Types of Cooling in Transformers: How They Work and Why They Matter

What is Transformer Cooling？

Transformer cooling refers to the process of dissipating heat produced by iron cores and windings through the cooling medium to the external environment. This helps keep the temperatures of key components such as the core and windings within the allowable range. Effective cooling ensures stable insulation performance of the transformer, extends its service life, and avoids faults.

Basic Principles of Transformer Cooling

The basic principle of transformer cooling is quite simple. The core is to promptly remove the heat generated during operation to prevent the equipment from overheating. The key to cooling is to use a cooling medium (such as oil or air) to transfer the heat from the heated coils and cores to the surrounding environment.

For example, the interior of a common oil-immersed transformer is filled with insulating oil. The oil absorbs the heat from the coils and cores. The hot oil flows along the pipes or radiators in the oil tank and cools down after coming into contact with the outside air. The cooled oil then flows back to continue absorbing heat, forming a cycle.

Another example is a dry-type transformer: it uses air as the cooling medium. Either the heat is removed by the natural flow of air, or fans are installed to force air circulation to accelerate heat dissipation.

In simple terms, transformer cooling is a cycle process of “absorbing heat – transporting heat – dissipating heat”. The aim of the process is to keep the temperature within a safe range and allow the transformer to operate stably.

Why is transformer cooling so important?

During transformer operation, two types of losses will occur: one is core loss (caused by the alternating magnetic field, including hysteresis and eddy current losses), and the other is winding loss (Joule heat generated when current passes through the winding resistance).

All these losses are converted into heat. If they cannot be dissipated in time, the temperatures of the components will continue to rise. This will accelerate the aging of the insulation materials, and sometimes it may cause insulation breakdown, winding burnout and other accidents.

Classification of Transformer Cooling Methods

Cooling methods are generally denoted by two or four letters.

What are internal cooling and external cooling?

1. Internal Cooling

Internal transformer cooling refers to the heat transfer process inside a transformer , with the core being the circulation mode of the internal cooling medium (such as oil, insulating liquid, and gas).

Function: Transfer the heat generated by windings and core to the internal cooling medium.

Key Elements:

Internal medium: mineral oil (O), gas (G), insulating liquid (L/K).

Circulation method: natural convection (N), forced circulation (F), forced directed circulation (D).

2. External Cooling

It refers to the process of dissipating heat of the cooling medium to the external environment.

Function: The heat carried by the internal cooling medium will be transferred to external environment.

Key elements:

External medium: air (A), water (W). Most transformers use air as cooling medium. Only in some special cases we use water.

ONAN:

Internal cooling: O (mineral oil) + N (natural convection) → Heat from windings is naturally carried away by oil.

External cooling: A (air) + N (natural convection) → Heat on the surface of the oil tank is naturally dissipated into the air.

Cooling Methods of Oil Type Transformers

Oil-immersed transformers use mineral oil as both the internal cooling medium and insulating medium.

Their cooling methods comply with the four-letter naming standard: internal medium – internal circulation – external medium – external circulation. Common types are as follows:

1. ONAN (Oil-immersed Natural Air Cooling)

Heat generated by windings and iron cores is transferred to the mineral oil in the transformer. As temperature rises, the density of oil decreases, and thus will flow upward. When the heat is gone, the temperature falls, the density of oil increases, and the oil will flow downwards. This process is called natural air convection. The heat is transferred to tube walls of the radiators, and then dissipated into air. No extra cooling device is needed in this process.

This cooling method is suitable for small-capacity transformers, usually below 800Kva, and low load density scenarios. For example, residential area distribution transformer, small commercial places and rural power grid. This method features a simple structure, low maintenance costs, and low operating noise.

2. ONAF (Oil-immersed Forced Air Cooling)

The internal oil circulation of the transformer still relies on natural convection. The difference is that axial flow fans are installed on the radiators. The fans force air to flow rapidly across the radiator surfaces, significantly improving the heat exchange efficiency between the radiators and the air. When the transformer load drops to a certain level, the fans can be turned off to switch to ONAN operation mode.

Application: It is applicable to medium-capacity transformers (usually 800 kVA ~ 10 MVA), such as the main transformers in medium-sized industrial parks and regional substations. Its heat dissipation capacity is about 30% higher than that of ONAN, enabling flexible adjustment of cooling intensity according to load changes.

