90MVA, 66/33 kV Tranformer switching with one 43.5MVA Gas turbine Generator connected to 66kV busbar

[sramesh]

We are evaluating whether 90MVA, 66/33 kV Tranformer switching is possible with one 43.5MVA Gas turbine Generator connected to 66kV busbar.The question is whether one 43.5 MVA Generaor will be able to withstand the switching inrush current of the 90MVA transformer(inrush is 5.3 times of I fl as per document)

Request your expert opinion on this

 

[Power0020]

The inrush current is usually of very short duration (less than a second)that shouldn't impact the generator capability and relays are designed with 2nd harmonic block for inrush current (usually 2 seconds).

To limit the inrush current, the transformer should be switched with no load on secondary.

 

[Skogsgurra]

Not so sure about the no load switching.

A loaded secondary reduces available volt-seconds and can reduce saturation, which reduces inrush. If you are lucky, that reduction will win over the increased current caused by the load. Did you run it in ETAP or some other trustworthy simulation program?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[prc]

If you are using gas turbine generator,why cant you raise generator voltage slowly to rated value so that in rush current is completely eliminated.More than generator Iam concerned about the step up transformewr used for raising the generator voltage to 66kV.It can get overloaded by inrush current.

 

[sramesh]

Thanks for the reply - Power0020, Skogsgurra and prc
On PRC comment, I forget to mention GTG is having a 55MVA transformer before connecting it to the 66kV bus, which is softstarts along with Generator. 90MVA transformer is distribution transformer from the BUS. We have 3 other STGs of 42MW capacity connected in 66kV bus. We are looking for restart after black out. until now we had another power source for black out re-start. This power source will be discontinued shortly. Just to add is we are currently experiencing voltage dip up to 11% while energising 90MVA transformer with 3 Generators online.
Skogsgurra: We would like to have a preliminary assessment before engaging a consultant.
Power0020: Can you please confirm about your point of view , if you have the similar experience. it will be helpful if you can explain a bit more as most of the engineers have opinion that Generator can Trip due to the high reactive inrush current requirement.

 

[ScottyUK]

I'd take the opposite view to Power0020 - switching a heavy reactive load onto a lightly loaded generator during a black start is inviting a major voltage excursion.

You'll find that the AVR will boost the field as it tries to recover the severe transient depression. It may hit the over-excitation limiter which in some installations is an instantaneous trip and in others is a delayed trip. You will quite possibly see the AVR then swing onto the under-excitation limiter as it tries to suppress the resultant overshoot. Your AVR topology will make a big difference to how the machine behaves: a 2-quadrant (regenerative) rectifier connected via sliprings can actively suppress the field by feeding the excess energy back into the source, but a brushless machine can't do this and has to rely on the resistance of the rotor to dissipate the stored energy over a time determined by the rotor R-L time constant, during which time the UEL may well be operational. Normally this isn't a trip, just a warning that the machine is operating near the stability limit, which isn't the best place to be especially if you are going to be adding large blocks of load.

Can you arrange the system transformer to be softly energised at the same time as the GSU transformer? It may require modification to switching interlocks, and you probably don't have synchronising capability other than at the GSU transformer breakers but in a blackout this probably doesn't matter too much.

 

[sramesh]

Thanks for your post ScottyUK, noted your suggestion for further evaluation
Regards

 

[Power0020]

I have never heard of transformer inrush problem with generators, the AVR action if controlled by a PID controller may not be that fast to act on a milliseconds event.

The saturation check function in generator protection relay should block the trip signal, the only concern here is the UV setting on generator.

 

[Bronzeado]

sramesh,

Why not using a transformer controlled switching to mitigate the inrush current?

Please, find attached a paper on controlled switching of a 100 MVA, 230/138 kV transformer in which is showed the success of this strategy.

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro