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

Abstract

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 Insulated
Switchgears (GIS) to the outdoor switchyard, from where twin moose transmission lines link to the
Indian 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 on
June 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 a
distance of 0.5km from Tala on transmission line 1. In the second case, a simulation was conducted
to replicate conditions during the energization of feeder 2 when circuit breakers (CBs) were closed
sequentially at Siliguri and THP ends. Both cases indicated that TOVs occurred during fault but
were 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 remain
within safe operational limits, validating their effectiveness. With TOV eliminated, the study
suggests other degradations such as thermal aging, improper termination practices, or moisture
ingress as potential contributors to the failure. As the first known EMT based study of such failures
at THP, the findings offer broader insights for similar HV cable termination of hydropower plants
in Bhutan with comparable configurations.