Transformer Synchronization

As the transformer is power transferring devise there is no special conditions for connecting in parallel and the word synchronization is not correct. Provision of proper protection is necessary

Thanks for your reply. You correct that synchronization is not the correct word.

Here in lies a problem. For the protection of both transformers they are better off run separately but with the facility to inter connect them if needs be. Running the transformers permanently in parallel causes problems with the fault withstand of the existing panels.

The new transformer will need it's own switchboard so a way forward is to transfer half the load from Tx1 switchboard to Tx2 switchboard.

If your goal is to have the transformers run in parallel it is important that they be as matched as possible. It is best if they are the same model number from the same manufacturer, at the very least they should have same rating and impedance to minimize the chance for less than perfect load sharing and the possibility of circulating currents due to minor manufacturing differences in the windings and core configurations/materials. Also keep in mind that as you parallel all these impedances that you don't create a dangerous situation where you expose everything to unanticipated high fault currents.

If the transformers have on-load tap changers (OLTC), then you should make sure they have controls installed to perform paralleling as well. If they don't the load tap changer controls and paralleling modules from Beckwith Electric are excellent. See their web site for info (I have no connection with Beckwith, but I have used their products with good results):

http://www.beckwithelectric.com/products/controls-ltc.html

As another reply stated, you also need to look at the combined fault current capability of the two transformers, and whether your bus, breakers, switchgear, etc., have adequate capability to withstand that. A 4MVA transformer at 11kV has only about 210amps full-load, so assuming 8%Z, it has available fault current of about 2625amps. Two transformers feeding into a fault would potentially double that, so your equipment may handle it. Now if you wanted to parallel two 40MVA transformers, that's another story...

Parallel operation of transformers in three-phase system: Parallel operation means direct terminal-to-terminal connection between transformers in the same installations.

For successful parallel operation, three basic condition must be satisfied,

1) Transformers have the same phase-angle relation.

2) The same ratio with some tolerance and similar tapping range.

3) The same relative short-circuit impedance - percentage impedance - with some

tolerance.

_{(Reference: IEC-60076-8 Power transformers - Application guide})

It is to be noted that, If two transformers with slightly different ratios are energized in parallel, this will give rise to a circulating current between the transformers.

So, If you have planned to add one same capacity transformer, then there will be no issue.

If two or more transformer are connected in parallel, then load sharing % between them is according to their rating. If all are of same rating, they will share equal loads.

Vinu_Answers Sure_answers

In parallel operated transformers of same capacity, load sharing is rarely a problem, if their terminal volatges is taken care of. The problems in load sharing crop up in dynamic machines like generators. But, do please take care of their percentage impedances and vector group.

And, though alreday corrected, I would like to re-emphasise that for using the term 'synchronisation', at least one of the equipment should be a rotating electric machine. So, two transformers are never "synchronised" but only operated in parallel.