Generator Response to a Change in Grid Frequency 1

[scott88]

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

 

[LSpark]

The under excitation probably came up due to a change in power factor which pushed the machine (further) in to the lead. This will have happened due to changes on the grid which you AVR responded to so it maintained GT terminal volts.

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.

 

[dpc]

What happened to the grid voltage when the frequency dropped? And does this unit have a Power System Stabilizer?

 

[Skogsgurra]

I am amazed at the "instantaneous drop in grid frequency from 49.97 Hz to 49.60 Hz"!

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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[xnuke]

I think I know why the current increased instantaneously when the frequency dropped:

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.



xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[mgtrp]

Any chance that there is a large generator nearby that imports a lot of reactive power under normal circumstances? This tripping offline could cause both the underfrequency event observed and an increase in system voltage that would result in the generator attempting to import more reactive power and possibly hitting an excitation limiter.

 

[ScottyUK]

Depending on how sophisticated the AVR is, the UEL may follow the capability curve of the machine rather than be a fixed value. At higher MW loads the stability limit moves to the right of the capability chart (i.e in a lagging direction) and the minimum permissible excitation current increases. As the MW output momentarily increased during the system disturbance the UEL may have become active to prevent the machine moving toward a pole-slip (increasing MW with insufficient excitation).

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.

 

[scott88]

dpc
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.

 

[dpc]

I would be suspicious of your "real-time" data. Could be some sampling rate issues? An instantaneous drop in frequency of UK grid seems unlikely. Also, an instantaneous increase in generator current without a change in generator terminal voltage seems equally improbable. With grid disturbances, the rate of change of frequency is generally a key factor in system response. But I'm not a stability expert, so I await further education.

 

[QBplanner]

Why did the generator current increase immediately after the drop in grid freqeuncy

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.