# Power detection technology / Power detection

## Detection Technology

### The measured resistance value exceeds the limit

time：2021/1/15   source：华天电力  reading：804 time

The measurement of the DC resistance tester is to check the quality of the winding welding and the short circuit fault between the windings, whether the tapping power switch position is appropriate, whether the specific position is consistent with the label, whether the wire is broken or loose, and whether the winding is in series There is no broken strands in the windings. The measurement of the DC resistance tester is not only the maintenance of the transformer, the basic test report after the predictive analysis test and the change of the tap power switch position, but also the key inspection item after common failures.

During the use of the DC resistance tester, when the measured resistance value of the device exceeds the limit, 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, and the bridge is Is the battery voltage insufficient?

Secondly, it is necessary to understand that 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.

Furthermore, the three-phase distribution transformer 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.

It is common to analyze whether the tap changer has poor contact, leading 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. High-end. At this time, the tap changer cover should be opened and rotated several times, usually it can be eliminated.

If the limit is still exceeded after the above four points of inspection, it may be due to false welding, welding, wire breakage, or layer short circuit or winding burnout of the winding and lead. The user can deal with it according to the actual situation.

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