What Is Droop Control for a Generator

The droop speed is the drop in speed with load and is a characteristic of the governor or speed regulator for the prime mover. When used as a standalone generator this will determine the drop in frequency. It is typically 2-4% at full load.

When used in conjunction with other generators (in parallel) then the generators sets will share the total load based on the droop characteristic. If they are identical then they will share equally.

Two generators using droop control to provide max power at 60Hz. One has a 63.4 Hz reference and the other 61.8Hz reference for both to deliver full power at 60 Hz.

Thus the aim of droop control is to adjust the governor speed references to firstly achieve the correct system frequency and secondly share load in accordance with the respective generators contribution in terms of its generating capacity. This allows generators of different sizes to share the total load unlike isochronous operation which does not.

Voltage droop control- For a device that drives a load a series resistor is used to actually decrease output voltage. Seems counter productive. It's to keep output voltage at a bare minimum if output current goes up(voltage drops due to Ohm's Law) and at just below the max voltage if load were to drop(Ohm's again) It's better regulation for variances and uses less decoupling capacitors.

Frequency(speed) Droop Control- basically this is load sharing, generating units running in parallel. For multiple generators on the same electrical grid they will be operating on the same frequency. Changes in frequencies or speed seen as power output, will be reacted to by the generators and adjusted for, based on(I believe) the droop control settings. I've seen other people discussing 3 or different “set-ups” which I leave to them.

Reactive Droop Compensation - A circuit arrangement that creates a droop in generator output voltage that is proportional to the reactive load current. This circuit arrangement makes paralleled generators share the reactive component of the load current in proportion to the generator ratings. It also can essentially eliminate the reactive component of circulating current which may flow between paralleled generators. 

The Reactive Droop Compensation arrangement is commonly called the Generator Paralleling Module.