Operation of Transformers

Why Parallel Operation of Transformers is required ?

It is economical to installed numbers of smaller rated transformers in parallel than installing a bigger rated electrical power transformer. This has mainly the following advantages,

1) To maximize electrical power system efficiency: Generally electrical power transformer gives the maximum efficiency at full load. If we run numbers of transformers in parallel, we can switch on only those transformers which will give the total demand by running nearer to its full load rating for that time. When load increases we can switch no one by one other transformer connected in parallel to fulfill the total demand. In this way we can run the system with maximum efficiency.

2) To maximize electrical power system availability: If numbers of transformers run in parallel we can take shutdown any one of them for maintenance purpose. Other parallel transformers in system will serve the load without total interruption of power.

Condition for Parallel Operation of Transformer:

• For parallel connection of transformers, primary windings of the Transformers are connected to source bus-bars and secondary windings are connected to the load bus-bars.
• Various conditions that must be fulfilled for the successful parallel operation of transformers:

1.  Same voltage Ratio & Turns Ratio (both primary and secondary Voltage Rating is same).
2. Same Percentage Impedance and X/R ratio.
3. Identical Position of Tap changer.
4. Same KVA ratings.
5. Same Phase angle shift (vector group are same).
6. Same Frequency rating.

• Some of these conditions are convenient and some are mandatory.
• The convenient are: Same voltage Ratio & Turns Ratio, Same Percentage Impedance, Same KVA Rating, Same Position of Tap changer.
• The mandatory conditions are: Same Phase Angle Shift, Same Polarity, Same Phase Sequence and Same Frequency.
• When the convenient conditions are not met paralleled operation is possible but not optimal.