| 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. |
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