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3 rules for parallel transformers //

The following rules must be obeyed in order to successfully connect two or more transformers in parallel with each other:

Matching transformers for parallel operation
Matching transformers for parallel operation (photo credit: ABB)
  1. The turns ratios of all of the transformers must be nearly equal.
  2. The phase angle displacements of all of the transformers must be identical.
  3. The series impedances of all transformers must be nearly equal, when expressed as ‘%Z‘ using the transformer impedance base.

The first two rules are required so that the open-circuit secondary voltages of the transformers are closely matched in order to avoid excessive circulating currents when the parallel connections are made. The last rule is based on the fact that for a given voltage rating and %Z, the ohmic impedance of a transformer is inversely proportional to its KVA rating.

When transformers having the same %Z are connected in parallel, the load currents will split in proportion to the KVA ratings of the units.

Therefore, transformers with different KVA ratings can be successfully operated in parallel as long as their %Z values are all approximately the same.

Example of parallel transformers and loud BANG!

Two three-phase 10,000 KVA 66,000∆ – 12,470Y volt transformers were in parallel operation in a substation. The primaries of the two transformers are connected to a 66 kV transmission line through a single air break switch. This switch is designed to interrupt magnetizing current only, which is less than 1 A.

Circuit diagram for example showing circulating currents for parallel transformers with different turns ratios
Figure 1 – Circuit diagram for example showing circulating currents for parallel transformers with different turns ratios

The transformers were being removed from service and the secondary loads had been removed. A switchman then started to open the air break switch, expecting to see a small arc as the magnetizing current was interrupted.

Instead, there was a loud ‘BANG’ and there was a ball of flame where the air break switch contacts had vaporized. Something was obviously wrong. Upon closer inspection, it was revealed that the two transformers had been set on widely different taps:

The first transformer was on the 62,700 V primary tap and the second transformer was on the 69,300 V primary tap. Both transformers had a 7% impedance. Because the turns ratios were unequal, a circulating current was set up even without any secondary load. The open-circuit secondary voltage difference, assuming 66 kV at the transformer primaries, is calculated below.

The open-circuit secondary voltage difference
The open-circuit secondary voltage difference

The per-unit circulating current in the secondary loop is equal to ∆Es divided by the sum of the per-unit impedances of the two transformers //

The per-unit circulating current in the secondary loop

Converting Ic into amperes //

Converting Ic into amperes

Since Ic flows in a loop in the secondary circuit, the current out of the secondary of the first transformer equals the current into the secondary of the second transformer. But since the turns ratios are not equal, Ic does not get transformed into equal and opposite currents at the primaries.

Primary current of the first and second transformer

The net current through the air break switch, IAB, is the difference in the primary currents //

The net current through the air break switch

What actually happened?

The current through the air break switch supplies the Ic2Xs reactive losses of both transformers and therefore lags the primary voltage by 90°. The resulting current exceeded the interrupting rating of the switch, causing it to fail. The conditions described in this example are diagramed in Figure 1 above.

Reference // Power Transformers Principles and Applications by John J. Winders, Jr. (Purchase from Amazon)

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Lineil Pimentel
    Mar 06, 2018

    Hi Sir Good Day, will this be the same if the two power transformers came from two different sources with two different voltages, the one from 480V generator and the other from 4160V generator

  2. Doug Mayes
    Feb 11, 2017

    Would feeder protection remain the same once you parallel transformers? Should extra protection be installed?
    I leave a backup 10mVA transformer heated up but isolated because I fear a major fault on my plant’s feeders may take out both my main and backup transformers.

  3. Eduardo Lutz
    Nov 19, 2016

    Thank you a lot , really good posts.
    I have a question , what is the pourpose of having different types of connections like Dy5 or Dy7??

    Mar 24, 2016

    Thank you so much,
    Your articles are really useful to so many of us
    And the quality of the article is too good ,keep updated us,
    In any means if I could support your service Please let me know

    Best Regards / Cordialement,


  5. Hoang long
    Mar 18, 2016

    Wow so you great thanks

  6. Danielle
    Mar 17, 2016

    Great discussion.

  7. Imelda
    Mar 17, 2016

    Thank you so much, I learn something new every day.

  8. Naeem Hussain
    Mar 17, 2016

    Dear Edvard Csanyi,
    you are doing a great job.

    Mar 16, 2016

    Thanks to Edvard Csanyi .
    Electrical Wikipedia

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