The Function of the Conservator Tank

That horizontal cylinder mounted above an oil-filled transformer is not a spare fuel tank; it is the conservator tank, a simple solution to an inevitable physical problem: oil expands and contracts with temperature.

The Problem It Solves

The volume of oil changes with temperature: it expands under load and in hot weather, and contracts when cold or when the load decreases. If the main tank were completely full and sealed, the pressure would rise during expansion to the point of damaging the gaskets or the tank itself, and a vacuum would form during contraction, drawing in air and moisture.

How Does the Conservator Tank Work?

• It is mounted above the main tank and connected to it by a pipe, and is partially filled with oil.
• During expansion, the excess oil rises into the conservator tank, and during contraction it returns from there to the main tank.
• This way the main tank remains always full of oil, with no air pockets around the live parts.
• Air breathing in and out of the conservator tank passes through the silica gel breather unit to be dried.

Relation to Protection and Monitoring

• A Buchholz relay is mounted in the pipe connecting the two tanks to detect gases and oil surges.
• The oil level gauge on the conservator tank is one of the most important checkpoints in the daily inspection round; a continuous drop in level means a leak.