Question

For two three-phase transformers to operate in parallel, which of these is an absolute requirement?

• A. Same KVA rating
• B. Same cooling method
• C. Same phase sequence
• D. Same percentage impedance

Hint:

Never parallel three-phase transformers belonging to different phasor groups (e.g., a Y-Y group with a Y-Delta group) because the resulting \(30^\circ\) phase shift creates a large voltage difference, leading to catastrophic short-circuit currents.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to identify the absolute (mandatory) condition required to safely operate two three-phase transformers in parallel without causing severe damage.

Step 2: Detailed Explanation:

• Parallel operation of transformers is used to increase system capacity and reliability. To operate successfully, certain conditions must be met. These are classified into "absolute requirements" and "desirable requirements".

• Absolute Requirements:
1. Identical Polarities (for 1-phase) Same Phase Sequence (for 3-phase): The phase sequence of the secondary terminals of both transformers must be identical (e.g., both must be A-B-C). If the phase sequence is incorrect, it results in a dead short-circuit when the switches are closed, producing massive currents that will destroy the windings. This is an absolute requirement.
2. Zero Relative Phase Displacement: The transformers must belong to the same phasor group (e.g., both Dy1 or both Dy11) to avoid phase differences between their output voltages.

• Desirable Requirements (Not Absolute):
1. Same voltage ratio: Slight differences cause small circulating currents, which are tolerable but not ideal.
2. Same percentage impedance: Necessary to ensure that the load is shared in proportion to their kVA ratings, but parallel operation is still physically possible without it (though one might overload).
3. Same kVA rating: Transformers of different ratings can operate in parallel as long as their impedances allow proportional load sharing.
4. Same cooling method: Unrelated to electrical compatibility.

Step 4: Final Answer:
The absolute requirement is having the same phase sequence.