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A transmission line is a vital backbone for electrical power delivery. Most of the electrical transmission in Java and Bali, Indonesia is dominated by a 150kV transmission line network. Java and Bali are considered volcanic islands, so the number of lightning strokes to the transmission line is more than two million every year. It was recorded that negative lightning stroke was dominated 93.03% of the lightning faults and positive lightning stroke took 6.97% of faults. A fault caused by direct lightning stroke to tower transmission may cause an electrical shutdown, so it is interesting to make an estimation of voltage breakdown at insulation string installed on the tower as lightning hits the ground wire on the top of the tower. This paper proposes the use of a tower model adapting the insulator string model attached to the surge impedance of tower transmission. The implementation of several different lightning waveforms to hits the tower model during simulation is aimed to calculate the maximum amplitude of voltage along the insulator string. If the voltage peak is above the critical flashover voltage of the insulator, then back flashover occurs and initiates the ground fault. The results and analysis of breakdown voltage obtained from this work give significant information to improve the performance of insulation string at electrical tower transmission. [ABSTRACT FROM AUTHOR]