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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  2. 3 phase Transformer Security Using Small Operator Included system Venture Description: Generally most of the transformers are suffering from the environmental stress, conditions and line in currents. Because of the greater...

  3. Anonymous7/28/2012

    how does unequal voltage ratio leads to circulating current ??

    1. its simple! if 2 transformers of different voltage ratio are connected in parallel across the same supply voltage, the secondary side will have different voltage. So when they will be connected to the same bus bar, then voltage difference will be there, which will give rise to current (circulating current) both in primary and secondary.

    2. what is the value of circulating current

  4. Anonymous9/21/2012

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  5. Can Electrical power transformer be operated at voltages other than nameplate voltages? I

    1. yes it can be operated at a voltage lower than the nameplate voltage ratings, but it should not be operated at voltages above the nameplate voltage ratings.

  6. Anonymous11/12/2012

    Thanks for the info which is well explained. From a practical view, I guess there is a few things to watch out for when operating transformers in parallel. To name one would be the configuration protection configuration. This includes the physical protection scheme configurations and settings as well as the intertripping configuration (HV==>LV).

    I have come across various installations where a poor intertripping installation (also poorly maintained), has caused potentially dangerous situations. More specifically when there is a earth fault on one of the paralleled transformer's HV terminals (delta primary configured transformers with NER HV system) and it is cleared by the feeding HV circuit breaker.

    In one specific situation, the intertrip did not function and it left an energised transformer, with no fault current on the LV system. The tech that investigated the fault came dangerously close to the faulted cable/termination that is still energised via the LV system.


  7. Informative Post, as you described the specifications of 3 phase transformers and difference between 1 phase transformer its really amazing to read the articles. it can help to transformer manufacturers in future.

  8. what will happen if phase displacement is not same

  9. thanks for it. what will happen if phase displacement is not same?

  10. what is dead short circuit?

  11. Yeah! There are some of the general terms and necessary conditions which are associated with the parallel operation of transformers.

    Power Transformers in India | Transformer Manufacturer in India


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