AI-generated illustration for transformer winding resistance report review. The disconnected equipment is not a HEAKOR test, instrument specification, verified four-wire circuit or testing procedure.
Compare transformer winding resistance only after confirming the same temperature basis, tap position and electrical measurement basis. A factory value corrected to a reference temperature is not directly comparable with an uncorrected site reading. Nor is a terminal-to-terminal result necessarily the same quantity as an individual phase-winding value. For a liquid-immersed substation transformer, these distinctions should be resolved before a reported difference becomes an acceptance decision.
This guide is for EPC buyers and owners’ engineers preparing factory-to-site records for three-phase liquid-immersed substation transformers on IEC-based projects. It concerns what the reports must explain, not how to connect instruments, operate tap changers or authorize energization.
Start with the basis of the number
A resistance value needs more than a unit identifier and an ohm symbol. The reviewer must know which electrical path it describes, under which tap condition, and whether temperature correction has already been applied.
For the contractual test basis, identify the applicable standards and editions alongside the purchaser’s specification. IEC 60076-1:2011 covers general requirements for power transformers within its stated scope. Use the complete governing documents to establish the actual test requirements and acceptance criteria; the comparison record below is a proposed purchasing aid, not an IEC-mandated form.
Keep measured and temperature-corrected resistance separate
Temperature correction puts resistance values on a common reference basis; it does not repair missing measurement information. Resistance changes with conductor temperature. Instrument-maker technical guidance on factory comparisons therefore distinguishes measured results from temperature-compensated results.
Make the temperature-correction inputs visible in separate fields:
- The original measured resistance and its unit.
- The winding temperature associated with that measurement, including how the test organization established it.
- The conductor material and correction method used.
- The selected reference temperature and the resulting corrected resistance.
Do not overwrite the original value with the corrected value. If software performs the correction, preserve the input settings or a readable calculation record so the reviewer can identify what happened.
For example, a factory column headed “resistance at reference temperature” and a site column headed “measured resistance” are not ready for a percentage comparison. First establish a common basis using the accepted method. A familiar reference temperature must not be inserted merely because another project used it. An ambient-temperature entry also needs clarification if the report does not explain how winding temperature was established.
Confirm terminals before comparing phases
Terminal-pair resistance and individual phase-winding resistance are different reporting bases. The connection arrangement matters: an instrument-maker’s discussion of delta-winding results specifically distinguishes manufacturer data expressed per winding from data expressed between terminal pairs.
Request the actual terminal designations for each result and a statement of whether the reported number is directly measured or calculated. A heading such as “HV phase A” does not, by itself, tell the receiving engineer which terminals were involved.
Where factory and site reports use different naming conventions, have the responsible technical parties provide an explicit mapping to the accepted connection drawing. Where they report different electrical quantities, request an engineering-approved reconciliation rather than applying a generic conversion factor in the procurement spreadsheet.
Also preserve units during data transfer. Converting milliohms to ohms is a unit conversion; converting terminal-pair results to phase-winding values is a circuit-basis question. Treating the two as the same administrative task can hide a substantive mismatch.
Match the tap position, not just the winding name
Compare results at corresponding tap positions, with the relevant tap arrangements identified. The measured current path can include tap-changer contacts and other connections, not only winding conductor. Power-transformer resistance guidance discusses these components and the need to retain tap-specific information.
Ask the factory and site teams to use the accepted tap schedule when labelling results. If more than one tap-changing arrangement affects the configuration, identify the position of each relevant arrangement. “Rated tap” or “normal position” needs a clear correspondence to the approved schedule.
Agree the required tap coverage before testing is arranged. A site table with fewer positions than the factory table should show which comparisons are available and which are absent; a blank row must not become a satisfactory result. This is a reporting requirement to agree, not an instruction to move a tap changer.
Use one compact comparison record
The following fields help the engineering reviewer decide whether a comparison can be made. They do not supply a pass/fail threshold.
Electrical quantity
Factory record: Winding, terminal pair and measured/calculated basis
Site record: Corresponding quantity or documented reconciliation
Tap condition
Factory record: Identified position of each relevant changer
Site record: Matching position or explicit difference
Original result
Factory record: Resistance, unit and associated winding-temperature record
Site record: Same information for the site measurement
Correction
Factory record: Material, method, reference temperature and corrected value
Site record: Common accepted basis, with inputs retained
Measurement context
Factory record: Test-current record and relevant stability or method notes
Site record: Corresponding information and declared deviations
Keep the comparison with its source reports. The technical document centre provides a starting point for specifying the requested document package; it is not a repository of automatically applicable test evidence.
What should happen to an unexplained difference?
An unexplained difference needs technical assessment, not an automatic defect label or an automatic pass. Separate missing information from a confirmed like-for-like difference. Ask the reviewer to state what can be concluded, what remains uncertain and which clarification or further assessment is required.
Do not borrow a percentage limit from a general instrument article and call it the project’s acceptance criterion. Retain the governing requirement and authorized disposition. A comparable winding-resistance result does not, on its own, establish the transformer’s complete acceptance status.
Frequently asked questions
Does “resistance at 75 °C” mean the transformer was tested at 75 °C?
No. The heading may identify a corrected reference value rather than the actual measurement temperature. Check the raw result, original winding-temperature record and correction details before interpreting that column.
Does a small difference between phases prove agreement with the factory baseline?
No. Comparing phases within one test and comparing factory and site results answer different questions. The latter still needs corresponding terminals, tap conditions and a common temperature basis.
Does a static resistance table establish OLTC transition performance?
Not by itself. Static resistance and dynamic tap-transition measurements provide different information. Where the project requires an OLTC transition assessment, specify that evidence separately rather than inferring it from a table of stationary tap results.
Does ordering a factory report include site testing?
No inclusion should be assumed. Identify who performs and reviews any required site work, what records are delivered and whether that work is included in the accepted quotation or arranged separately.
Include the comparison requirements in your HEAKOR enquiry
Start with HEAKOR’s substation transformer enquiry page. Include rating, winding voltages, phase/frequency, vector group, tap schedule, destination and the governing test specification. Identify the requested factory and site records and the intended comparison basis.
Request a Quotation addressing the equipment and document scope. Testing availability, performing party, report content, site services and review responsibility require written confirmation for the proposed offer.