Transformers play a critical role in voltage transformation, power distribution and transmission. The winding structures are complex, and they must meet high reliability requirements.
Different types of transformers place different emphasis on test items, testing methods and maintenance priorities, but DC resistance testing remains a fundamental and essential procedure for both factory acceptance and operational maintenance.
Why measure DC resistance?
Inspect winding joint soldering quality
The quality of soldering at winding joints directly affects transformer performance and reliability. Poor soldering can increase contact resistance, causing localized overheating during operation and potentially leading to failures.
DC resistance measurement is a straightforward way to detect poor soldering, cold joints or detached joints.
When a joint is sound, its DC resistance should fall within the normal range and the three phases should be balanced.
If the resistance of one phase is abnormally high, that phase likely has a soldering or joint defect.
Detect inter-turn short circuits
Inter-turn short circuits are among the common failures in transformers.
When an inter-turn short occurs, large circulating currents form in the shorted turns, producing local overheating, accelerating insulation aging, and in severe cases burning out the transformer.
DC resistance measurement can effectively indicate inter-turn shorts.
The presence of shorted turns reduces the effective number of turns and therefore reduces the winding resistance.
By comparing the DC resistance of the three phases, if one phase’s resistance is significantly lower than the other two and exceeds allowable tolerance, this can be an initial indication of an inter-turn short in that phase.
Evaluate on-load tap changer (OLTC) contact condition
The on-load tap changer (or general tap changer) is used to adjust transformer output voltage. Good contact at the tap positions is essential for stable operation.
Poor contact increases contact resistance, causing local heating and affecting transformer performance.
Measuring DC resistance across different tap positions allows assessment of contact resistance at each tap.
After switching taps, the DC resistance at each position should meet technical requirements and maintain phase balance. An abnormal value at a particular tap suggests contact issues at that position.
Methods for measuring DC resistance
Bridge methods
Wheatstone bridge is suitable for measuring medium and higher resistances (typically ≥ 1 Ω).
It works by adjusting bridge arm resistances until the bridge is balanced and the detector reads zero; the unknown resistance is then calculated from the known arms.
The Wheatstone bridge offers high accuracy, but low resistances are susceptible to lead and contact resistance errors.
For transformer winding measurements, the winding under test is placed in one arm of the bridge and the remaining arms are adjusted to achieve balance.
The Kelvin bridge is designed for low resistance measurements (typically < 1 Ω).
Using a four-terminal connection it eliminates the effects of lead and contact resistance, making it the recommended choice for low-voltage windings or neutral loops in transformers.
During measurement, the current and potential terminals are separated; the bridge is balanced by adjusting arms to obtain an accurate low-resistance value.
Voltage-drop method (Volt-ampere method)
In this method, a known DC current is injected into the winding and the voltage drop across the winding is measured; resistance is then calculated from Ohm’s law R = V / I .
This method is simple and convenient. In practice you must select a suitable DC source and meters.
To reduce measurement error, use as large a current as safely possible without exceeding the winding’s rated current.
Pay attention to instrument accuracy and internal resistances, which affect results.
The voltage-drop method is applicable across a wide range of resistances and is especially useful in field situations where bridge instruments are not available.
Constant-current source method
The constant-current method uses a stable current source to apply a steady DC current to the winding.
Because the output current is constant and not affected by winding inductance, stable current is established quickly and measurement time is reduced.
Combined with high-precision voltage measurement, the DC resistance is calculated via Ohm’s law.
This method is particularly suitable for large-capacity transformers and improves measurement efficiency.
DC resistance testers —— ( left ) ZWR3210A & ( right ) ZW5320
Modern DC resistance testers on the market commonly use constant-current sources for high accuracy, speed and ease of use.
Simultaneous three-phase excitation method
In this method, all three windings are energized simultaneously so the three phase resistances can be measured at once, improving efficiency.
It requires the applied three–phase voltages to be symmetrical and stable, and strict control of wiring (neutral/ground handling) to avoid closed loops that could circulate currents or locally saturate windings.
