Two three phase transformers are said to be connected in parallel if their primary windings are connected to supply bus bars and secondary windings are connected to the load bus bars.
While connecting two or more than two three phase transformers for parallel operation, it is essential that their terminals of similar polarities are joined to the same bus bars because the improper connections may cause a short-circuit and the primary side of the transformers may get damaged unless protected by fuses or circuit breakers.
There are some principal reasons for connecting the three phase transformers in parallel.
• If any transformer damaged, the continuation of supply can be maintained through other transformers.
• When the load on the substation becomes more than the capacity of the existing transformers, then another transformer can be added in parallel.
• Any transformer can be taken out of the circuit for repair or routine maintenance without interrupting supply to the consumers.
Conditions for proper parallel operation of three phase transformers:
In order that the transformers work satisfactorily in parallel, the following conditions should be satisfied:
• Transformers should be properly connected according to their polarities.
• The voltage ratings and voltage ratios of the transformers should be the same.
• The per unit impedances of the transformers should be equal.
• The reactance or resistance ratios of the transformers should be identical.