Delta Star Connection in Transformer

What about the V1/V2 ratings of the transformers?

When you connect the three transformers either in star or in delta, of course you pass on the KVAs to the secondary too but at the expense of the secondary voltage/ current and also provided you have the primary 3 phase.

I'm afraid the right answer here seems to be: having united in a 3 ph supply (and in any a way) three 1 ph transformers of 5 kVA each, we cannot get one 3 ph transformer of 15kVA, we can get the 5kVA*root3, that is, 8.66 kVA 3 ph transformer.

Do you have a 3 phase supply?

You cannot as (simply put) three 5KVA single phase transformers can only be connected up to give approximately 8KVA 3 phase supply, and that is assuming you have a three phase supply to begin with as you cannot connect transformers to a single phase supply and get a three phase supply out.

http://en.wikipedia.org/wiki/Three-phase_electric_power

I strongly suspect you don't understand the concept of single and three phase. What's the application?

Hi connecting three 1-p transformers to produce a 3-p transformer is generally feasible but with a few drawbacks and restrictions. For example you have 220V 2:1 ratio transformer. Connected to 220V will give about 110V at rated load.Connecting 3 transformers' prime windings to 3-p 220V in star it will give equivalent results (110V)in output star and about 190V in output delta with triple added power Connecting input in 220V delta will soon burnout because the prime winding will see some 380V and will briefly give secondary 190V star and 328V delta To connect these transformers in their normal delta you would need 3-p 127V. But that's almost obvious whats not obvious is that in common 3-p transformers all three windings (because cores are in magnetic contact) are interacting with each other even at star with one burned prime winding to give you the worst case scenario and still will give output (not equal) to all three out windings. That's not he case with three different transformers at adequate distance. Actually that's partly good because it balances somewhat the load but has it's drawbacks.Once had a fridge compressor burnout because of a 1 phase drop in my area and the poor motor tried to start at 50-60V Slow death Hope helped some

You answered your own question and this is done all the time! Why, if you lose one transformer you can reconnect them in open delta, lose 1/3 of your capacity and keep on ticking with a reduction in load!

It is interesting to see this kind of question in this forum.After many years and many transformers I use to simply say no ,we cannot have 3 phase supply comming from a single phase supply source simply because there is no phase sequence.The answer in this context is still no.

In situation where there are available 3 phase supply and many 1 phase transformers and 3 phase devices waiting to be operated,this exercise may be needed.I think the rating of the KVA and the voltages will change accoding to a 3 phase machine,and there will be a lot of magnetising losses as they are individual and indipendent eventhough we can connect them up electrically D-Yn ,externally.

I believe this is a theoritical question ( more so it is 5MVA ).In real life we definitely do not need to think about this configuration which is very wasteful in terms of material use and non standard equipment involved.

Regards,

Khor

Think nobody implied 1 phase input would give 3-ph output. It's clear this guy asked if three different 1-ph transformers could be connected to 3-ph input and whats the logics of this connection eg equivalency with actual 3-ph transformer

This was already replied in #1.

If three single phase transformers are connected to the 3 phase line, then of course you pass on the KVA ratings to the secondary.

In a 3 phase line, the individual phases will be handling the V,I ratios and hence the individual phase will be handle the load KVA.

The Voltage of Pri/ Voltage on secondary depends on the insulation. and is converted based on its turn ratio.

The currents on pri &Sec depends on copper and also is equivalent whether it is single phase or 3 phase.

Think for a moment as the three phases as three independent loads and the loads are conneceted between the neutral and phase. Now can each transformer take the rated (ie 5KVA?)

The respective currents or voltages are automatically compensated.

Yes the secondary power is √3VI CosΦ (or secondary KVA = √3VI) but so is the primary. To take care of the load the primary draws the √3I current(say), but the copper is upto it.

Let us see it this way

You have 3x5KVA (secondary = 1KV for simplification) Transformer so each secondary current = 5A

Connect this in star

The voltage is now = √3 KV,

Current capacity = 5A so it can now transmit √3*√3*5 = 15KVA

in case of the delta connection, rememver though the current is √3*5A, but it is being shared by two phases.

Similar logic will exist at the primary side.

So 3 single phases can be connected on Star or delta in secondary to get the required secondary voltage (may be important for the load voltage rating)

Say we have 200V/100V transformer (of course we don't have, but to be simple )

The primary is 346V 3 phase (for 200V L-N)

Connect both in Y (Pri and secondary) you get 100V at L-N ie 173V Phase-Phase

Connect Pri in Y and secondary in Δ and you get 100V Phase-Phase

Connect Pri in Δ and secondary in Y and you get 300V (P-P)

Connect both in Δ and you get 173V (P-P)

KVA rating is not compromised.

However a simple 3 phase transformers would have needed much less iron, and losses.

A few links, got a bit late to edit and add - time to search them out.

http://www.ece.msstate.edu/~donohoe/ece3414three_phase_transformers.pdf

(page-4)

and these were not exactly on the OP requirement but talks about the bank.

http://www.federalpacific.com/university/transbasics/chapter3.html

http://www.elec-toolbox.com/usefulinfo/xfmr-3ph.htm

Just connect the high side and the low side of the 3 transformers using any of this configuration:

1. delta-delta

2. wye-wye

3. delta-wye

4. wye-delta

When you bank 3 single phase transformers, the total kva of this bank, is just the sum of the the three kVA ratings. Every electrical engineer knows this.

Just wondering, are you an electrical engineer? This is a basic thing for electrical engineers.