What is the Cooling of Transformers? Types, Working, Methods & Formulas

What is the Cooling of Transformers?

Cooling of transformers is the process by which the heat generated in the transformer windings and core is safely removed to prevent overheating, insulation damage, and failure.
This heat is mainly produced due to I²R losses, hysteresis losses, and eddy-current losses.

Transformer Cooling Methods

The main methods used to cool transformers are:
Dry-type cooling, Air-blast cooling, Oil-immersed cooling (ONAN/ONAF), Forced-oil cooling (OFAF/OFWF), and Gas-vapor cooling.
The selection of the method depends on the transformer’s rating and installation conditions.

Dry-Type Transformer Cooling

Dry-type transformers are cooled by natural air convection.
Their windings are placed inside a ventilated enclosure, and they are used in indoor applications because they are fire-safe.

Oil-Immersed Cooling

In oil-immersed transformers, the windings remain immersed in insulating oil.
The oil absorbs heat and carries it to the radiators, where heat is dissipated into the air by natural convection.

ONAN vs ONAF

• ONAN (Oil Natural Air Natural):
  Both oil and air circulate naturally to dissipate heat.
• ONAF (Oil Natural Air Forced):
  Fans on the radiators increase the rate of heat removal.

Forced-Oil Cooling

In forced-oil cooling, a pump circulates oil through the radiators, and fans or water systems remove heat.
This method is used in large power transformers.

Gas-Vapor Cooling

In gas-vapor cooling, a vaporizable liquid absorbs heat and converts into vapor, which then condenses in tubes and returns as liquid to circulate again.
This method provides very high-efficiency cooling.

Why is cooling necessary?

Cooling is essential because excess heat can cause insulation damage, increased losses, and transformer failure.
Effective cooling increases the reliability and life of the transformer.

Role of Transformer Oil

Transformer oil provides insulation, absorbs heat, increases dielectric strength, and transfers heat to the radiators to improve cooling.

Fire-Safe Liquids

Askarel was banned due to environmental hazards.
Now UL-listed silicone-based and less-flammable liquids are used (as per NEC 450.23).

Introduction

A portion of the electrical energy supplied to a transformer always gets converted into heat.
This heat is mainly produced due to I²R losses in the windings and hysteresis and eddy-current losses in the iron core.
If this heat is not removed properly, the transformer temperature may rise to dangerous levels, which can damage the insulation and reduce the life of the transformer.

Therefore, different special transformer cooling methods are used. These methods depend on the transformer’s size, rating, and installation conditions.

Types of Transformer Cooling

The main types of transformer cooling are Dry-Type Cooling, Air-Blast Cooling, Oil-Immersed Cooling (ONAN, ONAF), Forced-Oil Cooling (OFAF, OFWF), and Gas-Vapor Cooling.
The cooling method is selected based on the transformer’s size, rating, and installation environment.

Self-Air–Cooled Transformers (Dry-Type)

In self-air-cooled transformers, the windings are surrounded only by atmospheric air.
Heat is released into the air by natural convection and radiation.
Such transformers have been used for small ratings for many years.

Modern insulating materials like:

• porcelain
• mica
• glass
• asbestos

can withstand high temperatures.
Because of these materials, air-cooling is now possible even in larger transformers.

Except for small transformers, most dry-type units are placed in sheet-metal enclosures with louvers or gratings so that air can enter and exit easily.

Dry-type transformers are available today up to 3000 kVA and 15,000 V class.

Air-Blast Cooled Transformers

In this cooling method, the core and windings are placed inside a metal enclosure.
A blower forces air inside the enclosure and circulates it forcefully.
Heat is removed quickly, and the unit cools faster.

Air-blast transformers are generally used for large ratings —
up to 15,000 kVA capacity and
up to 35,000 V voltage levels.

Liquid-Immersed, Self-Cooled Transformers

In these transformers, the core and windings are completely immersed in an insulating liquid (usually oil).
This liquid performs two functions:

1️⃣ Provides insulation
2️⃣ Absorbs and carries heat

Oil absorbs heat from the windings and core and carries it to the metal surface of the tank.
From the tank surface, heat dissipates into the air by natural convection and radiation.

• In small transformers, the tanks are smooth (Fig. 5.8).
• Medium-size units have corrugated or finned tanks (Fig. 5.9 & 5.10).
• Very large units have external radiators (Fig. 5.11), where oil circulates naturally by convection.

