Effect of shielded distribution cables on lightning-induced overvoltages in a distribution system

Lightning strikes have current risetimes ranging from 0.1 to several mus and can produce transient overvoltages substantially greater than the BIL of many distribution systems in spite of lightning arresters, the value of which is often compromised by excessive lead lengths. We compute the lightning induced overvoltages over distribution transformers connected through cables to an overhead distribution system as a function of ZnO arrester lead length, lightning current rising time, the type of cable, and the length of cable employed. We also compute the voltage across the top 10% of turns of distribution transformers resulting from the high dV/dt of the voltage incident on the transformers. Two types of cable, TR-XLPE and EPR, were considered. The attenuations of the cables were measured as a function of frequency and these propagation characteristics were modeled in the ATP-EMTP program. The computed data indicate that the arrester lead length, lightning rising time, the type of cable, and the length of cable employed have substantial impact on the overvoltage on transformer and the voltage across the top few turns of the transformer winding. The greater high frequency losses of EPR cable can reduce substantially the lightning-induced overvoltages to which distribution transformers are exposed.