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Home / Technical Articles / Parallel Operation Of Transformers
Parallel transformers - Siemens
Parallel transformers - Siemens

Most transformers installed in parallel have the same kVA, turn ratios, and impedances, which can make it difficult for power engineers in industrial and commercial facilities to understand circulating currents and load sharing.

As systems change over time, and transformers are replaced or added, users need to know the impact of paralleling transformers using different parameters.


The need for operation of two or more transformers in parallel often arises due to:

Load growth, which exceeds the capactiy of an existing transformer
– Lack of space (height) for one large transformer
– A measure of security (the probability of two transformers failing at the same time is very small)
– The adoption of a standard size of transformer throughout an installation

Total power (kVA)

The total power (kVA) available when two or more transformers of the same kVA rating are connected in parallel, is equal to the sum of the individual ratings, providing that the percentage impedances are all equal and the voltage ratios are identical.

Transformers of unequal kVA ratings will share a load practically (but not exactly) in proportion to their ratings, providing that the voltage ratios are identical and the percentage impedances (at their own kVA rating) are identical, or very nearly so.

In these cases, a total of more than 90% of the sum of the two ratings is normally available. It is recommended that transformers, the kVA ratings of which differ by more than 2:1, should not be operated permanently in parallel.

Conditions necessary for parallel operation

All paralleled units must be supplied from the same network. The inevitable circulating currents exchanged between the secondary circuits of paralleled transformers will be negligibly small providing that:

  • Secondary cabling from the transformers to the point of paralleling have approximately equal lengths and characteristics
  • The transformer manufacturer is fully informed of the duty intended for the transformers, so that:
    – The winding configurations (star, delta, zigzag star) of the several transformers have the same phase change between primary and secondary voltages
    – The short-circuit impedances are equal, or differ by less than 10%
    – Voltage differences between corresponding phases must not exceed 0.4%
    – All possible information on the conditions of use, expected load cycles, etc. should be given to the manufacturer with a view to optimizing load and no-load losses

Common winding arrangements

Fig. B21 : Phase change through a Dyn 11 transformer
Fig. B21 : Phase change through a Dyn 11 transformer

As described in 4.4 “Electrical characteristics-winding configurations” the relationships between primary, secondary, and tertiary windings depend on:

  • Type of windings (delta, star, zigzag)
  • Connection of the phase windings

Depending on which ends of the windings form the star point (for example), a star winding will produce voltages which are 180° displaced with respect to those produced if the opposite ends had been joined to form the star point.

Similar 180° changes occur in the two possible ways of connecting phase-to-phase coils to form delta windings, while four different combinations of zigzag connections are possible.

  • The phase displacement of the secondary phase voltages with respect to the corresponding primary phase voltages.

As previously noted, this displacement (if not zero) will always be a multiple of 30° and will depend on the two factors mentioned above, viz type of windings and connection (i.e. polarity) of the phase windings.

By far the most common type of distribution transformer winding configuration is the Dyn 11 connection (see Fig. B21).

SOURCE: Schneider Electric

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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. Abhishek chauhan
    Feb 14, 2016

    I want to know that on which factors the parallel operation of 4transformer can be done n which pair of transformer will be connect parallel to other pair like star-star!!delta-delta connections like that?. I hope u replay me soon.

    Thank you.

  2. tweety
    Mar 25, 2015

    hi, i’ve ever seen in oil and gas company that all transformers aren’t allowed to work in parallel. it’ll give impact its impadance value, they said. is that right?

  3. pranav
    Feb 19, 2015

    Dear edvard in parralele operation of tranasformer both operate on differant load how can it possible

  4. ankit
    Jan 22, 2015

    I am hungry about technology
    Pls feed me

  5. damico89
    Oct 12, 2012

    Hello, I was wondering if there was any danger in the transformers blowing up when put in paralleled…. my boss asked us the question the other day on the job site, he him self said something about if they are additive or subtractive they may explode…..

  6. Albeno
    May 09, 2012

    I want to know what the advantages or disadvantages are of operating instrument voltage transformers in parallel, the voltage ratings are as follow: 33 kV / 110 VAC, 200 VA per phase, class 1.0 voltage factor = 1.2 We are currently paralleling three phase VTs in our distribution system, is it better to use 3 x single phase VTs. I’m unsure what the phase shifts are for single phase vs. three phase VTs as we are using the VTs for metering purposes, which serves as reference to our compensating equipment. Any help would be appreciated! Regards

  7. Majid Erfanian
    Sep 13, 2011

    Dear Eduard ,as I know , parallel operation of power transformers are clearly showed in IEC 60076.
    Are you going to open a new discussion in this field ?

    • Edvard
      Sep 13, 2011

      Yes, paralel operation of power transformers are well described in IEC 60076. This technical article is an excerpt from mentioned IEC standard and if anybody wants to comment or discuss on it, go ahead!

      • Er.Lalit Rai
        Sep 01, 2013

        Kindly recheck IEC 60076 is not clear.Pleasesend link.

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