By measuring the DC resistance of the transformer winding and the bushing, you can check the welding quality of the internal wire joints of the winding, the welding quality of the lead wire and the winding joint, the tap positions of the voltage tap switch and the contact between the lead wire and the bushing, and the parallel branch circuit Whether the connection is correct, 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 temperature rise test, in order to provide accurate winding resistance, it is also necessary to measure the DC resistance. Therefore, the measurement of the winding DC resistance is the main item of the transformer test. The handover test standard stipulates that it is a mandatory item; the preventive test procedure stipulates that after the transformer has been in operation for 1-3 years, after the non-excited voltage-regulating transformer changes the tap position, after the on-load tap-changer transformer is overhauled (on all tapping sides) ) And after major repairs and when necessary, this test must be done.
The voltage drop method or the bridge method is used to measure the DC resistance of the transformer winding. Although the measurement method is simple, there are many factors that affect the accuracy of the measurement. It is necessary to select a suitable instrument and perform measurement in accordance with relevant regulations to obtain more accurate results. To ensure the accuracy of measurement, the safety of measuring equipment and personnel, and to speed up the test process. The matters needing attention when measuring are as follows:
1) For transformers with voltage taps, measurements should be made at all tap positions.
2) When the three-phase transformer has a neutral lead wire, the resistance of each phase winding should be measured; when there is no neutral lead wire, the resistance between the wires can be measured, and then the phase resistance can be calculated.
3) The measurement must be carried out when the temperature of the winding is stable, and the temperature difference between the winding and the environment should not exceed 3°C. When the temperature is stable, the upper oil temperature of the transformer can generally be used as the winding temperature, and the measurement should be recorded.
4) Due to the large inductance of the transformer, it takes longer for the current to stabilize. In order to measure accurately, you must wait for the meter to indicate stable before reading. If necessary, take measures to shorten the stabilization time.
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 temperature measurement, the connection contact status and the current stability, etc., it is necessary to recheck after the measurement. When in doubt, re-test to obtain accurate measurement results.
6) During measurement, all windings that are not tested should be open and not short-circuited. When measuring low-voltage windings, a higher voltage will be induced in the high-voltage windings at the moment of power switching, so 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 the power is turned on or off. Therefore, in order to prevent damage to the instrument, pay special attention to the operation sequence. When turning on the power, first turn on the power circuit, then turn on the voltmeter or galvanometer, and then disconnect the power circuit.
8) The influence of the lead wire should be corrected when measuring the resistance value.
9) In order to compare with the factory value or the previous measured value, the DC resistance value measured at any temperature should be converted to the same temperature. When it is converted to 20℃, please refer to the attached table.
