Transformer Cooling Methods and Systems

Transformer Cooling Methods

A transformer in operation develops heat as a result of the copper and core losses during operation. Heat is not produced rapidly in small transformers, but the rate is much higher in large transformers. The rate of heat production determines how high the operating temperature rises above the ambient temperature. There are many  transformer cooling methods discuss below.

Getting rid of heat is a key element in keeping the temperature of a transformer as low as possible. The necessary rate of heat dissipation influences the size and construction of a transformer. Transformers are classified in two groups according to the method of cooling dry-type and oil immersed.

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Dry Type Transformer cooling Methods

Dry-type transformers are cooled by air circulating over the outside housing. These transformers are usually rated no higher than 500 kW. Dry type transformers are commonly wall-mounted, and used for lighting. Unlike oil-immersed transformers, they may be used indoors without a fireproof vault. Dry type transformer has further two types.

  1. Air Natural
  2. Air Blast or Air forced

    Transformer Cooling Methods and types
    Transformer Cooling Types

Oil Immersed Transformer cooling Methods

Oil immersed transformers are suspended in oil filled tanks. The oil absorbs the heat from transformer and delivers it to the environment outside the transformer.

Transformers Types: Oil Immersed cooling

Oil Natural Air Natural Type (ONAN)

Air Forced Oil Natural  Type (ONAF)

Oil Forced Air Natural Type (OFAN)


Oil Forced Air Forced Type (OFAF)

Transformers Types: Oil Immersed Water-Cooled

  1. Oil Natural Water Forced (ONWF)
  2. Oil Forced Water Forced (OFWF)

Some transformers are cooled by convection. The oil expands as it becomes warm from contact with the transformer coils. It then rises to the top of the tank and releases the heat to the surrounding air. The cool oil sinks back to the bottom of the tank. This circulation continues as long as the transform reduces heat.

The tank of an oil-immersed transformer is designed to remove heat from the oil as rapidly as possible. The surface of the tank may have fins to expose more surface area to the surrounding air and increase the rate of cooling. If the transformer requires faster cooling, it can be equipped with fans to force air over the tank.

Some transformers are cooled by circulating water. A coil of pipe, usually made of copper, is placed inside the tank just below the surface of the oil. The ends of the pipe extend out through the tank wall and are connected to a pump that forces cool water through the pipe while the transformer is operating.

The oil can also be cooled by pumping it through a radiator exposed to the air. For faster cooling, fans may force air over the radiator.

An effective cooling system for the transformer can increase transformer capacity by 25 to 50%. For example, a 2000 kW transformer, if cooled efficiently, can operate at 3000 kW without damage.

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