Electrical Power Systems

Symmetrical ault 225
8.7 ig. 8.31 shows a sample power system network and a three phase fault occurs at point . Calculate
the fault MVA at . The per unit values of reactance all refer to a base of 50 MVA.
ig. 8.31
[Ans: 1.4 × 714 MVA
= 1000 MVA]
8.8 ind the reactance per phase external to a generator with 0.06 pu internal reactance voltage such
that the steady current on short-circuit shall not exceed 8 times the full load current. The generator
rating is three phase, 10 MVA, 0.8 kf and 11 KV [Ans: 0.786 W]
8.9 The main bus-bars in a generating station are divided into three sections, each section being
connected to a tic-bar lay a similar reactor. One 20 MVA, three phase, 50 Hz, 11KV generator,
having a short circuit reactance of 0.15 pu, is connected to each section bus-bar. When a short
circuit takes place between the phases of one of the section bus-bars, the voltage on the remaining
sections falls to 60 percent of the normal value. Calculate the reactance of each reactor.
[Ans: 0.454 W each]
8.10 ig. 8.32 shows a sample power system. If the circuit breaker used is of 2000 MVA, calculate the
capacity of the reactor in case of the fault occurring at point , so that the circuit breaker could be
used quite safely.
ig. 8.32
[Ans: 4.67 W]