Analysis of Transient Characteristics in the 110kV Cable Joint System during Switch Closing

Abstract

In recent years, there occur quite a few breakdowns of cable joint when circuit breakers are closed in the 110kV cable lines. It's necessary to pay attention to the influence of cable joint in cable line without load during closing because of the different wave impedance between cable body and joint. In this paper, a RLC equivalent model of cable joint is established, and then an 110kV cable line including 14 joints is simulated by PSCAD. To study the effects of closing angle on overvoltage at cable joint core and metal shielding, the circuit breaker was closed 360 times in one power frequency cycle by using Multiple Run in PSCAD. Additionally, Fast Fourier Transform (FFT) is used to analyze the frequency characteristic of overvoltage at joint metal shielding. Then to study the effects of cable length on joint overvoltage, three different cable line lengths with the number of joint unchanged are simulated. Then electric field distribution inside the joint which suffered the most serious overvoltage is simulated by ANSYS. The simulation results show that cable line overvoltage is a 180° periodic function of the closing angle. The ideal closing angle and the closing angle matched the most serious overvoltage are both at the intersections of overvoltage curve of the phase B and the phase C. When breaker is closed at 0°, the maximum overvoltage of cable is 1.23p.u. While when breaker is closed at 90°, the maximum overvoltage of cable line is 1.85p.u. In addition, the shielding overvoltage of the 4 straight-through joints (3#, 6#, 9# and 12#) is more severe than that of other insulated joints. The maximum shielding overvoltage 5.23kV is at 6# joint of phase A with breaker closed at 90°. And FFT analysis of overvoltage at joint metal shielding shows that overvoltage at joint metal shielding contains high-frequency voltage component of 12 kHz to 14 kHz and 4 kHz with their amplitude below 70V. What's more, the line overvoltage increases with the increase of cable length, and the location of maximum overvoltage gradually moves from the end joint to the middle joint with the increase of the cable length when the cable line length is longer than 7500m. What's more, the electric field distribution inside two joints, the 6# joint whose metal shielding suffered the most severe overvoltage in all joints and the 12# joint whose core suffered the most severe overvoltage in all joints, are simulated by ANSYS. The results show that the maximum field strength in the joint 6# and 12# are more than that in joint without overvoltage, and the maximum field strength is always at the root of stress cone. The field strength at interface between silicone rubber and cable insulation in 12# joint is −1.4kV/mm, and the maximum field strength at the high-voltage shielding tube is 5.7kV/mm, both of which are doubled than those in joint without overvoltage.