What is winding resistance and why do we measure it?

The winding resistance is the DC resistance of the copper (or aluminum) wire in the transformer windings, including connections, joints, and tap/selector switches (OLTCs) where relevant. This resistance is low: from micro- and milliohms on the low-voltage side to a few ohms on the high-voltage side.

Because resistance is temperature-dependent (for copper, the temperature coefficient is ~0.004/°C), temperature correction must always be considered when comparing measurements.

Why measure this resistance?

The winding resistance test (also referred to as the “DC winding resistance test”) is a form of continuity check. Problems that can be detected include:

• Loose or poorly contacting connections (e.g. clamps, welds)
• Poor contacts in the tap changer (OLTC), or wear/internal issues across different tap positions
• Indications of shorted turns or unintended parallel paths. A turn-to-turn short often changes the DC resistance only slightly, so confirm it with a turns ratio (TTR) or excitation current test
• Changes in resistance over time, which may indicate degradation
• Unequal resistance values between phases in three-phase transformers

According to common guidelines, an unexpectedly high or low resistance value may indicate defects or abnormalities in the current path (conductors).

Relation to other tests

• The winding resistance test is often performed together with the turns ratio and excitation current tests.
• Because the transformer has a magnetic core, the core must be saturated to obtain a stable measurement.
• After the DC test, demagnetization is strongly recommended to avoid high inrush currents when returning the transformer to service.

Measurement principle – Ohm’s law

The principle is simple: inject a known DC current (I) into the winding and measure the voltage (U) across it. Resistance = U / I. Use a test current of no more than about 10 % of the rated current of the winding, so the winding does not heat up during the measurement.
In practice, however, the process is more complex due to:

• Inductance of the winding
• Magnetization of the core
• Energy dissipation and time constants
• Sensitivity to disturbances or accidental lead disconnections

Core saturation

Before a stable resistance can be measured, the core must be saturated (so flux variations or magnetic effects no longer influence the result).

• Too low a test current may lead to incomplete saturation ⇒ incorrect values
• The time required depends on inductance, applied voltage/current, and winding resistance

Inductive effects and voltage spikes

A critical safety factor: if a current lead disconnects unexpectedly during the test, the inductance may generate a high voltage spike – posing a risk.
Therefore, test instruments must include a safe discharge path and protection against current interruption.

Temperature and correction

Resistance values are highly dependent on temperature. To compare measurements under different conditions, results are corrected to a reference temperature. Report at the same temperature as the factory report, so you can compare the values directly. International standards (e.g. IEEE, IEC) provide correction formulas for copper or aluminum.

Test voltage compliance

The test instrument must provide sufficient voltage compliance (ability to supply voltage) to drive current through the winding despite resistance and inductance. Instruments are typically designed for voltages of 50–60 Vdc.

How HighTest instruments support this

HighTest offers several models in their winding resistance tester range. Key features include:

Thanks to these features, HighTest equipment is well-suited to meet the challenges of winding resistance testing in real transformer diagnostics.

Phase-to-phase comparison

For three-phase transformers, resistance balance between the three phases is critical. A deviation of more than 2 % between phases, or more than 1 % from the previous measurement, may indicate connection imbalance or contact wear.

Comparison with reference values

Compare measurements with factory specifications or historical data. Since temperature strongly affects resistance, always apply correction to the same reference temperature (using the correction formula).

Temperature correction formula (copper):

• Rs​ = Temperature compensated resistance
• Rm​ = Measured resistance (uncompensated)
• Ts​ = Standard temperature (°C)
• Tm​ = Measured temperature (°C) during test
• Tk​ = Temperature constant (234.5 °C for copper, 225 °C for aluminium)

Tap resistance profiles (OLTC)

For transformers with on-load tap changers (OLTC), you can measure the resistance in each tap position and generate a resistance vs. tap position graph. Deviations or irregularities in the profile often indicate poor contacts or contact wear.

You can also perform a dynamic measurement (monitor resistance during the switching operation) to detect transition issues.

Diagnostic signals

Limits and warnings

• A small deviation in resistance does not automatically indicate a defect – tolerances or standards may apply.
• A large phase-to-phase difference or deviation from historical data are stronger indicators of issues.
• Temperature correction is essential to make measurements comparable.
• After a DC test, demagnetization is strongly recommended to avoid residual magnetization and high inrush current at re-energization.

Practical considerations

Cable quality and connections

Ensure test leads are of low resistance and high quality, with solid, reliable connections at both transformer bushings and instrument side.

Discharge first, then disconnect

The dangerous voltage spike occurs when the current is interrupted. Only disconnect the test leads once the instrument has fully discharged the winding.

Full disconnection from voltage sources

Make sure the transformer is de-energized during DC testing, and that other circuits are disconnected.

Stabilization period & monitoring

Wait until the measured value is stable (based on intelligent criteria such as those in HighTest instruments) before recording the result.

Demagnetization after test

Ensure the instrument demagnetizes the core after the test to remove residual flux and minimize inrush currents at re-energization.

Test environment and temperature recording

Record the temperature at the time of measurement (or use sensors) so correction can be applied.

Logging & trending

Store results and compare them with previous measurements as part of condition-based maintenance.

Safety procedures

• Ensure test leads are properly secured and cannot disconnect during testing
• The instrument should provide automatic discharge in case of power loss or fault
• Be aware of inductive voltage spikes when disconnecting

Winding resistance testing is often an integral part of the heat-run test for transformers.