actual droop of generator


Thread Starter


I have a real curves of generator reaction of frequency disturbance in real system. How to calculate the actual droop of generator?

first, you should know that there is no such thing as generator droop. Droop is something that is determined by turbine control system (turbine controller).
Droop is given in % and means how much turbine speed will change (in %) when there is a load change from 0 to 100%.


I just want to ask, have you ever seen the speed of a generator decrease as it is loaded (presuming it's a stable grid or a stable "island")? Because the speed of the generator is directly proportional to the frequency. And, if the generator is mechanically coupled to the prime mover, then the speed of the prime mover is directly proportional to the frequency of the generator.


If the unit (generator and prime mover) was not operating at or near rated output during the disturbance, you should be able to calculate droop from the data. A unit with 4% droop would change output by 25% of rated load (prime mover rating, not generator rating!) with a 1% change in frequency. A unit with 5% droop would change output by 20% of rated load (prime mover rating, not generator rating!) with a 1% change in frequency. So, take a look a the change in rated load with the change in frequency. Droop is proportional.

But, Mik is right about droop: Droop isn't related to the generator's characteristics. It's a prime governor setting.
Yes, I understand that. But my question is, if I have the curves f(t) frequency (time), P(t) load (time), or P(f) load (frequency) is it possible to calculate a droop.

If I understood correctly
R(droop)=deltaF(frequency change)/deltaP(load change).

But the question is from what second of disturbance beginning have I take the change.

I have never seen change in speed when turbogenerator is loaded or unloaded. Couple of time I had an opportunity to observe speed change when unit was loaded from 60% to 100% of nominal load. However, during this change (which was gradually in 15 min time) speed changed about + or - 3 rpm (nominal speed is 3000 rpm) which was caused mainly due to process noise.

I don't have a feeling for droop setting of the prime mover yet. I've read a lot about it on, but still not really sure how would I measure it or see it.
There may be some ramp rate limiter on the energy control admission valve to the prime mover, and we don't know how short or long the frequency disturbance was or how unstable it was.

You need to contact the supplier of the prime mover governor (control system) and ask them this question. They should be able to point to a setting, setpoint, or variable that defines the droop characteristic for the prime mover.

I believe you have the formula correct, but, again, if there is some kind of ramp rate limiter on the fuel or steam (we don't know what kind of prime mover you have) control valve that may also affect the time interval.

There are just too many variables to be able to do this calculation as you are suggesting.

If your prime mover uses speed reference for controlling load below rated load, look to see how much the load changes (as a percentage of prime mover rated load) for a given change in speed reference (in percent of rated speed). Since frequency and speed are directly proportional (assuming the prime move and generator are mechanically coupled), you can correlate the change in speed reference with a change in frequency.

Again, there are just too many variables to try to do with the data you might have.

Bruce Durdle

I have "measured" a droop curve in the past using a combination of off-line and on-line measurements.

First, before synchronising. Alter the speed control setting and note the no-load speed for a range of different settings. Then, after synchronising, and with the machine running at its nominal supply frequency, repeat the exercise. This time, note the power corresponding to each setting rather than the speed. For each setting, calculate a speed offset as (no-load speed from test 1 - running speed from test 2). Plot the speed offset on the vertical axis against power output on the horizontal axis.

Thank you all for the answers.

I try to describe the problem. I work for TSO, I am analyst. We are synchronously connected with "infinite" grid. We performed the tests of the separation from with "infinite" grid and imitated the real failures in the system that were caused by the switch-offs of individual generating blocks or by network (line openings) or switch-offs of the HDVC link. My purpose is to analyze the characteristic numbers of primary control.

For calculating the network power characteristic λ.

λ= sum(1/sGi x PGi,r/fr),

sGi is the droop or regulation of generator i
fr is the nominal rated frequency
PGi,r is the nominal rated power of generator i

I have to calculate sGi (the droop or regulation of generator i)

λ is usually calculated in timeframe 10-30sek (primary control duration time).

So, my question is: is sGi the droop speed control (what are you talking about) or it is something else. And one more think, my curve P(f) active power(frequency) is a curve, not a straight line.