A transformer selected for a temperate, low-altitude site may not deliver the same continuous output at a hot mountain substation. For the owner, that mismatch can reduce available capacity or require changes to cooling equipment and electrical clearances. Selecting a power transformer therefore starts with site conditions, alongside voltage ratio and MVA rating.
This guide focuses on air-cooled, liquid-immersed transformers used in grid substations.
Define the Site's Temperature Profile and Installation Altitude
Specify maximum and minimum temperatures, seasonal averages, and elevation above sea level. A single annual average hides summer peaks and winter starting conditions.
Common IEC normal-service reference conditions include:
| Parameter | Reference condition |
|---|---|
| Maximum cooling-air temperature | 40°C |
| Average temperature of the hottest month | 30°C |
| Annual average temperature | 20°C |
| Installation altitude | Up to 1,000 m |
These are reference conditions, not a guarantee that every transformer suits every site within them. Higher temperatures or elevations require explicit design consideration. [1]
For the main transformer, use conditions at the cooling-air inlet. Nearby buildings and other equipment can make this temperature different from the weather-station reading.
Assess How Ambient Temperature Affects Oil and Winding Temperatures
Temperature rise measures how much hotter a transformer component becomes than its cooling medium. A temperature-rise value in kelvin is a temperature difference, not an absolute operating temperature.
Consider a simplified example with an assumed top-oil rise of 55 K:
| Cooling-air temperature | Assumed top-oil rise | Calculated top-oil temperature |
|---|---|---|
| 35°C | 55 K | 90°C |
| 45°C | 55 K | 100°C |
This arithmetic illustrates the loss of thermal margin. Actual rise varies with loading and cooling performance.
The winding hot spot-the hottest region of the winding insulation-also requires assessment. A satisfactory top-oil reading alone cannot establish acceptable insulation ageing. IEC 60076-7 addresses ambient temperature, loading, and thermal ageing for mineral-oil-immersed transformers. [2]
Evaluate the Effect of Reduced Air Density on Transformer Cooling
At higher elevations, lower air density changes heat transfer between radiator surfaces and the surrounding air. A fan delivering the same volumetric airflow also moves less air mass when density decreases.
Ask the supplier to evaluate radiator performance at the installation altitude. The assessment should cover transformer losses, oil circulation, radiator arrangement, and fan performance where applicable.
Adding panels without reviewing spacing and airflow is not a complete solution. Closely packed banks or obstructed outlets can limit the benefit of additional surface area.
For a new power transformer, require the quoted continuous rating to apply at the actual elevation. A general statement that the unit is "suitable for high altitude" leaves the available capacity unclear.
Check External Insulation and Electrical Clearances at High Altitudes
Altitude also affects the dielectric strength of air: its ability to withstand voltage without electrical breakdown. This requires a separate review from the cooling calculation.
Check bushing selection, phase-to-phase clearances, phase-to-earth clearances, and connections to surrounding equipment. IEC 60076-3 addresses transformer insulation levels, dielectric testing, and external clearances in air. [3]
Do not treat creepage distance and air clearance as interchangeable. Creepage follows an insulating surface; clearance is the shortest distance through air. Extending one does not automatically satisfy the other.
Require the transformer designer to document altitude corrections and coordinate them with the substation layout. Larger bushings may affect terminal positions, transport dimensions, and maintenance access.
Compare ONAN and ONAF Cooling for the Actual Site Conditions
ONAN uses natural oil circulation and natural airflow. ONAF retains natural oil circulation but adds fans to force air across the cooling surfaces. [4]
ONAF can provide a higher designed cooling stage, but the supplier must confirm its capacity at the specified temperature and altitude. Fans do not remove the need for altitude assessment.
Compare guaranteed MVA ratings for each cooling stage, fan controls, auxiliary power demand, noise, and maintenance access. Ask what load remains permissible if a fan group or its power supply fails.
For the main transformer, the usable rating during the required contingency can matter more than the highest rating available with every fan operating.
Determine Whether Derating or a Revised Transformer Design Is Required
Derating means reducing the permitted output so the transformer remains within its operating limits. It may be appropriate for an existing unit, but a new unit can instead be designed around the required site duty.
Options may include lower losses, revised winding cooling, additional radiator capacity, or a different cooling arrangement. Ask the manufacturer to compare designs against the same temperature limits and load cycle.
Avoid applying an unsupported percentage reduction per degree or per kilometre of altitude. The result depends on the design and cooling method.
Include expected load growth and emergency loading separately. IEC 60076-7 provides loading guidance; temporary overload capability should not substitute for the required continuous rating. [2]
Account for Solar Exposure, Enclosures, and Restricted Airflow
An open-yard transformer and an identical unit behind acoustic screens may experience different cooling conditions. Firewalls, enclosures, nearby radiator banks, and restricted discharge paths can cause warm air to recirculate.
Provide the supplier with the installation layout, solar exposure, ventilation openings, and surrounding obstructions. Keep radiator inlet and outlet paths clear while retaining required fire and electrical separations.
Cold conditions also need attention. Confirm insulating-liquid suitability, starting requirements, fan operation, and temperature limits for accessories. Include cold starting in the operating schedule; the minimum storage temperature does not by itself establish acceptable energisation and loading conditions.
Review the complete installation, including seasonal extremes, before accepting the cooling arrangement.
Specify Site Conditions, Performance Guarantees, and Verification Requirements in the RFQ
The request for quotation should state site temperatures, altitude, load cycle, cooling stages, and installation restrictions. Require guaranteed losses, applicable temperature-rise limits, and continuous ratings for the specified conditions.
Agree how factory results will demonstrate site performance. IEC 60076-2 covers temperature-rise limits and testing for liquid-immersed transformers; factory altitude and ambient conditions must be considered when establishing the applicable acceptance criteria. [5]
For the main transformer, request the thermal assessment, insulation coordination details, cooling drawings, and a clear list of deviations.
Send every bidder the same site schedule. Before ordering, attach the agreed site ratings, test requirements, and cooling-system scope to the purchase specification.
Sources
Schneider Electric, normal service conditions and ambient temperatures:
https://www.se.com/uk/en/faqs/FA133122/
https://www.se.com/nl/en/faqs/FA132693/
IEC 60076-7:2018, Loading guide for mineral-oil-immersed power transformers: https://webstore.iec.ch/en/publication/34351
IEC 60076-3:2013+AMD1:2018, Insulation levels, dielectric tests and external clearances in air: https://webstore.iec.ch/en/publication/62880
Eaton, Secondary Unit Substations Design Guide, cooling-class definitions: https://www.eaton.com/content/dam/eaton/products/design-guides---consultant-audience/eaton-secondary-unit-substation-design-guide-dg022017en.pdf
IEC 60076-2:2011, Temperature rise for liquid-immersed transformers: https://webstore.iec.ch/en/publication/599
