What do ONAN, ONAF and OFAF mean?
The four letters identify the internal medium and circulation, followed by the external medium and circulation. For these oil/air designations, N indicates natural circulation and F indicates forced circulation.
| Code | Internal circulation | External cooling |
|---|---|---|
| ONAN | Natural oil circulation | Natural air circulation |
| ONAF | Natural oil circulation | Forced air circulation |
| OFAF | Forced oil circulation | Forced air circulation |
These are selected oil/air codes, not a complete list of fluids or cooling systems. Use the actual unit's nameplate and manufacturer records.
How should buyers read a 40/53.3 MVA or 66/88/110 MVA rating?
Keep the stages together and match each to its documented cooling arrangement. J&J's rebuilt ABB 230/13.8 kV record states 40/53.3 MVA at 55 C and 44.8/59.7 MVA at 65 C. The Kolektor ETRA 138/34.5 kV GSU record states 66/88/110 MVA. Review the corresponding nameplate and technical package before using a stage in a project comparison.
Which cooling equipment should the quotation identify?
Request the included radiator, fan, pump, control and auxiliary-power scope as applicable to the actual design. Identify missing or loose-shipped components and any field assembly, inspection or testing needed to establish the offered condition.
- State the continuous and contingency MVA required at the site's operating conditions.
- Confirm the cooling arrangement and temperature-rise basis tied to each proposed rating.
- Identify fan/pump supplies, controls, alarms and the project's redundancy requirements.
- Have the project engineer assess ambient, altitude, load profile and operation with cooling equipment unavailable.
- Keep any operating-direction or winding-duty limit separate from the cooling-stage label.
Does the highest cooling rating apply in both operating directions?
Do not assume it. The Royal SMIT 525/69 kV offer distinguishes 240/320/400 MVA step-up stages from a 250 MVA maximum step-down duty. Cooling-stage MVA and operating-direction limits answer different questions. Preserve both in the inquiry and quotation.
Request transformer capacity on the correct cooling basis
Email tommy@jjtransformers.com with the required MVA and cooling duty, high- and low-side voltages, connection, frequency, quantity, site conditions and required date. Include the existing nameplate or controlling specification. J&J can compare published equipment records and review unlisted requirements; final suitability and current availability are confirmed separately.
Common questions
Written quotes and supporting records control unit-specific ratings, availability, condition, price, and delivery.
Does ONAF mean any ONAN transformer can be uprated by adding fans?
No. Use the manufacturer's documented design and rating. Adding cooling equipment does not itself establish an approved higher MVA rating.
Is the highest nameplate MVA always the rating I can use?
No. The required cooling equipment, temperature-rise basis, winding duty, operating direction and approved site conditions must support it.
Is a 40/53.3 MVA transformer an exact 50 MVA unit?
No. Keep its actual cooling-stage ratings. Whether one stage meets a 50 MVA project requirement requires the responsible engineer's review of the full duty and conditions.