{"id":2408,"date":"2026-09-29T17:00:00","date_gmt":"2026-09-29T09:00:00","guid":{"rendered":"https:\/\/shenhengglobal.com\/?p=2408"},"modified":"2026-09-27T19:57:35","modified_gmt":"2026-09-27T11:57:35","slug":"onan-vs-onaf-transformer-cooling","status":"publish","type":"post","link":"https:\/\/shenhengglobal.com\/pt\/blog\/onan-vs-onaf-transformer-cooling\/","title":{"rendered":"ONAN vs ONAF Transformer Cooling: Ratings and Applications"},"content":{"rendered":"
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ONAN vs ONAF<\/strong> requires a clear engineering boundary before it can support a purchase, inspection or commissioning decision. ONAN vs ONAF describes two oil-immersed transformer cooling arrangements. ONAN uses oil circulation by natural convection and air circulation by natural movement. ONAF adds fans to force air across the radiators for a higher rating or more thermal margin. The selection depends on load profile, ambient, noise, auxiliary power, reliability, maintenance and the complete tested transformer\u2014not simply the presence of fans.<\/p>\n

Define the cooling designation<\/h2>\n

Record the transformer\u2019s cooling designation, rated power at each cooling stage, temperature-rise basis, ambient and altitude. A nameplate may provide more than one rating, but the permissible load depends on which fans or pumps are operating.<\/p>\n

Ask for the manufacturer\u2019s cooling diagram and control logic. Confirm radiator area, fan arrangement, fan duty, sensor points and the alarm or trip response to fan failure. Do not infer an ONAF rating from a visually similar fan bank.<\/p>\n

Compare natural and forced air paths<\/h2>\n

ONAN relies on radiator surface and natural air movement, so room or yard layout, solar exposure and fouling affect heat rejection. ONAF improves air movement but adds motors, guards, controls, noise and auxiliary power.<\/p>\n

Check the airflow direction, clearances, fan guards, filters, ambient temperature and maintenance access. Prevent recirculation of hot air or blockage by cable trays, walls or storage. The forced-air stage must have a clear thermal path.<\/p>\n

Relate ratings to load profile<\/h2>\n

For a steady load that remains within the ONAN rating, fans may be unnecessary. For a sustained high load or a compact installation, ONAF can provide useful additional capacity. Short peaks still need a thermal model and the manufacturer\u2019s guidance.<\/p>\n

Trend load, top-oil temperature, winding or hottest-spot indication, ambient and fan state together. A larger ONAF nameplate does not authorize overload when fans are unavailable or the room is hotter than the design basis.<\/p>\n

Check auxiliary power and failure behavior<\/h2>\n

Fans require control power, starters or drives, protection, alarms and a defined failure response. If the auxiliary source is lost, the transformer may revert to a lower rating or require load shedding.<\/p>\n

Test fan start, staging, rotation, current, overload, alarm, remote indication and loss-of-control-power behavior. Coordinate the fan supply with the station service system and emergency operating plan.<\/p>\n

Review noise, maintenance and environment<\/h2>\n

Forced fans can increase acoustic noise and draw dust or salt into the radiator area. Bearings, guards, motors, contactors and sensors create additional maintenance points.<\/p>\n

Specify sound limits, cleaning intervals, spare fans, access, corrosion protection and inspection records. Do not add a fan package without reviewing electromagnetic, mechanical and fire interfaces.<\/p>\n

Make the project decision<\/h2>\n

Create a matrix for ONAN rating, ONAF rating, ambient, altitude, temperature rise, fan quantity, auxiliary power, noise, alarms, maintenance and contingency. Mark each item confirmed, excluded or requiring clarification.<\/p>\n

Choose the cooling arrangement that matches the actual load, environment and operator capability. Retain the approved cooling schedule, wiring, control settings and commissioning record with the transformer file.<\/p>\n

Define the equipment boundary<\/h2>\n

Start with the exact asset, voltage class, construction and released drawing. A useful engineering answer is tied to a physical boundary and a defined operating state. Record where the calculation, inspection or comparison starts and ends, and identify equipment intentionally excluded. This prevents a general catalog description from being mistaken for a project acceptance value.<\/p>\n

Keep the product identity consistent through design, procurement and commissioning. Note the supplied model, accessories, enclosure, cable interface and environmental assumptions. If the installed arrangement differs from the drawing, stop and obtain an approved disposition instead of filling a missing value from another product family.<\/p>\n

