With reference to load sharing of a.c. generators: (a) state the meaning of the term speed droop;(3) (b) explain the effect of running generators in parallel with different speed droop settings for EACH of the following: (i) when the generators are sharing the load equally;(3) (ii) when the load suddenly increases.(4)
(a) Meaning of the term speed droop (3 marks)
Speed droop is the intentional reduction in generator speed as load increases, controlled by the governor.
It is expressed as a percentage decrease in speed (or frequency) between no load and full load.
It allows stable load sharing between generators in parallel, because each generator’s governor adjusts its fuel input in proportion to load changes.
(b)(i) Generators sharing the load equally with different droop settings (3 marks)
When droop settings differ but load is steady and equally shared:
The generators may still run stably but at different governor positions.
The machine with the lower droop setting will tend to take slightly more load if any imbalance occurs.
This can cause unequal frequency-power response, making the system less stable in response to changes.
(b)(ii) When the load suddenly increases (4 marks)
If total load demand increases suddenly:
Both governors respond by increasing fuel, but the generator with lower droop setting will pick up more of the additional load.
The generator with the higher droop setting will take less proportion of the increased demand.
This results in unequal load sharing of kW, depending on the droop settings.
If the difference in droop is large, it may cause overloading of one generator or instability in frequency regulation.
