Three units of single phase transformers and one single three-phase transformer rating

Question

Examoogle · Accepted Answer

will be same for one rating

Explanation: A bank of three single-phase transformers and one three-phase transformer are functionally equivalent in terms of power rating and transformation ratio for a given load application. Both configurations are designed to handle the same total kVA rating while providing the same primary and secondary voltage levels.

Working Principle: The total capacity of a three-phase system is $S_{3\phi} = 3 	imes S_{1\phi}$. When three single-phase transformers are connected in a bank (e.g., Star-Delta), they simulate the operation of a single three-phase transformer unit. The internal magnetic coupling and physical arrangement differ, but the external electrical performance parameters remain identical for the same specified rating.

Key Formulas: $S_{3\phi} = 3 	imes S_{1\phi}$ — Total power capacity of three single-phase units; $V_{L} = \sqrt{3} V_{ph}$ — Line voltage for Star connection

Key Points: Three-phase transformers offer reduced floor space and lower weight compared to a bank of three single-phase transformers.; Single-phase transformers are easier to transport and replace individually in case of failure.; Electrical rating (kVA, Voltage, Frequency) is independent of whether the iron core is shared or separate.

Advantages: Modular replacement for single-phase banks; Identical electrical characteristics for same rating

Disadvantages: Three-phase units are difficult to repair on-site; Three-phase units lack redundancy compared to separate single-phase units

Applications: Distribution systems; Power grid transmission substations

Additional Info: Option B is incorrect because physical configuration does not dictate the electrical rating capacity.. One three-phase transformer is essentially three single-phase windings on a common magnetic core.

Answer: A is correct — The rating of a three-phase transformer is equivalent to a bank of three single-phase transformers of the same total capacity.

Exam Tip: Always remember that for an ideal transformer bank or 3-phase unit, the efficiency and regulation are primarily defined by the winding resistance and leakage reactance, not the physical core structure.

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