Simulation study on lightning impulse voltage of metal sheath under high voltage power frequency

作者： GOZ Electric 时间：2024-07-31 09:19:51 阅读：13

    With the development of urban power transmission and distribution lines, high-voltage single-core cables have been widely used due to their excellent performance. However, unlike three-core cables, when high-voltage single-core cables are invaded by overvoltage waves, their metal sheaths will generate very high impulse voltages due to electromagnetic induction. Once the outer sheath is broken down and multiple points are grounded, a large circulating current loss will be formed during operation, and its current carrying capacity and service life will be greatly reduced. At present, the metal sheath impulse voltage that occurs during the operation of high-voltage single-core cables is mainly limited by reasonably setting the grounding method and sheath voltage protector. In order to find factors and methods to limit the metal sheath impulse voltage, domestic and foreign scholars have conducted relevant research on the metal sheath impulse voltage.

    During the cable switching process, there have been many studies on the metal sheath impulse voltage of high-voltage single-core cables caused by operational overvoltage. Related literature studies have shown that the operation impulse voltage of the metal sheath of the cable intermediate joint caused by the opening and closing of the circuit breaker has a high-frequency component of more than 40kHz, which will be detrimental to the cable insulation. Related literature shows that the impulse voltages at both ends of the cable joint sheath are in opposite phases. Higher impulse voltages at both ends of the sheath may break down the insulation on both sides of the single joint sheath and cause high-temperature damage. Relevant literature research suggests that the cable cross-connection length and the distance from the circuit breaker are the main factors affecting the metal sheath operating impulse voltage.

    At present, the following studies have been conducted on the lightning impulse voltage of the metal sheath of high-voltage single-core cables generated by external overvoltage. Research in relevant literature shows that the lightning impulse voltage of the metal sheath of the ungrounded point of the 220kV high-voltage single-core cable far exceeds the lightning impulse withstand voltage of the outer sheath of 47.5 kV, and needs to be limited by a voltage protector. After simulation and testing, relevant literature proposed that the cable line where lightning waves can directly invade needs to be reasonably equipped with a voltage protector on the metal sheath. Relevant literature proposes that the intrusion wave generated by the lightning shielding overhead line or the lightning strike near the cable grounding electrode will cause the metal sheath lightning impulse voltage. The simulation of relevant literature shows that the lightning shielding line and the lightning tower will increase the voltage of the cable metal sheath to the ground. Relevant literature analyzes the factors affecting the amplitude of the lightning impulse voltage of the metal sheath of the high-voltage single-core cable and concludes that the amplitude is greatly affected by the cable length and load.

    However, relevant literature suggests that power frequency voltage will affect the transient overvoltage characteristics of the cable core of a single-core cable. Related literature studies have shown that the transient characteristics of the cable core voltage will change with the lightning waveform. Because the metal sheath impulse voltage is derived from the electromagnetic induction of the cable core current, the transient characteristics of the cable core voltage will cause the transient characteristics of the metal sheath impulse voltage to change. In practice, the power frequency phase of the lightning intrusion cable is random, and the lightning current waveform parameters are also uncertain.

    Therefore, in the above-mentioned study on the lightning impulse voltage of the metal sheath of the single-core cable, only the single effect of the lightning impulse voltage is considered, and the power frequency operation of the cable and the influence of the change of the lightning current parameters on the lightning impulse voltage of the metal sheath have not been considered. Therefore, when studying the lightning impulse voltage of the metal sheath, it is more comprehensive and in line with the actual situation to consider the conditions of power frequency operation and the changes in the lightning current waveform parameters.

    In this study, a 220kV overhead line-cable line model was built in PSCAD to simulate the metal sheath lightning impulse voltage caused by lightning shielding overhead line conductors invading the cable. Under power frequency operation conditions, by changing cable parameters such as cable length, laying method, cross-interconnection segment length, and lightning current parameters such as lightning current waveform and intrusion phase, the amplitude change of metal sheath lightning impulse voltage was studied, which can provide a reference for the external overvoltage protection and protector configuration of 220kV cables.

    In this study, PSCAD was used to simulate the metal sheath lightning impulse voltage amplitude of high-voltage single-core cables in power frequency operation, and its influencing factors were studied. The following conclusions were drawn:

1) Compared with the end grounding method and the cross-interconnection grounding method, the first-end grounding method can significantly reduce the metal sheath lightning impulse voltage at the ungrounded end. The first-end grounding method can reduce the external overvoltage protection cost. When the cross-interconnection grounding method is used, the metal sheath lightning impulse voltage amplitude maximum phase will change from the intrusion phase to the non-intrusion phase at the second interconnection point due to the cross-interconnection of the metal sheath at the intersection.

2) Changing cable parameters such as reducing cable length, improving the segment uniformity of cross-connected grounding cables, and using a herringbone or compact arrangement does not significantly reduce the lightning impulse voltage of the metal sheath.

3) The amplitude of the lightning impulse voltage of the metal sheath is positively correlated with the steepness of the lightning current and the duration of the lightning. The amplitude of the lightning impulse voltage of the metal sheath is reduced by about 80% when the same lightning current invades the power frequency voltage trough than when it invades the peak. If this characteristic can be used, it may provide new ideas for the protection of cable external overvoltage.