Transformer Reactive Power Requirement 1

[prc]

Let me refer to the following thread where rcwilson explained nicely about the above aspect of transformers. thread238-301672 (2011)
When a transformer of 100 MVA 10 % impedance is feeding power to grid from generator, there will be a drop (reduction ) of 10 MVAR in transformer. It results in improved power factor at transformer output, compared to the pF at generator output due to the MVAR "consumed" by the transformer.
There can be 3 situations. (a) the transformer HV voltage (say 132 kV) same as grid voltage of 132 kV (b) Transformer voltage at the time of synchronizing at +10 % voltage (145 kV) (c) Transformer tap at -10 % voltage. Now I have a question. How the MVAR consumed in transformer will vary under (b) & (c) and how much. As I understood, under (b) more MVAR will be used in transformer and under (c) more MVAR will be taken from grid. But how much? Under what conditions of grid these options will be required?

 

[bacon4life]

Generators usually match voltage and frequency prior to synchronizing, so very little should change at the instant of synchronizing.

When calculating the voltage drop, you have to use phasors rather than magnitude. At 1.0PU load using, the voltage magnitude only changes by about .005PU do to the series reactance.

As the transformer terminal voltage decreases, the series reactive losses increase, but the excitation losses decrease. Typically the series losses are much larger than the excitation losses. Transformers are subject to saturation, so the excitation losses dramatically increase above 1.05 PU.

 

[ScottyUK]

Hi prc,

The interaction between transmission system, GSU transformer and generator AVR is quite complex. Generally the GSU transformer voltage of the incoming machine would match that of the grid at the time of synchronising. Once the GCB is closed then the OLTC is used regulate system voltage and/or to flex the reactive capability of the generator. in my experience the reactive demand of the transformer doesn't change significantly over the normal tapping range, but as you note the reactive demand can be met from the generator or from the system or the reactive demand shared between both, all depending on the combination of system voltage, OLTC position and generator terminal voltage.

In the UK I haven't yet seen a large station despatched to a leading pf, where the GSU transformer Vars would be supplied by the system. Typically our stations either sit Var-neutral on the system at the HV side of the GSU transformer with the generator meeting the reactive demand of the transformer and no net reactive power flow at the GSU transformer, or they operate with the GSU transformer's OLTC at a below-nominal tap position which increases the step-up ratio. In this latter case the generator's AVR operates to restore the generator terminal voltage which would otherwise be forced low by the tap setting, causing reactive export from the machine onto the system. I'm not going to to try to describe in words what is far better shown on a vector diagram of the generator and transformer, but there are some good explanations in some machines texts. I will see if I can find a couple when back at work tomorrow.

 

[prc]

Thank you all. I have taken actual case from site. The GSU is with OLTC and the running condition is with transformer at 145 kV tap and grid at 132 kV with a large reactive drop across the transformer. I am curious what is happening then inside transformer to pull the 145 kV induced in transformer to 132 kV, the grid voltage. There is no generator circuit breaker and so the synchronization to grid is at GSU out put end. Probably utility may be synchronizing at 132 kV tap and then raising tapping on transformer to 145 kV to absorb (?) reactive power from grid. rcwilson explained something like that in this forum in 2013.

Scotty UK I will wait for your guidance on books.

 

[ScottyUK]

Raising the tap to 145kV against a system voltage of 132kV (or considered another way, forcing the generator voltage down by increasing the transformer ratio) will cause the generator to export reactive power, not absorb it, if the AVR is set to regulate the generator terminal voltage. The AVR will boost the field current to restore the terminal voltage to setpoint. At the most simplistic level, if the generator + GSU transformer are trying to raise the system voltage but can't because the generator is smaller than the system, then the system will absorb Vars.

There are some worked examples in the attachment to this post. I've uploaded this before, so maybe you have a copy. Appendix C is perhaps the most useful in understanding how the machine capability relates to the tapchanger.

I apologise for not getting a chance to look on my bookshelf today, it has been a little hectic. I will try again tomorrow.

 

[prc]

Thank you Scotty for that manual.