Understanding the Working of an Oil-Cooled Transformer

Electrical transformers are mandatory devices used in industries. Their effective functioning is very much essential to carry the activities at a smoother pace. Among the different technical aspects, the cooling mechanism of the transformer is also one that determines the transformer function. Oil-cooled transformers are common electrical devices with outstanding benefits and features.

Oil-Cooled Transformer – What is it?

An oil-cooled transformer is so-called as it uses oil as a coolant. Its design is made in such a way that there is no corrosion, damage, or disruption in the physical and functional elements. To protect the transformer tank, an oil-resistant the coating is made.

Oil-cooled transformers are used for various industrial applications – power stations, substations, solar, wind, power plants, automobile equipment, metal processing equipment, elevators, CNC machines, and so on. Based on the mode of application and size, the cooling arrangements of the oil-cooled transformers will vary. Wherever necessary, air blast fans, forced oil circulation, water-cooled heat exchangers, or other things are enabled to ensure better cooling through oil.

Oil-based Cooling – A Significant Mode of Transformer Cooling

The oil-based cooling mode is a common type of transformer cooling. Surrounding the core and the windings, oil is placed that captures the heat and transfers outside. Through the oil, the heat transfers to the air by radiation and convection modes. As the oil provides more conduction capacity than the air, oil-based cooling is advantageous than the air-based cooling. It also enables a high coefficient of conduction that promotes the natural circulation of oil through radiators.

Though oil is the primary element used for cooling in these oil-cooled transformers, different modes of cooling means are possible.

Oil Natural Air Natural (ONAN)

In oil-cooled transformers, the heat generated is taken in by the oil present in the tank. This happens naturally through the conduction process. This heated oil passes to the top and the cool oil replaces the place from the bottom. Through the heated oil, the tank walls heat by convection and then, the heat pass to the air by radiation and convection. As the transformer cooling occurs through the natural processes of conduction, convection, and radiation through the oil means, it is called the Oil Natural Air Natural type of cooling. It is mostly useful for transformers with capacities ranging up to 20MVA.

Oil Natural Air Forced (ONAF)

In this type, heat is transferred to the oil by conduction first. Thereby, the tank gets heated by the heated oil. If there is a more dissipating surface, then the cooling can be much faster. So, instead of natural cooling by air, forced air is provided by fans. Automatic switch on and off of the fans is enabled based on the increased heat levels of the oil or the tank. As the cooling is done naturally by oil and forced air blown, it is called Oil Natural Air Forced (ONAF) mode.

Oil Forced Air Natural (OFAN)

Now, by the name itself you can understand that the cooling through oil is forced. In this mode of cooling, water circulates inside the coils and enables cooling. Then, the heated transformer tank gets cooled by the natural air through convection and radiation processes. However, there is a chance of water entry inside the transformer. The transformer oil can get contaminated creating a problem for cooling.

Oil Forced Air Forced (OFAF)

In this type of cooling, the oil is cooled by the air blast with fans. High and less number of fans are automatically switched on and off based on the loads. When there are lesser loads, the cooling will be similar to that of ONAN. When there are high loads, with automatic means, the fans and pumps will be switched on. Special sensing elements are used to detect the estimated temperature level and switch on the fans. Mostly, this mode of cooling is used in transformers with capacities above 30 MVA.

Different Types of Oil Used

The oil used in transformers (like mineral oil and synthetic oil) for cooling has specific characteristics. It acts as a coolant and insulation means. It also avoids sludge build-up and protects paper from chemical attacks.

Mineral Oil– Petroleum-based products, naphthenic oil, and paraffinic oil are used for transformer cooling. Naphthenic oil will be oxidized easily when compared to paraffinic oil. However, sludge is produced during the cooling process. The so formed sludge by using paraffinic oil does not get precipitated at the bottom – so there won’t be any change in the cooling system. But, the sludge formed by paraffinic oil gets settled at the bottom and this may interfere in the cooling process.

Synthetic Oil – Chemical products like silicon oil are used for transformer cooling. Synthetic oils are generally used in fire-prone areas as they are fire-retardant.

Oil-cooled transformers of different sizes, models, and capacities are widely used in diversified industries. Various Transformer Manufacturers like Servomax Limited manufacture and supply standardized, reliable, and safe models for industries with customized specifications.