It will be utilizing EMTP as analytic tool, performing overvoltage analysis,regarding 345kV voltage grade level of the Mai-Liao Power Plant; especially its EHV No. 1 Station, concerning switching surge and lightening surge. The overall purpose is to evaluate whether the insulation strength of the 345kV system, is strong enough to withstand overvoltage threats of lightening surge and switching surge. Based on suggestions of ANSI/IEEE Std. C62.22 and outcomes of analysis of this research, relevant assessment will be made in regards to whether or not, the circuit breaker and transformer installations of the GIS system, possess sufficient protection margin; further alternation suggestions will also be made concerning the simulation methodologies and respective models to be applied, for lightening surge and switching surge. Various models for lightening surge simulation include: transmission line model, tower model, lightening current source model and substation model. Various models for switching surge simulation include: transmission line model, time-controlled switching model, statistic switching model and transformer model.