What Are the Cooling Methods of a Transformer? . Transformers play a critical role in the transmission and distribution of electrical power. However, their efficiency and longevity heavily depend on proper cooling methods. Transformers generate substantial heat during operation, and failure to manage this heat can lead to reduced performance, equipment failure, or safety hazards. In this article, we’ll explore the main cooling methods for transformers, their applications, and their benefits.
Introduction to Transformer Cooling
When the transformer is in operation, heat is generated due to copper losses in the windings and iron losses in the core. The removal of heat from the transformer is known as cooling of the transformer.
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Why Cooling is Crucial for Transformers
Excess heat can degrade transformer insulation, increase power losses, and even lead to catastrophic failures. Efficient cooling:
- Prolongs the lifespan of transformers.
- Enhances performance by maintaining operational temperatures.
- Prevents overheating and subsequent damage to components.
Transformer Cooling Methods
A transformer is a static electromagnetic device that is used to convert electrical energy from one voltage level to another voltage level. During this process, some amount of energy is lost in the winding and core of the transformer.
The energy lost in a transformer is dissipated as heat. The amount of energy that is lost in a transformer is very small compared to the amount of energy is transferred. But it becomes very large for a higher kVA rating. And for a large kVA rating of a transformer, heat dissipation is a difficult task. The temperature of a transformer will rise and it may cause damage to the insulation if the heat dissipation is not done properly. Hence, it is necessary to maintain a temperature within a permissible limit to avoid thermal degradation of insulation.
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The method used for the transformer cooling depends on the type, size of a transformer, and suitable conditions at the site where it is installed. The transformers can be dry or oil-immersed type. There are two types of coolants are used in the transformer;
For the dry type transformers
- Air Natural (AN) Cooling
- Air Forced (AF) or Air Blast (AB) Cooling
For oil immersed type transformers
- Oil Natural Cooling
- Oil Natural Air Natural (ONAN) Cooling
- Oil Natural Air Forced (ONAF) Cooling
- Oil Forced Air Forced (OFAF) Cooling
- Oil Forced Water Forced (OFWF) Cooling
Air Natural (AN) Cooling
The air natural (AN) cooling is used in dry type, self-cooled transformers. In this method, the natural circulation of surrounding air is used for cooling of the transformer. The windings of the transformer are protected against the mechanical damages by a sheet metal enclosure. The air natural cooling is used for low voltage small transformers upto a few kVA.
Air Forced (AF) or Air Blast (AB) Cooling
The air forced (AF) cooling is used in the dry type, forced-air cooled transformers. In this method, a continuous blast of filtered cool air is forced with the help of a fan through the core and windings of the transformer for cooling. This method is used for cooling of the transformers upto 15 MVA
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Oil Natural Cooling
In this method of cooling, the winding and core of a transformer are immersed in the oil. An oil-filled iron tank is used as an enclosure. The heat produced by the core and winding is transferred to the oil by the process of conduction. The oil temperature rises as it comes in contact with heated parts. And the place of heated oil is taken by the cool oil. The heated oil transfers heat to the tank surface and heat dissipates to its surrounding.
The power transformer with a capacity of up to 10 MVA has a cooling radiator system with natural cooling. This method of cooling is the most used method for oil-immersed transformers because of the below advantages;
- Freedom from dust
- Easy heat dissipation from the inner part
- There is no effect of moisture
- The oil has a high coefficient of volume expansion. Hence a good amount of heat is dissipated by natural convection.
For a large rating of transformer, the smooth surface of the transformer tank is not sufficient for proper heat dissipation. In this condition, fins are used to increase heat dissipation area. In many cases, radiators and tubes are provided on the walls of the tank to further increase the heat dissipating capacity.
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Oil Natural Air Natural (ONAN) Cooling
The oil natural air natural cooling is used in oil immerged type transformers. The most of the transformers of the medium and large ratings have their core and windings immerged in dielectric oil, which acts both as cooling medium and insulating medium.
