Cooling of Transformer — Methods, Types & Comparison
A transformer is a static electrical machine — unlike motors and generators, it has no rotating parts to drive a cooling fan. Yet during operation, significant heat is generated due to copper losses (I²R) in windings and iron losses (hysteresis + eddy currents) in the core. If this heat is not removed efficiently, the insulation degrades, oil breaks down, and the transformer's lifespan reduces drastically.
The removal of heat from the windings and core of a transformer to maintain safe operating temperature is called cooling of transformer.
Why Cooling is Necessary
- Prevents insulation breakdown — Class A insulation fails above 105°C
- Maintains transformer oil properties (flash point ~140°C)
- Increases efficiency and extends operational life (every 6°C rise above rated temperature halves insulation life)
- Prevents hot-spot formation in windings
- Allows higher loading capacity without derating
Sources of Heat in a Transformer
Copper Loss (Pcu) = I²R (varies with load)
Iron Loss (Pi) = Hysteresis Loss + Eddy Current Loss (constant)
At full load, copper losses dominate and generate maximum heat in the windings. Iron losses remain constant regardless of load and heat the core continuously. The cooling system must handle both sources effectively.
Cooling Methods for Dry-Type Transformers
Dry-type transformers do not use any liquid coolant. They are preferred for indoor installations, areas with fire hazard concerns, and voltage ratings typically up to 33 kV.
1. Air Natural (AN) Cooling
Used for small transformers up to 3 MVA. Natural convection of surrounding air removes heat from the core and windings. The transformer body is designed with fins and corrugations to increase the surface area exposed to air.
Fins on transformer tank increase surface area for natural air cooling.
- Mechanism: Hot air rises, cool air replaces it (natural convection)
- Application: Small distribution transformers (up to 10–15 kV)
- Advantage: No moving parts, zero maintenance
- Limitation: Insufficient for large transformers with high losses
2. Air Forced (AF) / Air Blast Cooling
For transformers rated above 3 MVA in dry-type construction, forced air circulation using fans or blowers is employed. The transformer is placed over an air chamber, and pressurized air is directed through the core and winding ducts.
Air Blast Cooling — fans force air through transformer windings.
- Mechanism: Blowers force air at sufficient pressure through core and windings
- Application: Substations in densely populated areas where oil is a fire hazard
- Advantage: 30–50% more cooling capacity than natural air
- Limitation: Requires continuous power supply for fans; dust accumulation
Cooling Methods for Oil-Immersed Transformers
Oil-immersed transformers are used for higher voltage ratings (above 33 kV). The core and windings are submerged in transformer oil which serves dual purpose — electrical insulation and heat transfer medium.
1. Oil Natural Air Natural (ONAN)
The most common cooling method for distribution transformers up to 30 MVA. Transformer oil absorbs heat from windings, becomes lighter, rises to the top, flows through radiator tubes/fins, transfers heat to surrounding air, cools down, and sinks back — creating a natural thermosiphon circulation.
Oil Immersed Natural Cooling — oil circulates naturally through radiator tubes.
- Application: Distribution transformers up to 30 MVA
- Advantage: Simple, reliable, no auxiliary power needed
- Limitation: Limited cooling capacity for very large transformers
2. Oil Natural Air Forced (ONAF)
Same as ONAN but with fans blowing air over the radiator surfaces. This increases heat dissipation from oil to air by 40–50%. Fans are thermostatically controlled — they switch on when oil temperature exceeds a set threshold (typically 65°C).
- Application: Power transformers 30–60 MVA
- Advantage: Higher loading without increasing transformer size
- Limitation: Fan failure during peak load can cause overheating
3. Oil Natural Water Forced (ONWF)
Used for transformers above 500 kVA where water supply is available. Water is circulated through copper coils or spiral pipes immersed in the oil. Water absorbs heat from the oil efficiently due to its high specific heat capacity.
Oil Immersed Water Cooling — water pipes inside the oil tank.
- Application: Large power transformers near water sources (hydroelectric stations)
- Advantage: Very effective heat removal
- Limitation: Risk of water leakage contaminating oil; requires water treatment
4. Oil Forced Air Forced (OFAF)
For very large power transformers (above 60 MVA), both oil and air are forced. Motor-driven pumps circulate oil through external heat exchangers where fans blow air over the oil coolers. This provides maximum cooling efficiency.
- Application: Large power transformers above 60 MVA (grid substations)
- Advantage: Highest cooling efficiency; compact design for given rating
- Limitation: Complex system; pump/fan failure is critical; higher maintenance cost
5. Oil Forced Water Forced (OFWF)
The most powerful cooling arrangement used for the largest transformers (above 100 MVA). Oil is pumped through external water-cooled heat exchangers. Both oil and water are force-circulated for maximum heat transfer.
- Application: Generator transformers at power stations (100+ MVA)
- Advantage: Handles extremely high heat loads
- Limitation: Most complex and expensive; water contamination risk
Comparison Table — All Cooling Methods
IEC Designation Codes (IS 2026 / IEC 60076)
The four-letter code describes the cooling system:
Letter 2: Internal circulation (N = Natural, F = Forced, D = Directed)
Letter 3: External coolant (A = Air, W = Water)
Letter 4: External circulation (N = Natural, F = Forced)
Example: ONAN = Oil Natural, Air Natural. ODAF = Oil Directed, Air Forced (oil is directed through specific winding ducts using baffles).
How to Select the Right Cooling Method
- Indoor/fire-sensitive areas: Dry-type (AN or AF)
- Distribution (up to 30 MVA): ONAN — simple and reliable
- Sub-transmission (30–60 MVA): ONAN/ONAF dual-rated
- Transmission (60–100 MVA): OFAF with redundant fans/pumps
- Generator transformers (100+ MVA): OFWF at power stations with water supply
- Near water sources: ONWF or OFWF
Frequently Asked Questions
Q1. Why is cooling necessary in a transformer?
Cooling removes heat generated by copper losses (I²R) and iron losses (hysteresis + eddy currents). Without adequate cooling, insulation degrades, oil decomposes, and the transformer's life reduces significantly — every 6°C rise above rated temperature halves insulation life.
Q2. What is the most common cooling method for distribution transformers?
ONAN (Oil Natural Air Natural) is the most widely used method for distribution transformers up to 30 MVA. Oil circulates naturally by thermosiphon effect and dissipates heat to surrounding air through radiator fins.
Q3. What does ONAN/ONAF dual rating mean?
Many transformers are rated for two cooling modes — for example, 40/50 MVA ONAN/ONAF. At normal load, natural cooling (ONAN) is sufficient. When load increases and oil temperature rises above 65°C, fans switch on (ONAF mode) providing additional 25–33% capacity.
Q4. Why is oil used in transformers instead of water?
Transformer oil serves dual purpose — cooling AND electrical insulation. Water is a conductor and would cause short circuits between windings. Oil has high dielectric strength (~30 kV/cm), good thermal conductivity, and remains liquid over a wide temperature range (-30°C to 140°C).
Q5. What happens if the cooling system fails?
If cooling fails, the transformer temperature rises rapidly. Protection relays (oil temperature indicator, winding temperature indicator) trigger alarms at 85°C and trip the transformer at 95°C. Prolonged overheating causes oil decomposition (producing combustible gases), insulation failure, and potentially fire or explosion.