Abstract: This paper first introduces the principle of series resonant withstand voltage circuits and presents the process for calculating test parameters. It details the series resonant withstand voltage test process for 220kV GIS and GIL at a power plant, analyzing and addressing the low Q-value issue in the test circuit. The study further examines the discharge flashover phenomenon observed in phase B during the test, accurately pinpointing the location of the flashover within the gas compartment. Subsequent investigations into the installation process, inspection records, and X-ray/visual magnification of the defect location collectively determine that the flashover discharge was caused by residual foreign objects during on-site installation, leading to local field strength distortion and subsequent flashover. Repeated withstand voltage tests resulted in localized overheating, carbonization, and cracking of the insulator surface. Finally, the paper proposes precautions and recommendations for GIS and GIL equipment installation as well as series resonant withstand voltage testing.