The effect of change in fuel supply when generators are in parallel

 

Since the speeds of the two machines are tied together by their synchronous bond, the increase in fuel to the machine 1 does not make it run faster. 

Alternatively, it utilises the increased power input for carrying more load than machine 2.

This is possible because of angular advance of machine 1 with respect to machine 2. 

Here E1 advances E2 by an angle α. The resultant voltage Er sets up a current Isy which is almost in phase with E1. 

Hence, power per phase of machine 1 is increased by an amount =E1 Isy whereas that of machine 2 is decreased by the same amount. 

Since Isy has no reactive component, the increase in fuel supply does not disturb the division of KVAR but it increases KW of machine 1 and decreases that of machine 2.

The effect of change in excitation / different generator voltages when in parallel

Suppose the initial operating conditions of the two identical alternators in parallel with KW and KVAR load being shared equally, thus operating in same power factor. Each machine supplies a load current I so that total output current is 2I. 

If excitation of alternator No.1 increases, then E1 increases than E2. This difference sets up a circulating current Ic = E1 - E2 / 2Zs which is confined to the local path through the stator and the busbars. 

This current is superimposed on the original current. The Ic is added as a vector to the machine No.1 current I and subtracted from machine No.2 current I. 

Now the two machines deliver I1 and I2 current at respective power factors cosø1 and cosø2. The cosø1 reduces and cosø2 increases. 

So, the effect of KW loading is negligible but KVAR1 & current supplied by machine No.1 increases and KVAR2 & current supplied by machine No.2 decreases.