Zero reactive power function

[ELEP]

Hello Eevryone!

I have come across one of a 'Zero reactive power function' while reviewing AVR document for 776MVA Generator. In AVR manual this functions is not described in detail.

After googling few websites I found this function is incorporated in Power controller(VAR controller for Power Plant) also which is used as Unload function.

I would be very much greatful if anybody clarify more about above mentioned function in this thread?

Thanks in advance!

 

[tlrols]

Presumably this is part of a power factor control function...as compared to a voltage control function (i.e. an automatic voltage regulator). I would be shocked if the transmission provider for that very large machine would ever allow the use of power factor control. Typically speaking generators are provided with a voltage schedule that they are expected to meet--this requires the use of automatic voltage regulators in the AVR mode, NOT the power factor control mode. The zero reactive power function is suitable for use only in testing...it should never be used in operations without the express permission of the transmission provider.

 

[ELEP]

Thanks!
tlrols,
there is separate power factor control function in addition to Zero reactive power command in AVR.
I have encountered this(Zero pf) command in 270MVA as well as 776MVA Generator. I am doubtfull if they are usually using this function.

 

[j3n5]

I believe this is a standard function in excitation systems of generators. Also known as "Q-unload" command. It may be used by the control system in "unit stop" sequences. Similar command to governor would be "P-unload", usually commanded at the same time in the stop sequence.

 

[ELEP]

j3n5, you are right!
the manual says that,
'An unloading facility is available
which, when activated, will reduce
the generator reactive power to
zero or the power factor to 1,0.'

perhaps it may be generator open condition.

thank you so much!

 

[esee135]

... I use this function to dump the VARs so you decrease the wear on the breaker contacts during shutdown. The idea of machine unload during shutdown is to limit load rejection, open with near zero stator current, save the wear on the breaker contacts etc. Kind of like shutting off your car at 6000RPM, you can, but it's not the best thing to do.

+1 on the system operator issue, AVR's are there for a reason. Fast cos-phi control response will handicap the ability of your generator on the power system.

 

[ELEP]

thanks !
esee135

What I understand is that before isolating generator from system during fault or shutdown condidions Zero reactive power function will be activated so that only active power current will be interupted by circuitbreaker,
Which can save wear on breaker.

One more question, is this function inbuilt in your AVR or it is separately incorporated in VAR controller?.

 

[esee135]

It's built in. Actually I did it in an ABB Unitrol F. All you have to do is have a digital input fire the VAR unload as I called it, or what it did internally is the input triggered the PF/VAR controller on with a fixed setpoint for 0 MQ or unity pf. In the unit controls it watched for a deadband of the MQ, then initiated the shutdown.

If you don't have it, you can do it with a PLC start/stop chain, or you could make a slow tuned outer closed loop that you unload watching MQ's and feed the AVR summing junction. That error would be available only during the shutdown sequence.

 

[esee135]

...one more note,

----
What I understand is that before isolating generator from system during fault or shutdown condidions Zero reactive power function will be activated so that only active power current will be interupted by circuitbreaker,"-----

static exciter shutdown during an 86 and during a controlled shutdown differ drastically. On 86, the bridge goes to negative retard, field breaker trips, and the crow bar asserts.

On a controlled shutdown (stop signal given), the field breaker usually stays closed, and the bridge goes to negative retard. Very different shutdown curves. Prior to this is where the pf unload would occur.

The difference is on a controlled situation you don't want the di/dt voltage on the rotor/causing havoc on the stator as well. Hence, negative forcing during shutdown to arrest the field current momentum.

 

[ELEP]

Thank you so much 'esee135' for posting detail explaination.
Now my doubt is clear, with present 776MVA generator we have this function, I have investigated few older generator where this function is not there, as you suggested we can retrofit old AVR with new one.

 

[janiliewa]

Great post ,thanks for the info.

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