In mid-July, a substation engineer stands at the transformer yard of a hot and dusty Middle Eastern port city, monitoring an alarming signal on the screen. The winding temperature has reached 118ºC and is continuing to rise on a 65 k temperature rise transformer with a capacity of 50 MVA. It has been less than two years since the installation and the transformer has been under high load during relentlessly hot and record-breaking summer. It appears that the oil-natural cooling system of the transformer is unable to eliminate the heat from the transformer tank. The engineer is faced with the dilemma that any large transformer owner has to solve one day: he could either retrofit the cooling system of the transformer, derate it or be prepared to a shorter lifespan of the unit. It should be noted that cooling is not a mere maintenance issue. It is an engineering consideration determining the capacity of the transformer tank and electrical insulation lifespan.
The article present all the available transformer cooling technologies including ONAN cooling, ONAF cooling, OFAF cooling, ODWF cooling, and new solutions such as air and heat-pipe retrofit solutions.
Briefly put: Cooling methods for transformers are defined by the four-letter codes of IEC standard: ONAN, ONAF, OFAF, OFWF and ODWF. The thermal limit is determined by the maximum average rise in winding temperature, usually 65 K for oil-submerged transformers according to IEC 60076-2 and IEEE C57.12.00. Installing forced-air fans in an ONAN transformer usually gives a continuous capacity increase of around 20-33%; full application of forced oil cooling can boost electricity supply by 40-50% as compared to a self-cooled device.
Why Cooling Determines Transformer Output
Transformers transform electrical energy very efficiently but even the 1 to 3 % of power that gets converted to heat implies an enormous amount of loss. For instance, a transformer rated at 50 MVA and running at the efficiency of 99 % has heat loss of about 500 kW continuously, which is enough to heat a small house. The heat generated has only one path to go: that is through the insulation system to the oil or air, then to the wall and radiators of the tank then finally to the surrounding water or air. This heat removal process gives an upper limit on the continuous performance of the transformer measured in kVA.
Heat is an enemy of transformers. The main cause of insulation aging follows the Arrhenius principle: every increase of the temperature of 6-10 K above the expected hot spot temperature halves the expected life of insulation. A transformer designed for 65 K average rise of temperature and working at 80 K will not only worse performance significantly but also consume its design life very fast causing it to lose years of life each month.
Decoding the Cooling Code and Main Cooling Methods
The cooling process is identified according to standard four-letter nomenclature defined in IEC 60076-2 and IEEE C57.12.00: the first letter states the extremely important medium which comes into contact with the windings (o = oil, k = liquid with the flash point above 300°, a = air), the second letter tells us about the way this medium is circulated (n = naturally, f = artificially forced circulation, d = directed circulation), the third letter refers to the medium with which the apparatus interacts (a = air, w = water) and the final letter refers to the way circulation is carried out (n = naturally, f = artificially).
| Cooling Code | How It Works | Typical Rating Boost vs. ONAN | Typical Transformer Size |
|---|---|---|---|
| ONAN | Oil circulates by natural convection; heat radiates to still air | Baseline | Up to ~5 MVA (single stage); large units are rated ONAN at reduced load |
| ONAF | Adds radiator fans; air blown across the cooling surfaces | +20 to +33% | 5-100 MVA |
| OFAF | Oil pumps force circulation; fans force air over radiators | +40 to +50% | 50-300 MVA |
| OFWF | Oil pumped through a water-cooled heat exchanger | +40 to +50%, compact footprint | Large units where water is available |
| ODWF / ODAN / ODAF | Directed oil flow through winding channels; fans or water externally | Up to +60% on the directed design | 100 MVA and above |
Multi-stage ratings are routinely utilized in the purchase of power transformers, for instance ONAN/ONAF/ONAF or the combination of ONAN/ONAF/ONAF/ONAF, and allow operators to switch on the fans as the load increases. For example, a transformer with a power of 60 MVA can be rated at 40 MVA as ONAN, at 50 MVA as ONAF, and at 60 MVA, with all fans active. The fans used are usually axial fans consuming from 0.75 kW to 3 kW of power, a negligible amount given the loads that they support, amounting to several hundred to thousands of dollars per installation, fan included.
