The original temperature-rise report is missing, but the nameplate survives. A refurbishment proposal can retain that historical rating as a reference. It cannot assume that the rebuilt transformer has the same thermal capability if winding geometry, losses, insulation or cooling paths have changed. [1]
The practical task is to write a rating statement that the evidence can support. Begin by identifying the intended duty and the components retained, replaced or redesigned. A restored appearance and a successful ratio test answer different questions from thermal capability.
Separate three possible claims
A historical rating describes what the original unit was declared to do. A repaired-design rating describes the duty justified by the current engineering design. A demonstrated duty describes conditions covered by measurements or operation. These categories may agree, but their agreement must be established rather than assumed.
| Proposed statement | Evidence needed | Limit when evidence is missing |
|---|---|---|
| Original nameplate rating retained as reference | Identified original plate and unit identity | Historical declaration only |
| Repaired assembly suitable for stated continuous duty | Loss, cooling and insulation design assessment | No unrestricted capability claim |
| Temperature rise demonstrated under test | Defined load, losses, cooling state and ambient conditions | Result applies to recorded conditions |
| Temporary restricted service proposed | Assessed limits and monitoring basis | Does not establish a permanent rating |
| Increased rating requested | Revised complete design and verification | No uprating from appearance or conductor area alone |
Reconstruct the thermal questions
Identify where heat is generated and how it reaches the cooling medium and surroundings. Total load loss matters, but so does its location. A replacement lead or changed conductor arrangement can create a local heat source that the old bulk design did not include.
Review cooling geometry as measured in the rebuilt assembly. Blocked passages, altered spacers or replacement cooling equipment can change the path even when the radiator outline remains familiar. The unknown original heat-run result should be a named evidence gap, not silently replaced by a generic temperature-rise value.
Use calculations and measurements for their own purposes
A thermal calculation can assess a proposed duty using stated properties and boundary conditions. Its uncertainty grows where material, loss distribution or flow data are unknown. A measurement can verify the assembly under its recorded condition; it does not automatically cover every ambient, cooling outage or overload scenario.
If service records are used, align loading, ambient conditions and cooling status. Evidence of satisfactory operation at moderate load cannot validate a higher duty that was never encountered. Likewise, an external temperature does not by itself establish the winding’s internal hottest location.
A rebuilt liquid-immersed transformer’s temperature-rise assessment must address the actual cooling configuration and recorded test conditions. IEC 60076-2 addresses temperature rise; a maintenance record alone does not demonstrate the proposed thermal duty. [2]
Rebuild the declared loss budget
Assumptions: the proposed rated operating point has 5 kW no-load loss and 40 kW load loss at its reference temperature, unchanged voltage/frequency and constant winding resistance for this screen. At 0.8 pu current, losses are 5 + 40×0.8² = 30.6 kW; at rated current they are 45 kW. The rated point requires 45/30.6 = 1.471 times the total heat rejection, or 47.1% more. A heat demonstration at the lower duty leaves that increment unverified. Retain the original nameplate only as a historical reference and withhold the proposed full-duty thermal claim until the rebuilt configuration’s loss distribution, temperatures and cooling are assessed. [1][2]
References
- IEC, IEC 60076-7:2018, Power transformers – Part 7: Loading guide for mineral-oil-immersed power transformers.
- IEC, IEC 60076-2:2011, Power transformers — Part 2: Temperature rise for liquid-immersed transformers.