# Power detection technology / Power detection

## Detection Technology

### Method of measuring transformer resistance

The measurement of DC resistance is to check the quality of transformer winding welding and short circuit between windings, whether the tap switch position is correct, whether the actual position is consistent with the indication, whether the lead wire is broken or loose, and the winding of the parallel winding has no broken strands. The measurement of DC resistance is not only a basic test item after transformer maintenance, pre-test and tap changer position change, but also an important inspection item after failure.

Generally, it is a time-consuming and laborious task to measure the DC resistance of transformer windings and high-power inductance equipment by traditional methods (bridge method and voltage drop method). In order to change this situation, shorten the measurement time and reduce the workload of the tester, Developed a fast DC resistance tester, which is a professional equipment for measuring the DC resistance of transformer windings and high-power inductance equipment.

Therefore, the DC resistance must be carefully measured and tested to minimize measurement errors. According to regulations, for transformers above 160kVA, the difference between phase resistance and phase resistance is usually less than 2% of the three-phase average value, and the difference between lines is usually less than 1% of the three-phase average value. For transformers of 160kVA and below, the difference between phase resistance and phase resistance is usually less than 4% of the three-phase average value, and the difference between lines is usually less than 2% of the three-phase average value. Compared with the difference between the measurement phases in the corresponding parts, the difference between the measurement phases should not be greater than 2%.

When the measured resistance value of the DC resistance tester exceeds the limit:

(1) First consider whether there is a measurement error (such as whether the external lead is connected, whether the test lead is too long or too thin, whether the contact is good, whether the battery voltage in the bridge is insufficient).

(2) The resistance value of DC resistance is greatly affected by temperature, so it must be converted to the same temperature (usually 20℃, R20=(T+20)/(T+T), T copper=235). Comparison, and generally based on the oil temperature of the upper layer.

(3) The three-phase distribution transformer currently in use, the high-voltage winding adopts Y-shaped connection, when the resistance value exceeds the ji limit, the following formula can also be used [RA=(RAB+rac-rbc)/2, RB=(RAB) +rac-rac)/2, RC(RBC+rac-rab)/2] in order to find the defect phase.

(4) Poor contact of the tap changer leads to high resistance. For example, the switch is clean and electroplated, insufficient spring pressure, uneven stress, and carbon deposits on the overvoltage contacts will cause the resistance to conduct. At this time, the tap changer cover should be opened and rotated several times, usually it can be eliminated.

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