Introduction
The DC winding resistance test is one of the simplest yet most revealing diagnostic tests for power transformers. By driving a controlled DC current through each winding and observing the resulting voltage drop, it gives direct access to the real condition of conductors, joints and tap-changer contacts. When the results are corrected for temperature and trended over time, even small internal defects can be detected long before they lead to failures. This makes winding resistance testing a core element of modern transformer commissioning and maintenance programs.
1. Purpose and Diagnostic Role of the Winding Resistance Test
In the winding resistance (DC resistance) test, the ohmic resistance of the transformer windings is measured. Based on the test results it is possible to:
- detect internal faults and short circuits inside the transformer,
- identify turn-to-turn faults and closed loops in a winding,
- spot high-resistance joints on the current path,
- evaluate the condition of tap-changer and diverter-switch contacts, and
- check the quality of connections at bushings and terminal leads.
Because all of these issues directly affect reliability and can easily remain hidden, utilities treat the winding resistance test as a routine test during commissioning, periodic maintenance and fault investigation.
The following sections describe the test circuit, numerical requirements and interpretation notes in detail.
2. Test Circuit and Basic Measurement Requirements
For both star and delta windings the resistance is measured using a DC source, ammeter and voltmeter.
- In a star-connected winding, the DC current is injected between a line terminal and the neutral point; the voltmeter measures the voltage between the same points.
- In a delta-connected winding, the DC source is connected between two-line terminals so that the current flows through one side of the delta and the voltage across that side is measured.
These two arrangements correspond to the standard star and delta test circuits.
To obtain reliable measurements, the current level must satisfy the following conditions:
- The injected current shall be greater than 1.2 times the no-load current of the winding.
- In practice it is recommended to perform the test at about 10 A.
- At the same time, the injected current must not exceed 10% of the rated current of the winding, so that the conductor temperature does not increase significantly during the test.
The DC voltage is applied to the winding for long enough to allow the current to reach its final, steady value. The voltage across the winding and the current through it are only recorded once the current has stabilised. The winding resistance is then obtained from:
3. Temperature Correction and Thermal Conditions
By convention, the winding resistance of a power transformer is expressed at a reference temperature of 75 °C during factory routine tests. The measured resistance therefore has to be converted to this reference using:
where:
- (
): resistance corrected to the reference temperature (T_s),
- (
): measured resistance,
- (
): reference temperature (normally 75 °C),
- (
): temperature of the winding during measurement (°C),
- (M): material constant
- (M = 234.5) for copper windings,
- (M = 225) for aluminum windings.
The winding temperature (Tm) is determined as follows:
- For transformers that have been out of service for a long time, the whole unit is near ambient, so (Tm) can be taken as the ambient temperature.
- If the transformer has been de-energized for a shorter period (less than about three hours), (Tm) should be taken as the average of the readings of the winding and oil thermometers, provided that these readings are reasonably close to one another.
Correcting winding resistance while the transformer is cooling down can otherwise introduce large errors. International standards therefore emphasize that winding resistance should be measured at least three hours after the transformer has been taken out of service, so that temperature gradients have largely decayed and the transformer has approximately uniform temperature.
4. Execution on All Taps, Measurement Technique, and Trending
The resistance test is not limited to a single operating point. For proper assessment of the windings and the on-load tap-changer (OLTC), the following procedure is recommended:
- The test should be carried out on all tap positions of the transformer.
- During commissioning, periodic maintenance and troubleshooting—especially when a fault related to the OLTC is suspected—the resistance should be measured across all taps in the raising direction of the tap-changer, and then again across all taps in the lowering direction.
- If, during periodic tests, it is not practical to perform measurements in both directions, resistance shall at least be measured for all taps in one direction.
Special notes and good practices:
- To verify the correct behavior of the tap-changer and its diverter switch, the current should be observed while changing taps. If large oscillations or interruptions of current are seen during a tap change, this is a sign that further investigation is required, for example by performing a dynamic resistance test of the tap-changer.
- When the test is performed with a battery as the DC source, the battery capacity should be at least 60 Ah, so that the necessary current can be sustained throughout the test.
- When using an digital winding-resistance meter, the instrument should continue measuring after the current has stabilized. A recommended approach is:
- record the resistance continuously for about 10 seconds,
- determine the maximum, minimum and average resistance during this interval,
- if the difference between the maximum and minimum values, divided by the average value (Deviation), is less than 0.3%, the measurement is considered valid and the average resistance during this period is taken as the result.
To make comparison easier and to check the reproducibility of results:
- The measured resistances for all phases and all taps should be recorded in a way that allows easy visual comparison; drawing them on paper or plotting them with software is strongly recommended.
- Just as in dynamic resistance testing, it is very useful to use instruments that, in addition to resistance, also provide indicators such as ripple and slope of the resistance trace. Abnormal values of ripple or slope can reveal problems in the tap-changer and help plan timely maintenance.
- For each tap position, the corrected resistance values should be plotted on a graph (resistance versus tap position), which allows the operator to quickly see irregular steps, asymmetry between phases or changes compared with historical tests.
From a diagnostic point of view, the trend of resistance over time is extremely important. To identify poor connections at bushings or tap-changers, or to detect the development of turn faults, engineers should look at:
- how the resistance of each phase changes from one test campaign to another, and
- how the resistance of one phase differs from the other phases at the same tap.
This requires that resistance measurements be accurate and repeatable, and that test data be carefully preserved.
Figure 1: Transformer winding test circuit using ALLINA T1.
5. Evaluation Criteria, Limits, and Safety Considerations
When interpreting results, several numerical limits are commonly applied:
- The difference between measured resistance and factory test values is ideally not more than 1%, but deviations up to 3% are generally acceptable.
- The maximum difference between phases on the same tap should be about 2%.
- The difference between current and previous field tests on the same transformer should not exceed roughly 3%.
Values outside these ranges may indicate:
- poor or loose connections at bushings or leads,
- increased contact resistance in the tap-changer or diverter switch,
- partial turn-to-turn short circuits or localized conductor damage.
In addition to electrical accuracy, the test has important safety and operational implications:
- The DC current used in the resistance test magnetizes the transformer core, leaving a residual flux after the test.
- If the transformer is energized immediately without removing this residual magnetism, a large inrush current may occur. This can stress the transformer mechanically and thermally and may cause undesired operation of protection relays.
- Therefore, after completing the winding resistance test, it is essential to demagnetize the transformer core—for example by applying decreasing DC currents of alternating polarity—so that no excessive inrush current is produced when the transformer is returned to service.
Finally, for each tap position and phase, the corrected resistance value should be plotted and retained. When these graphs are compared over years of operation, even small changes in resistance or growing discrepancies between phases stand out clearly, allowing utilities to recognize developing problems early and take preventive action.
Figure 2: The result of a sample transformer winding resistance test using ALLINA T1.
6. Conclusion
DC winding resistance testing translates a very simple measurement into deep insight about the internal condition of a power transformer. When carried out on all phases and taps, corrected for temperature, and carefully trended, it exposes weak joints, tap-changer defects and emerging winding faults that cannot be seen from the outside. Integrating this test into routine maintenance and post-fault investigations improves reliability and reduces the risk of unexpected outages. With proper demagnetization and data management, it remains one of the most cost-effective tools in the transformer engineer’s toolbox.
