This study describes analysis of tower grounding resistance effected the back flashover voltage across insulator string in a transmission system. This paper studies the 500 kV transmission lines from Mae Moh, Lampang to Nong Chok, Bangkok, Thailand, which is double circuit in the same steel tower with two overhead ground wires. The factor of this study includes magnitude of lightning stroke, and front time of lightning stroke. Steel tower uses multistory tower model. The assumption of studies based on the return stroke current ranged 1%200 kA, front time of lightning stroke between 1 (s to 3 (s. The simulations study the effect of varying tower grounding resistance that affect the lightning current. Simulation results are analyzed lightning over voltage that causes back flashover at insulator strings. This study helps to know causes of problems of back flashover the transmission line system, and also be as a guideline solving the problem for 500 kV transmission line systems, as well. Tower grounding resistance, back flashover, multistory tower model, lightning stroke current. I. INTRODUCTION HAILAND is tropical country which having a lot of thunderstorm days and lightning discharge activities in one year. The back bone of power system transmission is overhead transmission system. So, it is essential to investigate a lightning surge for a reliable operation of a power system, because the lightning surge over voltage is one of dominant factors for the insulation design of the power system and the protection of equipments in power transmissions and substations. When lightning strikes the top of a transmission tower, a lightning current flows down to the bottom of the tower and causes a tower voltage rise which results in a back% flashover across an insulator string. At present, Electric Generation Authority of Thailand (EGAT) has expanded to total about 2,600 circuits%km of 500 kV, as shown in Fig. 1[1]. Switching over voltage in EGAT system has been conducted by some researchers [1%5]. However, the lightning over voltage estimation of overall B. Marungsri is with Suranaree University of Technology, Nakhon Ratchasima, 30000, Thailand (phone: +66 4422 4366; fax: +66 4422 4601; e% mail: bmshvee@ sut.ac.th). S. Boonpoke, A. Rawangpai and A. Oonsivilai are with Suranaree University of Technology, Nakhon Ratchasima, 30000, Thailand. C. Kritayakornupong is with Electricity Generating Authority of Thailand (EGAT). system which may occur and reach the lightning withstand voltage has not yet been conducted. EGAT requires analyzing the voltage level of lightning over voltage of existing 500 kV transmission lines. Mae Moh Tha Tako Nong Chok Pluak Deang Ratchaburi Chom Bung Sai Noi Wang Noi Fig. 1 Existing EGAT 500 kV Transmission System EGAT have been reported the statistic of lightning occurs in northern region during March%August 2005[6]. The report concluded that the lightning often occurs during April to May. But the most severe lightning takes place in June. The positive lightning is about 5% with magnitude between 11 kA and 171 kA. But the negative lightning is about 95% with magnitude between %10 kA to % 139 kA. The most lightning magnitude is between %10 kA and %50 kA. When lightning stroke strikes the top of a tower, a traveling voltage is generated which travels back and forth along the tower, being reflected at the tower grounding and at the tower top, thus raising the voltage at the cross%arms and stressing the insulator strings. Flashover will occurs across an insulator string if this lightning transient voltage exceeds its lightning withstand voltage level. Such flashover called back flashover Back flashover voltages are generated by multiple reflections along the struck tower and also along the shield wire for shield lines at the adjacent towers. The lightning induced over voltage across an insulator string for the struck tower is not a Study of Tower Grounding Resistance Effected Back Flashover to 500 kV Transmission Line in Thailand by using ATP/EMTP B. Marungsri, S. Boonpoke, A. Rawangpai, A. Oonsivilai, and C. Kritayakornupong T International Journal of Electrical Power and Energy Systems Engineering 2:2 2009 97