Conditions for parallel operation of transformers

Conditions for parallel operation of transformers.

If you are in electrical engineering, then you may know the importance of parallel operation of transformers at various power generating stations and substations. To operate 2 or more transformers in parallel we need to take care of few necessary conditions.

Some of the required conditions are

  1. For single phase transformers:
    • Same polarity of transformers
    • Same voltage ratio
  2. For 3 phase transformers:
    • Same polarity
    • Zero relative phase displacement
    • Same phase sequence
    • Same voltage ratio

In this post we are supposed to discuss some of the above mentioned conditions. Let us take a look over them:
  1. Polarity: The polarity of the transformers connected in parallel should be same otherwise it may lead to dead short circuit.
  2. Voltage Ratio: The voltage ratio of the 2 transformers should be kept equal in order to avoid losses occurring in transformers due to load circulating currents. If unequal voltage ratio is used it will give rise to circulating current in the closed circuit formed by the secondary of the transformer even at no load condition. The maximum permissible no-load circulating current should be 10% of its rated value.
  3. Zero Relative Phase Displacement: This is the necessary condition for the 3 phase transformers. As the name suggests, the relative phase displacement between the two transformers must be zero.
  4. Phase Sequence: This is also an important condition for 3 phase transformers which needs the phase sequence of the 2 transformers to be same otherwise it may lead to short circuit of the each phase pairs.

The above mentioned conditions were the general thumb rule conditions. Apart from these, some other requirements are also there which ensures that the transformer possesses better load sharing and operating power factor.

To meet those requirements, we can also make use of these two ways which are related to load side:
  1. Equal per unit impedances
  2. Equal “resistance to reactance” ratio

These are some of the general terms and necessary conditions associated with the parallel operation of transformers. If you have any queries and suggestion regarding this post or anything else, then do comment below.

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