How does a transformer DC resistance tester calculate resistance?
A DC resistance rapid tester is an essential test for transformers after handover, major overhaul, and tap changes. This article introduces how to calculate the resistance of a transformer using a DC resistance tester.
Measuring the DC resistance of transformer windings and high-power inductors using traditional bridge and voltage drop methods is typically a time-consuming and labor-intensive task. To address this, shorten measurement time, and reduce the workload of testing personnel, Guodian Zhongxing's DC resistance tester employs new power supply technology. It features rapid measurement, compact size, ease of use, and high measurement accuracy, making it an ideal device for measuring the DC resistance of transformer windings and high-power inductors. This rapid DC resistance tester complies with DL/T845.3-2004 and national standards GB6587 "General Outline of Environmental Testing for Electronic Measuring Instruments" and GB6593 "Rules for Quality Verification of Electronic Instruments".
The conversion of transformer DC resistance is mainly used for longitudinal comparison of on-site measured values with factory or historical test data.
RA = Ra * (T + tA/T + ta)
RA: Resistance value measured at temperature tA (usually factory data or historical data).
tA: Temperature measured during RA measurement
The resistance measured at Ra:ta temperature;
ta: Temperature during Ra measurement (°C); DC resistance (ohms).
T is calculated using constants; for copper wires, use 235; for aluminum wires, use 225.
Deviation rate = (R<sub>converted</sub> - R<sub>factory</sub>) / R<sub>factory</sub> * 100%
The calculated resistance value should not differ from the factory value by more than 2% (the deviation rate in the above formula).
Issues related to the conversion between measured values and factory values
1. In actual field tests of oil-immersed transformers, it is frequently found that the difference between the oil level temperature readings and the factory specifications often exceeds +2%.
2. Generally, oil-immersed transformers of 66kV and above are equipped with oil level temperature gauges and winding temperature gauges, which can be seen on the transformer itself. The most common type is the pointer-type temperature gauge. The transformer's non-electrical protection panel usually has a digital temperature display, accurate to one decimal place.
3. In preventive testing, for the period immediately after shutdown (within 1 hour of shutdown), we also consider the oil surface temperature to be the winding temperature, and the temperature difference between the two should not exceed 5°C. Therefore, the oil temperature can be considered to be the winding temperature.
4. For newly installed transformers that are not yet in operation, it is recommended to conduct tests in the morning; direct sunlight can cause damage to the upper and lower oil layers.
The temperature difference is relatively large.
5. For dry-type transformers that are not in operation or have been out of operation for more than 12 hours, we generally consider the winding temperature to be the ambient temperature at the time of testing. If there is any doubt about the temperature, a temperature gun can be used for measurement.
6. In cases where the winding temperature cannot be accurately measured or the transformer is not equipped with a temperature gauge, the measurement results should be judged and compared according to the three-phase imbalance rate.
Three-phase resistance imbalance rate calculation: Imbalance rate = (Three-phase measured peak value - Three-phase measured low value) / Three-phase average value * 100%
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