Paralel operation of transformers

Two or more transformers are said to be connected parallel when both primary and secondary sides of each transformer are connected to common buses on the supply and load sides respectively as shown in the figures below.

a) b)
Parallel operation of: a) 1-phase, b) 3-phase transformers
Description: Odbiornik - Receiver

Parallel operation of transformers is correct when:

  • no transformer secondary current flows in the no-load state.
  • transformers are loaded proportionally to their rated power.
  • the respective currents of individual transformers are in phase with each other

In order to fulfill the above conditions transformers which are to operate in parallel must meet the following requirements:

  • Rated voltages, both primary and secondary, should be the same (within ±0,5%). 
  • Transformers should have the same group of connections with the same hourly shift 
  • Short-circuit voltages of the transformers should not differ by more than 10% 
  • The operational power ratio S1/S2 (continuous/short-time) should not be bigger than 1/3 for any transformer. 

If any of the above conditions are not fulfilled, equalizing currents will flow in the secondary windings of the transformers, resulting in additional losses. This has an adverse influence on the load distribution which prevents the transformers from operating at their full rated power.

Parallel operation of transformers has many advantages, both operational and economical. For example, where the power demand varies over a wide range, fitting one large transformer may be less economical than fitting multiple transformers which are switched to suit the load. During periods of low power demand the core losses of one large transformer would contribute to high inefficiency.

An additional advantage of parallel operation is the fact that the power of a single transformer intended for parallel operation is lower than the power of a transformer working alone with the same load.
Therefore the power and costs are lower in case when reserve transformer is applied.

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