Case Study on the Failure of 400kv XLPE Cables Termination of Tala Hydropower Plant through Electromagnetic Transient Analysis Using Mipower Software

This paper presents an investigation into the failure of 400kV XLPE (Crossed-Link Polyethylene)cable terminations at the Tala Hydropower Plant (THP) using Electromagnetic Transient (EMT)analysis in MiPower software. Four XLPE cables connect the underground Gas InsulatedSwitchgears (GIS) to the outdoor switchyard, from where twin moose transmission lines link to theIndian grid via Siliguri Substation. There have been frequent issues with cable terminations.Noteworthy incidents include a flashover in the outdoor cable terminations of Y phase feeder 1 onJune 4, 2021 and another flashover in the indoor cable termination of B phase feeder 2 on March 17,2022. Despite these recurring failures, the root causes remain inconclusive. EMT analysis was employed to examine the Temporary Over-Voltages (TOV), during various operations. The THP-Siliguri network was modelled in MiPower, and two case studies were analyzed, considering the impact of lightning arrestors (LAs). In the first case, a three-phase to earth fault was simulated at adistance of 0.5km from Tala on transmission line 1. In the second case, a simulation was conductedto replicate conditions during the energization of feeder 2 when circuit breakers (CBs) were closedsequentially at Siliguri and THP ends. Both cases indicated that TOVs occurred during fault butwere effectively suppressed by LAs. The study confirms that the LAs at THP are functioning reliably,as simulations show TOVs suppression at cable terminations. With LAs in place, TOV levels remainwithin safe operational limits, validating their effectiveness. With TOV eliminated, the studysuggests other degradations such as thermal aging, improper termination practices, or moistureingress as potential contributors to the failure. As the first known EMT based study of such failuresat THP, the findings offer broader insights for similar HV cable termination of hydropower plantsin Bhutan with comparable configurations.