This approach is suitable for three-phase transformers and can provide a more comprehensive assessment for transformers with complex winding connections.
Factors affecting measurement results
Temperature effects
Winding resistance varies significantly with temperature: as temperature rises, resistance increases.
Always record the winding temperature during measurement and convert readings to a common reference temperature before comparing with factory or historical values.
The commonly used conversion formula is:
R2 = R1 *｛ (T + t2) / (T + t1) ｝
where R1 and R2 are resistances measured at temperatures t1 and t2 (°C), and T is a material constant (use T = 235 for copper windings and T = 225 for aluminum windings).
Use a high-accuracy thermometer to measure the winding temperature during actual measurements, and try to make the measurement temperature as close as possible to the transformer’s operating temperature to minimize errors introduced by temperature conversion.
Winding inductance
Transformer windings have substantial inductance. When the test source is switched on or off, induced EMFs from self-inductance affect current stabilization and therefore measurement accuracy.
For large transformers the time required for current to reach steady state can be long, increasing measurement duration.
To mitigate inductance effects, use a constant-current source, or demagnetize/assist magnetization to allow stored energy in the inductance to dissipate before taking readings. Always wait until current is stable before reading results.
Instrument accuracy
The accuracy of test instruments directly determines measurement accuracy. Choose instruments (bridges, DC resistance testers) with an appropriate accuracy class for the expected winding resistance and measurement requirements. Instruments should be regularly calibrated and maintained to ensure they meet accuracy specifications.
Also minimize effects from instrument internal resistance, lead resistance and contact resistance during measurement.
Measurement precautions of DC resistance
Safety precautions
- Follow electrical safety rules and standard operating procedures.
- Do not touch the circuit under test while it is energized; wear insulating gloves and other protective gear.
- Avoid forming uncontrolled closed loops or grounded loops during the test.
- After measurements, promptly disconnect the test source and restore transformer terminals to their normal condition.
Pre-test preparations
- Ensure the transformer is de-energized, grounded and fully discharged. Hang warning tags and establish safe isolation zones.
- Check and record the transformer’s nameplate data (rated capacity, voltage levels, winding material, winding connection type, etc.).
- Verify test instrument calibration certificates, inspect test leads and clamps; allow instruments to warm up and perform a short-circuit (zero) check.
- Confirm required personal protective equipment (insulating gloves, eye protection, etc.) is available.
Key operational points during measurement
- Measure and record winding surface or terminal temperature (for temperature compensation).
- Perform lead compensation: short the measurement leads to correct the zero offset.
- Measure phase by phase unless simultaneous three-phase measurement is safely possible and appropriate; use four-wire connections for low-resistance tests.
- For each measurement perform two runs with current reversed and take the average to cancel thermoelectric and polarity errors.
- Wait until current and voltage are stable (consider device L/R time constant) before reading values.
- Record raw current and voltage, calculate resistance, then convert resistance to the reference temperature.
- Compare results with factory or historical values and compute percentage deviations.
- Draw conclusions and produce a written report documenting procedures, readings, temperature corrections and diagnostic judgments.
Common abnormality analysis
- Three-phase balance: standards vary, but if one phase deviates from the other two by several percent (common thresholds: 1–3%), further inspection is required.
- Phase resistance significantly higher: possible poor joint soldering, bad contact, local corrosion or an open circuit.
- Phase resistance significantly lower: possible inter-turn short or overlaps (note: DC resistance alone cannot definitively locate inter-turn shorts; use complementary tests).
- Continuous upward trend in resistance: suggests contact surface oxidation/corrosion or mechanical loosening; consider replacing joints or carrying out maintenance.
Conclusion
Measuring DC resistance is essential to ensure transformer quality and safe operation, to improve maintenance efficiency and accuracy, and to comply with industry standards. There are multiple test methods, this article summarizes the basic ones and the key factors that influence results.
If you need transformer DC resistance testing services or a tailored test plan, please contact us for technical support.
