The purpose of this study is to simulate the VFT overvoltages within the 727138-8DQ1 550 kVgas-insulated substation caused by energizing different GIS sections by means ofdisconnectors. The simulation model comprises:▪ 550 kV gas-insulated substation 8DQ1▪ Two 550 kV power transformers connected by GIS “transformer connection modules”▪ Two bays comprising gas-to-air bushings