Failure of Electromagnetic Voltage Transformer Due to Sustained Over-Voltage on Switching - an Indepth Field Investigation and Analytical Study

An electromagnetic voltage transformer (EMVT) was damaged when the 220KV bus bar was being de-energised by a circuit breaker fitted with grading capacitors across its contacts. This paper describes the field investigations and analyses the phenomenon using the principle of harmonic balance. It is established that the failure occurred due to fero-resonance between CB grading capacitors and non-linear EMVT inductance causing sustained over-voltage and consequently much higher than usual current. It could be a thermal or insulation failure or both. The analytical results obtained are shown to be similar to those determined by actual field. tests.