# Parallel Operation of Two Synchronous Generators

Lets us assume there is a Parallel Operation of Two Synchronous Generators then they have an inherent tendency to remain in step if any changes in their armature currents by a divergence of phase. Let’s assume two identical machines in parallel and work on the same load with respect to the load, their e.m.fs are generally in phase with respect to the local circuit formed by the two armature windings, however, their e.m.fs are in phase-opposition to each other.

Assume two identical machines 1 and 2 are connected in parallel and there will no external load. If machine 1 accelerates, it’s e.m.f. will lead to that of machine 2. The resulting phase difference 2 \delta causes e.m.fs to lose phase-opposition in the circuit so that there is in effect a local e.m.f Es which will flow a current IS in the local circuit of the two armatures.

The current IS flows in the synchronous machine impedance Z_s of the two machines together so that it lags by \theta =arc\tan { (x/r) } \approx 90^0 on Es on account of the leading of reactance in ZSIS, therefore, flows out of the machine 1 nearly in phase with the e.m.f. and enters 2 machines is in opposition to the e.m.f. continuously, machine 1 produces a power PS=  E1IS as a generator and supplies it in form of I2R losses excepted to machine 2 as a synchronous motor. The synchronizing powerPS=  E1IS tends to retard the faster machine 1 and accelerate the slower, machine 2, pulling the two back into step.

Within the limits of maximum power, therefore, it is not possible to disturb the synchronous running of two synchronous generators in parallel, for a divergence of their angular positions results in the production of synchronizing power of both the machines, which increases the loads on one machine the forward machine and accelerates the backward machine to return the two to synchronous running.

When both synchronous machines have equal loaded to an external circuit, the synchronizing power is developed in the same as on no load, the effect is to decrease the load of the slower machine at the same time increase the load on the faster machine.

The conditions as shown in the above Fig. where I1, I2 is the equal load currents of the two machines before the appearance of phase displacement, and I1, I2 are that currents changed by the circulation of the synchronizing current IS.

As the above statement has been applied to identical machines. Actually, it is not necessary for them to be identical, not required to have equal excitations nor power supplies. In general, every machine will have different synchronous impedance ZS1, ZS2, and different e.m.f.’s E1 and E2and different speed regulations.

The governors of prime movers are mostly arranged so that a reduction of the speed of the prime mover is compulsory for the increase of the power developed.

The speed/load characteristics of governors are identical the machines can never share the total load according to their ratings. The governor characteristics take the form as shown in the above Fig. If the two machines are not identical, the load will be shared according to the relative load values at the running speed, for synchronous machines they must necessarily run at same speeds.