Evaluate transformer winding resistance test results by first confirming the reading is stable, then correcting it to a common temperature and comparing phases, tap positions and prior baselines under identical conditions. Use the reference criteria that apply to the specific transformer, standard and contract rather than a blind universal threshold, and treat anomaly patterns as evidence for targeted follow-up, not as a final diagnosis.
Wait for a Stable Reading Before Comparing Results
The only valid resistance value is a stable one. Transformer windings are inductive, so after the test current is applied the reading decays toward the true resistance over a settling period. Comparing an early transient value with a historical baseline creates false alarms, so define stability in your procedure: for example, a defined number of consecutive readings within a small spread over a fixed period, recorded by the instrument.
Record the stabilisation time together with the reading. A winding that takes unusually long to stabilise can itself indicate a connection issue or a magnetic-circuit condition, so the time-to-stable value belongs in the report as evidence, not just as a procedural detail. If the reading never stabilises, resolve the cause before any comparison.
Use the instrument’s stability display or repeated-readings feature rather than relying on a single screen value. Some instruments record the measured value only when the drift falls below a threshold for a defined period; confirm what that threshold is, because it defines the repeatability of every result in the file. If the instrument does not offer such a feature, capture readings at fixed intervals and note the spread in the report.
Correct Resistance Values to a Common Temperature
Winding resistance changes with temperature, so raw readings from different dates cannot be compared directly. Correct each measurement to a common reference temperature using the winding’s actual temperature and the temperature coefficient of the conductor material, and apply the reference temperature defined by the applicable standard, contract or manufacturer documentation.
Measure the winding temperature reliably. Top-oil temperature is a useful input for oil-immersed units, but the winding itself may be warmer or cooler depending on load history, so follow the temperature measurement method required by your procedure and record both the method and the value. State the correction formula and the reference temperature in the report, because a corrected value without its assumptions cannot be reproduced by another engineer.
The table below shows an illustrative normalisation record. The values are examples of the method only; replace the reference temperature and comparison basis with the ones that apply to your transformer and procedure.
| Test date | Tap | Measured resistance | Winding temperature | Corrected value at reference | Baseline corrected value | Change vs baseline |
|---|---|---|---|---|---|---|
| 2025-03-10 | 3 | 0.426 mΩ | 24 °C | 0.481 mΩ | 0.478 mΩ | +0.6% |
| 2025-09-14 | 3 | 0.431 mΩ | 27 °C | 0.480 mΩ | 0.478 mΩ | +0.4% |
| 2026-05-02 | 3 | 0.447 mΩ | 25 °C | 0.499 mΩ | 0.478 mΩ | +4.4% |
In this illustrative example, the trend shows a rising corrected resistance against the baseline, which would justify checking the connection and tap changer contacts at that position and scheduling a repeat measurement. The interpretation step is the same for any unit: normalise, compare with the baseline, and judge the change against the applicable criterion.
Compare Phases Without Using a Blind Universal Threshold
Phase comparison is the most sensitive way to spot anomalies, because phases of the same winding are expected to behave alike. Compare each phase under the same current, temperature and tap position, and evaluate the spread between phases against the criterion applicable to the transformer. Reference criteria may come from the maintenance specification, the OEM documentation or the contract; confirm the applicability of the source before using it.
Do not rely on a single round number quoted from an unrelated context. A phase spread that is acceptable for one transformer class and voltage level may be significant for another. The defensible approach is to compare the current phase pattern with the transformer’s own baseline and with the criterion that applies to that unit, and to record all three in the report.
Where your programme follows a maintenance specification such as ANSI/NETA MTS, confirm the edition and the exact table that applies to the transformer type and test method before using it. Maintenance specifications define test methods and acceptance guidance for a broad range of equipment; the value in a specific record comes from applying the correct section to the correct asset and documenting the reference used.
Evaluate Every Tap Position as a Trend
For transformers with tap changers, treat each tap position as its own trend. Resistance at a specific tap reflects the contact and transition path at that position, so an increase confined to one tap points to that tap’s contacts, while a change across all taps points to the winding or the main connections. Plot the resistance profile across taps for each phase and compare profiles over time.
On-load tap changer contacts degrade gradually with operation count, and the resistance at frequently used taps can drift before it reaches a level that triggers an alarm. A tap-by-tap trend is therefore more useful than a single overall value for scheduling OLTC maintenance, because it shows which positions are changing and at what rate.
Plot the resistance against tap position for each phase and compare the shape of the profile with previous tests. A healthy profile is smooth; a single elevated point or a step between adjacent taps identifies the position that needs inspection. When a tap changer has been serviced, the resistance profile before and after the service provides direct evidence that the contact work was effective.
Recognise Patterns Linked to Leads, Joints and OLTC Contacts
Resistance patterns help localise the problem, even though they do not name the mechanism. An increase on all phases of one winding suggests a common path issue, such as a bushing connection, a winding lead or an internal joint. An increase on one phase only points to that phase’s lead, joint or winding connection. An increase at one tap position on all phases points to the tap changer selector and transition contacts at that position.
Consider the thermal history too: hot spots from a bad joint can oxidise the connection and raise resistance further over time, so a rising trend with load cycles is more serious than a single high reading. Record load and temperature history alongside the electrical results so the reviewer can judge whether the pattern is stable, improving or deteriorating.
Where available, combine resistance evidence with infrared inspection of bushings and connections. A hot spot at the same location as a high-resistance phase is strong corroborating evidence, while a hot spot without a resistance anomaly may point to a different mechanism. The two datasets together localise the problem more reliably than either alone.
Record Discharge, Test Conditions and Retest Decisions
Complete the record with the test conditions that make the result reproducible: instrument and calibration date, test current, stabilisation time, winding temperature, tap position, lead arrangement and connection diagram. Also record the discharge step, because it confirms that the procedure was followed safely and explains any interruption before a retest.
Define the retest decision in the report. If a pattern is borderline or inconsistent with the baseline, state the retest interval and the conditions for the retest rather than leaving it to memory. If the pattern is confirmed, escalate with the full evidence package, including the normalised values, the phase spread and the tap profile, and let the responsible engineer decide whether additional tests such as excitation current or ratio verification are required.
Retests must use the same test current, lead arrangement and temperature measurement method as the original reading, because any change in conditions can explain a change in result. If a retest confirms the anomaly, document the repeatability and the escalation; if it does not, document the probable cause of the first reading, such as a poor connection or an unstable stabilisation period, so the record remains honest and auditable.
Frequently Asked Questions
Why must resistance values be temperature-corrected?
Because conductor resistance changes with temperature, readings taken on different dates are only comparable after correction to a common reference temperature. Use the winding’s actual temperature, the correct temperature coefficient for the conductor, and the reference temperature defined by your standard, contract or manufacturer documentation.
How should phases be compared?
Compare phases at the same current, temperature and tap position, and evaluate the spread against the criterion applicable to that transformer. Treat the transformer’s own baseline and trend as the primary reference, and record the criterion source so the comparison can be audited.
What does an OLTC tap trend tell us?
A tap-by-tap resistance trend shows which tap changer positions are changing and how quickly, which supports condition-based maintenance of selector and transition contacts. An increase at one tap position is more localised than a change across all taps, and the trend rate helps set the maintenance interval.
For the full lifecycle test framework, see the complete power transformer testing checklist. To qualify a winding resistance test system for your transformer fleet, review power transformer testing equipment and request a technical proposal with your winding data and test procedure.