What an air cooled transformer Really Is

An air cooled transformer is a transformer whose heat is removed by air rather than by a dedicated liquid cooling circuit or forced water loop. In practice the term covers two large families that engineers meet every day on distribution networks, industrial plants and commercial buildings: dry-type transformers, where the windings are cooled directly by surrounding air, and oil-immersed transformers that rely on natural oil circulation plus radiator surfaces to move heat into the air. Both are, in the broadest sense, air cooled transformer designs — the cooling medium that finally absorbs the heat is ambient air.

The distinction matters because it drives almost every purchasing decision: fire safety, installation space, maintenance routine, altitude and humidity limits, and of course price. Understanding the basics before you send an enquiry is the fastest way to avoid overpaying for capacity you do not need, or discovering too late that the unit you ordered cannot be installed where you planned.

How Air Cooling Works in Practice

Heat in a transformer comes from two sources: no-load loss in the magnetic core and load loss in the windings. Air cooling removes that heat through three common mechanisms, usually described by the standard four-letter cooling codes stamped on the nameplate.

• AN (air natural): convection only. Windings and radiators give heat to still air. Quiet, zero auxiliary power, ideal for light and medium loads.
• AF (air forced): fans push air across cooling ducts. The same frame can carry a higher rating, typically with a capacity boost applied only during peak periods.
• ONAN (oil natural, air natural): the classic oil-immersed arrangement — oil rises through the windings, circulates into radiators, and gives up heat to the air by natural convection.
• ONAF (oil natural, air forced): fans over the radiators increase dissipation, allowing a higher loading stage on the same tank.

Dry-type units are further classified by insulation system, usually Class F or Class H, with defined temperature-rise limits that keep the winding well below its thermal class under rated load. Oil-immersed units follow the conventional limits of 65 K average winding rise and 60 K top-oil rise under standard conditions. These numbers are not marketing decoration — they are the boundary between a transformer that lasts thirty years and one that ages prematurely.

The Two Main Types of air cooled transformer

Dry-type air cooled transformer. Cast-resin models such as SCB10, SCB13, SCB14 and SCB18 are the standard choice for indoor installation: substations inside buildings, hospitals, data centres, metro stations, shopping malls and production halls where oil leakage or fire load is unacceptable. They are self-extinguishing, need no oil sampling, and are easy to inspect. The trade-off is a higher purchase price at the same rating and a greater sensitivity to dust, humidity and poor ventilation — the enclosure must breathe.

Oil-immersed air cooled transformer. Models such as S11, S13, S20 and S22, together with amorphous alloy SH15 units, use oil for insulation and heat transfer and air to finally reject the heat through radiators or corrugated tank walls. They offer the best cost per kVA, excellent overload capability and strong performance outdoors or in standalone substations. S13 and S22 grades reduce no-load loss substantially compared with older S9 designs, which matters on networks that run energised around the clock.

For projects with strict loss evaluation or renewable energy integration, amorphous alloy cores and low-loss S20/S22 designs are usually worth the small price premium; on a 24-hour load profile the energy saving pays back the difference within a few years.

Specifications That Decide Quality and Price

Whether you are buying a 30 kVA unit for a workshop or a 6300 kVA unit for a plant substation, the same specification sheet drives both performance and quotation. Insist that every item is stated explicitly rather than left to the supplier's discretion.

• Rated capacity and voltage class: 30–6300 kVA is the common distribution range; 10 kV and 35 kV are the usual voltage classes.
• Core material and loss values: grain-oriented silicon steel grade or amorphous alloy, with guaranteed no-load and load losses in watts.
• Winding conductor: copper or aluminium, foil or wire — this is the single largest cost variable and the most common source of misleading quotations.
• Impedance voltage: typically 4–6 % for distribution transformers; specified values affect short-circuit withstand.
• Vector group: Dyn11 and Yyn0 are standard; confirm the requirement for your earthing and unbalanced-load philosophy.
• Cooling class and temperature rise: AN, AF, ONAN or ONAF, matched to ambient temperature and altitude.
• Enclosure protection: IP20 to IP54 for dry-type, plus ventilation design that prevents hot spots.
• Tap changer: off-circuit or on-load, and the tapping range on the high-voltage winding.
• Standards and testing: IEC 60076 or GB 1094 compliance, routine tests, and type-test reports on request.

Mistakes Buyers Make — and How to Avoid Them

The most expensive errors are rarely technical mysteries. They are small omissions that surface after delivery.

• Choosing on price alone. A quotation that quietly swaps copper for aluminium or cuts core grade can be 20–30 % cheaper and considerably more costly over the service life.
• Ignoring installation conditions. A dry-type transformer squeezed into a sealed room will overheat; an oil-immersed unit placed where oil containment is prohibited will fail inspection.
• Forgetting future load growth. Specify normal and emergency loading, not just today's demand, and ask for the AF or ONAF rating.
• Over-specifying. Buying two sizes larger than needed wastes capital and runs the unit at low efficiency.
• Missing documentation. Manuals, routine test certificates, loss measurement reports and drawings are part of the product, not a favour.

Buying Factory-Direct

Because the air cooled transformer is a mature, standards-driven product, the difference between suppliers lies in material honesty, process control and after-sales traceability rather than in novel technology. Buying directly from the manufacturing plant removes trading margins and, more importantly, puts you in contact with the people who wound the coils and ran the tests. That is why experienced procurement teams ask for the factory's test bench records and a lifelong quality tracking commitment before signing.

Runsheng Transformer Co., Ltd. manufactures S11, S13, S20 and S22 oil-immersed transformers, SCB10, SCB13, SCB14 and SCB18 dry-type transformers, SH15 amorphous alloy units, 35 kV and below preassembled substations, and custom special transformers for mining, rectifier and renewable energy applications. Units comply with national standards and ship with manuals, inspection reports and test certificates, supported by lifelong quality tracking.

If you are comparing offers, send your specification sheet — capacity, voltage ratio, vector group, cooling class and installation environment — and ask for a quotation that states conductor material, core grade and guaranteed losses line by line. That single habit is usually worth more than any discount.

Contact: phone +86 13406394988 | e-mail runsheng@7813transformer.com | WhatsApp 008613508930968