OEL and generator capability curve don't seem to match 2

[mmuel]

Hello Gentlemen,
We have a 50MW gas turbine that is used for peaking and system stability. The problem we are experiencing is when we are exporting VARS (over exciting the generator) we hit the volt/hertz limit (1.05pu) before the generator capability curve indicates we should.
Here are the specifics: 65MW unit. It is limited to 50MW because of emissions. 13.8KV terminal voltage and goes to GSU and to 161KV. The 161KV line is usually around 163KV, so generator voltage is approximately 14KV.
The most we have gotten was about 9 MVAR before we hit the 1.05pu limit but the capability curve shows we should be able to get at least 20 MVAR. Attached is the generator curve. Any explanation would be appreciated.

 

[Kiribanda]

mmuel,

What is the set operating mode of your generator AVR?Is it on Voltage controlled mode or Constant Power Factor mode?

(Note: My understnding is when a generator is exporting VARS,it is in under excited state and not over excited like a sync. motor)

 

[davidbeach]

Overexcited is exporting VArs; underexcited is importing VArs.

 

[mmuel]

The AVR is in the Power factor mode. I’m not sure if that is the correct terminology but when the operator sets the VARS at a set point, it will hold that VAR set point.
I am trying to also figure out how to do the math from the other gages (PF, Amps, and Voltage) to check if the VAR meter is at all close.

 

[rcw retired EE]

You can't fight Ohm's law.

The capability curve assumes that the generator is pushing MW and MVAR's into a system with a constant voltage and that the system impedance is low enough to accept it.

In your case, the system voltage is too high for the present generator step up transformer tap setting.

MVAR's always flow from high to low voltage. To push MVAR's to the utility, the effective generator voltage at the 161 kV level has to be higher than the system voltage. If the generator hits 105% voltage limit before it's at the capability limit, that means the system voltage is too high as seen by the generator.

Take the unit off line and change the transformer tap to one with a higher 161 kV nominal voltage (lower generator bus volts). That will raise the effective generator voltage at 161 kV and allow you to push more MVAR's to the system.

If the system impedance between the generator bushings and the "infinite bus" of the utility is too large, generator current flowing through that impedance raises the voltage at the transformer bushings. The more MVAR's the generator tries to push, the higher the voltage rises and the voltage limit occurs before t reaches the thermal reactive capability limits. Note that the step up transformer impedance is usually the largest part of that impedance.

 

[Kiribanda]

mmuel,

So my suggestion is to change the AVR operating mode from PF control mode to "Voltage Controlled Mode".Then it will control the voltage at the 13.8 kV gen bus.From the capability curve then you should be able to export 25 MVARS at 40 deg C, provided 25 MVARS is reduced from another generator or there is a fresh demand from the network.Otherwise if you try to push MVARS to the system manually without any demand from the system then it will increase the bus voltage

Does it make sense?

 

[waross]

Amps times volts/1000 gives KVA.
KVA times PF gives kW.
KVARS squared equals KVA squared minus kW squared.

Bill
--------------------
"Why not the best?"
Jimmy Carter