Surges on Chicago 12-Kv System

The highest transient overvoltages on the Chicago 12-kv system occur at the inception of single line-to-ground faults. They may be thought of as resulting from the superposition of a ``cancellation wave´´ on the normal steady-state voltage of the system. The transient is not a high- frequency phenomenon and is not generated by an ``intermittent-arcing´´ mechanism. The maximum crest values reached are in the neighborhood of 3.5E and persist for only the first cycle or two of the fault. For the remainder of the fault, including the clearance switching, the maximum overvoltage is ¿3E. These overvoltages may cause secondary (simultaneous) failures in any part of the connected system where the insulation is unable to withstand them for the short times they persist.