What is the purpose and standard of DC resistance measurement of dry-type transformer windings?

1, 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 connection between the pinout and the housing, the pinout and the tap-changer is good.

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

1.4 Whether the wire specifications and resistivity meet the requirements.

1.5 Are the resistances 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.

2, the measuring conditions

2.1 The temperature of the winding under test and the resistance between the terminals of the winding should be recorded, and the measurement should be direct current.

2.2 Care should be taken to minimize the effect of self-inductance in the measurement.

2.3 Measure money, the transformer in the constant temperature environment static time should not be less than 3h.

2.4 The winding temperature should be measured at the same time as the winding resistance.

3. Measuring standards

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

4, the calculation of the measurement results

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

4.2 Interline resistance conversion △ connected to Y.

5. Precautions when measuring

5.1 The current used to measure the DC resistance should be 2%-10% of the rated current, not more than 20% of the rated current, 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, the transformer core magnetic field will produce high voltage when disconnected, which may endanger personal safety and damage the instrument. Therefore, a discharge circuit is required to slowly flow current from I through the losses on the resistor. Disconnect the line after an hour.

Each time the resistance value on the side must be converted to the same temperature for comparison, if the DC resistance in the comparison results does not exceed the standard, but the value of each amount has increased, it should also attract enough attention. If the neutral point of the transformer has no lead out, when the imbalance value of the three-phase wire resistance exceeds 2%, the line resistance needs to be converted into a phase resistance in order to find the defect.

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