Generator Response to a Change in Grid Frequency
Generator Response to a Change in Grid Frequency
(OP)
Hi,
I am looking for some clarification on my understanding of generator frequency response following a recent sequence of events.
The generator is an 280MW machine with a gas turbine prime mover connected to the national grid. Following an instantaneous drop in grid frequency fromn 49.97hz to 49.60 the following sequence of events occured;
1. The Generator current rose from 6.03kA to 8.78kA
2. Under Excitation Limit warning came in
0.05 seconds later (Generator controller responds to the change in frequency)
3. Generator current fell to 5.80kA
4. Excitation current increases from 694A to 804A
5. Power factor shifted from -0.86 to -0.84
0.05 seconds later (GT controller responds to the generator)
6. IGV position opened from 40% to 41%
7. Active power rose from 181MW to 202MW
System stabilised.
My questions and understanding are regarding;
- Why did the underexcitation limit warning come in immediately after the drop in grid frequency with no change to the excitation current?
- Why did the generator current increase immediately after the drop in grid freqeuncy?
I appreciate that a drop in grid frequency will result in the generator controller responding and the requirement to increase active power/excitation current/IGV position, its the immediate effect that confuses me.
Any guidance appreciated,
Thanks
I am looking for some clarification on my understanding of generator frequency response following a recent sequence of events.
The generator is an 280MW machine with a gas turbine prime mover connected to the national grid. Following an instantaneous drop in grid frequency fromn 49.97hz to 49.60 the following sequence of events occured;
1. The Generator current rose from 6.03kA to 8.78kA
2. Under Excitation Limit warning came in
0.05 seconds later (Generator controller responds to the change in frequency)
3. Generator current fell to 5.80kA
4. Excitation current increases from 694A to 804A
5. Power factor shifted from -0.86 to -0.84
0.05 seconds later (GT controller responds to the generator)
6. IGV position opened from 40% to 41%
7. Active power rose from 181MW to 202MW
System stabilised.
My questions and understanding are regarding;
- Why did the underexcitation limit warning come in immediately after the drop in grid frequency with no change to the excitation current?
- Why did the generator current increase immediately after the drop in grid freqeuncy?
I appreciate that a drop in grid frequency will result in the generator controller responding and the requirement to increase active power/excitation current/IGV position, its the immediate effect that confuses me.
Any guidance appreciated,
Thanks






RE: Generator Response to a Change in Grid Frequency
Under excitation does not mean low excitation, it means you are approaching the stability limit beyond which a pole slip will occur.
What mode was your machine operating in? If it was frequency sensitive I would expect the load to increase on falling frequency.
RE: Generator Response to a Change in Grid Frequency
RE: Generator Response to a Change in Grid Frequency
How instantaneous is that? One second? Two seconds? Is that at all possible in a national grid? Small nation, perhaps?
You seem to have followed this event on a high speed recorder (0.05 seconds later) so there should be some interesting recordings.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Generator Response to a Change in Grid Frequency
The rotor magnetic field leads the stator magnetic field in a generator. The angle between them is the power angle. When the grid frequency dropped, the stator field began lagging the rotor field even more, thus the power angle got larger and the power out of the machine increased. Since the excitation was held constant initially, the armature voltage remained the same. For an increase in power output with a constant voltage, the current increased.
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RE: Generator Response to a Change in Grid Frequency
RE: Generator Response to a Change in Grid Frequency
How do you measure grid frequency - implied from shaft speed, or direct measurement? If the former then the machine would be in a dynamic state as the load angle changes relative to the grid, so implied frequency measurement can be misleading.
RE: Generator Response to a Change in Grid Frequency
Generator terminal voltage remained constant when the frequency change occurred, once the increased current had stabilised at 8.78kA it fell from 20.29kV to 19.94kV. Excitation current then rose and returned the generator terminal voltage to 20.64kV, it eventually stabilised at 20.3kV.
Skogsgurra
The drop in frequency was over 0.15 Seconds. It was the UK national grid.
ScottyUK
The frequency is measured at the grid entry point direct measurement but we also have a measurement of the shaft speed. The shaft speed was the figures I have given, grid frequency trends show an ever so slightly less drop in frequency.
Thanks for everyones input, it has been a big help and clarified a few points.
RE: Generator Response to a Change in Grid Frequency
RE: Generator Response to a Change in Grid Frequency
teh generator active current increases following the frequency changes is beause the kinetic energy in teh rotor and the synchronizing torque.
please note, your definition of "immediately" could cause confusion. if you can show the generator current recorded curve, it would be more helpful.
I would guess the generator active current increase in a "slow" way it should not be too fast let's in a cycle or two. I would guess it increase from 6.03kA to 8.78kA with a slope.
I can not help you on exciter respnose because it could be many reasons.