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Parallel operation of transformers

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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

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).


Fig. B21: Phase change through a Dyn 11 transformer