Parallel Operation of 3-Phase Transformer
Definition:
If two or more transformers are connected to the same supply on the primary side and to the same load on the secondary side, then it is called as parallel operation of transformers.
Condition for Parallel Operation of Transformer:
Some of these conditions are convenient and some are mandatory.
Convenient conditions:
• Same voltage ratio and turns ratio (both primary and secondary voltage rating is same)
• Same percentage impedance and X/R ratio.
• Identical position of tap changer
• Same KVA ratings
Mandatory conditions:
• Same phase angle shift (vector groups are same)
• Same frequency rating
• Same polarity
• Same phase sequence.
NOTE: When the convenient conditions are not met, the parallel operation is possible but not optimal.
EXAMPLE:
Q: Two 3-phase transformers cannot be operated in parallel if their:
(A) KVAs are different
(B) % Impedances are different
(C) Voltage ratios are different
(D) Phase sequences are different
ANSWER: (D)
Explanation: When the convenient conditions are not met, the parallel operation is still possible but not optimal.
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