Principles of Transformers in Parallel Connection

For supplying a load in excess of the rating of an existing transformer, two or more transformers may be connected in parallel with the existing transformer. The transformers are connected in parallel when load on one of the transformers is more than its capacity. The reliability is increased with parallel operation than to have single larger unit. The cost associated with maintaining the spares is less when two transformers are connected in parallel.

It is usually economical to install another transformer in parallel instead of replacing the existing transformer by a single larger unit. The cost of a spare unit in the case of two parallel transformers (of equal rating) is also lower than that of a single large transformer. In addition, it is preferable to have a parallel transformer for the reason of reliability.

With this at least half the load can be supplied with one transformer out of service.

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 and 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
7. Same Polarity
8. Same Phase sequence

Some of these conditions are convenient and some are mandatory.

The convenient conditions are: Same voltage Ratio and Turns Ratio, Same Percentage Impedance, Same KVA Rating, Same Position of Tap changer.

The mandatory conditions 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.

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Contents

• Introduction
• Condition for Parallel Operation of Transformer
• Summary of Parallel Operation of Transformer
• The combinations that will operate in parallel
• The combinations that will not operate in parallel
• How to check Synchronization of Transformers
• Advantages of Transformer Parallel Operation
• Disadvantages of Transformer Parallel Operation
• Conclusions

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