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Member
Engineering Fields - Power Engineering - New Member

Join Date: Jun 2011
Posts: 7

Droop of a Generator

06/21/2011 12:07 AM

What is the exact meaning of Generator Droop? How does it help in load sharing? pls explain.

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Guru
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#1

Re: Droop of a Generator

06/21/2011 12:35 AM

I haven't yet searched on Woodward Governors, but I suspect they have a good description of isochronous vs droop governor control. (Basically, any deviation from matching speed "pulls" the deviating generator back into line with the single isochronous generator in the paralleled group--I think.)

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Guru
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#2

Re: Droop of a Generator

06/21/2011 4:26 AM

Droop refers to the generator Governor, when the system speed varies the Governor responds, Governor droop refers to a dead band whereby the Governor does not respond, usually set at 5%, this means that the Governor will not respond until the system speed varies by up to 5%, this prevents generates that are sharing a load from hunting. In other words the system can vary by 5% before a generator tries to recover the frequency.

Regards JD

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#3

Re: Droop of a Generator

06/21/2011 6:17 AM

Droop is defined as a decrease in speed setting of generator as the load increases. Droop is expressed as a percentage of the original speed setting from no load to full load. The normal recommended percent of droop is 3% to 5.

Droop is calculated with the formula:

%Droop = [{((No Load speed - Full Load Rated Speed)/Full load Rated Speed}/100]

Why Droop is Necessary?

If a engine system is running without droop, a load increase will cause the engine to slow down. Then, Governor will respond by increasing the fuel until the engine speed has returned to the original speed.

Due to combined properties of inertia and power lag, the engine speed will continue to increase beyond the original speed setting, causing an overspeed of the engine. The governor will again respond to decrease the speed to correct for the overshoot of the speed. It will over-correct the speed in other direction causing an undershoot of the engine speed. This over-correction of the speed in both directions will amlify until the engine trips on overspeed.

This instability of the system can be eliminated with the droop. As the load increases, the speed setting of the engine is decreased. When the governor moves to correct the drop caused by the increase of the load, It will be correcting to a lower speed setting. This lower speed setting prevents the engine speed from overshooting. Normally a droop governor lowers the speed reference from 3% to 5% of the reference, from no load to full load.

Governor can run in two mode:

Isolated or Isochronous Mode:

an isolated system is an application where two or more engines are driving a common load. This load could be electrical generators, pumps, ship propellers, etc. These isolated systems are not connected to any other systems or to a utility. If two engines operating in the isochronous mode without load sharing controls and supplying to the same load, one of the unis will try to carry the entire load and the other will drop all of its load. In order for the two units to share equal load, Some additional means must be used to keep them stable.

Droop Mode:

In droop mode, All the engines will share the load as per their droop setting. If all the engines in a droop mode have the same setting, they will share equal load with the same speed setting and vice versa.

Vinu_answers Sure_answers

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#4

Re: Droop of a Generator

06/21/2011 6:57 AM

http://www.annainc.com/pdfs/genBasics.PDF

http://www.canadiancontrols.com/documents/technical/Speed%20Droop%20and%20Power%20Generation.pdf

and an interesting discussion here and a tutorial here

All these along with above should fairly sum up the whole.

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#5

Re: Droop of a Generator

02/20/2014 1:45 AM

Dear M. Menon,

When we talk of DROOP - it is of Two Types - one is SPEED DROOP, pertaining to Prime Mover Speed and its variation and the other one is VOLTAGE DROOP for the Generator/Alternator. Since you referred "GENERATOR DROOP" - in your posting, it relates to VOLTAGE DROOP of alternator.

Theoritically the Voltage Drop from No-Load to Full-Load it it should be Zero. But in practice/actual working, of an alternator it is not so, since as the Current flows through the winding the Voltage DROP takes place depending upon the Winding resistance. This Drop will be highest at Full-Load and normally it will be 5% average.

The Power Factor should be LAGGING in the range of 0.8 to 0.92 or so. If the Power Factor is leading, the terminal Voltage will be higher than the Induced Voltage.

Why Droop is Important.? This Droop Character will decide the PARALLEL OPERATION of 2 or MORE GENERATORS and LOAD SHARING of the GENERATORS are GOVERNED by this DROOP CHARACTER and hence we cannot and should NOT blindly attempt to synchronize with out matching the DROOP CHARACTER.

The same Logic applies to SPEED DROOP also.

DHAYANANDHAN.S

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