ETO ETO Exam papers 📅 Aug 2025

Exam Question

With Reference to 3-phase Transformers:

(a) Discuss the essential and desirable conditions to be fulfil for operating two three-phase transformers in parallel. (5)

(b) What are the advantages of a transformer bank of three single-phase transformers over a two three-phase transformer of the same kVA rating? (5)

(c) In a 25kVA, 3300/235V, 1-phase transformer, the iron and full load copper losses are 350 watts and 400 watts respectively. Calculate the efficiency at half load, 0.8 p.f. (6)

Reference Answer

(a) Essential and Desirable Conditions for Parallel Operation of Two Three-Phase Transformers
When operating two three-phase transformers in parallel, both essential and desirable conditions must be met to ensure safe, stable, and efficient load sharing.
Essential Conditions: These conditions, if not met, can lead to dangerous circulating currents, short circuits, or system imbalances.
Same Voltage Ratio (Turns Ratio): Both transformers must have an identical primary to secondary voltage ratio. Any difference will cause circulating currents to flow even under no-load conditions.

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