ONAN vs. ONAF vs. OFAF vs. ODWF
| Recurso | ONAN | ONAF | OFAF | ODWF |
|---|---|---|---|---|
| Moving parts | None | Fans only | Pumps + fans | Pumps + water system |
| Faixa de classificação típica | Up to 5-10 MVA | 5-100 MVA | 50-300 MVA | 100 MVA and above |
| Cost per MVA | Lowest | Moderate | Higher | Highest |
| Reliability / maintenance | Highest; nothing to fail | Fan motors need attention | Pump seals and motors added | Water leaks and fouling risk |
| Área ocupada | Large radiator banks | Compact radiators + fans | Compact with pumps | Most compact |
| Ruído | Lowest | Fan noise | Pump + fan noise | Pump + water noise |
It’s simple to understand the engineering trade-offs here: every additional forced cooling stage provides additional MVA per unit of tank volume, but results in adding equipment that may fail, consume power or produce noise. ONAN is still the most common option, since it has no failure modes besides the transformer, while ONAF is the default option due to affordable, tested and easily retrofitted fans; OFAF and ODWF are only used for large transformers, when tanks and heat density leave no other option.
Temperature Rise Classes and Limits
| Parâmetro | IEC 60076-2 Typical Value | IEEE C57.12.00 Typical Value |
|---|---|---|
| Max average winding rise (oil-immersed) | 65 K | 55 K (OA), 65 K (FA) |
| Max hot-spot winding rise | 78 K (85°C hot-spot, 7 K ambient allowance) | 80 K |
| Max top-oil rise | 60 K | 55-65 K by class |
| Design ambient temperature | Average 30°C, max 40°C | 30°C average, 40°C max |
| Max hot-spot temperature limit | 98°C (winding), 105°C (oil) | 110°C continuous |
These figures are part of an agreement or contract with a format. If a transformer does not pass the temperature-rise evaluation test at the factory, then it has not followed the conditions of the specification and the certification needs to include the measured temperature rise. When altitude is taken into account (1,000 m or higher) or high ambient temperature (plus or minus 40 degrees Celsius), then the factors are to be checked in IEC 60076-2, the reason for which is that the devices will have to be derated in the hotter climate and thus will be different from the devices that will be used in a moderate climate.
In order to understand how the different stages of cooling influence the process | the same 25-MVA transformer has been considered with different conditions and processes, so that one can realize the peculiarity when the suppliers provide unrealistic offers:
| Cooling Mode | Continuous Rating (25 MVA Tank) | Rise at 25°C Ambient | Rise at 40°C Ambient | Derating Needed at 40°C |
|---|---|---|---|---|
| ONAN (natural) | ~15 MVA | 52 K top-oil | 67 K top-oil | ~12% |
| ONAF (fans on) | ~20 MVA | 48 K top-oil | 63 K top-oil | ~8% |
| OFAF (pumps + fans) | ~25 MVA | 44 K top-oil | 59 K top-oil | ~5% |
| ODWF (directed + water) | ~30 MVA | 40 K top-oil | 55 K top-oil | ~5% |
Retrofitting and Uprating Existing Units
Converting from ONAN to ONAF service is often the preferred upgrade method because it’s usually cheaper than buying a bigger transformer.
Check thermal design. Get confirmation from the manufacturer about whether or not the radiators and hot-spot in the windings can cope with the extra heat. Some tanks will require extra radiator surface area even with the installation of fans.
- Install fan bank. The cost will usually fall within $1500 to $8000 and will include 2-6 axial fans, guards, mounting frames, wiring in addition to contactors, while installation will require 2-4 days of outage.
- Equip with temperature-controlled switching. The fans should start working from a top-oil temperature signal while turning off automatically based on either a timer or a temperature drop in order to avoid quick cyclical operation.
- Update the nameplate and protection. The new combined rating must be included in relay settings as well as maintenance records.
- Commission with heat run. It would be advisable to repeat the temperature-rise test at the new rating in order to ensure that the winding rise of 65 K is followed.
