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Resistance exceeds DC resistance tester

time:2020/12/15   source:华天电力  reading:416 time

The DC resistance tester is an indispensable experimental item after the transformer is handed over, overhauled, and changed on the tap switch. Under normal conditions, using traditional methods (bridge method and voltage drop method) to measure the DC resistance of transformer windings and high-power inductive equipment is a time-consuming and labor-intensive work. It is to measure the DC resistance of transformer windings and high-power inductive equipment. Ideal equipment.

It uses a typical four-wire measurement method to improve the accuracy of measuring resistance (especially low resistance). Program-controlled constant current source, program-controlled preamplifier, and A/D converter constitute the main body of the measurement circuit. The central control unit applies a constant, high-precision current to the external load to be tested by controlling the constant current source, and then processes the obtained data (including test voltage, current test current, etc.) to obtain the actual resistance value.

Transformer DC Winding Resistance Tester.png

When using the DC resistance tester, when the measured resistance 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 above layer oil temperature.

2. 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).

3. Poor contact of the tap changer leads to a relatively high resistance value. For example, the switch is clean and electroplated, and the spring pressure is insufficient. Uneven stress and carbon deposits on the overvoltage contacts will cause the resistance value to be turned on. At this time, the tap changer cover should be opened and rotated several times, which can generally 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 out the defect phase.

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