AVR control on generators 2

[bigdog50]

Question is if a large industrial turbine is running on Automatic voltage control can you then run in var control or power factor control on top of that? and if so what are the benefits of running var or pf control on top of AVR?

THanks for any imput!

FLORIDA,
BIGDOG50

 

[dpc]

You would actually run VAR/PF control **instead of** voltage control.

With a regulator in normal voltage control mode, it is adjusting the excitation to try to maintain a setpoint voltage on the generator output (or at whatever point you connect to the voltage input). If the generator is running isolated, off the grid, this makes sense.

However, consider the situation of a generator tied to the power grid or another larger generator. The single generator is not large enough to change the grid voltage in most cases. So even a very small voltage error signal will cause the regulator to drive the excitation to maximum (or minimum) trying to get the voltage input back to the setpoint, but the generator just can't do it. So the generator excitation will swing from min to max as the voltage varies. For these situations, you must switch from voltage control to Var or PF control. In these modes, the setpoint is the kVAR output of the machine or its output power factor. Now the voltage can vary (within limits) and the regulator will be happy and making excitation adjustments to maintain a constant generator power factor or VAR output.

 

[electpav]

Hi DPC,
but wt will happen if suddenly Grid Parallel info is missed.
Then AVR will Autometic will come to Voltage Droop mode.
We can chose either VARmode or PF mode.
in PF mofe Basically AVR Export the KVAR to maintain the
PF of parrelel system. It calulate the KVAR on the basis of PF value.
while in VAR mode it directly upply the KVAR as per the setting values

bye

 

[RRaghunath]

dpc has explained the AVR function very well. I only want to add a bit to it.

It is important that the AVR switches back to Voltage droop / control mode whenever the connection with grid is lost and the generator is islanded with the plant load.
If my understanding is right, the modern day AVRs can perform this act if an apprpriate digital input (representing loss of grid connection) is wired to it.

Hope the above is useful.

 

[bigdog50]

THANK YOU ALL FOR YOUR RESPONSES. THE ISSUE WERE HAVING HERE IS THAT WHEN SHUTTING THE PLANT DOWN FROM SERVICE AND THE STEAMER AT TIMES ATTEMPTS TO INCREASE IN MVARS OR LAGGING. WHEN THIS HAPPENS THE STEAMER GENERATOR ATTEMPTS TO GO BEYOND THE CAPABILITY CURVE OF .85 LEADING OR LAGGING AND ALL TIMES I HAVE BEEN ON SHIFT IT WAS LAGGING VARS. WHEN THIS HAPPENS AND THE STEAMER IS NEAR REVERSE POWER -5 IT TRIPS ON AVR TROUBLE! NORMALLY ON TRENDED LOGS I SEE THAT THE GENERATOR WENT BEYOND IT'S CAPABILITY CURVE AND THIS IS WHAT IT'S SUPPOSE TO DO TO PROTECT THE MECHINE BUT WHY IS IT NOT BACKING DOWN TOWARDS UNITY WHEN LOAD IS LOWERING? COULD IT BE BECAUSE OF THE UTILITY WERE SUPPLYING OF SYSTEM DEMAND CAUSING THIS? NOTE ARE CT'S TURBINES WE HAVE NEVER HAD AND AVR TROUBLE? ANY RESPONSE APPRECIATED.

FLORIDA,
BIGDOG50

 

[dpc]

As rraghunath indicates, the AVR requires a contact input that tells it when the unit is synchronized to the grid and when it is not. Var/PF control is only used when tied to the grid and the AVR should automatically revert to voltage control when separated from the grid. The contact inputs are normally auxiliary contacts from circuit breakers indicating when the grid tie breaker and generator breakers are open and closed.

You should refer to the instruction manual for your AVR for more information on the required inputs.

But the AVR should not remain in VAR/PF mode when off-line.

 

[GigawattClub]

For the love of god leave it in voltage control! Plants in VAR and PF control mode have caused/contributed to system voltage collapses in the past. The purpose of the AVR is to support system voltage. If you are in VAR/PF control you will do nothing to prevent a voltage collapse.
As for DPC's comment about the AVR maxing for small changes in system voltage, that is what the Line Drop Compensator or Voltage Droop function prevents. This is from a electrical utility perspective so if you are talking about a 50 kW unit do whatever you want. If you are from an electrical utility I would advise against having this conversation with someone from the WECC as it is a requirment for all interconnected generators above 10MW to run with the AVR in voltage control mode.

 

[rconnett]

WECC is the operating council of utilities in the Western United States - for those who aren't aware.

 

[dpc]

GigawattClub,

He did say it was an industrial turbine. pf/var control is the norm for cogen and other smaller generators tied to the grid. Industrial generators are interested in using their generators to improve their overall pf, not in helping the utility maintain their grid voltage.

The AVR can be kicked back to voltage control on system voltage excursions through appropriate relaying if required.

In the case of a system voltage collapse, the industrial generator is going to be rapidly tripped off the grid by the undervoltage relay that the utility made them put in anyway.

 

[GigawattClub]

I must correct my original statement that there is a 10MW cutoff. It actually is any syncro generator connected to the transmission grid can not be run in PF/VAR control. I am not sure what type of interrconnection we are dealing with, but in the WECC area regardless if it is Co-Gen plant or not, if it is interconnected at the transmission level it will be run in Voltage Control mode.

 

[Rodmcm]

Older AVR's often don't have the capability to switch back to volts mode, one technique is to include over/undervoltage, over/under frequency cut outs on the pf/Var controller.
However, Bigdogs problem is shutting down. It seems that the AVR is set to control al all times the breaker is in, it could be that it is set to constant VARs and not pf. Thus as the turbine is backed off the VARs stay constant with reducing load. Reverse power has nothing to do with the AVR only the governor. I would suggest that you switchout the VAR controller on shut down.

 

[bigdog50]

Thank you all for your feed back. It's been very helpfull. I do know for sure after reading are interconnect agreement and they do require us to run in AVR control, and we have, since first commissioning of last year. Again thanks for the helpful tips..

BIGDOG50

FLORIDA,
BIGDOG50