The DC resistance tester under UHV power can help many power workers conduct various power tests more conveniently.
1. The accuracy level of a general transformer DC resistance tester should be (0.2): the output current of transformers below 120MVA should be greater than 10A, the output current of transformers above 120MVA should be greater than 20A, the output current of transformers above 180 MVA should be greater than 40A, and the output voltage should be as high as possible. The output voltage of the 10A transformer DC resistance tester shall not be less than 18V, and the output voltage of the 20A transformer DC resistance tester shall not be less than 25V.
The output current of the instrument determines the measurement accuracy, and the output voltage of the instrument determines the measurement speed. Therefore, when choosing an instrument, it is important to understand the output voltage of the selected instrument. The instrument should have the function of directly converting the resistance measurement value at room temperature into a resistance value at 20 ℃, which is more convenient and fast in actual measurement.
2. Testing method:
(1) Measure with a transformer DC resistance tester.
(2) Test principle wiring diagram (refer to the test wiring of various DC resistance testers).
3. Test steps
(1) Measure and record the top oil temperature, ambient temperature, and humidity.
(2) Connect the measuring device or instrument effectively to the measured winding through the probe and start measuring.
(3) After the experiment is completed, the "discharge" or "reset" button on the measuring equipment or instrument should be used to completely discharge the measured winding.
4. Basis (or method) for judging test results
(1) Convert the measured value into the same temperature using the formula R2=R1 (T+t2)/(T+t1) (where R1 and R2 are the resistance values at t1 and t2 temperatures, respectively, and t1 can be taken as the transformer winding temperature during the handover test); T is the temperature constant of resistance, 235 for copper wire and 225 for aluminum wire.
(2) For transformers above 1.6MVA, the difference in resistance between each phase winding should not exceed 2% of the average value of the three phases; For windings without a neutral point, the line to line difference should not exceed 1% of the three-phase average value.
(3) For transformers of 1.6MVA and above, the phase difference should generally not exceed 4% of the average three-phase value; The line to line difference should generally not exceed 2% of the average value of the three phases.
(4) There should be no significant difference in the resistance of each phase winding at the same position and temperature compared to the previous results.
(5) The three-phase unbalance rate is an important criterion for judgment, and various standards and regulations have detailed and clear provisions. There should be no significant change in the three-phase imbalance ratio during handover compared to when leaving the factory; Otherwise, even if it is less than the specified value, it cannot be simply judged as qualified.
5. Precautions
(1) Generally, measurements should be taken after the oil temperature has stabilized. Only when the oil temperature is stable, can the oil temperature be equal to the winding temperature, and the measurement results will not have temperature conversion errors due to temperature differences.
(2) Due to the long charging process during the measurement of large transformers, sufficient attention should be paid to demagnetization or auxiliary methods.
(3) After measurement, attention should be paid to fully discharging the tested winding.
