Métodos de resfriamento de transformadores para operação segura e eficiente

Transformer Cooling Methods for Safe and Efficient Operation

June 26, 2024 Luciano Bertene

Transformer cooling is crucial to the reliable operation of power transformers. Transformers are used to increase or decrease voltage in electrical power systems and can generate significant amounts of heat during their operation. Excessive heat can lead to deterioration of insulating materials, reduced transformer efficiency, and even catastrophic failure. To avoid these problems, various cooling methods are used to dissipate heat and keep the transformer temperature within safe limits.

Because there are no rotating parts to create a draft, transformers are more difficult to cool than rotating machines. For small powers up to, for example, 20 kW, the external surface is sufficient to dissipate the heat generated by radiation losses. In other words, simple tanks large enough to accommodate the transformer are not sufficient for the heat generated inside due to the various losses. However, as the size of the transformer increases, a normal single tank may not be able to dissipate its losses. In this case, cooling tubes are provided that increase the heat dissipation area.

Cooling Methods for Transformers

During the energy transfer process, losses occur in the transformer winding and core. These losses appear as heat. As for cooling methods, there are the following two types of transformers:

Dry type : Transformers that use air as coolant are called dry type transformers.
Oil-immersed type : Transformers that use oil as a coolant are called oil-filled transformers.

There are some cooling methods for transformers . The choice of cooling methods for transformers depends on the size, type of application and site conditions of the transformer to be installed.

The letter symbols used to denote these methods depend on the following.

Medium cooling is used.
Type of circulation used.

Average

Refrigerants ( refrigerant ) for transformers and the symbols used to designate them are:

Air – A, Gas – G, Water – W, Mineral Oil – O, Synthetic Oil – L, Solid Insulation – S

Traffic

The refrigerant can circulate naturally. Forced refrigerant circulation . Thus, the following symbols are used:

Wild – N, Forced – F

Signs indicate transformer cooling methods

The order of letters used to designate cooling processes without an external heat exchanger is:

The medium comes into contact with the windings.
The circulation of the refrigerant in contact with the windings

Dry type transformers

Small transformers up to 25 KVA are dry type transformers and have the following cooling devices.

Natural Air – ON

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

Air explosion – AB

Sometimes cooling through natural air circulation is not enough and then air jet cooling is used. In this method, the transformer is cooled by a continuous jet of air passing through the cores and windings. The air blast is generated by the external surfaces.

Oil Transformers

Generally, most transformers are immersed in oil. Oil provides better insulation than air because it is a better conductor of heat. Mineral oil is used for this purpose.

Oil-cooled transformer – ON

Oil, a better conductor of heat than air, is used in almost all transformers (except transformers for special applications, such as fire-hazardous mines).

Transformers with a power of approximately 5 MVA or a loss of up to 50 kW use tube boilers. The tubes are generally round, have a diameter of 50 mm and are arranged in one to three rows.

Oil and Air Cooled Transformer – ONAF

In their method, circulating oil transfers heat to the tank walls under natural pressure. The transformer tank is hollow and air is blown through the open space to cool the transformers.

Transformer with forced water and oil cooling – ONWF

In this method, the cooling coils are mounted above the transformer core but below the ground surface. To cool the transformer, water is circulated through the cooling coils.

This method is economical if a natural water source already exists.

Oil-cooled transformers – OF

In large transformers, the natural circulation of the oil is not sufficient to cool the transformer. Therefore, forced circulation is used. The oil is circulated by a motorized pump at the top of the transformer.

It depends on the heat exchange Oil forced Transformer as:

Oil-cooled transformer with natural air cooling – OFAN

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

Oil-cooled and air-cooled transformer – OFAF

The oil is cooled in the external heat exchanger container through blasts of air from the fans. Generally mixed cooling conditions are used with EM conditions up to 50% of transformer power and UM conditions at higher loads.

OFWF oil and water cooled transformer

Heating oil is cooled in water heat exchangers. This cooling method is suitable for transformer batteries. OFWF is used in transformers in hydroelectric plants.

Air cooling

Air-cooled transformers use natural convection or forced air circulation to dissipate heat. They are generally smaller and suitable for lower power applications. Air cooling is economical and requires less maintenance than oil cooling.

Liquid-cooled refrigeration

In liquid-filled cooling processes, a dielectric liquid other than oil is used as a coolant, for example synthetic esters or silicone liquids. These fluids have better thermal properties than traditional transformer oils and are more environmentally friendly. Liquid-filled transformers are often used in sensitive or environmentally regulated areas.

Oil and air cooling

Forced cooling methods are used for high power transformers. Pumps or fans are used to allow oil or air to circulate in a controlled manner and thus improve cooling performance. Forced cooling is essential for transformers operating at higher loads to prevent overheating.

Conclusion

In summary, adequate transformer cooling is essential to maintaining the reliability and longevity of transformers in electrical power systems. Depending on the size, power and operating conditions of the transformer, different cooling methods are chosen to prevent overheating and ensure safe and efficient operation.

Common questions

Why is cooling necessary for transformers?

Cooling is necessary to maintain a safe operating temperature for transformers. Transformers generate heat due to electrical losses in their windings and core. Failure to dissipate heat effectively can result in insulation degradation and transformer failure.

How to choose the cooling method for a transformer?

The cooling method of a transformer is generally selected based on factors such as power rating, operating conditions, and environmental considerations. Larger transformers often use oil cooling, while smaller transformers may use air or liquid cooling.

What are the advantages of liquid cooling?

Liquid cooling provides better heat dissipation and fire protection than traditional oil cooling. It is also more environmentally friendly and therefore suitable for sensitive areas.