Coordination between Transformers OLTC and Generators avr in power station

[ironmaan]

Hi all,

In power station, it's possible to use the interaction between OLTC and Generator avr to control the reactive power. I was wondering if someone have some reference explaining more about the coordination between them and what kind of possibilities can be used.

Thank you in advance !

 

[waross]

I was under the impression that OLTCs were mostly used at the receiving ends of transmission lines to compensate for line losses with varying loads.
But if you have an OLTC at the generating station, all else being equal, changing the tap will change the reactive power transfer.
But all else may not be equal.
If the AVR is in power factor mode, it will compensate for the changed tap and after a possible switching disturbance the reactive power transfer will return to the original value.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

OLTCs (LTC in my company’s vernacular) are rarely used in conventional synchronous machines, at least in the United States. The generator voltage regulator effectively controls the voltage on the generating bus when the machine is running, so additional voltage control and adjustment is unnecessary.
LTCs seem almost universal in distributed generator applications such as wind farms and solar plants. Originally when wind farms were built with type I and II induction generators it made sense as the individual components could not control voltage. Now with inverter based generation, which has voltage control capability, I’m not sure why this practice has remained, other than possibly adjusting bus voltage when the inverters are all offline. At our only IBR plant I’m pretty sure they basically never operate. I’m assuming it’s just left over from earlier design, but I plan to ask.

 

[dpc]

I've run into this situation at paper mills where the local generators are not always running. We generally use fairly wide and slow setting on the LTC controller and try to regulate with the generator excitation. If you are operating in parallel with the grid, the utility may have requirements for voltage support that would preclude, or at least restrict your ability to do power factor control. The two systems will definitely fight each other if not properly configured. It's probably much better to use the generator to regulate the voltage or power factor rather than the transformer LTC.

 

[waross]

A question, David.
Are the LTCs used mostly when the mill generation is off-line.
Would this put the LTCs in the category of line voltage drop compensation on the load end of a transmission line?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dpc]

Bill,

I think the LTC was more of a belt and suspenders approach to cover all possible operating conditions. The incoming transmission voltage fluctuated a fair amount when the generators were offline. It's been a while, but I think we ended up recommending that they consider switching the LTC to manual when the generators were running. It's also possible that the transformer and LTC pre-dated the turbine-generators.

Dave

 

[waross]

Thanks Dave.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

OLTC AVRs come with the option of Voltage control as well as Reactive power control.
In case of Generator transformer, it is best to use Reactive power control so as to ensure that the generator is always pumping reactive power in to the grid. This means generator is working at lagging power factor which ensures the generator field is strong.
A strong field results in improved stability for generator during grid disturbances.