Lightning simulation of a combined overhead line/cable connected GIS

The 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 thorugh overhead lines. The main purpose is to investigate whether overvoltage protection is necessary at the GIS busbar. Here, the price for a GIS SA is significantly more expensive than the price for an AIS SA. The analysis is conducted by implementing a simulation model in PSCAD/EMTDC.
Simulations are conducted for both SF and BFO where the overvoltage at the transformer are evaluated as this component has the lowest insulation strength. The overvoltages are evaluated for different front imes of the lightning surge, different soil resistivities at the surge arrester grounding in the overhead line/cable transition and different length of the connection cable between the transformer and the GIS busbar with a SA implemented. Those simulations are conducted for different positions of the circuit breaker present at the GIS busbar. The lightning current required to cause inadmissible voltages at the transformer is evaluated and the MTBF is found for different cases. With the circuit breaker at the GIS busbar in closed position results indicate that SF does not cause inadmissible voltages to appear at the trasformer. However, BFO caused by a lightning stroke of extremely high magnitude can cause inadmissible voltages to appear at the transformer. With the circuit breaker at the GIS busbar in open position results indicate that both SF and BFO can cause inadmissible voltages to appear at the tranformer. A risk assessment based on the simulation results and statistical for lightning, yields the conclusion that implementation of a surge arrester at the GIS busbar is not necessary.