Measurement of transient overvoltages from fast switching of an inductive load using open-air capacitive sensors

Energization and de-energization of electric power components cause transient overvoltages that potentially can damage installations. Particularly, fast switching of inductive loads is a concern. Two measurement expeditions were conducted related to shunt reactors connected to the tertiary winding of a 380/220kV transformer, switched either using an SF 6 or a vacuum circuit breaker. The transient waveforms were measured accurately by means of capacitive sensors picking up the stray electric field from the conductors. For a 50 kV shunt reactor, connected by means of overhead lines to an SF 6 switch, the largest overvoltage occurred during energization with a value just over 3 pu. The overvoltages were lower for switching a 33 kV shunt reactor connected to a vacuum switch by power cables. However, in most cases, escalating restrikes occurred during de-energization. Frequency components of about 500 kHz, excited upon switching, were transmitted through an earthing/auxiliary transformer causing voltage peaks exceeding 5 kV in the local low-voltage network. The paper shows that the methodology reaches a stage beyond “a proof of principle” and that its results can contribute to finding solutions for situations where actual failures have occurred.