Parallel Operation of Transformers

                           
                                                   The transformer is said to be in parallel operation when their primary

windings are connected to a common voltage supply and the secondary windings are connected to a

common load.

Advantages of Parallel Operation

  

• It increases the reliability of supply system.
•  The size of transformer increases with the increase of its rating,its transportation and instalation is difficult.On the other hand transportation and instalation of small size transformers are easy.
• One ore more transformers may be taken under maintenance and the remaining will supply the load.

 Disadvantages of Parallel Operation 

• Increasing short circuit current that increase breaker capacity.
• Bus rating could be high.
• Parallel operation reduces the transformer impedence which creates the high short circuit current.
• The control and protection is complex. 

Conditions of Parallel Operation 

• The line voltage ratio of two transformers must be equal.
• The per unit impedence of each transformer should be equal and the ratio of equivalent leakage reaactance to the equal resistance X/R .
• The transformers should have same secondary winding polarity.
• The transformers have same phase sequence.
• The transformerss have the zero relative phase replacement between the secondary line voltages