GSU Transformer 5

[wesles24]

Hello,
I am putting together a specification for a 115/2400 GSU transformer and I was wondering if anyone on here may know if this type of transformer should follow IEEE Std. C57.116-2014: IEEE Guide for Transformers Directly Connected to Generators? Also, what is the difference between normal substation transformers and GSU transformers? From my understanding, the GSU Transformers experience much more thermal stress. I have heard that they may should be wye-delta connected??? Should anything different be stated in the GSU spec that is not in a normal transformer spec?

Any feedback and information will be appreciated.

Thank you!

 

[krisys]

GSU Transformers are generally subject to higher voltage variations (rather steady state over voltage) in order to control the reactive power to be pushed to the grid. Also during the start up, the voltage build up process may encounter the variation in frequency (due to speed build up). Thus over-fluxing is one of the very important criteria to be considered. Thus it is a special design.

There could be many other aspects, which I do not readily recall.

 

[ScottyUK]

Usually with very heavy bracing to withstand faults. Usually generously rated to allow for continuous or prolonged periods of running at base load.

 

[prc]

As far as a user is concerned, not much difference-not even more thermal stress (in case of net work transformers depending on load, usually the coolers will be switched on-off to maintain the same copper temperature ) or more bracing (the fault currents will be less in GSU as the generator impedance will limit it). The designer has to take care of extra stray losses, tank/LV turret heating from the LV side leads and bushings that may be carrying very high currents of the order of 5-20kA.He must take care of the transferred surges to LV (more than nominal BIL value) during back charging.

 

[arrehman1]

you might also want to consider higher rated bushing CTs due to high X/R ratio'd faults near GSU.

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[ScottyUK]

Hi prc,

'Generously rated' in relation to the turbine rating - gas turbines like cold dry air, so it is possible for a machine to produce more output than its nameplate rating at standard conditions. Turbine capability rises faster than transformer capability as temperature drops, and most operators don't want their turbine constrained by the transformer capability so the transformer is typically 10% larger in rating than the alternator it is coupled to.

'Heavily braced' is usually necessary due to the very high assymetric peak currents during a fault close to the generator which imposes maximum stress on the bracing. Generator subtransient reactance isn't all that large in terms of limiting the fault, plus in a multi-machine station there is a lot of infeed from both the grid and neighbouring units without much system impedance to calm things down.

 

[m3ntosan]

Transformers intended to be connected directly to generators in such a way that they may be subjected to load rejection conditions shall be able to withstand 1,4 times rated voltage for 5 s at the transformer terminals to which the generator is to be connected.

May you grow up to be righteous, may you grow up to be true...

 

[wesles24]

Thank you all for your input

 

[Power0020]

@m3ntosan, can you please mention the reference for this requirement of 1.4 p.u voltage for 5 s @ generator side?

 

[prc]

Scotty, the generously rated MVA comes in to play only for GSUs for gas turbine generators.
That is one part of GSU range.Otherwise it is less than alternator rating as part of power will be taken by auxiliaries. Some special requirements will be there for GSUs for pumped storage hydro stations, or units with generator circuit breakers.

A 3L fault on GSU primary or generator terminals is remote as the connections are of isolated phase bus duct.

Transformer engineers believe that transformers designed with a core flux density of 1.7 T at rated voltage and frequency will withstand 110 % overfluxing continuously, 125 % for 1 minute and 140% for 5 sec. No one has tested it though. These are the extent of overfluxing noticed in thermal power stations with large generators and GSU specifications normally call for this range of overflux with stand capability.

 

[m3ntosan]

@Power0020 IEC 60076-1

May you grow up to be righteous, may you grow up to be true...