How to Evaluate Losses in a Three Phase Oil Immersed Transformer

Loss figures often decide which three phase oil immersed transformer wins a tender—even when first cost looks similar. EPC and utility buyers need a repeatable way to read no-load loss, load loss, and total losses on the same test basis before ranking quotations.

This guide focuses on loss evaluation for outdoor distribution and industrial feeders. Use it as a procurement checklist alongside the approved single-line diagram; it does not replace project-specific engineering approval or utility evaluation rules.

Part 1. Why transformer losses matter in awards

A three phase oil immersed transformer runs for decades. No-load loss is present whenever the unit is energized, while load loss scales with current. Small guaranteed-loss differences therefore compound into large lifetime energy costs.

Procurement teams that compare only nameplate kVA and delivery date often miss that two bids used different reference temperatures or incomplete loss tables. Aligning loss data early prevents late commercial disputes after factory tests.

Public product information on Shenheng Power is a useful starting point, but final guarantees must match the purchase order revision and the utility evaluation method.

Part 2. Loss data buyers must lock in the RFQ

Capture these loss-related inputs before normalizing bids:

Request the guaranteed loss table and the factory test method in the same RFQ revision that freezes kVA, impedance, and vector group.

Part 3. Operating profile and evaluation boundaries

Load factor changes which loss component dominates. Lightly loaded feeders emphasise no-load loss; high-duty industrial feeders make load loss more visible in the evaluation. Record expected load duration curves when the owner uses a capitalised-loss or total-owning-cost formula.

Harmonic-rich loads, frequent overload, or elevated ambient temperatures can invalidate a simple catalogue comparison. State altitude, average ambient, and any known harmonic content so vendors quote realistic guarantees.

Indoor fire-sensitive buildings may prefer dry type equipment instead. When the project stays with liquid-filled outdoor distribution, keep the loss discussion inside the oil immersed design envelope rather than mixing technologies mid-tender.

Part 4. Documentation and loss RFQ checklist

Attach this checklist to every RFQ so vendors return comparable loss packages instead of marketing adjectives alone.

Part 5. Factory tests and site verification notes

Factory routine tests should report measured no-load loss and load loss against the guaranteed values in the contract. Review tolerances, reference temperature, and tap position recorded on the test sheet before accepting the unit for shipment.

At site, confirm nameplate data, vector group, and protection settings against the approved design before energization. Loss values are not re-proven by a simple insulation resistance check; keep the factory report in the turnover package for future energy audits.

Maintenance teams should also track oil condition and cooling performance, because degraded cooling can raise winding temperature and effective losses over time even when the original factory numbers were acceptable.

Part 6. Product recommendation and Fit Boundary

For many three phase oil immersed transformer distribution scopes, Shenheng Power publishes a relevant catalog starting point at S11-M oil immersed distribution transformer.

Fit boundary: This recommendation supports typical outdoor distribution and industrial feeder scopes documented on the product page. It is not a substitute for utility-mandated loss capitalisation formulas, unusual harmonic duty, altitude extremes, or national grid-code loss limits.

If your study points to a different kVA, voltage combination, or cooling class, review the broader oil immersed transformer category and submit rated data plus required loss guarantees through Contact Shenheng Power.

Related reading on the same cluster includes the oil immersed transformer buyers guide, the oil sampling guide, and the S11-M commissioning checklist.

Part 7. Common buyer mistakes on loss comparison

• Comparing load loss without stating the same reference temperature.
• Ranking bids on first cost while ignoring no-load loss on lightly loaded feeders.
• Accepting catalogue typical losses instead of contractually guaranteed values.
• Omitting factory test report review before shipment release.
• Mixing dry type and oil immersed loss tables inside one evaluation sheet without adjusting the method.

FAQ

What is the difference between no-load loss and load loss?

No-load loss is mainly core loss present whenever the transformer is energized. Load loss is mainly winding loss and increases with load current.

Why do buyers care about transformer losses in tenders?

Losses drive lifetime energy cost. Many utilities therefore capitalise guaranteed losses when ranking three phase oil immersed transformer quotations.

How are losses verified on a factory test report?

Routine tests measure no-load and load loss against the guaranteed table, with the reference temperature and tap position recorded on the report.

Should loss capitalization or total owning cost be used?

Use the owner or utility method stated in the tender. Do not invent a capitalisation rate that the project documents do not authorize.

How does temperature reference affect loss comparison?

Load loss changes with winding temperature. Bids that use different references are not directly comparable until corrected to the project basis.

What loss data should appear in an RFQ for an oil immersed unit?

Request guaranteed no-load loss, load loss at rated current, total losses if required, reference temperature, and the factory test document list.

References

• IEC 60076-1 power transformers — general
• IEC 60076-2 temperature rise
• IEEE standards portal