A 138 kV/13.8 kV, 20 MVA, three-phase transformer has a reactance of 10% and a resistance of 0.4%. Calculate a reasonably close approximation the peak fault current in the low voltage winding under the worst conditions, if a three-phase fault occurs at the low voltage terminals. For the purpose of the calculation the impedance of the source can be considered negligible.
If the fault current is interrupted by a circuit breaker on the high voltage side of the transformer, and if the capacitance per phase of this winding is 5200 pF, determine the frequency of the transient recovery voltage seen by the breaker.
Refer again to Problem 3.8. Let us now assume that the impedance of the 138 kV source is not negligible and that it has the same X/R ration as the transformer itself. Let us further state that a symmetrical fault on the 138 kV bus would develop a fault current of 18 kA rms.
Determine the frequency and relative magnitude of the transient recovery voltage of the circuit breaker when it interrupts the fault described in Problem 3.2.
Assume that the stray capacitance on the source side of the breaker is 12,000 pF.