Abstract: The mechanism of this phenomenon was studied with a double tension porcelain insulator strings in a 500 kV converter station. Experiments including resistance testing and cross-sectional staining were conducted, and a simulation model of double insulator strings was established to obtain the distribution pattern of the space electric field. The result shows that water vapor intrusion into the cement adhesive gap resulted in the resistance decrease of several insulators and the formation of 24 zero-value insulators (20 on the west side and 4 on the east side). The 122kV voltage difference between the strings was the main cause of the inter string discharge.