Analysis of 115kv Circuit Switcher Damage During A Normal Switching Operation

Correct and rapid fault clearance and power outage minimization are high priorities for electric utility companies. The protection schemes should work properly to isolate faults quickly and minimize the impact on the transmission system. This paper examines the behavior of the protection schemes during the catastrophic failure of a circuit switcher protecting a tapped distribution transformer. The failure happened during a paralleling and load transfer switching operation between two parallel transmission lines. The failure was not detected by the line protection schemes but the tapped transformer was isolated by zero sequence overvoltage relaying (3V0). The events captured by analog signal triggers in the Digital Fault Recorders at the transmission line terminals and limited sequence of event and fault records at the tapped station provided valuable information to characterize the event. The extreme distortion of the voltage signals created an inaccurate estimate of the voltage waveform in the vendor provided analysis software and caused the loss of frequency tracking by the 3V0 relay's algorithm. After reading this paper, the reader will have a new perspective on how to pinpoint the root cause of an equipment damage from available and expected, but nonexistent, fault records.