For the following single line diagram of a three-phase system, a three-phase fault occurs at point P and just before the fault the line voltage is 18 kV at P. Find out the transient fault current in pu and original value at point P. Take, the given kVA and kV of the G1 as the new base. [hint: the transient and sub-transient reactances of the synchronous generators are assumed to be same. But for the motors, assume that the transient one is 1.7 times of the sub-transient reactance.]

A 600 KVA generator with X"a = 0.22 per unit is connected to a bus through a circuit breaker, as shown in the following figure. Connected through circuit breakers to the same bus are three synchronous motors rated 200 KVA, 2.5 KV, 1.0 power factor, with X"a = 0.15 per unit. The motors are operating at full load, unity power factor and rated voltage, with the load equally divided among the machines. (a) Find the rated symmetrical short-circuit current in Amperes, which must be interrupting breakers A and B for a fault at point P. Simplify the calculations by neglecting the Prefault current. M. GHA B

60 – MVA,15 – kV Problem 4 (Questions 21 through 23)-Two identical synchronous machines, each rated with a subtransient reactance of 0.1 py, are connected through a line of reactance 0.1 pu on the base of the machines rating. One machine is acting as a synchronous generator, while the other is working as a motor drawing 45-MW at 0.80 PF leading with a terminal voltage 14.5-kV. A symmetrical three-phase fault occurs at the motor bus. Calculate the following: 21. The generator fault current contribution (Ru.) is 22. The motor fault current contribution (p.u.) is 23. The total fault current is