Question

A balanced, symmetrical, round-rotor, 3-phase, 60 Hz, 2-pole, 100MVA, 13.8kV, synchronous generator is being driven at synchronous speed. During tests, an IFNL (field current required to produced rated terminal voltage under open circuit) of 150A and an IFSC (field current required to produced rated armature current under short circuit) of 100A were obtained. Use a linear model and neglect the armature resistance for the following calculations. a) What field current is needed to maintain rated terminal voltage when the generator is supplying 100MW power at unity power factor? b) What power factor would be obtained if the field current were maintained at the IFNL value when the generator supplies 100MW at rated voltage? c) What field current would be required to obtain a lagging power factor of 0.85 while maintaining the same power and terminal voltage?

Answer 1

(a) the field current required to maintain unity power factor at the load is equal to the field current under the no-load test required to produce the rated voltage.

(b) using linear model the rated armature current will be :

if the field current is maintained at no-load value giving the rated voltage, so the new armature current

then the power factor will be leading case it will happen

(3)under lagging case :

under the lagging case, the field current increases the armature current decreases are vice-versa.

then

.