ONAN vs ONAF Transformer Ratings: Which Capacity Should EPC Buyers Use?

Use the transformer rating that matches the cooling equipment available in the operating condition you are buying for. ONAN and ONAF figures are not interchangeable capacities. When reviewing an offer from Zisheng Electric, an EPC buyer should compare the required load with each declared cooling rating before selecting the headline MVA figure.

This is a procurement decision with a direct plant consequence. A transformer may meet the normal load with fans running but fall short of the required duty when their supply is unavailable. The right purchase specification makes both requirements visible.

What ONAN and ONAF mean

ONAN means Oil Natural Air Natural: oil circulates naturally inside the cooling circuit, and heat passes to naturally moving surrounding air. ONAF means Oil Natural Air Forced: oil circulation remains natural, while fans force air across the cooling surfaces. The fans help remove heat; they do not change the winding voltage ratio.

The higher rating of an ONAN/ONAF transformer belongs to its declared thermal design. It is not an automatic percentage increase that can be applied to any existing transformer. Siemens Energy’s power transformer portfolio lists different cooling arrangements; the purchased unit’s guaranteed schedule must state which arrangement supports each rating.

Compare the operating cases, not just the largest number

First separate normal operation from the project’s required contingency. A contingency is a defined loss of equipment or supply that the plant must accommodate. State the remaining load, expected duration and available cooling in that condition. The owner may require uninterrupted full output, controlled load reduction or a different response. Procurement cannot decide that requirement by selecting a cheaper transformer.

Consider a hypothetical tender with an 18 MVA continuous normal load and a requirement to retain 18 MVA when fan power is unavailable. Offer A declares 16/20 MVA ONAN/ONAF. Offer B declares 20/25 MVA at the same environmental conditions. Offer A meets the normal load only with the specified forced cooling, but its declared ONAN rating does not meet the stated 18 MVA contingency. Offer B’s declared natural-cooling capacity meets that numerical requirement, subject to the rest of the specification.

This example is a comparison exercise, not a Zisheng Electric delivery or test result. It assumes continuous ratings on the same basis. Any proposed temporary overload needs a separate manufacturer-reviewed thermal assessment; the example does not authorize one.

Make the environmental basis common

Two MVA values are comparable only when their conditions match. Ask each bidder to identify the ambient-temperature basis, installation altitude, cooling-air restrictions and specified temperature-rise limits. A radiator arrangement intended for open outdoor airflow cannot simply be assumed to perform identically inside a restricted enclosure.

Include the intended load profile where daily peaks matter. Also distinguish equipment loss guarantees from auxiliary consumption. Fan input power is a separate operating demand and supply-design input; a headline transformer efficiency claim may not describe it. Keep these items visible in the evaluation rather than merging them into an unexplained total.

Buy the complete capacity-enabling package

Clarify whether the offer includes installed fans or only provision for future fans. Ask for the actual motor supply voltage, phase arrangement and frequency, along with controls, protection, local isolation and remote indication scope. Specify which party supplies and terminates the incoming power circuit. The transformer auxiliary power specification guide explains these interfaces in more detail.

Do not translate one failed fan into a proportional loss of MVA. Fan grouping, shared supplies, radiator allocation and thermal behavior matter. Request the manufacturer’s declared operating limits for the contingency the owner actually requires. Where the capability is not declared, record an open technical clarification rather than assigning an invented capacity.

Keep performance evidence separate from control checks

The purchase schedule should connect every guaranteed cooling rating to the agreed design and test evidence. A short demonstration that fans start does not establish continuous thermal capability. Conversely, thermal evidence does not prove that the installed site’s alarm wiring and control sequence are correct.

After the procurement basis is approved, use the cooling stage acceptance guide for functional verification. That later work should test the purchased arrangement, not discover which capacity the plant intended to buy.

For a 110kV–115kV power transformer enquiry, send Zisheng Electric the normal and contingency MVA requirements, site conditions and available auxiliary supplies. Request a rating schedule with one row per cooling condition and explicit exclusions. It gives procurement and engineering a common basis for comparing price against usable capacity.