Parallel 2 Transformer

Sounds like a 20 volt discrepancy to me, with the higher voltage taking on more than 50% of the load.

Why on earth would you want to parallel two dissimilar transformers? One will take the bulk of the load while the other is happily just adding circulating currents.

Transformers of that size will normally have off load tap changing, use them to balance the voltage first.

Jackofalltrades is on the right lines....but it helps if you state your real problem at the start, so it does not look like a "homework" question!

My guess about your problem is :-

1. That you need to change the transfo taps de-energised.
2. To do that WITHOUT cutting off any loads, you must parallel the loads [and transfos] through the LV [380V] tie breaker.
3. You can then (since the loads are now in parallel) isolate one transfo and change its taps.
4. When you have changed the tap, you need to put that transfo back on line, before opening the tie breaker to split the load again.
5. Item 4 means you must parallel the transfos with different output voltages.

You will get circulating currents while the [not-equal] transfos are in parallel.

1. Since you have 20 V difference in 380V nominal, this is about 5% rated volts.
2. Each transfo has 6% impedance - so you have 5% volts across 6 + 6 =12% impedance. Since 5%/12% is 0.42, you will get 42% rated current circulating in a no-load situation.
3. If there were 100% load, the higher voltage transfo would take nearly all the load, with about 5% drop - to about the voltage of the lower transfo.

Since you are not exceeding 100% current, the transformers are in no danger of being overheated. But if you leave them in parallel you will be wasting energy and money supplying the losses due to the circulating currents. One transformer will wear out much quicker because it reaches full load instead of (ideally) half-load.

In a "normal" situation, one could have a split bus with 100% load on one transfo "side" and little on the other. Opening/closing the tie would cause 50% to 100% and 50% to about 0% transitions. But the size of these current changes is not very different to the case with "unequal" taps.

What I would worry about is that a transfo, carrying 50% or more of rated current through its inductive reactance, gets transferred onto almost zero load when the tie breaker is opened. The inductive current will keep flowing and might over-volt a load.

However, breakers do not really open instantly - in the normal action they have to burn off any inductive energy in arcs before current falls to zero - and as noted, 50% to 0% change could happen in the "equal taps" case with some systems.

I agree with you for circulation current in transformer.

But there shall be no problem in momentary paralleling such transformer.

In plant I worked we often switched source from source 1 at 100% to other source 2 at 0%. We never faced over voltage problem as such change over are done at LV side of system.

Dear all,

This case is now CLOSED. I already change the tap one by one without shutting down any loads, and when 380V and 400V were collided, transformer of 400V took about 970A of Ic and 400A of current-load. Things were going smoothly because of all suggestion you have given to me.

Tornados, you're simple and meaningfull. thanks a lot, sir.

Tonys, listening is fundamental and essential. Thanks a lot, mate.

Jack, your wisdom is unlimited source of power. Thanks a lot, mate.

Electricalexpert65, on now-earth everything need money, but money is not everything. Thank you very much, sir.

67model, it's awesome listening to you. Thanks a lot, sir.

Happy singh, experience is best teacher. There must be a place where our brain can explain. Thank a lot for sharing your experience, mr. singh.

cheers,