Short Circuit of Synchronous and Induction Machines
A three-phase short circuit on the terminals of a generator has twofold effects. One, large disruptive forces are brought into play in the machine itself and the machine should be designed to withstand these forces. Two, short circuits should be removed quickly to limit fault damage and improve stability of the interconnected systems. According to National Electrical Manufacturer's Association specifications, a synchronous machine shall be capable of withstanding, without injury, a 30 s, three-phase short circuit at its terminals when operating at rated kVA and power factor, at 5% overvoltage, with fixed excitation. Synchronous generators are major sources of short-circuit currents in power systems. A rotating machine can be studied in the steady state depending upon the specific-type of the machine, for example, synchronous, induction, or dc machine. The negative sequence reactance is the reactance encountered by a voltage of reverse-phase sequence applied to the stator, with the machine running.