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Detection Technology

Dc resistance tester measurement

time:2020/5/9   source:华天电力  reading:306 time

Dc resistance is measured to check the welding quality of the windings and the short circuit between the windings. Whether the position of the tap switch is good, and whether the actual position is consistent with the indication; Whether the wire is broken or loose; The winding of the strand is not broken, etc. The measurement of dc resistance is an essential test item after the overhaul of the transformer, the predictive test and the change of the position of the tap changer, as well as an important inspection item after the failure.

In general, it is a time-consuming and labor-intensive task to measure the transformer windings and the dc windings of high-power inductive equipment by the conventional methods (bridge method and voltage drop method).

Therefore, the dc resistance measurement test must be carried out carefully to minimize the measurement error. The regulation stipulates that for transformers above 160kVA, the difference of phase resistance is usually no more than 2% of the three-phase average. The difference in resistance between conductors is usually no more than 1% of the three-phase average. For transformers of 160kVA and below, the difference of phase resistance is usually no more than 3. Four percent of the average of the phases, and the difference between the lines is usually no more than two percent of the average of the three phases; The difference measured in the previous section should not be greater than 2%.

Transformer DC Winding Resistance Tester.png

When the resistance measured through the dc resistance exceeds the standard:

1. The first thing to consider is whether there is any measurement error (such as whether the external lead is connected, whether the test lead is too long or too thin, good contact, insufficient battery voltage in the bridge, etc.).

2. The resistance value of DC is greatly affected by temperature, so it must be converted to the same temperature (usually 20°C, R20 = (T + 20)/(T + T), and T Copper = 235) for comparison. And usually higher than the reservoir temperature is the base.

3. Currently used three-phase distribution transformer, the high-voltage winding adopts y-shaped wiring. When the resistance exceeds the standard value, the following formula [RA = (RAB + rac-rbc) / 2, RB = (RAB + rbc-rac) / 2, RC (RBC + rac-rab) / 2] can also be used to find the defect phase.

4, tap switch contact is poor, so the resistance value is high. If the switch is not clean, the coating is not clean, the spring pressure is insufficient, the force is not uniform, and the voltage is too high when the contact will accumulate carbon, the resistance will be high. At this point, the tap switch cover should be opened and rotated back and forth several times, which can usually be saved.

If the above inspection and treatment still exceed the standard, it indicates an internal fault, and the windings and leads may be welded, unwelded, broken, etc., or the interlayer short circuit or the windings may be burned. Unable to process the site, the repair room must be sent for core repairs.

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