GSU Tap changer control strategy - Review
GSU Tap changer control strategy - Review
(OP)
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
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






RE: GSU Tap changer control strategy - Review
RE: GSU Tap changer control strategy - Review
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.
RE: GSU Tap changer control strategy - Review
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.
RE: GSU Tap changer control strategy - Review
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!
RE: GSU Tap changer control strategy - Review
RE: GSU Tap changer control strategy - Review
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.
RE: GSU Tap changer control strategy - Review
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
RE: GSU Tap changer control strategy - Review
Sorry for repeating myself but are there any publications from TSO's or Dispatch centers where these practices are documented?
Thanks.