The significance and precautions of measuring the DC resistance of transformer windings
Release time：
2023-06-07
source：
network
Measuring the DC resistance of the transformer winding and the bushing can check the welding quality of the wire joint inside the winding, the welding quality of the lead and the winding joint, the tap position of the voltage tap changer and the contact between the lead and the bushing is good, the connection of the parallel branch is correct, the current carrying part of the transformer is short circuit and the winding is short circuit.
The measurement of DC resistance of transformer winding is a simple and important test item in transformer test.
Measuring the DC resistance of the transformer winding and the bushing can check the welding quality of the wire joint inside the winding, the welding quality of the lead and the winding joint, the tap position of the voltage tap changer and the contact between the lead and the bushing is good, the connection of the parallel branch is correct, the current carrying part of the transformer is short circuit and the winding is short circuit. In addition, in transformer short-circuit test and temperature rise test, in order to provide accurate winding resistance values, it is also necessary to measure DC resistance. Therefore, the measurement of winding DC resistance is the main item of transformer test. The handover test standard stipulates that the project must be done; The preventive test procedure stipulates that this test must be done after 1 to 3 years of transformer operation, after the tap position of the unexcited voltage regulator is changed, after the tap changer of the on-load voltage regulator is repaired (on all tap sides), and after major repair and if necessary.
The DC resistance of transformer winding is measured by voltage drop method or bridge method. Although the measurement method is simple, there are many factors affecting the accuracy of the measurement, we must choose the right instrument and measure according to the relevant regulations in order to get more accurate results. In order to ensure the accuracy of measurement, measurement equipment and speed up the test process. Points to note when measuring are as follows:
1) For transformers with voltage taps, measurements should be made at all tap locations.
2) When the three-phase transformer has a midpoint lead line, the resistance of each phase winding should be measured; When there is no midpoint lead line, the resistance between the lines can be measured and then the resistance of each phase can be calculated.
3) The measurement must be carried out under the condition that the winding temperature is stable, and the difference between the winding and the ambient temperature is required to be no more than 3℃. In the case of stable temperature, 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, the time required for current stability is longer. In order to measure accurately, it is necessary to wait for the indicator to be stable before reading, and measures should be taken to shorten the stabilization time if necessary.
5) Considering that there are many factors affecting the accuracy of the DC resistance measurement, such as the accuracy level of the instrument, the test wiring method, the accuracy of the temperature measurement, the connection contact condition and the current stability, etc., after the measurement is reviewed, and if there is any doubt, it should be retested to obtain accurate measurement results.
6) When measuring, the non-tested windings should be open and cannot be shorted. When measuring low-voltage windings, a higher voltage is induced in the high-voltage windings at the moment the power is turned on and off.
7) Because the transformer inductance is large, the power supply is on or off at the moment, the self-inductance electromotive force is high, so in order to prevent damage to the instrument, special attention should be paid to the operation sequence. When turning on the power supply, first turn on the power supply circuit, then turn on the voltmeter or galvanometer, and then turn off the power supply circuit.
8) The resistance value should be measured to correct the influence of the lead.
9) In order to compare with the factory value or previous measured values, the DC resistance value measured at any temperature should be reduced to the same temperature, when converted to 20℃, can refer to the attached table.
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Luoyang Xinghe Electric Co., Ltd.
Add：Luoyang Yiyang County industrial agglomeration zone west Zhuang Industrial Park
Tel：+86 13903880417
Mail：lyxinghe6688@163.com
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