Appropriate Grounding Distance of Overhead Ground Wire for MEA's 24 kV Distribution Line Lightning Protection

摘要

This paper proposes the method to improve the performance of lightning protection of Metropolitan Electricity Authority (MEA) 24 kV distribution lines considering the grounding distance of Overhead Ground wire (OHGW). The Alternative Transient Program-Electromagnetic Transient Program (ATP-EMTP) is employed as the main tool for system modeling and simulation. The study shows that when the average lightning strikes to OHGW in case of no Grounding of OHGW bring about the voltage across pin-post insulator more than 1500kV, the voltage values exceed the critical flashover (CFO) of 180 kV of the insulator; resulting in power outage later. This case has the backflashover rate of 2,722 times/year, or equivalent to the financial loss of about 6 million USD/year. The previous claim is based upon the 350/10 microsecond lightning current waveform, the average lightning current of 40 kA in Thailand, and ground resistance of 10 ohms (average in MEA). However, after OHGW have been grounded for every pole, every other poles, every other two poles, every other three poles and every other four poles or every 40, 80, 120, 160, 200 meters respectively (distance between pole 40 meters in MEA). The results show that the voltage across pin-post insulator reduce from 1500 kV (No grounded of OHGW) to 110, 130, 165, 218, 221 kV respectively. Therefore, Appropriate Grounding distance of OHGW is not more than 120 meters because the voltage across pin-post insulator is lower than the critical flashover (CFO) of 180 kV and also found that the grounding distance of 120 meters of OHGW can decrease the backflashover rate from 2,722 times/year (No Grounding of OHGW) to 1,215 times/year or 55% reduction, or equivalent to the reduced financial loss of about 3.37 million USD/year. This reduction compared with the cost of equipment installation equals a payback period of approx. 3.69 years.