Question

With reference to voltage variation profiles caused by load changes imposed on alternating current generators:

(a) Sketch a voltage dip, showing an acceptable recovery time;

(b) Outline the salient factors that cause the variation in Q.

(c) Outline the salient factors of recovery from the variation in Q.

Factors causing variation.

Sudden load current surges (e.g. due to motor starting) on a generator cause a corresponding change in its output voltage. This is due to an internal voltage drop in the generator windings and the effect is usually called voltage dip.

The size of the dip depends upon

Size of load. Power factor of load [lag is bad]

Generator internal reactance. [KVAr]

Factor of recovery.

Recovery is by AVR.

An AVR will control the generator's voltage to ±2.5% (or better) of its set value over the full load range. This is its steady-state voltage regulation. Transient voltage dip is usually limited to 15% for a specified sudden load change with recovery back to rated voltage within 1.5 seconds. In special cases where unusually large surges are expected (e.g. from heavy-duty cargo cranes) the generator/AVR performance limits may be extended.

Recovery depends upon.

The alternator. The excitation system. The AVR. The prime mover.