The a case study in Vietnam Pham Quang Huy creator Surapong Chiraratananon Chairperson Yu, Cun Yi Examination Committee Dhadbanjan, Thukaram Examination Committee Sida Scholarship donor text series technical report th Bangkok Asian Institute of Technology 1999 continuing eng 86 leaves The major problem of insulation level of GIS substation is the overvoltages due to lightning, especially with high value of lightning current. This study concerned with electromagnetic transients to evaluate the failure risk of insulation of equipment. It is more importance because this study is applied for 500kV GIS substation, the new equipment with many advantages of small space requirement, low maintenance cost, low environmental impact, etc. In this study, modeling of the power system and lightning source has been done in the transient state. By using EMTP program, the overvoltages due to lightning have been calculated in various cases of changing lightning current, point of lightning impact, phase angle of power frequency voltage, i.e .. The overvoltage results inside the 500kV GIS are collected for evaluating the risk of failure. A method for estimating the risk of failure of an apparatus as a result of lightning strikes on the transmission lines has been developed to meet the requirements of quick and low-variability calculation. The results shown that with tower footing resistance is 10 ohms· or less, the risk due to lightning can be accepted. Otherwise, the solution to reduce the risk of failure for 500kV GIS substation could be carried out by installing surge anester, reducing tower surge impedance, re-arrange the spans, etc. The calculation can be used efficiently as a calculated process of substation design. A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering. School of Environment Resources and Development Thesis (M.Eng.) - Asian Institute of Technology, 1999 Electric substations Vietnam Lightning protection Vietnam Asian Institute of Technology. http://203.159.5.9/ait-thesis/detail.php?q=B12781 http://203.159.5.9/ait-thesis/detail.php?q=B12781 220200 20260817162400.0