Transformer Cooling System: 5 Answers For Beginners

Transformer cooling is essential to protect all transformer types against temperature rise and failure. I have written an article about the impact of temperature rise, you can read it here.

In this article, I will answer 5 common questions about transformer cooling system. For more details about cooling system, please check my other article here. Let’s get starter.

Is transformer cooling system effective?

Transformer cooling is very effective, and a transformer with cooling system like ONAF is much effective than another one without cooling fans. Besides, Transformer rating increases with the increase of cooling stages

For example, if we have a multi stages ONAN/ONAF/ONAF cooling system for a 45/60/75 MVA transformer, then when the second stage of cooling fans operate, the rating becomes 60MVA, and by the operation of the third stage we get rating of 75MVA.

For a transformer to operate at its maximum efficiency, its important to keep the conversion losses at minimum possible level.  Every transformer has its load tolerance and beyond this level it starts to dissipate heat because of normal operation and overload factors.

In residential or commercial areas, it is very common that transformers are overloaded, particularly during the peak load hours. This increase in load results in increase of heat losses which raises the temperature of transformer’s windings and surface.

Considering all these issues, the role of a proper cooling system becomes really important for transformer’s longer life and better output and efficiency. It is understood that increased tolerance against overheat conditions enhances the load carrying capacity of transformer, which is a common occurrence in peak load hours.

Do dry type transformer needs cooling systems?

For similar rating of voltage and power capacity, dry type transformers are usually larger in size compared to oil-filled transformers. Furthermore, their heat resistance is not good, which makes it less ideal performer under heat.

This drawback is mainly due to the fact that the material used for surrounding the core and coil i.e. epoxy resin is less conductive under thermal conditions. However, these transformers are safer and therefore used in schools, hospitals and small-scale renewable energy stations. Since these transformers are designed for large power ratings, their electrical losses also increase with the increase in size. Hence, dry type transformers require effective cooling system to mitigate these losses and improve overall efficiency.

Cooling systems for dry type transformers

Dry type transformers use following methods for cooling systems.

Air Natural Cooling (AN):

This method involves circulation of cool air in the transformer based on the phenomenon of natural convection. When temperature of the inside of transformers is increased in comparison to surrounding temperature, heated air is cooled by the cooling. This self-cooled system is economical and preferred in smaller output transformers typically below the rating of 1.5 MVA.

Air Forced (AF) Cooling:

In comparison to self-cooled mechanism based on natural phenomenon, this type of cooling is done by forced circulation of air inside the transformer. High pressure fans and blowers are used for this purpose to ensure that high velocity of air is injected to the transformer’s core and windings.

To maintain the temperature level protection, an alarm is used which gets activated as soon as the temperature of the inside reaches the safe limit. This method is economically feasible in systems made for 15MVA or higher ratings.

Types of cooling system used in large transformers

For large transformers Oil Natural Air Forced Transformer (ONAF) is used for cooling. This method can typically handle transformers up to the rating of 60MVA. For larger transformers, (OFAF) is used. And a more enhanced cooling system is used for transformers of hundreds MVA rating, which is (OFWF).

ONAF: Traditional convection process is used and air circulation is done with forced process to dissipated from the surrounding areas to the core and winding of the inside of transformer

OFAF: is similar to ONAF but here, forced oil with pumps is used.

OFWF: oil forced with pumps and water is forced to cool down the tank from the out side.

By increasing the size of tank area, the cooling process can further be enhanced. Since the heat dissipation level increases, fans and blowers used for forced injection of air work more quickly and efficiently, making the entire process within the safety heat limits.

For more information about cooling system of transformers read my article here.

Is transformer cooling system controllable?

Yes, Transformer cooling system is controllable. The control system may be a simple control panel with temperature sensors, or it can be a PLC based control circuit, as I will mention below. Fans and pumps of the cooling system are controlled depending on the temperature rise of the transformer. The control system can be manual or automatic based on the transformer rating, criticality and the ambient temperature.

Transformer failure is mainly caused due to the issues or complete failing of cooling systems. To mitigate this issue, many cooling systems has been designed to ensure that transformer is well protected against any such issues.

Traditional cooling methods use air, water or oil as cooling agents, with preferences given on the basis of the capacity of transformer. However, since the development of electronic components, an automatic cooling system has been introduced which involves use of relay type protection with human involvement at each step.

This kind of protection has its limitations in particular cases because of complex wiring and tendency of isolating the transformer from other parts of electrical system.

Nowadays, an automatic control system is used with the help of Programmable logic circuit (PLC) and various sensing devices. The data gathered through these sensors is fed to the PLC which is additionally connected to a temperature controller.

This system has multiple cooling systems and each cooling system is used depending upon the limit of temperatures set through programs. Overall control of this cooling systems is managed using temperature comparators.