Transformer Cooling Methods: ONAN, ONAF, OFAF, ODAF, KNAN — Cooling Code Interpretation, Cooler Selection & Fan Control Strategy

Abstract

The IEC 60076-2 four-letter cooling code is the universal language for specifying how a transformer dissipates heat from its windings and core to the environment. Every cooling method represents a different combination of internal cooling medium (mineral oil, natural/synthetic ester, silicone fluid), circulation mechanism (natural convection vs. pumped/forced), external cooling medium (air or water), and external circulation mechanism. The choice of cooling method determines the transformer's rated capacity (a transformer with forced cooling can deliver 1.4-2.0× the ONAN rating), its noise profile, its auxiliary power consumption, and the complexity of its maintenance. This article decodes each letter of the IEC code, provides the thermal physics that governs the relationship between cooling and rating, and gives practical selection guidance including fan/pump start-stop control strategies that minimize contactor wear and thermal-mechanical stress on windings.

1. The IEC 60076-2 Cooling Code

The four-letter code defines the cooling circuit:

Position 1 — Internal Cooling Medium:

O = Mineral oil or synthetic insulating liquid with flash point ≤300 °C
K = Insulating liquid with flash point >300 °C (silicone fluid, synthetic ester, natural ester)
L = Insulating liquid with no measurable flash point (typically perfluorocarbon fluids)

Position 2 — Internal Circulation:

N = Natural convection (thermosiphon — hot oil rises, cold oil falls, no pump)
F = Forced circulation (oil pumped through the cooling circuit)
D = Forced and Directed circulation (oil is pumped AND directed into specific winding ducts)

Position 3 — External Cooling Medium:

A = Air (ambient)
W = Water (typically raw water from river/sea, or treated water from cooling tower)

Position 4 — External Circulation:

N = Natural convection (no fans, heat transfer by natural draft through radiators)
F = Forced circulation (fans force air through radiators, or pumps circulate cooling water)

1.1 Common Cooling Codes Decoded

Code	Internal Medium	Internal Flow	External Medium	External Flow	Typical Application
ONAN	Mineral oil	Natural thermosiphon	Air	Natural convection	Distribution transformers, small power transformers up to ~30 MVA
ONAF	Mineral oil	Natural thermosiphon	Air	Forced (fans)	Medium power transformers, 20-100 MVA
OFAF	Mineral oil	Forced (pumps)	Air	Forced (fans)	Large power transformers, >100 MVA
ODAF	Mineral oil	Forced + Directed	Air	Forced (fans)	Very large power transformers and GSU transformers, >200 MVA
OFWF	Mineral oil	Forced (pumps)	Water	Forced (pumps)	Indoor/underground transformers, generator transformers where water is available
ONWF	Mineral oil	Natural thermosiphon	Water	Forced (pumps)	Indoor transformers with limited space
KNAN	High-flash-point liquid	Natural thermosiphon	Air	Natural convection	Distribution transformers with enhanced fire safety (near buildings)
KNAF	High-flash-point liquid	Natural thermosiphon	Air	Forced (fans)	Medium power transformers with fire safety requirements

1.2 Multiple Cooling Ratings

A transformer with ONAF cooling typically has three rated capacities on the nameplate:

Example: 30/40/50 MVA (ONAN/ONAF1/ONAF2)

ONAN (30 MVA): All fans and pumps OFF. Natural cooling only. This is the base rating.
ONAF1 (40 MVA): First stage of fans ON (typically 50% of fans operating). Rating increases by approximately 33% (√(first-stage cooling enhancement)).
ONAF2 (50 MVA): All fans ON. Maximum rating. Rating increase over ONAN is approximately 40-65%.

The physics: The rated capacity under natural cooling (ONAN) is proportional to the surface-area heat dissipation capability of the tank and radiators. Adding forced air (ONAF) increases the convective heat transfer coefficient (h) by 3-10× on the radiator surface, allowing higher total dissipation and thus higher load current for the same temperature rise.

