Power transformer is designed to have maximum efficiency

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Option A: Near full load

Explanation & Concepts:
A power transformer is designed to operate at maximum efficiency near its rated full load to ensure minimal energy losses during continuous operation. Since power transformers serve base loads in transmission networks, they are optimized to keep constant losses (iron losses) equal to variable losses (copper losses) at or near full load.

Working Principle: The efficiency of a transformer is defined as the ratio of output power to input power. Efficiency is maximized when variable copper losses ($I^2R$) equal constant core/iron losses ($P_i$). Because power transformers are designed for consistent, high-utility operation, the copper losses are engineered to match the core losses specifically at the rated full-load current.

Key Formulas: $\eta = \frac{Output}{Output + P_i + I^2 R_{eq}}$ — Efficiency formula where $P_i$ is core loss and $I^2 R_{eq}$ is copper loss.; $P_i = I^2 R_{eq}$ — Condition for maximum efficiency where iron loss equals copper loss.

Key Points: Distribution transformers are designed for max efficiency at 50-70% load due to intermittent loading.; Power transformers are designed for max efficiency at 100% (full) load due to continuous loading.; Core losses remain constant regardless of load, while copper losses vary with the square of the current.

Advantages: Higher operational efficiency at rated capacity; Reduced thermal stress during steady-state full load operations

Disadvantages: Lower efficiency at light loads compared to distribution transformers; Requires stable grid demand to maintain peak efficiency

Applications: Grid Transmission; Step-up/Step-down substations

Additional Info: Option C (50% load) describes the typical design optimization for distribution transformers, which face variable load cycles.. The efficiency curve is relatively flat near the peak, but power transformers prioritize the full-load point to minimize long-term losses.

Answer: A is correct — Power transformers are designed to achieve maximum efficiency at or near full load because they operate continuously under high, stable demand.

Exam Tip: Always distinguish between Power Transformers (Full load max efficiency) and Distribution Transformers (50-75% load max efficiency) in exams.

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Option B: At 75% load

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Option C: At 50% load

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Option D: Between 50 to 75% load

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