# Power detection technology / Power detection

## Detection Technology

### The DC resistance tester exceeds its limit value

The DC resistance tester is mainly composed of precision small signal measurement circuit, high-power switching power supply circuit, protection circuit, high-speed single-chip control circuit and other components. Its function is to check the quality of winding welding and short circuit between windings, and tap switch position Whether it is correct, whether the lead wire is loose or broken, whether the actual position is consistent with the indication, and the winding of the parallel winding has no broken strands, etc. Measuring DC resistance is not only a basic test item for transformer maintenance, pre-testing and tap changer position change, but also a key inspection item after a failure.

When using the DC resistance tester, when the measured resistance value exceeds the limit, pay attention to the following points:

1. 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) for comparison , And generally based on the oil temperature of the upper layer.

2. First consider whether there is a measurement error (for example, 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).

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

4. For the three-phase distribution transformers currently in use, the high-voltage winding adopts Y-shaped wiring. When the resistance value exceeds the limit value, 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.

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