Approximate relationship: S_ONAF / S_ONAN ≈ √(1 + Δh/h_ONAN)

For a typical ONAF design with h increasing by a factor of 4: S_ONAF/S_ONAN ≈ √5 ≈ 2.24. In practice, 1.4-1.7× is achieved because:

The oil temperature rise in the windings (internal ΔT) is not reduced by external cooling — it depends on the oil flow rate, which is unchanged in ONAF (natural thermosiphon)
The radiator surface area is sized for the ONAN rating and is the limiting factor
The additional 33% ONAF1 step uses fewer fans and thus lower effective h

2. Oil Circulation: Natural vs. Forced vs. Directed

2.1 Natural Thermosiphon (ONAN, ONAF)

Hot oil from the top of the winding rises through the upper header into the radiator panels. As heat is transferred to the ambient air through the radiator surface, the oil cools and becomes denser, sinking to the bottom of the radiator and returning to the transformer tank at the bottom header. The driving force is the density difference between hot and cold oil:

ΔP_driving = (ρ_cold - ρ_hot) × g × H

Where H is the vertical height between the center of the winding (heat source) and the center of the radiator (heat sink). For mineral oil at 80 °C vs. 20 °C: ρ_cold ≈ 880 kg/m³, ρ_hot ≈ 850 kg/m³, g = 9.81 m/s², H ≈ 2-3 m → ΔP ≈ 600-900 Pa.

Advantages of natural circulation:

No pump — no auxiliary power, no pump failure, no pump maintenance
Inherently self-regulating — oil flow increases with temperature difference
Lower noise (no pump noise)
Lower cost

Disadvantages:

Oil flow rate limited by the thermosiphon head — typically 0.05-0.15 m/s in the winding ducts
Hot-spot-to-average-winding gradient is higher (Δθ_hs-to-avg ≈ 10-15 K) because oil flow is slower at the hot-spot location
Radiators must be above the transformer (height differential required)

2.2 Forced Oil Circulation (OFAF, OFWF)

Oil pumps (typically centrifugal, 1,500 or 3,000 RPM) circulate oil through the cooling circuit at a controlled flow rate. Typical oil flow rate: 1-3 m/s in the main cooling ducts.

Advantages:

Higher oil flow rate → lower hot-spot temperature (by 5-15 K at rated load compared to ONAN/ONAF)
More compact — radiators/coolers can be remote from the transformer (no thermosiphon head requirement)
Better temperature distribution — less oil temperature stratification in the tank

Disadvantages:

Pump reliability — pumps are single-point failure. Pump failure under load causes rapid temperature rise; pump flow monitoring and alarm are mandatory
Pump power consumption: Typically 5-15 kW per pump for a 200 MVA transformer (0.005-0.01% of rating — small but continuous)
Pump-induced static electrification — oil flow can generate electrostatic charge at the oil-cellulose interface, leading to partial discharge (most significant for forced oil flow >2 m/s)

2.3 Directed Oil Flow (ODAF, ODWF)

In directed-flow designs, the pumped oil is channeled through ducts that guide it directly to the winding hot-spot zones rather than allowing the oil to distribute freely in the tank. This provides:

Maximum hot-spot cooling for a given oil flow rate
HST reduction of 5-10 K compared to non-directed forced flow (OFAF)
Typical for generator step-up transformers (GSU) >200 MVA

3. Cooler Selection

3.1 Air Coolers (ONAF, OFAF)

Radiator panels: Pressed steel panels (typically 520 mm or 750 mm center-to-center spacing) with multiple fins per panel. Heat transfer area: 2-5 m² per panel. A 50 MVA transformer typically has 20-40 radiator panels (4-8 radiators with 5-6 panels each).

