Generator Capaility within voltage limits

[Rodmcm]

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?

 

[ELEP]

Yes, 11.5kV tap can be considered as nominal tap for transformer.
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.

 

[Kiribanda]

Rodmcm,

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.

 

[GTstartup]

Rememember that generator capability curves are only good for nominal voltage. The capability to deliver/Absorb Vars is reduced by the square of the delta from nominal voltage. i.e 0.9 Unom - 0.81 Vars

 

[ScottyUK]

AVR should be in voltage control.
OLTC in manual mode should be used to flex the VA[sub]R[/sub] 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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If we learn from our mistakes I'm getting a great education!

 

[rcw retired EE]

Ability to push or absorb MVARS is limited by the ability of the system to accept or deliver VARS without hitting voltage limits and the impedance of the step up transformer. If your utility voltage rises as you supply MVARs due to system impedance, the generator will hit its voltage limit before reaching the MVAr limit.

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.

 

[Rodmcm]

Thank you, I will try to find the references. I still cannot understand why, with a tap range capable of supplying the complete range of VArs to the system (under a defined +/-10 % voltage range at 132kV) why we need to generate at anything other than 11.5kV. The control is as per Scotty UK description at the moment, but i do not see why it has to be this way. Can someone give me a toddlers quide?

 

[ratz1]

To deliver the VAR's to the network you should have the generator voltage(Vg) higher than the system one (Vs). Vice versa, to absorb the VAR's the gen. voltage shall be lower.
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.

 

[Rodmcm]

The delivered VArs are at the point of connection at 132kV, hence the tap changer auto control will adjust to meet any change in conditions won't it?

 

[rcw retired EE]

I've never had much luck using auto tap changer control on a generator step up transformer. When I connect it to monitor and control the generator bus voltage, the generator AVR and tap changer controls fight each other. The generator ends up at 105% limit and the tap changer is on -16 tap or vice versa.

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.

 

[Rodmcm]

Thanks for that, as observed by us as well. I am just having trouble understanding why this should be so if you dial up constant kW and constant VArs then I would have thought that you could hold the KV constant at 11.5KV and the tap changer alters according to the 132kV conditions.

 

[waross]

Why don't you set the OLTC to equal the generator nominal voltage?
"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