What an ONAN Transformer Really Means

ONAN stands for Oil Natural Air Natural — a cooling designation defined in IEC 60076 and mirrored in GB 1094 and IEEE C57.12.00. It describes a liquid-immersed transformer in which the insulating oil circulates by natural convection inside the tank, and the heat carried by the oil is dissipated to ambient air by natural convection and radiation across the radiator surfaces. There are no pumps and no fans. The four letters are a complete professional term: they define the cooling medium and the circulation method for each stage of the heat path, so the term "ONAN transformer" should never be shortened, split or rearranged.

Because the cooling loop has no moving parts, ONAN transformers are the simplest, quietest and most maintenance-friendly class of distribution transformer — which is why the overwhelming majority of 30–6300 kVA oil-immersed distribution units in service worldwide are ONAN rated.

How the Natural Cooling Loop Works

Heat generated in the core and windings transfers to the surrounding oil. Warm oil becomes less dense and rises toward the tank top, while cooler oil descends along the tank walls and radiator panels. This thermosiphon effect produces a continuous, self-regulating circulation. Radiators — corrugated walls, tubes or panel banks — multiply the effective heat-transfer surface, and paint finish, panel spacing and free airflow around the unit all influence the final temperature rise.

Typical ONAN limits are 65 K top-oil temperature rise and 78 K winding average rise by the resistance method, with hot-spot limits around 98 °C for standard insulation systems. When load or ambient conditions exceed natural cooling capability, manufacturers add fans (ONAF) or oil pumps (OFAF/ODAF) — but that is a different cooling class and must be declared on the nameplate.

What Is Inside the Tank

• Core — laminated grain-oriented silicon steel, either a three-limb stacked core or a wound / three-dimensional magnetic path core for lower no-load loss.
• Windings — copper or aluminium conductors with oil ducts for axial and radial cooling.
• Insulation system — kraft paper, pressboard and transformer oil forming the oil-paper dielectric.
• Tank and radiators — corrugated, tubular or panel type, sized to the calculated loss budget.
• Accessories — silica-gel breather, conservator or sealed-tank design, Buchholz relay on larger units, pressure relief valve, oil level gauge, oil and winding temperature indicators, off-circuit tap changer (typically ±2 × 2.5%), lifting lugs and earthing terminals.

Key Parameters to Verify on the Datasheet

• Rated power (kVA) and voltage ratio, for example 10/0.4 kV or 35/0.4 kV.
• Vector group — Dyn11 and Yyn0 are the common distribution options.
• No-load loss and load loss, which largely determine the energy bill over a 25–30 year service life.
• Impedance voltage, usually 4–6% for distribution ratings.
• Cooling class, temperature rise and insulation level (LI/AC withstand).
• Rated ambient temperature and altitude assumptions used for the design.
• Sound pressure level, paint system, ingress protection and total transport weight.

Where ONAN Transformers Are Used

These units are the workhorses of utility distribution feeders, industrial plants, commercial buildings, hospitals and data facilities, rural electrification, mining and renewable energy collection systems. Common oil-immersed product families built to the ONAN principle include the S11, S13, S20 and S22 series, with the higher-number designs targeting progressively lower losses, and amorphous alloy units such as SH15 aimed at very low no-load loss for lightly loaded or intermittent duty.

Why the ONAN Class Dominates Distribution Networks

• No auxiliary power supply is required for cooling — the unit is grid-independent.
• Fewer failure modes: no fans, no pumps, no cooling control circuits.
• Lower sound levels than forced-cooled units of the same rating.
• Suitable for outdoor pole or pad mounting with minimal weather protection.
• Long service life with routine oil testing and simple visual inspection.

How to Size and Specify Correctly

• Estimate the real load profile rather than the connected load; a 20–30% design margin is typical.
• Check the ambient temperature and installation altitude — both derate natural cooling.
• Choose the loss level by lifecycle cost, not by purchase price alone.
• Confirm the vector group against your protection and earthing scheme.
• State the tap range, impedance tolerance and any harmonic or non-linear load requirements.
• Request the type test report for losses, temperature rise and impulse withstand.

What Actually Drives the Price

Two ONAN transformers with the same kVA rating can differ by 30% or more in price. The gap usually comes from winding material (copper versus aluminium), core grade and loss class, radiator area, tank construction, accessory quality, tap changer type and the extent of routine and type testing. A quote that looks cheap on day one can cost far more in no-load losses over twenty years of continuous energisation, which is why professional buyers always compare price and guaranteed losses together.

Common Procurement Pitfalls

• Loss figures quoted loosely, with no penalty clause for exceeding guaranteed values.
• Aluminium windings presented as copper in the commercial offer.
• No routine test certificate or factory inspection report in the delivery documents.
• Undersized radiators, so the unit runs hot at rated load in summer.
• Tap range, vector group or impedance left off the nameplate and drawing.
• Accessories claimed as "equivalent" without a stated make or model.

Factory-Direct Supply and Quality Assurance

A reliable manufacturer publishes its loss tables, insulation levels and radiator calculations instead of hiding them behind a generic datasheet. Runsheng Transformer Co., Ltd., a power transformer manufacturer based in China, produces S22, S20, S11 and S13 oil-immersed transformers, SCB13, SCB14 and SCB18 dry-type transformers, SH15 amorphous alloy transformers, mining and rectifier transformers, and 35 kV preassembled compact substations. Every oil-immersed transformer is supplied factory-direct with a manual, inspection report, test certificate and lifelong quality tracking, so the specification on paper is the specification delivered.

Frequently Asked Questions

Can an ONAN transformer be installed indoors? Yes, provided the room is ventilated to remove the heat load; the rating is based on free air circulation, so enclosed spaces require a calculated ventilation opening.

How often should the oil be tested? A dielectric strength and moisture check every one to two years, plus dissolved gas analysis for critical units.

Is a higher efficiency class worth the extra cost? For continuously energised distribution units, the reduced no-load loss usually repays the price difference within a few years of operation.

Can a custom ratio or special duty be built? Yes — mining transformers, rectifier transformers, lightning arrester duty units and capacity-regulating designs are routinely engineered to order.

Final Advice for Buyers

Treat the ONAN transformer as a long-term energy asset rather than a commodity. Specify the losses, demand the test reports, verify the winding material and radiator sizing, and compare total cost of ownership. Working with an experienced factory-direct supplier keeps both the price and the technical support under your control.

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