Going beyond ONAF and using forced oil circulation requires more effort, as it involves adding oil pumps and modifying the tank plumbing. This option is considered when dealing with transformers that are exceeding 30 MVA. Water-cooled conversions (OFWF) might be possible, but in reality, they don’t bring that many advantages since they are connected to the risk of leakages.
Cooling of Dry-Type Transformers
Dry-type transformers are characterized by their usage of air for cooling and compliance with particular codes which include AN (Air Natural) or AF (Air Forced). In terms of heat removal, dry-type transformers do not include oil, and so instead rely on the area of their encapsulated or wound coils. The capacity of dry-type transformers is relatively smaller than oil-filled models. For instance, typical cast-resin transformers may reach a maximum capacity of up to 3-10 MVA. Dry-type transformers may be categorized into two temperature rise classes according to the IEC 60076-11 standard: Class F (which rises by 100K to reach a temperature of 155°C) and Class H (which rises by 125K to reach a temperature of 180°C).
The implementation of forced-air cooling adds a couple of hundred dollars for the fan and increases the rated capacity of the transformer by approximately 25 to 30%. The key advantage of dry-type transformers is the absence of oil in the operation process. However, the usage of air as a cooling medium also poses certain disadvantages such as the need for proper ventilation and an expensive system for dust management.
Which Cooling Method for Which Application
- Distribution transformers more than 2.5 MVA – In this case, ONAN is the standard method used for most applications. It may be used, however, in a case when there is a presence of peaky industrial load, allowing for use of ONAF.
- Power transformers – At this point, ONAN method could be used with some fans to provide an appropriate cooling environment.
- Large power transformers (more than 100 MVA) are of an OFAF type, so there are oil pumps and forced flow systems being used for these devices.
- OFFSHORE DISTRICT AND CROWDED PLACES – An OFWF type should be preferred when there are water resources, reducing the size of the system.
- Indoor commercial buildings deal with dry type units while using AN and AF methods, as oil and the indoor fire code do not go well together.
- Hot climate and high altitude bases call for a need to implement additional cooling level and/or derating because surrounding air is not capable of taking much heat.
Comparação de Marcas e Preços
| Marca | Origem | Cooling Technologies Offered | Approx. Price Range (1 MVA ONAN) |
|---|---|---|---|
| ABB | Switzerland/Sweden | Full range, ODWF and directed-flow experience | $10,000-$25,000 |
| Siemens | Germany | Large power units, staged ONAF/OFAF | $11,000-$28,000 |
| Hitachi Energy | Suíça | Power transformers with advanced cooling | $12,000-$30,000 |
| Schneider Electric | France | Distribution and dry-type AF units | $9,000-$22,000 |
| Eaton / GE | USA | Distribution ONAN/ONAF | $7,000-$18,000 |
| Jiangsu Subian Electric Power | China | ONAN, ONAF, staged ratings, dry-type AN/AF | $5,000-$14,000 |
Market leaders such as ABB, Siemens, Hitachi Energy, and Schneider Electric are the defining players in the large-scale power transformer industry with their ODWF/OFAF designs that are used as benchmarks in the industry. Notably, manufacturers such as Jiangsu Subian Electric Power are known for their ONAN and staged ONAF units designed for the smaller 0.5-10 MVA classes that most customers are interested in. Their units have been tested in accordance with IEC 60076-2, and they receive certificates verifying that the temperature rise laboratories can measure the temperature rise of their transformers. Subian ships its products all over the world; it can produce tanks and radiators according to customer specifications at prices that are 40 to 60% lower than the prices quoted by its European competitors. Customers who need fan bank retrofitting should check with Subian about the technical specifications for radiator surface technology and pump choices, since when the ambient temperature is as high as 40 degrees Celsius, these specifications are more important than the price.
How to Choose the Right Cooling Method
- The first thing is to define the type of load profile. A load profile with constant use has a higher cooling requirement than a load profile with varying use. Therefore, a cooling stage may be optimized for running the fans alone during peak afternoon times.
- Next, you shall consider the effects of ambient. If your ambient is more than 40 degrees Celsius or you are located in an altitude of more than 1000 meters, you will need to derate it according to IEC 60076-2 or provide an additional cooling stage.
