Why transformer are connected in   parallel.

  • In order to increase the current rating to supply large load transformer need to be connected in parallel.
  • It is not practical to construct a large transformer which can single handedly supply large load.
  • If there is break down in one unit then the other unit can supply the load so we can have uninterrupted power supply.
  • Cost of stand by unit is reduced.
  • Transformer can be turn on and off depending upon power demand.
Necessary condition for parallel operation

                   Two transformer in parallel


  • Transformer should have equal voltage ratio otherwise a circulating current flow between the two transformer which increase the current supplied by one transformer and reduce the current supplied by other transformer which increase copper loss and reduce efficiency and can also damage over load transformer.
  • Polarities of both the transformer must be same otherwise it will give rise in circulating current earlier.
Necessary condition for parallel operation of three phase transformer 

  • Same phase sequence.
  • Same polarity.
  • Same voltage ratio.
  • Phase shift between primary and secondary must be same .
Desirable condition of parallel operation 

  • The load sharing between the transformer should be proportional to the kva rating of transformer .
  • It can be achieve if per unit impedance of transformer on their own base are equal.
The X/R ratio of both transformer should be equal in order to maintain same power factor for both the transformer.

Advantages:-

  1. Reliability increase.
  2. Economic spare or standby capacity.
  3. Transformer can be switched off when demand reduces.
Disadvantage:-

  1. Costly.
  2. Occupies more space.