Synchronous generators

When a short-circuit occurs downstream of a synchronous generator, the generator may continue to produce output voltage and current if the field excitation is maintained and the prime mover continues turning the generator at synchronous speed. The flow of short-circuit current from the generator into the fault is limited only by the generator impedance and downstream circuit impedance's . The magnitude of generator fault current depends on the armature and field characteristics, the time duration of the fault, and the load on the generator. The ability of a generator to supply current during a fault is a function of the excitation system.

(1) Some generator excitation systems do not have the ability to sustain short-circuit current. The magnitude of fault current is determined by the generator reluctance, and, for such systems, can be essentially zero in 1.0 to 1.5 seconds.

(2) Static exciters derive excitation voltage from the generator terminals. Since static exciters do not sustain short-circuit current, protective devices on the system will not operate properly, or at all. Static exciters, therefore, are not recommended. Static exciters with current boost should be specified for applications requiring static excitation.

(3) Round-rotor generators with brush less exciters, typically above 10 MVA, can sustain short-circuit

 current for several seconds. Salient-pole generators less than 10 MVA, also with brush less exciters

, will typically sustain short-circuit current at 300 percent of generator full load amperes.

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