What Is a Transformer Tank? Types, Materials and Design

What Is a Transformer Tank?

A transformer tank is the welded steel enclosure that houses the core-and-winding assembly of an oil-immersed transformer and holds the insulating oil. It performs three jobs: it supports and protects the active part, it contains the oil that provides electrical insulation and heat transfer, and it forms the mechanical interface for bushings, radiators, the conservator and protective devices. The tank must be strong enough to withstand the internal pressure created by oil expansion, and sealed well enough to keep moisture and oxygen away from the oil.

Functions of the Tank

Insulation support: the oil inside the tank insulates the windings from the tank wall and earth

Heat dissipation: oil carries heat from the windings to the tank wall and radiators, where it is released to air

Protection: the sealed enclosure keeps out moisture, dust, rodents and corrosive atmospheres

Structural base: the tank carries the weight of the active part and all accessories, and transfers it to the foundation

Main Tank Types

Sealed (Conventional) Tank

A sealed tank is a fully enclosed welded structure with no oil conservator. A gas cushion or elastic diaphragm absorbs the oil volume change caused by temperature swings, so the oil never contacts the outside air. These tanks are simple, compact and common in distribution transformers and smaller power transformers. Some designs fit a silica-gel breather where pressure relief is needed, filtering moisture from any air that enters.

Conservator Tank

A conservator tank is a small oil reservoir mounted above the main tank and connected by a pipe. When the oil heats and expands, the surplus flows up into the conservator; when it cools, the oil flows back. The main tank therefore remains completely full of oil at all times, and the oil surface in the conservator is protected by a silica-gel breather that removes moisture from the air drawn in during contraction. This design is standard for medium and large power transformers because it keeps the main tank free of gas space and slows oil ageing.

Materials and Fabrication

Most tanks are made from mild steel plate, formed, welded and reinforced with ribs and stiffeners. Mild steel is strong, economical and easy to fabricate; it is finished with a zinc-rich primer and weather-resistant top coat for outdoor service. Stainless steel is specified for coastal, chemical or high-humidity environments where corrosion resistance outweighs cost. Galvanised steel and aluminium appear in special lightweight designs. Every tank is hydrostatically or pneumatically leak-tested after welding, and the interior is cleaned and dried before assembly of the active part.

Cooling and Accessories

Cooling depends on the surface area of the tank and its radiators. Small units are naturally cooled (ONAN); larger units add detachable radiators, fans (ONAF) or oil pumps (OFAF) to raise the continuous rating. Typical accessories mounted on or beside the tank include:

High-voltage and low-voltage bushings

Oil conservator with silica-gel breather and oil-level gauge

Buchholz (gas) relay on the conservator pipe

Pressure relief valve and oil sampling valve

Oil thermometer and winding temperature indicator

Earthing terminals on the tank and cover

Design Considerations

The tank must allow the oil to expand without overpressure, so the conservator volume and breather capacity are calculated from the oil temperature range. The base frame must be rigid enough to lift the complete transformer, and the cover must be removable for inspection. Tank dimensions also have to respect transport limits, because a unit that cannot pass through the customer's gate or container door creates avoidable cost. Standard mechanical and thermal requirements follow IEC 60076-1 and GB 1094, with oil and pressure-related tests described in IEC 60076-1 and GB/T 6451.

Maintenance

Routine checks include inspecting the breather desiccant colour, verifying the oil level in the conservator, cleaning radiator fins, checking for seepage at welds and gaskets, and confirming the pressure relief device is unobstructed. Any oil leak must be repaired promptly: a slow leak lets moisture in and gradually degrades the dielectric strength of the oil.

FAQ

Why is the tank filled with oil instead of air?

Oil provides both insulation and cooling. Its dielectric strength is far higher than air, and its circulation carries heat from the windings to the tank surface. Air-filled dry-type transformers are used indoors where fire safety rules exclude oil.

What is the purpose of the silica gel in the breather?

The gel absorbs moisture from the air that enters the conservator as the oil contracts on cooling. Without it, humid air would reach the oil surface and reduce its dielectric strength over time.

Can a sealed tank be converted to a conservator design?

Conversion is generally not practical because the tank structure, cover and accessories differ. It is more economical to select the correct type at the specification stage.

Why do large transformers always use conservators?

Large units contain far more oil, so thermal expansion is large. A conservator keeps the main tank completely oil-filled, eliminates the gas space above the oil, and makes oil sampling and top-up routine operations safe and simple.

What is the difference between ONAN and ONAF cooling?

ONAN means natural oil circulation and natural air cooling, relying on the tank and radiators alone. ONAF adds fans that blow air over the radiators, increasing the cooling capacity and therefore the permissible continuous load.