Generator Capaility within voltage limits
Generator Capaility within voltage limits
(OP)
We have an 11.5kV 58MW generator with the usual capability curve showing 0.8 lag/0.9 lead at MCR. The stator voltage limits are given as 11kV +/- 5%.
We cannot deliver/absorb full VArs at 11.5kV, we always have to change the voltage up to +/-6% across the Var range. The connection to the 132kV grid is by a tap changing transformer which we have to manually control. We would of thought that we could leave the transformer in autotap at 11.5kV and deliver the full range of VArs at 58MW. Is this common, and why?
We cannot deliver/absorb full VArs at 11.5kV, we always have to change the voltage up to +/-6% across the Var range. The connection to the 132kV grid is by a tap changing transformer which we have to manually control. We would of thought that we could leave the transformer in autotap at 11.5kV and deliver the full range of VArs at 58MW. Is this common, and why?






RE: Generator Capaility within voltage limits
Many Power plats have ratings like this, Generator nominal voltage rating 11kV & Genr. Tfr. nominal voltage rating 132/11.55kV(5%more).
It is General practice to keep nominal rating of Gen. Tfr. 5% more to compensate full load voltage drop so that we get wide voltage control range in excitation side. But this is depends upon network requirements also, some Utilities do not insist to keep 5%more nominal rating of Gen. Tfr. if network is not heavily loaded.
RE: Generator Capaility within voltage limits
What is your operating mode of the Exciter? Is it in Voltage Regulating mode or Constant power factor mode.
Normally the opearting mode should be voltage regulating mode. The step transformer is at fixed tap.Then the export and import of VARS depends on the system voltage conditions while the gen bus voltage is kept at constant 11.5 kV.
But if you set the exciter to constant PF mode then, the 11.5 kV bus voltage has to be maintained by the LTC of the transformer.
Any comments please.
RE: Generator Capaility within voltage limits
RE: Generator Capaility within voltage limits
OLTC in manual mode should be used to flex the VAR import and export to the system.
Small variation of the AVR setpoint should allow the reactive despatch to be trimmed to values between the OLTC steps.
You could automate control of the OLTC based on the reactive despatch but this is fairly uncommon, at least in the UK.
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RE: Generator Capaility within voltage limits
Refer to ANSI/IEEE Standard C57.116.
Also, a good article from IEEE Transactions on Industry Applications, Vol 33, N0. 6 Nov.Dec 1997 - "Estimation of Reactive Power Export and Import Capability for Non-Utility Generators" discusses this very issue.
RE: Generator Capaility within voltage limits
RE: Generator Capaility within voltage limits
Assuming that the unit transformer ratio is 1:1 , the delivered VAR's are app:
VAR = ( Vg - Vg*Vs)/ Xe
Where Xe = unit transformer short circuit impedance.
RE: Generator Capaility within voltage limits
RE: Generator Capaility within voltage limits
When the tap changer control is on the high voltage (utility) side, it ends up trying to raise or lower the utility voltage. My 50MVA generator was no match for the large utility so the voltage would barely change and the tap changer ended up on the highest or lowest tap.
Maybe with proper reactive compensation settings you could get it to work.
I always end up running the tap changers manually with the plant DCS monitoring voltages and MVAR flows. It sounds an alarm and suggests tap changes to the operators.
RE: Generator Capaility within voltage limits
RE: Generator Capaility within voltage limits
"The stator voltage limits are given as 11kV +/- 5%."
This gives you a voltage limit of 11.55 kV. That is only 0.05 above 11.5 kV or 0.43% margin left for VAR correction.
Try a tap setting that corresponds to 11 kV or less.
Bill
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"Why not the best?"
Jimmy Carter