Fan configuration:

Axial flow fans (typically 560-800 mm diameter, 1,500 RPM, 0.5-1.5 kW each)
2-4 fans per radiator bank
Fans mounted below the radiators (blowing upward through the radiator fins) or on the side (blowing horizontally)

3.2 Water Coolers (OFWF, ONWF)

Shell-and-tube or plate heat exchangers. The oil flows on the shell side (or one side of the plate), cooling water flows on the tube side (or the opposite plate side). Oil-to-water heat transfer coefficient is typically 200-500 W/(m²·K) — much higher than air-cooled (10-30 W/(m²·K)), enabling very compact cooling.

Critical safety concern: Oil-to-water leakage. If the heat exchanger develops an internal leak (tube corrosion, gasket failure), water enters the oil at higher pressure (oil pressure in the cooler should be maintained below water pressure to prevent oil leakage into the water system — an environmental concern). Water in the oil drastically reduces the insulation strength. Mitigations:

Double-tube-sheet design with leak detection space between tube sheets
Oil-water differential pressure monitoring
Annual heat exchanger pressure testing

4. Fan and Pump Control Strategy

4.1 Temperature-Controlled Staging

Cooling fans and pumps are controlled by winding temperature or top-oil temperature:

Control Parameter	ONAN → ONAF1	ONAF1 → ONAF2	ONAF2 → OFF
Top-oil temperature	Start at 55-65 °C	Start at 65-75 °C	Stop at 45-55 °C
Winding temperature (calculated or measured)	Start at 75-85 °C	Start at 85-95 °C	Stop at 65-75 °C

Hysteresis: A minimum 5-10 °C difference between start and stop setpoints is essential to prevent short-cycling of fans/contactors. Contactor mechanical life is typically 100,000-300,000 operations — short-cycling can exhaust this within months.

4.2 Staggered Start

To avoid the starting current surge of all fans simultaneously drawing the inrush current on the auxiliary supply, fans are started in a staggered sequence with a 2-5 second delay between each fan (or group of 2-4 fans). This is implemented through the PLC/RTU controlling the cooling system or through time-delay relays.

4.3 Pump-Fan Interlocking

For OFAF/ODAF transformers: oil pumps must start before fans. Without oil flow, the radiator sections are thermal dead legs — running fans without oil flow cools the oil locally but does not circulate it, creating a dangerous temperature stratification where the winding is hot but the top-oil thermometer reads cool (because the measured oil is stagnant at the top). The interlocking:

Pumps ON → 10-30 s delay → Fans enabled
Fans OFF → Pump continues for 10-30 s (cooldown) → Pumps OFF

FAQ

Q: What is the difference between ONAF and OFAF cooling?

The critical difference is the internal circulation: ONAF uses natural oil convection (thermosiphon) — oil circulates by buoyancy force only, with fans on the radiators. OFAF uses oil pumps to force circulation. A transformer with ONAF cooling has no oil pumps, making it simpler and more reliable, but has a higher winding hot-spot temperature (5-15 K higher at full load). A transformer with OFAF cooling achieves lower hot-spot temperature and more compact design, but the oil pumps introduce additional failure points (pump motor, bearings, shaft seal, flow switch) and consume auxiliary power. For transformers >100 MVA, forced oil circulation is standard because the natural thermosiphon head is insufficient to achieve adequate oil flow through the large winding mass.

Q: Can I operate an ONAF transformer with the fans OFF continuously?

Yes — the ONAF transformer's ONAN rating is the continuous rating without fans. For a 40/50 MVA ONAF transformer, you can operate at up to 40 MVA indefinitely with fans off. However, operating above the ONAN rating requires the fans to be in service — the winding temperature rise at 50 MVA without fans will exceed the insulation thermal class. The transformer protection (winding temperature indicator, WTI) will alarm and eventually trip on overtemperature if the load exceeds the available cooling capacity.

Q: Why do ester-filled transformers use a "K" prefix (KNAN, KNAF) instead of "O"?