Use standards and manufacturer evidence together<\/h2>\n

Standards define the framework, test concepts and terminology, while manufacturer documents define the verified construction and operating limits of the supplied equipment. A standard link cannot prove that an unverified modification is acceptable, and a brochure cannot replace the project specification or safety rules.<\/p>\n

For procurement, request the standard edition, design verification or type-test scope, routine test records, installation limits and any altitude or environment correction. Store the evidence with the asset record. When a result is disputed, compare definitions, test conditions and boundaries before comparing numbers.<\/p>\n

Plan installation and commissioning<\/h2>\n

Installation quality often determines whether a sound design remains sound in the field. Confirm foundations, access, lifting, clearances, ventilation, cable bend radius, earthing, labels and control interfaces before energization. Use the approved method statement and permit system. A neat cabinet is not proof that every bond, interlock or sensor is connected correctly.<\/p>\n

Commissioning should move from document checks to visual inspection, mechanical operation, electrical continuity, functional logic, protection or control tests and final restoration. Record instrument identity, settings, raw values, exceptions and the person who performed the check. Keep equipment isolated whenever a test or inspection exposes a hazard.<\/p>\n

Control changes and handover<\/h2>\n

A change to a cable route, relay setting, tap position, cooling path or earthing connection can alter the original design basis. Use a controlled change process with an owner, technical review, drawing revision and acceptance evidence. Temporary test arrangements must be removed or clearly tagged before handover.<\/p>\n

The final package should include as-built drawings, test sheets, approved settings, equipment identity, maintenance instructions, deviations and training records. A future operator should be able to reproduce the decision without relying on memory or an unlabelled photograph.<\/p>\n

Safety note<\/h2>\n

Electrical equipment can retain energy and can be energized from more than one source. This article is not a switching instruction. Qualified personnel must follow local regulations, isolation, absence-of-voltage verification, grounding and supervision requirements. Never reach into an energized compartment or defeat an interlock to make a measurement.<\/p>\n

If a test fails, an enclosure is damaged, abnormal noise or discharge is observed, or the actual configuration is unclear, keep the asset in a safe state and escalate to the responsible engineer. Repeated energization without diagnosis can increase damage and risk.<\/p>\n

\"ONAN
Use the approved product drawing and site conditions when applying this guidance.<\/figcaption><\/figure>\n

How to use the result in a project<\/h2>\n

Start with the released design and final equipment identity. Note the operating state, measurement point, environmental conditions and decision the evidence must support. Then compare the record with the applicable standard, manufacturer instruction and project acceptance plan. If a prerequisite is missing, keep the item open rather than filling the gap with a generic value. This gives procurement, commissioning and operations teams the same reference point.<\/p>\n

For a supplier inquiry, ask for the calculation basis, verified arrangement, limits, test records and deviations. For site work, record the person, instrument, date, isolation boundary and restoration check. A concise record is more useful than a long report that hides the one fact needed to approve the next step.<\/p>\n

\"ONAN
Field verification should document the actual configuration, not only a general catalog illustration.<\/figcaption><\/figure>\n
\n\n\n\n\n\n\n\n\n\n
Project checks for ONAN vs ONAF<\/caption>\n
Check<\/th>\nEvidence<\/th>\nDecision use<\/th>\n<\/tr>\n<\/thead>\n
Definition<\/td>\nBoundary, equipment identity and operating state<\/td>\nPrevents different teams comparing different quantities.<\/td>\n<\/tr>\n
Design basis<\/td>\nRatings, environment and duty assumptions<\/td>\nConnects the answer to the real installation.<\/td>\n<\/tr>\n
Verification<\/td>\nApproved method, instrument, raw result and acceptance basis<\/td>\nCreates an auditable record.<\/td>\n<\/tr>\n
Change control<\/td>\nDeviation, review owner and as-built revision<\/td>\nStops hidden field changes becoming risk.<\/td>\n<\/tr>\n
Handover<\/td>\nSettings, tests, photos, maintenance and exceptions<\/td>\nSupports safe future operation.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

Related product and engineering guides<\/h2>\n

For the matching Shenheng product context, see the relevant product page<\/a>. For the broader topic, read the parent engineering guide<\/a>. Related reading: first related guide<\/a> and second related guide<\/a>.<\/p>\n

Sources and standards<\/h2>\n

Authority source 1<\/a> \u00b7 Authority source 2<\/a><\/p>\n

Video for system context<\/h2>\n

This neutral Practical Engineering explainer provides background on substations, transformers and switchgear. It is educational context, not a switching or test instruction.<\/p>\n