不直接接地端感应出高电压,而电缆外皮与电缆接地支架间存在2.5mm空气间隙,电缆护层外绝缘材料破损后绝缘外皮电压接近于护层电压,电缆外皮与接地支架间电位差大于空气间隙耐压,气隙击穿放电。(5)在电缆支架处放电抑制措施中,断路器装设合闸电阻能在合闸角较小时将护层电压降低到气隙击穿电压以下,但合闸角接近90°时气隙仍会击穿;电缆外皮并联电容对护层电压抑制效果显著,当电容达到1μF后,能将护层电压限制在气隙击穿电压以下;更换两种残压较低的电缆护层保护器,效果不理想,但采用更换保护器与600Ω合闸电阻相配合的方式能将护层电压限制在击穿电压以下。比较上述三种抑制措施,推荐电站采用电缆外皮并联电容的方式,该方案实施可行性高且抑制效果显著。更多还原

【Abstract】 High-voltage single-core XLPE cable is widely used in power systems,due to its stable performance,safe and reliable working.Single-core cable working in the normal operation,lightning and operating transient and system failure state,its metal sheath will produce the induced voltage,especially when single-phase ground fault occurs in the power system.The induced voltage of cable sheath may damage the outer insulation,resulting in multi-point grounding accident,which does a great harm to the cable.Therefore,it is very important to study the induced voltage of metal sheath and its suppression measures.The paper focus on the phenomenon of discharge at cable and GIS joint stand when closing no-load transformer.Firstly,according to the observation result of the discharge phenomenon,the discharge position is 2.5mm air gap between cable sheath layer and cable stand.Then,cable sheath induced voltage are analyzed,which is caused by inrush current and high frequency closing overvoltage when closing no-load transformer.Finally,the induced voltage of 500kV single-core cable is accurately calculated when closing no-load transformer,and proper measures are proposed to avoid the discharge phenomenon.After the above research work,the conclusions of this paper are as follows:(1)In the calculation of closing no-load transformer,three transformer models are compared.The deviation between calculation results and test results is large by saturation transformer model,calculation results are close to tests value by BCTRAN model,but because there is only one silicon steel sheet material to choose from,calculated hysteresis loop and transformer actual hysteresis loop are different,Hybrid model takes the core structures and winding arrangement types into account,it reflects the flow path of magnetic flux in iron column,iron yoke and air gap betterly,so it is more accurate in transformer no-load closing transient process simulation.(2)When closing angle changes from 0°～90°,the maximum inrush current of cable core is 4380A,which is 5 times of rated current,current frequency is almost within 1000Hz,but also contains a high-frequency component with a smaller amplitude.The overvoltage amplitude is 557kV,which is 1.37 times of the rated voltage,most of the voltage frequency are concentrated within 2kHz,but at the same time,there are 50～100kHz frequency components due to the reflection of the voltage wave between GIS devices.(3)The high frequency component of overvoltage causes high-frequency capacitor current flow through the cable core when GIS closing no-load transformer,the cable metal sheath is induced out of high voltage by this high-frequency capacitive current and inrush current.When closing angle changes from 0°～90°,cable sheath voltage is between 12.70kV and 20.50kV,its frequency is up to 600kHz.(4)Magnetizing inrush current and overvoltage with high frequency component are produced when GIS closing no-load transformer,if there is a cable in the closing circuit,magnetizing inrush current and capacitor current with high-frequency component caused by overvoltage result in high voltage in cable sheath,there is a 2.5mm air gap between cable sheath layer and cable bracket,cable outer insulation voltage is close to sheath voltage after the insulation material is damaged,the voltage between cable sheath insulation and bracket is greater than the air gap withstand voltage,air gap is breakdown.(5)In order to avoid the occurrence of discharge,three kinds of discharge suppression measures were studied,including adding closing resistance in circuit breaker,adding parallel capacitor in cable sheath insulation and replacing cable sheath protector,Compared with the above three kinds of suppression measures,it is recommended that the power station adopt adding the parallel capacitance in the cable sheath insulation,the scheme is feasible and has remarkable inhibition effect.更多还原