Effect of Unequal Voltages and Change in Input to Prime Mover on Parallel Operation and Load Sharing of Alternators
Q. What is the effect of unequal voltages and change in input to prime mover on parallel operation and load sharing of alternators.
Effect of Inequality of EMF / Unequal Voltage :
In a perfect synchronization Isy is zero. E1-E2 (magnitude) and in exact phase opposition suppose due to any reason emf E1, increases (i.e. E1 > E2), then resultant emf ER appears in the local circuit of 2 alternators. This ER, is in phase with E1.This ER, sends Isy, in local circuit of alternator.
Isy lags ER, by 90°, also Isy, lags E1 by 90° and lead E2, by 90°. We know effect of p.f. on armature reaction.
Lapping p.f : Demagnetising
Leading pf : Strong magnetising
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| Figure A |
Current Isy, lags behind E1, i.e. for alternator 1 p.f. is lagging so there is demagnetising effect, not flux reduce. Therefore emf E1, reduces. At the same time, Isy lead E2, so for alternator 2 p.f. is leading producing strong magnetizing effect hence emf ↑ ( E2). The system is thus restore to a perfect synchronism.
Effect of Change in Input to Prime – movers :
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| Figure B |
Keeping excitation constant and if the input (steam supply) to the prime mover of 1st alternator is increased then its rotor advances in angular positions as compared with second alternator. Thus position of vector E1, differs to E2, by an angle α. There exists a resultant voltage. Er, as shown in Figure B (ii) which produces circulating current Ic which is nearly θ = 90° to Er. It is therefore obvious that power of 1st machine is increased by E1, x Ic per phase and power of the second machine is decreased by the same amount provided load is same. The current Isy, (i.e. Ic) has no appreciable reactive component and therefore there is no change in RKVA ∴ RKVA1 = RKVA2, Figure B (iii)
KW1 > KW2; RKVA1, = RKVA2
KVA1, > KVA2; is decreased and increased.