GSU Tap changer control strategy - Review

[Adam1980]

Dear all,

I went through the forum to see the input regarded the control strategies and installation requirements for the generator ste-up unit transformer tap changer and I found some indications. However I would appreciate if anyone can provide some literature regarding this topic whether in national grid codes or scientific publications or from experience.

The main questions are:
- what are the recommended requirements for GSU transformer (tap-changer or no tap-changer, on-load or no-load)
- operation of the tap-changer (if on-load would it be automatic or manual)
- strategy of operation (is it the target of the tap changer to control the voltage at the connection point i.e. the point at which the generators feeds into the network, or to control the reactive power dispatch).

Thank you.
Adam

 

[ScottyUK]

The answers to most of those questions will be determined by the transmission system operator. TSO's typically publish a set of rules stipulating the requirements from generating plant connecting to their system. In the UK it is The Grid Code; similar documents are published by many other TSO's too.

http://www.nationalgrid.com/uk/Electricity/Codes/gridcode/gridcodedocs/

 

[Adam1980]

Hello Scotty,
thank you for your answer. As you know the grid codes are quite general in defining a control/operating scheme. From practice what is usual the operating strategy for the GSU transformer?


Thank you.

 

[rcw retired EE]

Depends on the system. In USA, New York City voltage is very stable and the standard +/-2.5%, +/-5% De-Energized Taps are all that is required. They are set once and seldom change. Generator voltage is controlled to maintain voltage or MVAR output as required by the system operator.

In areas where system voltage is less stable, +/-15% or 10% tap ranges with On Load Tap Changers are used, to match the fluctuating voltage. When the plant is off line, the local voltage may be so low that it is difficult to get the power plant started. The tap changer is operated manually or automatically to maintain the in plant voltage at nominal voltage.

When the generator comes online, the transformer and generator regulators will fight each other in Automatic and drive the tap changer to the maximum or minimum tap. Usual practice is to put the transformer regulator in manual and operate the generator voltage setpoint for MVAR or Voltage control. A plant DCS logic can suggest to the operator when a tap change is required. On many systems, a tap change can only be done if directed by the system operator.

 

[Adam1980]

Hello rcwilson,
thats a very nice summary of the operation strategy in New York. Thank you.

Do you know if such practices are published in some kind of guidelines or recommended operation practices documents for example.

A couple of questions:
- de-energized taps are different than off-load taps? but in both cases transformer de-energization is needed for a tap change.
- did you or anybody else reading this post know whether operating the GSU OLTC (in case it is installed) in automatic mode is a common practice?
In my opinion operating the OLTC in automatic mode would pose a great threat on system stability following large disturbances. However I can't find (till now) any literature that verifies/supports that.

Thank you!

 

[LSpark]

The OLTC on the plants (UK) I look at are usually operated manually in response to a request for a change in reactive power by the TSO. Where there are LV breakers and we import power back through the GSU the tap changers operate automatically to maintain the voltage on the unit auxiliary boards when the generator is off-load. The grid voltage is pretty stable so I doubt they are tapped in auto very often.

 

[Adam1980]

Dear all thanks for the replies.

In general is it possible to state the following:
- Tap changers are needed for the GSU especially when observing the grid code (ex. 0.9 - 0.85 pf for terminal voltage or PCC voltage of +/- 10%). Unless the generator operates within its capability for a +/- 10% terminal voltage range a tap changer is needed.
- If the tap changer is not operated in automatic mode that means for both cases of on-load or off-load tap changers a certain time is needed to change the tap position. This time should be scheduled especially when off-load tap changer is considered since the GSU should be de-energized. In stable networks this is acceptable since the voltage will not vary in a way which requires a tap changer action and any sudden dynamic event is faster than any tap changer action (usually in 5 sec. order).
- The last statement/questions would be: is it "recommended" to operate the OLTC of the GSU in automatic mode in a network where the voltage at the transmission level is not stable? will that help stabilizing the transmission system voltage or it may cause further oscillations since the OLTC operation will not be coordinated and could lead to controller hunting of the different OLTC's.

Here I am assuming that the GSU OLTC is controlling the generator's terminal voltage with the generator's acceptable oeprating range (+/-5%).

The main question of my inquiry is the question if the GSU tap changer could be used for daily network voltage control operations.

Thank you.

 

[ScottyUK]

Regulating the generator terminal voltage using the OLTC is an unusual operating mode, in the UK at least. The AVR will typically be used to control the voltage on the generator main terminals, and the OLTC will either regulate HV voltage or be used to flex the reactive power flow depending on how stiff the grid is at that location. The AVR may be used to trim a specific reactive despatch if the OLTC steps are too coarse, at the expense of a slight movement of terminal voltage.

UK regulations regarding CCGT stations require that a generating module of, say, two GTs and one ST is configured to tap simultaneously as though it were a single generator. This can introduce its own difficulties

 

[Adam1980]

Thanks again for the replies.

Sorry for repeating myself but are there any publications from TSO's or Dispatch centers where these practices are documented?

Thanks.