Dc resistance measurement of dry type transformer windings

First, the purpose of winding DC resistance measurement

1.1 Whether the welding or mechanical connection at the connection of the winding wire is good, whether there is welding or poor connection.

1.2 Whether the pin is properly connected to the shell and the pin is properly connected to the tap-changer.

1.3 Whether the welding or mechanical connection between the leads is good.

1.4 Whether the conductor specifications and resistivity meet the requirements.

1.5 Is the resistance of each phase winding balanced?

1.6 The temperature rise of the transformer winding is calculated according to the cold resistance of the winding before the temperature rise test and the instantaneous thermal resistance of the port power supply after the temperature rise test, so the temperature rise test needs to measure the resistance value.

GTB series dry test transformer

2. Measurement conditions

2.1 The temperature of the winding to be measured and the resistance between the winding terminals shall be recorded, and the measurement shall be direct current.

2.2 Care should be taken to minimize the influence of self-induction during measurement.

2.3 For testing, the static time of the transformer under constant temperature environment should not be less than 3 hours.

2.4 The winding temperature should be measured simultaneously with the winding resistance.

3. Measurement standards

For transformers below 1.6MVA, the difference between phases should generally not be greater than 4% of the three-phase average, and the difference between lines should generally not be greater than 2% of the three-phase average.

Iv. Calculation of measurement results

4.1 R0=Rm*(235+T1)/(235+T2) Copper 235 Aluminum 225.

4.2 Resistance conversion between lines △ connected to Y.

Precautions for measurement

5.1 The current used to measure the DC resistance should be 2%-10% of the rated current and not exceed 20% of the rated current, so as to avoid the error caused by the current causing the heating temperature of the winding.

5.2 When there is current I in the DC circuit, high voltage will be generated when the transformer core magnetic field is disconnected, which may endanger personal safety and damage the instrument. Therefore, a discharge circuit is needed to make the current from I slowly flow through the loss on the resistor. Disconnect the line in an hour.

The resistance value of each side must be converted to the same temperature for comparison, if the DC resistance in the comparison result is not exceeded, but the value of each quantity has increased, it should also cause enough attention. If the neutral point of the transformer does not lead the line, when the unbalanced value of the three-phase line resistance exceeds 2%, the line resistance needs to be converted to the phase resistance in order to find the defect.