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Anonymous Poster #1

# Parallel Operation of Two Transformers

06/02/2013 12:19 PM

Two Transformers having vector group DYn11 & DYn1 can be made parallel externally?

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#1

### Re: Parallel operation of two Transformers

06/02/2013 1:00 PM

No, they are out of phase. (Unless there is a typo in your description.)

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#2

### Re: Parallel operation of two Transformers

06/02/2013 3:55 PM

So, a little vaporized copper will only help to emphasize the value of a good engineering education.

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#3

### Re: Parallel operation of two Transformers

06/02/2013 5:38 PM

Parallel operation of Transformers

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#4

### Re: Parallel Operation of Two Transformers

06/03/2013 4:14 AM

Yes! But, by making one transformer a mirror image of the other. Pl. refer to the book "Design of Electrical Systems for Large Projects" by N. Balasubramanian.

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#5

### Re: Parallel Operation of Two Transformers

03/18/2014 7:27 AM

Dear All,

We had Planned to Parallel a 28 MVA and a 47 MVA Transformer with same Voltage Ratio, Polarity, Phase sequence and Vector Group.

But when it comes to Percentage Impedance, can 28 MVA Transformer be designed to have the same %Imp as a 47 MVA Trafo. Because the Definition of % Imp says that:

It is the percentage of the normal terminal voltage required to circulate full-load current under short circuit conditions.

So How can we maintain the same % Imp for these 2 transformers?? Also Please clarify whether any other Parameter needs to be considered prior to Paralleling. Any Specific recommendation for RTCC???

Earlier we had paralleled 2 No.s of 47 MVA Trafo of same parameters. But here the case is different. Kindly Help me in this regard.

Regards,

Raghavendran.R

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#6

### Re: Parallel Operation of Two Transformers

03/18/2014 9:50 AM

It is not impossible but critical. Between the two, the percentage impedance of the 28MVA Trf. would be lesser than that of the 47 MVA Trf. If you increase the percentage impedance of the 28MVA Trf. to match that of the 47MVA Trf., then the full load voltage drop would increase in the 28MVA Trf. and there could be circulating currents between the two trfs. in parallel.

Alternately, if you reduce the percentage impedance of the 47MVA Trf. to match that of the 28MVA Trf. then the fault level of the 47MVA Trf. would increase drastically, calling for special design of the trafo., to withstand the increased thermal & dynamic stresses of a short circuit.

I would like to quote from IS 10028-Part 2: Clause 12.0.1:

"12.0.1: Parallel Operation -for satisfactory operation in parallel, the concerned transformers shall have identical (a) phase displacement, (b) voltage ratio, and (c) equal percentage impedance within permissible tolerances. They shall be so connected that their polarity and phase rotation are identical.

Provided the polarities of the winding correspond,transformers with the following connections operate satisfactorily in parallel. Connections in Group A shall not be paralleled with those in Group B.

Group A - star/star, delta/delta, and delta/zig-zag.

Group B- delta/star, star/delta, and star/zig-zag.

12.0.1.1 Transformers designed for parallel operation shall not be required to divide the load within limits closer than to permit an individual variation from the rated output of any transformer of the group amounting to +/- 10 percent of such rated output when the total load on the group is equal to the sum of the rated outputs of all the transformers so connected. In addition, the rated outputs of the smallest transformer in the group should be not less than 33 percent of the rated output of the largest transformer in the group.

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#7

### Re: Parallel Operation of Two Transformers

03/21/2014 11:15 PM

Thank You Very Much Sir For that Update.

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