What is Droop on an AC alternator on a generator?

Droop is a method of controlling the reactive power of an alternator as the load increases. This is used in synchronizing applications, where multiple generators are in parallel. With all units set up to droop by the same percentage, the reactive load will be shared equally between them, because if one take more reactive load, the droop will work to counter this effect.

How does alternator droop work?

In a system in parallel, either between generators or the mains, the voltage of the system is locked together and all units will be at the same voltage. The reactive power of the system is shared between the units by varying the excitation (which in a standalone application would cause a variation in the voltage). Because the voltage of the system is locked together, instead of the voltage changing, the amount of reactive power absorbed by the alternator changes.

The droop system works by measuring the system load with a droop CT. This is connected to the AVR and as the load increases, it attempts to reduce the voltage by a small percentage (typically 3% from 0-full load at 0.8pf) and this therefore reduces the voltage.