How to Measure DC Resistance More Accurately？
Accurate measurement of DC resistance is critical in evaluating the performance and reliability of transformer windings. The quality of connections, balance of resistances, and temperature rise calculations all play vital roles in ensuring that transformers operate efficiently and safely. Adhering to established standards, such as IEC 60076-1, helps ensure that measurements are consistent and reliable. This article outlines the purpose of DC resistance measurement, the relevant standards, methods for measurement, and key considerations to improve accuracy. By following these guidelines, technicians and engineers can ensure that transformers remain in optimal working condition, thereby enhancing their longevity and performance.
Purpose of DC Resistance Measurement
1. **Check Connection Quality**
- Verify the quality of soldering and mechanical connections at the winding conductor joints.
- Confirm the integrity of connections between the leads and the bushing, as well as between the leads and the tap switch.
- Inspect the soldering and mechanical connections between the leads.
- Ensure that the wire specifications and resistivity meet the requirements.
2. **Resistance Balance**
- Check whether the resistances of the phases in the windings are balanced.
3. **Temperature Rise Calculation**
- The temperature rise of the transformer windings is calculated based on the cold resistance before the temperature rise test and the hot resistance measured immediately after disconnecting the power supply.
DC Resistance Measurement Standards
- **Conformance Standard:** IEC 60076-1.
- For transformers with a capacity of less than 1600 kVA, the phase imbalance rate for the windings: phase resistance is 4%, line resistance is 2%.
- For transformers with a capacity greater than 1600 kVA, the phase imbalance rate for the windings: phase resistance is 2%, line resistance is 1%.
Methods of DC Resistance Measurement
- According to IEC 60076-1. standards, there are two methods for measuring transformer DC resistance:
- **Bridge Method**
- **Volt-Ampere Method** (using a transformer DC resistance tester).
- The testing instrument should have an accuracy of at least 0.2 grade. Depending on the transformer capacity and resistance value, appropriate current ranges should be selected to measure the DC resistance of all windings.
- For transformers with tap windings, the DC resistance of all taps should be measured. For windings with a neutral point, the phase resistance should be measured, while for those without a neutral point, the line resistance should be measured.
Principles and Methods for Rapid DC Resistance Measurement
- Transformer windings have significant inductance and minimal resistance; as the capacity increases, so does the inductance, while the resistance decreases, resulting in a larger time constant.
- When measuring winding resistance, the charging current must undergo a transient process to reach a stable value after the DC power supply is turned on, making it crucial to measure the DC resistance quickly and accurately.
Considerations During Measurement
1. **Charging Time**
- It is essential to allow sufficient charging time for large-capacity transformers, as the charging duration is longer.
2. **Clamp Contact**
- Ensure that the clamp connections are secure to eliminate errors caused by poor contact.
3. **Temperature Deviation Impact**
- A 1°C temperature deviation in the three-phase coil will increase the measurement error to nearly 0.4% at room temperature.
4. **Tap Switch Operation**
- The no-load tap switch should ensure the positioning device is in the specified position; the on-load tap switch should be operated electrically.
5. **Environmental Records**
- During testing, record the ambient temperature and the temperature of the transformer oil.
6. **High Voltage Risk**
- When there is current \( I \) in the DC circuit, energy in the transformer core magnetic field can produce high voltage when disconnected. Therefore, a discharge circuit should be used to gradually reduce the current.
7. **Avoid Changing Taps**
- During resistance measurement, do not switch the no-load tap switch to change taps to avoid arcing and degradation of oil quality.
In summary, accurately measuring the DC resistance of transformer windings is essential for maintaining their reliability and efficiency. By following established standards and employing appropriate measurement techniques, engineers can ensure that all connections are secure, resistances are balanced, and temperature rises are correctly calculated. Understanding the various methods of measurement and considering factors such as charging time, contact quality, and environmental conditions will significantly enhance the precision of results. Ultimately, these practices contribute to the safe and effective operation of transformers, reducing the risk of failures and prolonging their operational lifespan.
JYR20S/40S/50S winding resistance tester
