Risk assessment of 170 kV GIS connected to combined cable/OHL network

This paper concerns different investigations of lightning simulation of a combined 170 kV overhead line/cable connected GIS. This is interesting due to the increasing amount of underground cables and GIS in the Danish transmission system. This creates a different system with respect to lightning performance, compared to a system consisting solely of AIS connected through overhead lines. The main purpose is to investigate whether overvoltage protection is necessary at the GIS busbar. The analysis is conducted by implementing a simulation model in PSCAD/EMTDC. Simulations are conducted for both SF and BFO. Overvoltages are evaluated for varying front times of the lightning surge, different soil resistivities at the surge arrester grounding in the overhead line/cable transition point and a varying length of the connection cable between the transformer and the GIS busbar with a SA implemented. Simulations are conducted for different positions of the GIS bus section breaker. The lighting current required to cause inadmissible voltages at the GIS connected transformer is evaluated and the MTBF is found for different cases. With the GIS bus CB in closed position results indicate that SF does not cause inadmissible voltages to appear at the transformer. However, BFO caused by a lightning stroke of extremely high magnitude can cause inadmissible voltages to appear at the transformer. With the GIS bus CB in open position results indicate that both SF and BFO can cause inadmissible voltages to appear at the transformer. A risk assessment based on the simulation results and statistical data for lightning, yields the conclusion that implementation of a surge arrester at the GIS busbar is not necessary.