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What to watch out for when using a DC resistance tester
There are many factors that affect the measurement accuracy of the DC resistance of the transformer windings when the DC resistance tester measures. For example, the accuracy level of the meter, the wiring method, and the current stability condition all affect the measurement accuracy. The DC resistance tester should take these factors into consideration before measurement to reduce or avoid the possible measurement error and obtain a more accurate measurement resistance value.
Test should pay attention to the following aspects:
1 The accuracy of the measuring instrument is not lower than 0.5.
2 The wires of the DC resistance tester must be in good connection with the meter and the test winding terminals. When using a single-arm bridge measurement, the lead resistance should be subtracted from the measurement result. The four cables (C1, C2, C3, C4) of the two-arm bridge shall be connected separately when measuring. The C1 and C2 leads shall be connected to the outside of the measured winding. P1 and P2 shall be connected to the inside of the measured winding to avoid C1 and C2. The contact resistance with the winding connection is measured.
3 Accurately record the temperature of the tested windings. The IEC international standard specifies the requirements and methods for measuring winding temperature. For dry-type transformers, the temperature shall be taken as an average of not less than three thermometers on the surface of the windings. For oil-type transformers, the temperature of the body's pours into oil shall be the difference between the upper oil temperature of the transformer minus the difference between the inlet and outlet oil temperatures of the cooling device. one. In order to facilitate comparison with the factory and previous measurement values, the winding resistance measured at different temperatures should be converted to a resistance of 75 degrees Celsius.
4 DC resistance tester When measuring the DC resistance of large-scale high-voltage transformer windings, the windings and other non-measured voltage levels should be disconnected from other equipment (such as arresters), and must not be grounded and must be prohibited from operation to prevent DC power supply. Inductive high voltages that may be generated when switching on or off endanger the safety, and grounding the non-tested windings can lead to large measurement errors.