This paper presents the effect of lightning waveform to the voltage at the pole top and back flashover rate (BFOR). The result found that short front time of lightning current waveform or high slope will cause high voltage and back flashover rate at the top pole. However the tail time of lightning current waveform will not effect to the top pole voltage and backflash over rate because the impulse resistance will only affect to the long front time of lightning current waveform. In case of short front time, impulse resistance will not affect to voltage at the pole top and backflash over rate. Moreover BFOR will vary with the event of thunder storms per year. ATP-EMTP is used to simulate cases in this paper to receive pole top voltage, critical current and backflash over rate. The 69 kV subtransmission line system of Metropolitan Electricity Authority (MEA) is demonstrated as the case study to improve its grounding standard of subtransmission and distribution line system in MEA's power system

This paper presents an external ground design in distribution system for improved lightning performance. Lightning protection is a set of features typically designed into grounding systems at locations subject to lightning damage. Basically, back flashover occurs when the amplitude of the lightning overvoltage experienced by insulator strings in response to direct strikes to a tower or to the shield wires at tower vicinities exceeds the insulation withstand of the line. The objective function is to reduce the effects of lightning overvoltage from back flashover is an attachment of external ground wire connected between the overhead ground wires and a ground rod. This paper analyzes the effects of lightning performance of 69 kV circuit in Metropolitan Electricity Authority (MEA)'s distribution system of Thailand. The proposed ATP-EMTP model for simulation and analysis of lightning in terms of pole top voltage, critical current and back flashover rate. The study results show that external grounding system design can reduce the effects of lightning and improve distribution system reliability.