Transformer

Transformer Cooling Methods for Safe and Efficient Operation

Transformer cooling is critical to ensuring the reliable operation of power transformers. Transformers are used to step up or down voltage levels in electrical power systems, and they can generate significant amounts of heat during their operation. Excessive heat can lead to the degradation of insulation materials, reduced transformer efficiency, and even catastrophic failure. Various cooling methods are employed to dissipate heat and maintain the transformer’s temperature within safe limits to prevent these issues.

Since no rotating parts 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 dissipate by radiation, i.e., plain tanks large enough to contain the transformer are inadequate to 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 this case, cooling tubes are provided, which increase the heat dissipating area.

Transformer Cooling Methods

Transformer Cooling Methods

During the energy transfer process, 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 the following two types;
  • Dry Type: Transformers using air as coolant are called dry-type transformers.
  • Oil-Immersed Type: Transformers using oil as coolant are called oil-immersed transformers.
There are several transformer cooling methods. The choice of transformer cooling methods depends upon the size, application type, and conditions obtained at the installed transformer site.

The letter symbols used for designating these methods depend upon the following.

  • Medium cooling is used.
  • Type of circulation employed.

Medium

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

Circulation

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

Characters Designate Transformer Cooling Methods

The orders in which letters are used to designate methods of cooling without external heat exchangers are:
  • The medium is in contact with the windings.
  • 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 ambient air as the cooling medium. This method is used for small, low-voltage transformers.

Air Blast – AB

Cooling by natural air circulation sometimes becomes inadequate, and the air blast type is used. The transformer is cooled by a continuous blast of cool air forced through cores and windings in this method. External faces produce the air blast.
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, a better conductor of heat than air, is used in almost all transformers (Except for those used for special applications like mines with a fire hazard).
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 diameter, and arranged in one to three rows.
onan-cooling-and-ofan-cooling

onaf-cooling-and-ofaf-cooling

Oil Natural Air Forced, Cooled Transformer – ONAF

In his method, the oil circulation under the natural head transfers heat to the tank walls. The transformer tank is hollow, and the air is blown through the open 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 the soil surface; 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 transformers, 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 the top of the 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 air circulation.

Oil Forced Air Forced – Cooled Transformer – OFAF

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

Oil Forced Water Forced – Cooled Transformer OFWF

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

Air Cooling

Air-cooled transformers use natural convection or forced air circulation to dissipate heat. They are often smaller in size and suitable for lower-power applications. Air cooling is cost-effective and requires less maintenance compared to oil-immersed cooling.

Liquid Immersed Cooling

Liquid Immersed Cooling

Liquid-immersed cooling methods use a dielectric liquid other than oil, such as synthetic esters or silicone fluids, as a cooling medium. These liquids have better thermal properties than traditional transformer oil and are more environmentally friendly. Liquid-immersed transformers are often used in sensitive or environmentally regulated areas.

Forced Oil and Forced Air Cooling

For high-power transformers, forced cooling methods are employed. This involves using pumps or fans to circulate oil or air at a controlled rate, enhancing the cooling efficiency. Forced cooling is essential for transformers operating at higher loads to prevent overheating.

Conclusion

In conclusion, proper transformer cooling is essential for maintaining the reliability and longevity of transformers in electrical power systems. Different cooling methods are chosen based on the transformer’s size, rating, and operating conditions to prevent overheating and ensure safe and efficient operation.

FAQs

Why is cooling necessary for transformers?

Cooling is necessary to maintain a safe operating temperature for transformers. Transformers generate heat due to the electrical losses in their windings and core. If the heat is not dissipated effectively, it can lead to insulation degradation and transformer failure.

How is the cooling method selected for a transformer?

The cooling method for a transformer is typically chosen based on factors such as its power rating, operating conditions, and environmental considerations. Larger transformers often use oil-immersed cooling, while smaller ones may use air or liquid-immersed cooling.

What are the advantages of liquid-immersed cooling?

Liquid-immersed cooling offers improved heat dissipation and better fire safety than traditional oil-immersed cooling. It is also more environmentally friendly, making it suitable for sensitive areas.

Leave a Reply

Your email address will not be published. Required fields are marked *