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On-Load Tap Changer (OLTC) Operating Philosophy

On-Load Tap Changer (OLTC) Operating Philosophy

On-Load Tap Changer (OLTC) Operating Philosophy

Dear all,

The power plant where I work is planning to integrate On-Load Tap Changing OLTC) Transformers onto the grid for a new power plant expansion consisting of Reciprocating engines.
There will be in total 3x 50MVA, 60kV/13.8kV Transformers, which are currently not equipped with an AVR to operate automatically. The operator will need to adjust as deemed necessary from the central control room.

I was wondering if members of this forum might provide reference information to an operating philosophy with regards to OLTC.

In advance, many thanks for any information.

Kind regards,


RE: On-Load Tap Changer (OLTC) Operating Philosophy

Power flow out of the generators will be controlled by the throttle or fuel settings on the prime movers.
Energy in equals energy out (including losses).
Voltage adjustment controls the flow of reactive power.
When there is a variation or change in the grid voltage there will be a corresponding change in the flow of reactive current.
With this change there is also a change in the power factor of the generators.
The flow of reactive current may be adjusted and/or controlled by the use of either the AVRs on the generators, the OLTCs or a combination.
On transmission lines the purpose may be different.
OLTCs are used to compensate for line losses.
The capacity of some long transmission lines may be limited more by the the limit of the compensation available from the OLTCs than by the ampacity of the line.
It depends.

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

RE: On-Load Tap Changer (OLTC) Operating Philosophy

Hi Bill,

Thanks for the reply.
Below you'll find a high-level schematic illustrating the situation.
With operation philosophy, I was referring to the procedures that the operator would take to manage the OLTC operation.
For example in what situation is the operator to use the OLTC? I understand it should be in close coordination with the generator output to the grid, but when shall the operator decide to use the OLTC for reactive power compensation to balance the grid voltage?
Thanks in advance for any information.

RE: On-Load Tap Changer (OLTC) Operating Philosophy

The 13.8kV bus voltage is reference to the Generator AVRs I suppose. In that case, the generator AVRs will try to maintain the 13.8kV bus voltage constant under varying load conditions. If you like to maintain power factor, I suppose the generator AVR will also have an option to select power factor mode and this suits better for captive power plant serving loads of a specific industry.
The transformer OLTC operation is not required / not desirable as long as the generator AVR is able to regulate the voltage.
If the variations are beyond the AVR range of generators, the transformer AVR can be set to come alive based on the trigger that says the generator AVR has reached its limit of regulation.
The other option is choosing dead band of transformer AVR beyond the available voltage range of regulation in generator AVRs. This ensures that transformer AVR cuts-in only after the generator AVR reaches its limit.
Just a few thoughts for you to work on, based on your specific requirements.
Generally speaking, Generator step-up transformer OLTC is manually operated in grid connected power stations.

RE: On-Load Tap Changer (OLTC) Operating Philosophy

Further to RRaghunath's post.
The generators will be maintaining the set voltage on the 13.8 kV bus in order to deliver a stable voltage at the 460 Volt bus.
Adding a tie to the grid, you will need OLTCs on the transformers to match the grid voltage and track changes in grid voltage.
Your agreement with the utility will generally include limits on the power factor and you will need the OLTCs to stay within the limits as the grid voltage changes.
You will not be able to control real power flow with the OLTCs.
Real power flow is controlled by the throttles or fuel rate of the prime movers.
As you go on-line and ramp your co-generation contribution, you may have to adjust the OLTCs to keep your PF between the allowable limits.

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

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