In modern day the entire generation and transmission is 3 phase in nature so the transformer required for 3 phase power system is also 3 phase.

3 phase transformer bank:-

If three single phase transformer are externally connected in order to realized a 3 phase transformer then it is called 3 phase transformer bank.

In 3 phase transformer bank 3 phase transformer are present so every part is repeated 3 times and thus costing is high.

If the three phase can be present on the same core then cost can be reduced.

3 phase transformer:- 

In the 3 phase transformer the 3 limbs of the core are oriented at 120 degree with respect to each other.

If all the 3 phase carry the same current then their flux are also balanced and sum of flux is zero.so flux does not required a return path.

Similar to 3 phase 3 wire system the flux when balanced do not need any return path to complete close loop.

The flux due to any phase complete it's close loop from the other two phases.

Difference between 3 phase transformer bank and 3 phase core type transformer:-

3 phase transformer bank:-

  1. More costly.
  2. 12 wires must be brought out (4 for each transformer)by the use of 12 bushing.
  3. Requirement more floor space.
  4. Offers more flexibility.
  5. If one unit is damage 1 single phase transformer can be replaced.
  6. One single phase transformer is required for standby.
  7. More iron loss due to more core volume.
  8. Less efficiency.
  9. Easy maintain.
3 phase core type transformer:-

  1. Less costly
  2. Star and delta connection is made inside the transformer so only six wire by use of six bushing need to be brought out.
  3. Required less floor space.
  4. Offer less flexibility.
  5. Entire unit needs to be replaced even if one winding is damage.
  6. Complete 3 phase unit is required for standby.
  7. Less volume of iron so less core loss.
  8. More efficiency.
  9. Difficult maintenance.
Comparison between core and shell type 3 phase transformer:-

Core type 3 phase transformer:-

  1. Less mechanical support.
  2. Less insulation required.
  3. More copper required.
  4. High voltage high power transformer.
  5. 3rd harmonic flux is absent.
  6. Emf is always sinusoidal.
Shell type 3 phase transformer:-

  1. More mechanical support.
  2. More insulation required.
  3. Less copper loss.
  4. Low voltage low power  transformer.
  5. 3rd harmonic flux is present.
  6. Emf is sinusoidal for delta connection.