- Make sure that you account for the space requirements of the equipment. Radiator banks require yard space but OFWF and ODWF technologies reduce the size of the footprint at the cost of water systems.
- Reliability is of high importance. For critical infrastructure, ONAN stages and simple fans should be favored over pumps whenever possible, as long as the rating allows.
- Be aware of the total cost of ownership. Motors and pumps consume energy and require maintenance. A large ONAN tank may have a lower cost than simple forced cooled motor.
- Do not forget to request a temperature-rise test with 65 K (or desired) winding increases being attested by the test certificate rather than being assumed.
Maintenance and Monitoring
The failure of cooling systems occurs in silence and is typically indicated by an alarm triggered by the drop in temperature. In sound cooling practice, a number of inspections of breakdown parts such as fan blades of the fan, fan motors, fan guards, and so forth are included. In addition to a monthly check of all parts mentioned, a quarterly check of pump seals and oil flow indicators should be performed. Together with annual thermography of radiator banks, there is also a need to carry out a written test of the auto-start of temperature switch controlled fan. As for the large cooling boards, the winding temperature section, and DGA should be analyzed together; if there is a rise of a hot spot with no fan malfunction, it points towards an issue within an internal part of the cooling unit.
The oil amount in the conservator should match the cooling design. The occurrence of low oil level can lead to a situation where cavitation happens and this can be of great danger compared to overheating.
Perguntas Frequentes
What do ONAN and ONAF mean on a transformer nameplate?
These are designations that are linked to cooling according to IEC 60076-2. ONAN stands for oil natural and air natural, which means the self-cooled standard design. The ONAF designation includes the radiator fans that help to cool the transformer, and so the ONAF designation denotes the increased operation capacity, which is 20% to 33% higher than that of the self-cooled design. For this reason, ONAN/ONAF transformers are rated twice.
How much does it cost to add forced-air cooling to a transformer?
The retrofit of an ONAF on a distribution-class transformer costs is 1500-8000 dollars since it usually involves the installation of 2-6 fans, as well as the installation of the guards, contactors, and temperature-control switches. Using forced air helps to raise the capacity of the transformer by at least 20-33.
Can I overload a transformer if I turn on the fans?
The overloads are permitted only up to the ONAF rating and only if the air temperature and elevation are within the limits prescribed in the concept. The actual increase of temperature is a requirement; thus, in case of the increase of the average temperature higher than 65K, the life span of the insulation will be reduced.
What is the difference between OFAF and ODWF cooling?
The difference consists in the fact that in OFAF, the oil is circulated through the radiator while in OFDA, the oil is pumped in winding cooling channels by means of water flow through the heat exchanger.
Does altitude really matter for transformer cooling?
Indeed, it does. Above 1000 meters, the air density is decreasing which makes cooling very inefficient.
Referências
- IEC 60076-1 — Power Transformers, General Requirements — Master specification for ratings and losses.
- IEC 60076-2 — Power Transformers, Temperature Rise — The standard that defines cooling codes and temperature-rise limits.
- IEEE C57.12.00 — General Requirements for Liquid-Immersed Transformers — North American cooling code and temperature-rise baseline.
- IEC 60076-11 — Dry-Type Transformers — Thermal classes for air-cooled units.
- Hitachi Energy — Transformer Products — Reference for staged cooling ratings on large power transformers.
- ABB Transformers — Industry reference for cooling technology options and datasheets.
- Siemens Energy Transformers — Cooling and uprating documentation for power transformers.
Conclusão
Coolings of transformers act as the important deciding factors determining the power and life span of the transformer. Whenever one reads the 4-letter code for cooling, understands the specifications of temperature rise, and makes a choice between natural cooling, forced cooling, or adding a fan to the existing cooling systems, all that info has practical use in engineering.
- ONAN is the basic standard; ONAF represents an improvement and consists of 20-33% over the ONAN with extra costs of retrofitting from $1,500 to $8,000.
- OFAF and ODWF are used for transformers of large sizes for the heat densities.
- Temperature shall be proven through the test and certified with the winding temperature rise of 65 K.
- In case of high altitude or temperatures, transformers shall be derated or the cooling shall be reinforced.