What is speed droop on a diesel engine? What about isochronous? Why and when is each useful?

Isochronous Vs Droop - What is the difference?

In isochronous mode, engine governor attempts to keep the engine speed fixed at its nominal speed - typically 1500RPM for 50Hz Power. This is true at all loads - the engine governor tries to keep the unit at this fixed nominal speed.

In droop mode the governor only targets to be at the nominal 1500RPM speed at full load. At no load the speed will be typically 3-5% higher, 1575RPM. As the load increases, the speed falls proportionally down to 1500RPM.

Droop Control Mode Uses

Droop was originally a feature of mechanical governors. Most mechanical governors still use droop. Engines like the Perkins 1103A-33G as standard still have mechanical governors and the speed droops as the load increases.

When paralleling diesel generators this change in speed and hence frequency allows multiple diesel generators to share the load in proportion to their power. This load share arrangement of using droop was very common - the more modern method is isochronous load share where an engine controller manages the engine speed electronically.

Why is droop necessary?

The best description we could find for the reason for this is from a Woodward manual located here. The extract is as follows from page 13:

In a system without droop, a load increase will cause the engine to slow down. The governor will respond by increasing the fuel until the engine speed has returned to the original speed. Due to the combined properties of inertia and power lag, the engine speed will continue to increase beyond the original speed setting, causing an overshoot in speed. The governor again will respond to decrease speed to correct for the overshoot. It will over-correct the speed in the other direction causing an undershoot. This overcorrection of speed in both directions (instability) will amplify until the engine trips out on overspeed.

Isochronous Control Mode Uses

For more modern engines with electronic governors Isochronous mode is typically used. It is common when a generator stands alone (island mode). Isochronous regulation can be very tight, reacting to small change in the load and environment to keep the speed stable.

And this means...?

For most engines with mechanical governors, the engine speed at no load is 2-3Hz higher than the full load speed setting, because as this droop effect takes place, the speed of the engine will fall and therefore the aim is to have the full load speed generating 50Hz (or 60Hz).

Article: 154, Version: 1