Understanding Oil-Immersed Transformer Cooling Methods: From ONAN to ODWF -

Oil-Immersed Transformer Cooling Methods

The cooling system of the power transformer consists of two parts: the internal cooling system, which dissipates heat generated by the windings and core into the insulating medium; and the external cooling system, which ensures that heat in the medium is dissipated outside the transformer. Depending on the transformer’s rated capacity, the type of insulating medium, and the circulation method, the transformer adopts different cooling methods. The most commonly used cooling methods in oil-immersed transformers include: oil-immersed self-cooling (ONAN), oil-immersed air cooling (ONAF), Oil Forced Air Forced (OFAF), forced oil circulation water cooling (OFWF), oil directed circulation air cooling (ODAF), and oil directed circulation water cooling(ODWF).

Table of Contents

According to the specifications of “IEC 60076-2: Temperature Rise Regulations for Oil-Immersed Power Transformers”, “IEEE C57.12.00: General Requirements for Liquid-Insulated Distribution, Power and Voltage-Regulating Transformers”, and “CSA C88.1: Power Transformers and Reactors”, the cooling methods of oil-immersed transformers are represented by four-letter codes, and their composition and meaning are as follows:

The first letter represents the internal cooling medium in contact with the winding:
O	Mineral oil or synthetic insulating liquid with a flash point of no more than 300°C;
K	Insulating liquid with a flash point greater than 300°C, synthetic ester oil, natural ester oil, etc.
L	SF₆ or other liquids and gases, not commonly used, mainly in specific environments

The second letter indicates the circulation mode of the internal cooling medium:
N	The oil flow through the cooling equipment and the winding is a natural thermal convection circulation.
F	The oil flow in the cooling equipment is a forced circulation, and the oil flow through the winding is a thermal convection circulation.
D	The oil flow in the cooling equipment is a forced circulation.

The third letter indicates the external cooling medium:
A	Air
W	Water

The fourth letter indicates the circulation mode of the external cooling medium:
N	Natural convection;
F	Forced circulation (fan, pump, etc.)

The oil-immersed self-cooling system has no special cooling equipment. The oil circulates naturally in the transformer. The heat generated by the core and winding is transferred to the oil tank wall or radiator by the convection of the oil. The transformer oil inside the transformer tank is heated by the transformer body, and the density is reduced. The oil flow rises inside the tank, and the heat is transferred out through the heat dissipation device or the tank wall. The temperature drops, the density increases, and the transformer oil flow drops in the heat dissipation device or the tank, and then it is heated by the body again, and the cycle continues. The oil circulation is driven entirely by natural buoyancy resulting from temperature-induced density differences. The oil-immersed self-cooling system has a simple structure and high reliability, and is widely used in transformers with a capacity of less than 10,000kVA.

In the transformer oil-immersed self-cooling system, the oil enters the winding at point A and flows upward after being heated, and flows out of the winding at point B. From point B to point C, the oil is slightly cooled by the tank cover and the tank wall, and enters the radiator from point C; from point C to point D, the oil is cooled and descends, and the oil flowing out from point D enters the tank, and then the oil enters the winding at point A.

Oil-immersed air-cooling system, also known as oil natural circulation and forced air-cooling system. The oil circulates naturally in the oil tank. One or more fans are installed next to each radiator of the transformer oil tank to change the natural convection of the air into forced convection to enhance the heat dissipation capacity of the radiator.

This method improves cooling efficiency by 150–200% compared to ONAN. As a result, the transformer’s load capacity can be increased by 20–40%. When the load is small, the fan can be stopped to make the transformer run in a self-cooling mode. When the load exceeds a certain specified value, such as 70% of the rated load, the fan can be automatically put into operation. This cooling method is widely used in medium-capacity transformers above 10,000kVA.

Forced oil circulation air cooling system is used for large-capacity transformers. This cooling system is based on the oil-immersed air cooling system, and a submersible oil pump is installed on the connecting pipe between the main shell of the oil tank and the radiator with a fan (also called a cooler). When the oil pump is running, the oil in the oil tank is forced to be sucked into the radiator from the top, and then enters the oil tank from the bottom of the transformer to achieve forced oil circulation. The cooling effect is related to the oil circulation speed. As shown in the figure, the cooling structure of the forced oil circulation air cooling system used in large transformers.

Forced oil circulation water cooling is a system that circulates transformer oil through an oil pump and uses a water cooler (such as a plate or shell and tube heat exchanger) for cooling. The hot oil of the transformer flows through the cooler under the action of the oil pump, exchanges heat with the cooling water, and then returns to the transformer to complete the cooling process. OFWF has a high-efficiency water cooling system and a small footprint, and is often used in hydropower plants, underground substations or places with limited space.

ODAF– Oil directed circulation air cooling

Oil directed circulation air cooling is an efficient cooling method that uses an oil pump to force the insulating oil through the hot spots in the winding (such as the oil channel between the coils) and exchanges heat with the air through the air-cooled cooler. This method combines the design of the guided oil channel with forced air flow to improve cooling uniformity and efficiency. The oil flow is designed to be directional through the key parts of the winding, and the thermal control is more precise.

Oil directed circulation water cooling is a high-end cooling system that combines guided oil flow technology with a water-cooled heat exchange system. The oil flow is controlled by the flow guide structure inside the transformer to pass through the high heat load area, and the heat is quickly taken away by the water cooler outside. It is suitable for transformer systems with extremely high cooling requirements. The oil flow path is controlled and the cooling is uniform. It is mostly used in large power plant step-up transformers, power grid main transformers and other occasions with extremely high thermal stability requirements.