Impact of Lightning Current to 115 kV Subtransmission Underground Cable in MEA's Power System

摘要

The majority of Metropolitan Electricity Authority (MEA) distribution networks are overhead lines. And they will be gradually converted into underground cables to improve the system reliability and beautify the cityscape. However, although the cables are buried underground but in some area especially in the suburban where the high rise buildings or any other tall constructions are not in the vicinity, there could be the possibility the lightning strike directly on the ground. The collected statistics cite that, one of the main causes that often results in power interruption is the direct lightning strike on the ground where there are the cables buried nearby, which will cause high lightning induced voltage on underground cables. This study aims to evaluate the impact of lightning current on the high voltage underground cables which laid in the vicinity of striking location. The conditions for evaluation comprise the number of thunderstorm days, grounding resistance, length of cables, and types of cable bonding. The Alternative Transients Program-Electromagnetic Transients Program (ATP-EMTP) is used as a tool to investigate the failure rate resulted from those conditions. Based on the basic impulse level (BIL) of the cable, the minimum value of lightning current causing insulation damage is quantified and the probability of stroke peak current exceeding minimum value is then evaluated. On the other hand, the number of thunderstorm days in Bangkok Metropolis will provide the ground flash density. The failure rate of cable caused by lightning stroke can be then obtained by manipulating the probability of stroke peak current exceeding minimum value against the ground flash density. Case studies are made on 115 kV subtransmission cable that directly buried underground. Three types of cable bonding which include case 1: single point bonding for short length cable, case 2: double point (both ends) bonding for medium length cable, and case 3: cross-bonding for long length cable are investigated. The simulation results indicate that, under the same conditions, the lowest risk of insulation failure due to the direct hit lighting onto the ground occur with case 2: the double points bonding. Furthermore, the results also suggest that the failure rate directly relates to the increases of the number of thunderstorm days, grounding resistance, length of cables. The outcome of the study could serve as a guideline for designing the grounding system of high voltage underground cable in MEA, which previously focusing mainly on the safety value of induced voltage on metallic sheath and the current carrying capacity on the main conductor, by taking into accounts the impact lightning induced voltage. And perhaps modification of existing grounding standards MEA's power distribution system if the statistics from implementation confirms the results of this study.