What's The Significance and Precautions for Measuring the DC Winding Resistance?

The measurement of the DC resistance of the transformer windings with DC winding resistance tester is a simple and important test item in the transformer test.

Measuring the DC resistance of the transformer windings and bushings, can help checking the welding quality of the wire joints in the windings, the welding quality of the leads and winding joints, the respective tap-changer positions of the voltage tap-changers and the contact between the leads and the bushings, whether the parallel branches are connected correctly, whether the current-carrying part of the transformer is short-circuited, and whether the windings are short-circuited; In addition, in the transformer short-circuit test and the temperature rise test, in order to provide accurate winding resistance values, DC resistance measurement is also required.

Therefore, the measurement of the DC resistance of the windings is the main item of the transformer test. The handover test standard stipulates that it is a mandatory test item; the preventive test procedure stipulates that after 1-3 years of transformer operation, after the non-excitation voltage transformer is replaced to the tap position, the on-load tap changer must be overhauled (all tap sides) and after the overhaul and when necessary, this test must be carried out.

The DC resistance of the transformer windings is measured using the voltage drop method or the bridge method. Although the measurement method is simple, there are many factors that affect the measurement accuracy. For more accurate results, it is necessary to select the appropriate instrument and make measurements as required. In order to ensure the accuracy of the measurement, the safety of the measuring equipment and personnel,speeds up the testing process,the following should be noted when measuring:

1) For transformers with voltage taps, measurements should be made at all tap positions.

2) When the three-phase transformer with neutral point, the resistance of each phase winding should be measured; when there is no neutral point, the resistance between the lines can be measured, and then the resistance of each phase is calculated.

3) The measurement must be carried out when the winding temperature is stable. The difference between the windings and the ambient temperature is required not to exceed 3 °C. In the case of stable temperature, the oil temperature of the transformer can generally be used as the winding temperature, and it is necessary to measure it when measuring resistance.

4) Due to the large inductance of the transformer, the time required for the current to stabilize is longer. For accurate measurements, you must wait for the gauge indicator to stabilize before read the data.

5) Considering that there are many factors that affect the accuracy of DC resistance measurement, such as the accuracy level of the instrument, the test wiring method, the accuracy of the temperature measurement, the connection state and the current stability, etc. After finish the measurement, it is necessary to review it again. If there's doubt, retest to get accurate measurements.

6) When measuring, the non-test winding should be open, not short-circuited. When measuring a low-voltage winding, the high-voltage winding induced a high voltage in the instantaneous power-on split gate, and personal safety should be paid attention to.

7) Due to the large inductance of the transformer, the self-induced electromotive force is very high at the moment when the power supply is turned on or off. Therefore, in order to prevent damage to the instrument, special attention should be paid to the sequence of operations..

8) Measure the resistance value to correct the effect of the lead.