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During the transient operation of a synchronous generator, a lot of variations in the operation of the machine appear, which are unavoidable and can cause significant oscillations of the load angle ș and the electromagnetic torque. These oscillations can lead the generator to pole slipping. A fault, which is possible to occur, is a short-circuit at the supply of the excitation system of the synchronous generator. In this paper we investigate the performance of the machine during a shortcircuit at the excitation, while the synchronous machine is connected to the grid. Particularly, the waveforms of all the currents, stator, field and damper, the electromagnetic torque and the field in the air-gap are derived and studied.

Abstract. The maintenance of synchronous generators is very significant for the energy production. Being aware of the performance of a synchronous machine during different kinds of faults, such as phase to phase, phase to ground or field shortcircuits, could provide useful information about their protection. A short-circuit in the field excitation is not a common fault, but it is severe enough to create high oscillations to both electrical and mechanical magnitudes (field, stator and damper currents, rotor shaft torque, load angle etc). Such kind of faults is difficult to examine and provide experimental results because of the kinds of generator used for energy production. The size of these machines, either hydrogenerator or turbogenerator, is enormous for experimental laboratory studies. Another important issue that arises is the difficulties in the accurate calculation of the damper currents during transient operation. The Finite Element Method analysis gives solution to this kind of problems, as it can calculate as well the damper, stator and field currents as the electromagnetic field distribution inside the air gap.

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