The ac generator voltage is controlled by the field current and the speed, but the frequency is controlled only by the speed.
Since the generator power ultimately comes from the prime mover, the prime mover speed and the generators frequency both AC Generator droop at the same rate with increasing load on the generator.
Two ship ac generators sharing load
With two ac generators sharing load in parallel, their terminal voltages and frequencies both must be equal, and the frequency droop lines of two ac generators determine their share of the total load.
The frequencies of Generator 1 and Generator 2 at their own load
powers P1 and P2 can be written in terms of their respective frequency droop rates as follows:
where f = operating frequency, fno-load = no-load frequency, and FDR = frequency droop rate.
By parallel connection, we impose on the bus that f1 = f2 = fbus, that is,
Solving Equations and simultaneously for P1 and P2 (with all other
parameters known), we get the load shared by Generators 1 and 2.
Load sharing on two ship generators
It is noteworthy that the stiffer generator with low FDR (flatter line) shares the heavier load, and the weaker generator with high FDR (more drooping) shares the lighter load. This is analogous to load sharing between two mechanical springs in parallel. The stiffer spring, which droops less with load, shares greater load. Also, the machine with higher no-load speed (droop line shifted upward) would share greater load.
When we change the governor setting, we essentially shift the fuelinput
rate, and hence the speed line and the frequency line up and down, whereas the governor droop rate remains the same.
Controlling load share on ship generators
In practice, load sharing is controlled manually or automatically by adjusting the prime mover’s governor setting, which controls the input valve of the fuel (steam or diesel). The governor’s automatic control system
varies the fuel input rate and the speed directly proportional to the load.
When the load increases, the fuel is increased, and vice versa.
Slow down of ship generator
With two generators working in parallel, if one generator momentarily slows down for any reason, it delivers less power and subsequently speeds up. The other generator takes the greater load and slows down.
Such adjustment takes place until both generators run at the exact same speed to generate the exact same frequency and the exact same terminal voltage.
We can say that parallel generators have a great team spirit in helping each other run at the same speed as determined collectively by the total load demand on the team.
Sharing and adjusting reactive power kVAR on ship generators
In parallel operation, it is important that each machine shares the load (both in kW and in kVAR) within its own rated limit. The kW load can be balanced by adjusting the governor speed regulation, and the kVAR load can be balanced by the field excitation of each machine by adjusting the field rheostat in the voltage regulator.
After such adjustments in both machines, the armature currents should
be about the same percentage of their individual rated values and at about equal power factors.