The oil immerged transformers are enclosed in a sheet steel tank. The heat generated in the core and windings being transferred to the oil. The heated oil becomes lighter and rises to the top and the cool oil takes its place from the bottom of the transformer tank.
The heat of the oil is transferred to the walls of the tank by natural circulation of the oil and the heat is then transferred to the surrounding air through the natural radiation and convection. Hence, the oil gets cooler and falls to the bottom. Therefore, a natural circulation of oil takes place for the cooling of the transformer.
Oil Natural Air Forced (ONAF) Cooling
In oil natural air forced cooling of the transformer, the heat generated by the core and windings of the transformer is transferred to the walls of the tank and to the radiator through the natural circulation of the oil. Now, the forced air is directed over cooling elements (tank, radiator, tubes, fins etc.) of the transformer. Hence, the transformer is cooled by the natural circulation of oil and blast of air.
This method of cooling is suitable for the large transformers upto 60 MVA.
Oil Forced Air Forced (OFAF) Cooling
In this method of cooling, the heated oil is circulated from the top of the transformer tank to a heat exchanger and the blast of air is forced through the heat exchanger by turning on a fan. Then, the cool oil is returned to the bottom of the transformer tank.
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This method of cooling is used for higher rating transformers like transformers which are used in substations and generating stations.
Oil Forced Water Forced (OFWF) Cooling
In the oil forced water forced cooling method, the heated oil is circulated from the top of the transformer tank to a heat exchanger where the pressurised water is used to separate the heat from the oil. The cool oil is then returned to the bottom of the transformer tank.
This type of cooling is mainly used for very large transformers with a 100 MVA ratings like at generating stations.
Dry vs Oil Type Transformers:
Oil and air are the primary cooling materials used in a transformer.
Dry-type transformers, are normally cooled by air. The following two transformer cooling methods are adopted in dry-type transformers.
- Air Natural (AN) cooling – Cooled by surrounding air. Heat transfer by natural air convection.
- Air Force (AF) cooling – Forced air circulation using fans and blowers.
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Oil-type transformers are cooled using oil-air cooling or oil-water cooling method. There is a wider range of cooling methods for oil-type transformers.
- (Mineral) Oil Natural Air Natural (ONAN) – The core and coils are cooled by surrounding in oil. Heat transfer of oil by natural air convection.
- (Non-Mineral) Oil Natural Air Natural (KNAN) – The core and coils are cooled by surrounding in synthetic oil. Heat transfer of oil by natural air convection.
- Oil Natural Air Forced (ONAF) – Cooled by surrounding in oil. Forced air circulation using pumps, fans and blowers.
- Oil Forced Air Forced (OFAF) – Forced oil and air circulation using fans and blowers.
- Oil Natural Water Forced (ONWF) – Cooled by surrounding in oil. Forced water circulation using heat exchanges.
- Oil Forced Water Forced (OFWF) – Forced oil and water circulation using oil-to-water heat exchanges.
Transformer cooling class letter descriptions
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Applications of Cooling Methods
Each cooling method is chosen based on:
- Load capacity: Larger transformers need advanced cooling techniques.
- Environment: Coastal areas prefer OFWF for its efficiency.
- Operational requirements: Variable loads demand flexible systems like ONAF.
Benefits of Efficient Cooling Systems
- Enhanced Reliability: Proper cooling minimizes the risk of failure.
- Cost Savings: Reduces maintenance and operational expenses.
- Eco-Friendly: Efficient cooling prevents resource wastage and extends equipment life.
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FAQs
1. Why is transformer cooling important?
Transformer cooling prevents overheating, which can degrade performance and lead to equipment failure.
2. What is the most efficient cooling method for transformers?
Oil-Forced Water-Forced Cooling (OFWF) is the most efficient for high-capacity and critical applications.
3. What is ONAN cooling?
ONAN stands for Oil Natural Air Natural. It uses natural convection of oil and air to dissipate heat.
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