Review of the basic insulation level for 400 kV oil-filled cable systems under lightning events

The lightning impulse duty for 400 kV underground power cables using ATP is investigated and the recommended design level based on a probabilistic analysis is reassessed. A comprehensive study of the transient overvoltages experienced by cables within the UK National Grid´s transmission system has been performed. The paper investigates the effects of transient overvoltages due to lightning. To determine the peak voltages that a cable would experience, direct lightning strikes and backflashovers onto the overhead-line phase conductor were simulated both near a cable termination and at air-insulated substation (AIS) and gas-insulated substation (GIS) zones. A second paper will present the investigation into transient overvoltages due to switching, faults and other temporary overvoltages (TOV). A probabilistic approach to argue for a reduction of the basic insulation level (BIL) for 400 kV cables to a value of 1050 kV_p is used.