Posted by: SATYASRINIVAS | January 18, 2015

Effect of Change in Mechanical input of Alternator on Load Explanation with Phasor Diagram

Phasor Diagram:

The two alternators are running in parallel with their excitations constant. The armature currents I1 and I2 are also equal so that total load current is 2I1 or 2I2. The terminal voltage V is constant. Each alternator is sharing a load equal to P

Now when mechanical torque of alternator 1 is increased, its output will also increase. But E1 (Induced E.M.F), V and Xs are constant. So to increase power angle must be increased from δ to δ1 so new Ewill be ahead of previous position. The alternator 1 shares greater load than P. Therefore for constant load of 2P the load on alternator 2 must be less than P. This will make new E2 to fall back from its previous position. Due to the different positions of Eand E2, resulting voltage AB appears in the local circuit which will send a circulating current ISY lagging behind the voltage by 90o. This current ISY must be added to I1 and subtracted from I2.
The alternator 1 carries increased current I’1    (Note the Angle between V and I’ is Φ1  . This is why there is change in Power factor) and alternator 2 carries decreased current I’but total load current remains same  (Ī = Ī’1 + Ī’2). The power factor of alternator 1 is improved from cosΦ to cosΦ1 whereas it is reduced from cosΦ to cosΦfor alternator 2. But the load power factor remains unaffected.
Thus increase in mechanical torque in case of alternator will increase armature current and improve the power factor. The alternator will share increased load whose driving torque is increased whereas the other alternator which is in parallel is relieved from the load whereas the reactive power distribution remains unaffected.
     To consider the effect of change in input on corresponding power triangles of the two alternators we will assume that the two alternators are turbo alternators whose prime mover are supplied with steam.
       Now the excitations for the two alternators are kept constant where steam supply i.e. power input to prime mover of alternator 1 is increased. The two alternators are running in synchronism. So machine 1 cannot overrun machine 2. The increased power input for alternator 1 makes it possible for carrying more load. This will make rotor fort machine 1 advancing its angular position by an angle δ.
       The resultant e.m.f. Er is produced in the local circuit which will setup a circulating current ISY which lags Eby 90and almost in phase with E1. The power per phase fort alternator 1 is increased by an amount E1ISY whereas it is decreased by same amount for alternator 2. This current ISY has no appreciable reactive component and it will not disturb the reactive power distribution but active power output of alternator 1 will increase and that of 2 will decrease.

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