Which of the following types of cooling is employed in small and medium distribution transformers?

Question

Examoogle · Accepted Answer

The correct answer is Option A: Oil filled self-cooled. Oil-filled self-cooled transformers utilize natural convection currents within the insulating oil to transfer heat from the transformer core and windings to the tank walls, from where it is dissipated into the ambient air by radiation and convection. This method is the standard cooling technique for small and medium capacity distribution transformers due to its simplicity, reliability, and lack of requirement for external cooling components.

Step-by-Step Solution & Concepts:
Oil-filled self-cooled transformers utilize natural convection currents within the insulating oil to transfer heat from the transformer core and windings to the tank walls, from where it is dissipated into the ambient air by radiation and convection. This method is the standard cooling technique for small and medium capacity distribution transformers due to its simplicity, reliability, and lack of requirement for external cooling components.

Working Principle: As heat is generated in the windings and core, the oil in contact with these parts becomes hot and less dense. This heated oil rises to the top and moves towards the tank walls through convection. The cooling fins or corrugated tanks increase the surface area for efficient heat dissipation into the surrounding air.

Key Formulas: $Q = h A \Delta T$ — where $Q$ is the heat dissipation rate, $h$ is the convective heat transfer coefficient, $A$ is the cooling surface area, and $\Delta T$ is the temperature difference between the tank surface and ambient air.

Key Points: Used typically for transformers rated up to 5 MVA.; No external pumps or fans required for oil circulation or air movement.; Reliability is high due to the absence of moving parts.; Design relies on natural cooling provided by tank walls and radiators.

Advantages: Simple construction and low maintenance.; Extremely reliable due to no auxiliary power requirement.; Silent operation as no forced cooling equipment is used.

Disadvantages: Limited heat dissipation capacity.; Requires larger tank dimensions compared to forced cooling methods for the same rating.; Not suitable for very large capacity power transformers.

Applications: Pole-mounted distribution transformers.; Substation auxiliary distribution units.; Rural electrical networks.

Additional Info: For larger capacities, forced oil cooling (OFAF) or forced air cooling (ONAF) is preferred.. Option B (Water filled self-cooled) is technically non-standard for transformer insulation due to dielectric requirements of oil.. Options C and D involve water-cooling, which is typically reserved for very large power transformers or furnace transformers to manage high heat densities.

Answer: A is correct — Oil filled self-cooled (ONAN) is the standard method employed in small and medium distribution transformers to facilitate natural convective heat dissipation.

Exam Tip: Remember the cooling classification notation: ONAN (Oil Natural Air Natural) corresponds to Oil filled self-cooled, while ONAF refers to Oil Natural Air Forced.

Examoogle · Answer

Option B: Water filled self-cooled

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Option C: Water filled oil cooled

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Option D: Oil filled water cooled

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