This cooling method can be used for transformers of any rating, but very large transformers become bulky and expensive.

The most common liquid is insulating oil.

Nonflammable & Nonexplosive Liquids (Askarel & Silicone Fluids)

For electrical equipment, nonflammable and nonexplosive insulating liquids have also been developed.
These are used where fire safety is very important, such as:

• buildings
• indoor substations

The advantage of these liquids is that the transformer can be installed without a fireproof vault.

Transformers filled with such liquids were earlier called Askarel-insulated transformers.
However, the U.S. Environmental Protection Agency has banned Askarel.

Now, less-flammable, nonpropagating liquids are used, approved according to NEC Section 450.23.

Self-Cooling + Air-Blast Combination (Large Oil-Immersed Transformers)

Large oil-immersed transformers often use two cooling systems:

• Natural self-cooling
• Air-blast (forced cooling)

Construction is similar to a self-cooled transformer, but an additional motor-driven blower is installed.
The blower forces air through the radiators, increasing the cooling capacity many times compared to natural convection alone.

Blower motors are usually controlled by a thermostat:

• When the temperature rises, → blower turns ON
• When the temperature drops, → blower turns OFF

This makes the cooling system automatic.

Gas-Vapor Transformers –

This system is used in some large transformers.
A specific amount of gas is filled inside the transformer, along with a liquid that vaporizes when it absorbs heat.

During operation:

• A pump lifts the liquid from the bottom and distributes it over the core and windings.
• Under low pressure, the liquid spreads over the coils and current-carrying parts.
• Heat converts the liquid into vapor.
• The heavy vapor moves downward and enters the cooler-tube headers at the bottom of the tank.

Since the bottom of the cooling tubes is hotter than the top, the vapor condenses back into liquid and transfers its heat to the cooler surfaces.

The condensed liquid returns to the sump by gravity and is recirculated by the pump.

Cooling of Transformers Formulas + Examples

1. Transformer Temperature Rise Calculation

The temperature rise of a transformer =
Core loss + Copper loss (at load)

Formula:

ΔT = P_loss / (A × h)

Where:

• ΔT = Temperature rise (°C)
• Pₗₒₛₛ = Total losses (W)
• A = Cooling surface area (m²)
• h = Heat dissipation coefficient (W/m²·°C)

Typical values:

• Dry-type: 5–8 W/m²·°C
• Oil-immersed: 8–12 W/m²·°C
• Radiator type: 12–18 W/m²·°C

Example:

Total losses = 1500 W
Area = 10 m²
Coefficient = 10 W/m²·°C
ΔT = 1500 / (10 × 10) = 15°C

2. Oil Circulation Flow Rate (Forced-Oil Cooling)

Formula:

Q = P_loss / (Cp × ΔT_oil)
Where:

ΔTₒᵢₗ = Oil temperature difference between inlet and outlet (°C)

Q = Oil flow rate (kg/s)

Cₚ = Heat capacity of oil (approx. 1.8 kJ/kg°C)

Example:

P_loss = 50,000 W
Cp = 1.8 kJ/kg°C
ΔT_oil = 20°C
Q = 50,000 / (1.8 × 20) = 1.38 kg/s

3. Radiator Heat Dissipation Capacity

Formula:

Q_h = A_r × h_r × ΔT

Where:

• Aᵣ = Radiator area (m²)
• hᵣ = Heat dissipation factor (W/m²·°C)
• ΔT = Oil-to-air temperature difference

Example:

Radiator Area = 40 m²
Dissipation Factor = 14 W/m²·°C
ΔT = 30°C
Q_h = 40 × 14 × 30 = 16,800 W

4. Air Flow Requirement for Blower Cooling

Formula:

Air Flow (CFM) = (3.16 × P_loss) / ΔT_air

Example:

P_loss = 10,000 W
ΔT_air = 15°C
CFM = (3.16 × 10,000) / 15 = 2106 CFM

5. Gas-Vapor Transformer Heat Transfer

Formula:

Q = m × L

Where:

• m = Vapor generation rate (kg/s)
• L = Latent heat of vaporization (typically 200–300 kJ/kg)

Example:

m = 0.2 kg/s
L = 250 kJ/kg
Q = 0.2 × 250 = 50 kW