3. OFAF(Forced Oil Circulation Forced Air Cooling)

On the basis of ONAF, an oil pump is added inside the transformer to force the mineral oil to flow rapidly through the windings, core, and radiators, achieving forced heat transfer inside the transformer. Meanwhile, the radiators are equipped with fans to force air flow, completing efficient heat exchange between the inside and outside of the transformer. The oil circulation speed is controlled by the oil pump, and the heat exchange efficiency is much higher than that of natural convection.

Application: It is suitable for large-capacity transformers (10 MVA ~ 63 MVA), such as the main transformers in large industrial enterprises and hub substations. This method has strong heat dissipation capacity and can meet the high-load operation requirements of transformers, but it requires regular maintenance of the oil pumps and fans.

4. ODWF (Forced Directed Oil Circulation Water Cooling)

Forced directed oil circulation is adopted internally.

Application: It is suitable for extra-large-capacity transformers (≥ 63 MVA) or special scenarios with high ambient temperatures and high load rates, such as large thermal/hydropower stations and ultra-high voltage substations. Water cooling has extremely high heat exchange efficiency, but it needs to be equipped with a dedicated cooling water system, which has strict requirements for water quality and equipment tightness.

Cooling Methods of Dry-type Transformers

Dry-type transformers contain no oil and use air or special gas as the cooling medium. Structurally, they are pided into two types: open-type and enclosed-type. The coding of their cooling methods is mostly two or four letters. Common types are as follows:

1. AN (Air Natural Cooling)

This method realize heat dissipation through natural air convection. The heat generated by the windings and core is directly dissipated into the surrounding air through the transformer casing and winding surfaces without any auxiliary cooling equipment. The windings of dry-type transformers usually adopt epoxy resin casting or impregnation technology, which has good insulation performance and can be in direct contact with air.

Application: It is suitable for small-capacity dry-type transformers (generally ≤ 1600 kVA), such as power distribution systems in high-rise buildings, power distribution rooms of commercial complexes, and auxiliary power supply systems of data centers. This cooling method is rather safe, with good fireproof ability, which makes it a good choice for places with high security requirements.

2. AF (Air Forced Cooling)

Centrifugal fans are installed around the transformer body to force air to blow across the surfaces of the windings and core, accelerating air flow to remove heat. The fans can automatically start and stop according to the temperature rise of the transformer, balancing heat dissipation efficiency and operating noise.

Application: It is applicable to medium and large-capacity dry-type transformers (1600 kVA ~ 20 MVA), such as the main transformers in large shopping malls, subways, and hospitals. Its heat dissipation capacity is about 50% higher than that of AN, which can meet the high-load requirements while retaining the fire and explosion-proof advantages of dry-type transformers.

How to Select the Appropriate Cooling Method

The following factors affect the selection of transformer cooling.

Transformer type

First, we should determine the transformer type. This should be judged based on the requirements for fire protection, explosion protection, and installation environment.

Scenarios where oil-immersed transformers are preferred: Outdoor installation, large-capacity power supply, and cost-sensitive applications (e.g., urban and rural power distribution networks, main transformers in large industrial plants). Oil-immersed transformers feature high heat dissipation efficiency and low cost, are among the most popular transformer type in the power system. Before you decide the hermetically sealed vs conservator type, deep dive into side-by-side comparison of the 7 key differences.

Scenarios where dry-type transformers are preferred: They are suitable for places with strict safety requirements for fire and explosion protection. For example, high-rise buildings, shopping malls, schools, railway stations, laboratories.

Capacity and Load Conditions

Once the transformer type is determined, the cooling method should be further matched according to the capacity, load rate and operating temperature.

ONAN: ONAN is commonly used for small to medium capacity transformers operating under moderate load conditions, such as rural power distribution networks and dedicated transformers for small factories.

ONAF: Medium to large capacity scenarios with large load fluctuations. The cooling mode can be switched by starting or stopping the fans, balancing energy conservation and heat dissipation, such as main transformers in regional substations and medium-sized industrial parks.

OFAF: Large-capacity, high load rate scenarios requiring stable and efficient heat dissipation, such as main transformers in large industrial enterprises and hub substations.

ODWF: Extra-large-capacity scenarios or those with high ambient temperatures (summer ambient temperature ≥ 35℃) that require supporting cooling water systems, such as main transformers in hydropower/thermal power stations and ultra-high voltage substations.

AN: Small-capacity scenarios with good indoor ventilation that do not require additional maintenance of cooling equipment, such as power distribution systems in high-rise buildings and power distribution rooms of commercial complexes

AF: Medium and large-capacity scenarios with limited indoor space or insufficient ventilation. The fans can start and stop automatically, such as main power supply transformers in subways and hospitals