Question

With reference to the operation of governors fitted to diesel alternators that generally operate in parallel:

Explain why an isochronous characteristic is undesirable;

State what is meant by droop and give an analogy of droop in control engineering terms;

Sketch a load/frequency diagram showing how two generators share the electrical load.

In a generator with isochronous characteristics the frequency stays constant as KW load changes but the machines cannot run in parallel

Hence an isochronous characteristic is undesirable

With an isochronous characteristic the load swing repeatedly from one machine to another because the characteristic have no definite cross over point.

Droop.

If 2 generator are to share load their governor characteristic must have a definite crossing point. That is why governor droop is necessary.

Governor and AVR characteristic must have a small amount of droop to ensure stable parallel operation. A change in speed from full load to no load is called as droop.

After the incoming generator has been synchronised it is now ready to take up load. It will be recalled that the generator will have to provide two types of ac power, kW and kVAr load sharing.

After synchronising, GEN1 is still supplying all the load kW while GEN2 supplies zero kW. The governors of both machines are producing 60 Hz. This situation can be depicted graphically below.

The kW of GEN2 are measured right to left. It can be seen above that both machines are producing 60Hx, GEN1 is supplying 100% of the load kW and GEN2 is supplying 0 kW.

GEN2 can be made to supply Kw by adjusting its speed trimmer to increase the set point of the governor as shown above. This has the effect of ‘lifting’ the whole characteristic which results in GEN2 taking load and GEN1 losing load. A problem is that the system frequency increases.

Now the speed trimmer of GEN1 is adjusted to reduce the set point of the governor see above. This lowers the characteristics of GEN1 allowing GEN2 to take up more of the load and brings the frequency back to 60Hz. This load balancing is monitored on the kW meters of each machine.

If two generators are to share load their governor characteristics must have a definite ‘crossing point’ that is why governor ‘droop’ is necessary.

If the characteristics are flat (isochronous) as shown above then system accuracy is good because frequency stays constant as kW load changes, but machines cannot be run in parallel. With flat characteristics the load swings repeatedly from one machine to the other because the characteristics have no definite crossing point.

The amount of governor droop is a compromise between accuracy and stability. If the governor droop is large then the system is stable but the frequency will change slightly as kW load changes as shown below.