Jun 27, 2016

Transformer Cooling Methods


Since there are no rotating parts which induce a ventilating draught, transformers are more difficult to cool than rotating machines.

For small outputs, up to, say,, 20KW, the external surface is sufficient to enable the heat produced by the losses to be dissipated by radiation, i.e. plain tanks larges large enough to contain the transformer are sufficient to dissipate the heat developed inside because of the various losses.

However, as the size of the transformer increases a normal plain tank may not be able to dissipate its losses. In the case cooling tubes are provided which increase the heat dissipating area.

Transformer Cooling Methods

During the process of energy transfer, losses occur in the winding and core of the transformer. These losses appear as heat. As far as cooling methods are concerned, the transformers are of following two types;
  1. Dry Type: Transformers using air as coolant are called dry type transformers.
  2. Oil Immersed Type: Transformers using oil as coolant are called oil immersed transformers.
There are number of transformer cooling methods. The choice of transformer cooling methods depends upon the size, type of applications and type of conditions obtained at the site where transformer is installed.

The letter symbols used for designating these methods depending upon:
  1. Medium of cooling used.
  2. Type of circulation employed.


The cooling mediums (Coolants) used for transformers along with the symbols used for designating them are:
Air – A, Gas – G, Water – W, Mineral Oil – O, Synthetic oil – L, Solid insulation – S


The circulation of the cooling medium may be through natural means there may be forced circulation of coolant. Accordingly the symbols used are:
Natural – N,   Forced – F

Transformer cooling methods are designated by symbols.

The orders in which letters are used to designate methods of cooling without external heat exchangers are:
(a) The medium in contact with the windings.
(b) The circulation of the coolant in contact with windings

Dry type Transformers

Small transformers up to 25 KVA size are of dry type and have the following cooling arrangements

Air Natural – AN

This method uses the ambient air as the cooling medium. This method is used for small low voltage transformers.

Air Blast – AB

Cooling by natural circulation of air sometimes becomes inadequate, and then air blast type is used. In this method, the transformer is cooled by continues blast of cool air forced through cores and windings. The air blast is produced by external faces.
Air blast cooling of transformer

Oil Immersed transformers

In general most transformers are of oil immersed type. The oil provides better insulation than air as it is a better conductor of heat than air. Mineral oil is used for this purpose.

Oil Natural Cooled Transformer – ON

Oil being better conductor of heat than air is used in almost all transformers (Except for the transformers used for special applications like mines where there is a fire hazard) are oil immersed.
Transformers up to a capacity of about 5MVA or a loss up to 50KW use tanks with tubes. The tubes are usually round and are 50mm in dia and are arranged in one to three rows.
ONAN Cooling system and OFAN cooling system

ONAF Cooling system and OFAF cooling system

Oil Natural Air Forced, Cooled Transformer – ONAF

In his method, the oil circulation under natural head transfers heat to the tank walls. The transformer tank is made to hollow and air is blown through the hollow space to cool the transformers.

Oil Natural Water Forced – Cooled transformer – ONWF

In this method, cooling coils are mounted above the transformer core but below surface of oil, Water is circulated through the cooling coils to cool the transformer.
This method proves cheap where a natural water head is already available.

Oil Forced – Cooled Transformers – OF

In large transformer, the natural circulation of oil is insufficient for cooling the transformer and forced circulation is employed. Oil is circulated by a motor driven pump from top of transformer.
Depending upon heat exchange oil forced transformer as:

Oil Forced Air Natural Cooled Transformer – OFAN

In this method, oil is circulated through the transformer with the help of a pump and cooled in a heat exchanger by natural circulation of air.

Oil Forced Air Forced - Cooled Transformer – OFAF

The oil is cooled in the external heat exchangers using air blast produced by fans. Generally mixed cooling conditions are used with ONAN conditions up to 50% transformer rating and OFAN condition at higher loads.

Oil Forced Water Forced – Cooled Transformer OFWF

The heat oil is cooled in water heat exchangers. This cooling method is suitable for banks of transformers. OFWF is used for transformers designed for hydro-electric plants.


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