Failure of Electromagnetic Voltage Transformer Due to Sustained Over-Voltage on Switch off - Preventive Measures

The damage to a 220 KV electromagnetic voltage transformer (EMVT) due to sustained over-voltage on switch-off by a circuit breaker (CB) fitted with grading capacitors, and subsequent studies for the steady-state period have already been reported [1]. In this paper, the pre-steady state period is analyzed and excellent agreement shown between field and analytical results. The concerned parameters for various makes of CB's, EMVT's and bus-bar capacitance have been collected and a. number of studies conducted. It is concluded that the control of. the EMVT saturation characteristic is superior compared to the measures su, ggested earlier to prevent such over-voltages. A suitable saturation characteristic for the worst case has been identified.