Toru Tanimizu

Field test and EMTP simulation of transient voltages when clearing a transformer secondary fault

A field test was carried out to investigate a fault clearing transient due to current interruption of a three-phase to ground fault at the secondary side of a 3-MVA, 22/6.6-kV transformer by a vacuum circuit breaker. EMTP simulations in comparison with the measured results have made it clear that the capacitance Cp of a CR divider to measure voltages reduces dv/dt of transient recovery voltage across the circuit breaker by the ratio of (Ct/(Ct + Cp) where Ct is the transformer stray capacitance. The leakage inductance evaluated from the transformer rating has been found to be noticeably greater than that determined from the measured fault current possibly due to the transformer saturation. Considering the transformer magnetizing conductance and selecting an appropriate interrupted current, EMTP simulation gives a satisfactory result compared with a field test.