Statistical Lightning Simulations for a HV "Mixed" Overhead-Cable Line: Preliminary Studies

The paper addresses the statistical simulation of the backflashover performance of an Italian 150 kV - 50 Hz mixed overhead-cable subtransmission line, focusing on direct lightning strokes to the overhead-cable transition tower. An ATP-EMTP-based Monte Carlo procedure, developed by the authors, has been applied to assess the backflashover rate (BFR) of the mixed line (ML). In addition, the procedure was applied to the statistical evaluation of some ML components' stresses, such as the energy duty of metal oxide surge arresters (MOSAs) installed at the transition tower, the energy duty of sheath voltage limiters (SVLs) connected to the cross-bonded cable sheaths and the cable sheath voltages near the cable entrance. The backflashover response of the overhead portion of the ML differs from an all overhead line only at the transition tower, so that the overall performance practically coincides with that of an all-overhead line. A non-negligible probability of damage to SVLs installed at the first minor cross-bonding section is found, whereas a much lesser energy stress is associated to MOSAs. Lastly, maximum cable sheath over-voltages are always well below 50 kV.