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Parallel connection transformers

For parallel connection of transformers, various conditions that must be fulfilled for the successful parallel operation of transformers are:
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 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.

If the loads are unequal on 2 parallel transformers then their impedances must not be identical. Ideally the impedances on the rating of each unit should be at the same and imbalance loads would only occur then if the rating of 1 unit was higher than the other which would be ok. What is not ideal is like rated units of different impedances making 1 unit take more than its share of load.

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