Dry-type and oil-immersed distribution transformers

A distribution transformer steps a medium-voltage supply, commonly 11 kV or 33 kV in India, down to the 433 V used by a building or plant. The two main construction types, oil-immersed and dry-type, do the same electrical job. They differ in how the windings are insulated and cooled, and those differences decide where each can be installed and how it is looked after.

Oil-immersed transformers

In an oil-immersed transformer the core and windings sit in a steel tank filled with insulating oil. The oil insulates the windings and carries heat to the tank walls and radiators, where it passes to the surrounding air. In the most common arrangement, oil natural, air natural (ONAN), both oil and air circulate by natural convection with no pumps or fans.

Characteristics

• Oil is an effective coolant and insulator, so oil-immersed designs are compact for their rating and handle overloads well because of the thermal mass of the oil.
• They are well established for outdoor substations and for ratings across the distribution range, commonly up to 2,500 kVA and beyond.
• The oil needs attention: periodic sampling and testing, a breather to keep moisture out, and checks for leaks.
• Because the oil is combustible, indoor installation needs fire separation, oil containment and the precautions required by the applicable regulations.

Dry-type transformers

A dry-type transformer has no liquid. The windings are insulated with solid materials, often cast resin or vacuum-impregnated insulation, and cooled directly by air. Fans can be added to increase the rating for short periods.

Characteristics

• With no oil there is no oil leak or oil fire risk, which makes dry-type units attractive inside buildings, close to the load and in places with strict fire requirements.
• They need clean, dry, well-ventilated rooms. Dust and condensation on the windings are the main concerns.
• For the same rating they are usually larger and heavier than an oil-immersed unit, and they are generally more expensive.
• They are more sensitive to sustained overload because air has less thermal capacity than oil.

Comparing the two

Losses and efficiency

Both types have no-load losses in the core and load losses in the windings. Efficiency at rated load is high for both, typically around 98–99% for distribution ratings. What differs between two quotations is usually the specified loss level, not the construction type. In India, distribution transformer energy-efficiency levels are set out in IS 1180, and a specification that names a loss level makes offers comparable. The article on transformer losses and energy cost explains how losses turn into running cost.

Choosing between them

The decision usually follows from the site rather than from the transformer:

1. Where will it stand? An outdoor yard or a dedicated substation favours oil-immersed construction. A transformer inside an occupied building, a basement or near an escape route often points to dry-type.
2. What do the fire and building requirements say? Local authority, insurer and project rules can decide the matter.
3. What is the load profile? Frequent overloads favour oil cooling.
4. What maintenance can the site support? Oil testing needs a routine and a provider. Dry-type units need clean rooms and ventilation that stays effective.
5. What is the environment? Dust, humidity and corrosive atmospheres affect dry-type units more directly.

Whichever type is chosen, the transformer room or yard should allow for access during maintenance, safe clearances to live parts, ventilation sized for the losses, and a route for removal and replacement over the transformer's life.

Information to share for a specification

• Rating in kVA and the expected load profile.
• Primary and secondary voltages, vector group and tapping requirement.
• Installation location: indoor or outdoor, room size, ventilation and access.
• Fire, building and utility requirements that apply to the project.
• Required loss level or efficiency class and the applicable standard.

The transformer class listed on this site is oil-immersed with ONAN cooling, typically 25–2,500 kVA. If a project specification calls for dry-type construction, state it in the enquiry so the requirement is reviewed against what can be offered.

Where to go next

• Electrical Power & Distribution Transformers
• Reading a transformer nameplate
• Discuss your requirement: share the rating, voltages and installation location.