DC Excitation System

DC Excitation System

The DC excitation system has two exciters – the main exciter and a pilot exciter. The exciter output is adjusted by an automatic voltage regulator (AVR) for controlling the output terminal voltage of the alternator. The current transformer input to the AVR ensures limiting of the alternator current during a fault.

When the field breaker is open, the field discharge resistor is connected across the field winding so as to dissipate the stored energy in the field winding which is highly inductive.

The main and the pilot exciters can be driven either by the main shaft or separately driven by the motor. Direct driven exciters are usually preferred as these preserve the unit system of operation, and the excitation is not excited by external disturbances.

The voltage rating of the main exciter is about 400 V, and its capacity is about 0.5% of the capacity of the alternator. Troubles in the exciters of turbo alternator are quite frequent because of their high speed and as such separate motor driven exciters are provided as standby exciter.