The measurement of the winding resistance of the transformer winding is a simple and important test item. The purpose is to check the quality of the welding joints of the windings, the various positions of the voltage taps-the contact between the lead and the bushing is good. Whether the connection of the parallel branch is correct, whether there is a short circuit or internal disconnection phenomenon, etc. Simultaneously. It is also an important data for transformer short-circuit characteristic test. Therefore, after handover, overhaul, and after changing the tap position during operation. The test must be carried out.
Winding temperature measurement
The dry-type transformer measures the temperature in the passage between the high and low voltage windings as the winding temperature;
The oil-immersed transformer measures the top and bottom oil temperatures, and the average value is used as the winding temperature.
Selection of measuring current
The measured current shall not be greater than 15% of the rated current of the winding under test;
The measuring current should be selected according to the range of measuring resistance specified by the instrument;
Transformer DC resistance tester:
5A 1mΩ—4Ω
10A 0.2mΩ—2Ω
20A 0.1mΩ—1Ω
Winding resistance measurement
Connect the test circuit in accordance with Article 7. When the magnetic assist method is used, the current direction of the magnetic assist winding and the end of the winding under test should be the same. Turn on the power switch of the instrument, press 9.2 according to the product technical conditions to select the measurement current, press the start key to measure the winding resistance, record the winding resistance after the measurement result is stable, and press the reset key to complete the measurement after the discharge is completed.
Judgment criteria
1. Oil-immersed power transformer
For products below 110kV, transformers with a capacity of 1600kVA and below, the DC resistance unbalance rate is 4%, and the line is 2%; for transformers with a capacity of 2000kVA and above, the DC resistance unbalance rate is phase (when there is a neutral point lead) 2%, the line (when there is no neutral point lead) is 1%. For products above 110kV, the DC resistance unbalance rate (when there is a neutral point lead) is 2%, and the line (when there is no neutral point lead) is 1%.
2. Dry-type power transformer
For distribution transformers with a capacity of 2500kVA and below, the DC resistance unbalance rate is 4%, and the line is 2%; for power transformers with a capacity of 630kVA and above, the DC resistance unbalance rate (when the neutral point is drawn) is 2 %, the line (when there is no neutral point lead) is 2%.
3. DC resistance unbalance rate
The DC resistance unbalance rate should be calculated based on the three-phase actual measured maximum value minus the minimum value as the numerator, and the three-phase actual measured average value as the denominator; if the DC resistance unbalance rate exceeds the specified due to the wire and lead structure, it should be tested in the factory In addition to the actual measured value recorded in the record, the reason for this deviation should also be stated. The unit of use should be compared with the factory-measured value at the same temperature, and the deviation should not exceed 2%.