The letter "O" in the IEC cooling code is specified for insulating liquids with a fire point (Cleveland open cup) ≤300 °C — this covers mineral oil (fire point 140-170 °C) and some synthetic hydrocarbons. The letter "K" covers liquids with a fire point >300 °C — natural esters (fire point 350-360 °C), synthetic esters (fire point 260-310 °C, some qualify under K), and silicone fluid (fire point 350-380 °C). The difference is safety-related — high-fire-point liquids reduce the fire hazard classification and may allow reduced separation distances per NFPA 850, lower insurance premiums, and compliance with environmental regulations that restrict mineral oil use near waterways. The cooling physics (thermosiphon, fan application) is analogous between ONAN and KNAN — the higher viscosity of esters (typically 2-4× that of mineral oil at 40 °C) results in slower natural circulation (reduced thermosiphon head), so radiator sizing is adjusted accordingly.

Q: How should I set the fan start/stop temperatures?

The standard practice: Winding temperature (calculated from WTI) — start at 75-80 °C, stop at 65-70 °C. Top-oil temperature — start at 55-60 °C, stop at 45-50 °C. The winding temperature is the primary control parameter because it reflects the actual insulation hot-spot. The top-oil temperature provides a backup start signal (in case of WTI sensor failure). The hysteresis must be at least 10 °C to prevent short-cycling. For transformers in very cold climates (ambient <-20 °C), fan start may be blocked or delayed to avoid thermal shock (cold ambient air hitting hot oil-filled radiators can cause rapid contraction, stressing welds and gaskets). A minimum top-oil temperature of 30-40 °C may be required before fans are enabled.

Q: What maintenance do transformer cooling fans require?

Fan maintenance interval: Every 1-2 years. Actions: (1) inspect fan blades for cracking, erosion, imbalance (causes vibration that damages motor bearings), (2) check motor bearing noise (replace bearings if grinding/rumbling detected), (3) verify fan rotation direction and airflow, (4) clean radiator fin surfaces of dust, insects, vegetation debris (fouled fin surfaces reduce airflow by 20-40% and increase winding temperature), (5) test the motor winding insulation resistance (>1 MΩ at 500 V DC), and (6) functionally test the start/stop sequence from the control panel. Failed fans on one radiator bank create uneven cooling — the transformer may still operate within temperature limits, but the winding hot-spot shifts toward the uncooled side, potentially exceeding the design hot-spot at that location.

Q: Why does directed oil flow (ODAF) provide better cooling than non-directed (OFAF)?

In a non-directed OFAF design, the pumped oil enters the tank and distributes freely — it tends to flow through the path of least resistance, which may bypass the winding hot-spot zones where cooling is most needed. In an ODAF design, the oil flow is channeled through ducts, baffles, and distribution headers that direct a specified fraction of the total flow into each winding duct, ensuring that the hottest winding sections receive a guaranteed oil flow. The result: the winding-to-oil temperature gradient is reduced by 5-10 K, and the hot-spot factor H (ratio of hot-spot rise to average winding rise) is reduced from approximately 1.3 to 1.1. The cost is more complex internal oil distribution ducts and higher pump head (pressure required to overcome the duct friction losses).

References / Standards

Reference	Title
IEC 60076-2:2011	Power transformers — Part 2: Temperature rise for liquid-immersed transformers
IEC 60076-7:2018	Power transformers — Part 7: Loading guide for mineral-oil-immersed power transformers
IEEE C57.12.00-2021	IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers
IEEE C57.91-2011	IEEE Guide for Loading Mineral-Oil-Immersed Transformers
CIGRE TB 659	Transformer Thermal Modelling and Monitoring

*Authored by Du Fu, Production Engineer at ZY POWER. Cooling system design is not an afterthought — it directly determines the transformer's continuous rating, overload capability, and long-term insulation life. Every rating on the nameplate must be matched by adequate and correctly controlled cooling.*