A transformer that rejects heat to the surrounding air, rather than to water, a chiller loop or a dedicated heat exchanger, is broadly described as an air cooled transformer. In practice the term covers two product families: dry-type units, where air moves directly across cast-resin or resin-impregnated windings, and oil-immersed units, where insulating oil carries heat from the windings to the tank and radiator surfaces before air removes it. Both are specified in the same way — voltage class, rated capacity, cooling class, loss figures and temperature rise — so understanding those five items is the fastest route to a correct and cost-effective purchase.

What defines an air cooled transformer

The cooling medium is atmospheric air, and there is no water circuit, no pump and no cooling tower to maintain. That single design decision produces the practical characteristics buyers care about: simple maintenance, no risk of liquid leakage into the environment, and dependable operation in locations where a water supply is unavailable or unreliable. The main commercial variants are:

• Cast-resin dry-type units — SCB10, SCB13, SCB14 and SCB18 series, typically 30 to 2500 kVA at 10 kV, 20 kV and 35 kV. No flammable liquid, so they are the usual choice for indoor substations in buildings, hospitals, tunnels and metro stations.
• Oil-immersed air cooled units — S11, S13, S20 and S22 distribution transformers from 30 to 6300 kVA, where oil transfers heat to radiators and air removes it from the radiator surface. These remain the most economical option per kVA for outdoor and pad-mounted installations.
• Amorphous alloy variants — SH15 and SCBH15, which cost more up front but cut no-load loss substantially, making them attractive on lightly loaded or continuously energised feeders.
• Special designs — mining transformers, rectifier transformers, capacity-regulating transformers and other custom special transformers, where the cooling class must be matched to duty cycle, harmonic content and starting current.

Cooling classes: what AN and AF really mean

Dry-type nameplates usually show AN (natural air circulation) or AF (forced air circulation with fans), while oil-immersed units show ONAN or ONAF. The distinction matters commercially, because the Fan-assisted rating is not a free upgrade. It typically represents 125 to 150 percent of the natural rating, but only while the fans are running and the auxiliary supply is available. Sizing a continuous load on the AF figure means the transformer is effectively overloaded whenever a fan fails or the auxiliary circuit is isolated for maintenance. Design the base load on the AN or ONAN rating and treat the higher figure as short-term headroom.

Temperature rise is the second half of the same story. Cast-resin dry-type units are commonly built with class F insulation at 100 K rise, with class H and 125 K rise available where ambient conditions demand it. Oil-immersed units are normally specified at 60 K top-oil rise and 65 K winding rise. Confirm the values printed on the nameplate rather than assuming them, because they directly affect service life and warranty position.

Specifications to lock down before requesting quotes

• Rated power — from 30 to 6300 kVA; size on measured demand plus a realistic growth allowance, not on habit.
• Voltage ratio and tapping — for example 35 plus or minus 2 by 2.5 percent / 0.4 kV, with off-circuit or on-load tap changing as required.
• Vector group — Dyn11, Yyn0 or another arrangement matched to the downstream protection scheme.
• Impedance — usually 4 to 6 percent for distribution units; it governs fault level and voltage regulation.
• Loss levels — no-load and load loss quoted against a specific efficiency tier, not just a headline figure.
• Enclosure and ingress protection — IP20 or IP23 for dry-type, IP54 and above for outdoor pad-mounted equipment.
• Sound level — typically 45 to 65 dB(A) at one metre, which becomes a planning constraint in residential and hospital sites.
• Ambient and altitude — above 1000 m, derating applies and must be stated in the offer.

Where air cooled designs earn their place

Dry-type air cooled transformers dominate indoor distribution in commercial towers, shopping centres, hospitals, airports, data centres and metro systems, where fire load and liquid containment are unacceptable. Oil-immersed air cooled units serve utility distribution networks, industrial plants, mining sites and renewable energy projects, including wind and solar step-up duty where wide ambient swings are normal. Compact substations and preassembled substations combine either type with high and low voltage switchgear into a single factory-tested package, which shortens site work considerably.

Five levers that cut the delivered price

• Size on real load. An oversized unit locks in higher no-load loss for twenty years; that cost usually exceeds the saving on the smaller nameplate.
• Evaluate total owning cost. Purchase price plus capitalised losses is the honest comparison. A low-loss design that costs slightly more often wins within three to five years.
• Choose the cooling class honestly. Buy AF only where peaks are genuinely short and the auxiliary supply is reliable.
• Buy factory direct. Removing one or two trading layers typically recovers 15 to 30 percent of the delivered price and shortens the lead time on spares.
• Standardise rather than customise. A proven catalogue design is cheaper, faster and better supported than a one-off unless the duty genuinely demands it.

Acceptance checks before payment

• Nameplate against the purchase order: capacity, voltages, vector group, cooling class and serial number.
• Routine tests: winding resistance, turns ratio, insulation resistance, power-frequency withstand, no-load and load loss, impedance.
• Type test reports where applicable: temperature rise, impulse, sound level, and partial discharge for cast-resin units.
• Documentation pack: operating manual, inspection report, test certificate and traceability records.
• Visual and functional checks: casting quality, core clamping, fan and thermostat operation, paint finish and terminal tightness.

Manufacturers such as Runsheng Transformer Co., Ltd. supply S11, S13, S20 and S22 oil-immersed units alongside SCB10, SCB13, SCB14 and SCB18 dry-type transformers, and ship each unit with a manual, inspection report, test certificate and lifelong quality tracking — which shortens the acceptance stage considerably, particularly for buyers ordering from overseas.

Common pitfalls worth avoiding

• Comparing quotations on price alone when loss figures differ by 20 percent or more.
• Treating a fan-assisted rating as continuous capacity.
• Ignoring altitude, ambient temperature or harmonic derating.
• Forgetting spare fans, thermostats and temperature controllers in the order.
• Accepting a delivery without partial discharge or impulse type test evidence.

An air cooled transformer is a simple product made complicated by loose specifications. Fix the capacity, voltage ratio, cooling class, loss tier and temperature rise in writing, ask for the type test evidence behind those numbers, and compare suppliers on total owning cost rather than invoice price. Buyers who follow that sequence usually find that factory-direct supply from an established transformer manufacturer delivers both the lower price and the shorter, more predictable lead time.

For technical selection support, drawings or a quotation on distribution transformers, dry-type transformers or compact substations, contact Runsheng Transformer Co., Ltd. at +86 13406394988, e-mail runsheng@7813transformer.com, or WhatsApp 008613508930968.