Transformer winding DC resistance measurement

In the power production, the transformer is a very important device, so whether its various indicators are qualified is the guarantee of its normal work. The DC resistance test is to check the quality of the winding welding and whether there is a short circuit between the windings; check the winding conductor or lead wire Whether there is a broken bone or an open circuit fault, check whether there is short circuit between the turns, between layers, and whether the tap switch is in good contact with each position and whether the actual position and indication position of the tap changer are consistent.

At present, DC resistance is mainly measured by using a DC resistance tester. The principle followed is Ohm's law. The instrument outputs a constant current as a constant current source and measures the voltage on the coil under test to calculate the current.

Note: 1. Before measuring, make sure the contact of the measuring terminal is good. When adjusting the tap changer, the contact should be rotated several times. The contact is good during the period. The non-tested winding cannot be short-circuited or grounded during measurement. The current line of the measuring line should be clamped. On the outside, the voltage lines are clamped inside, minimizing the effect of contact resistance on the result.

2. When measuring a small transformer, the current on both sides of the coil should be about 20% of the rated current of the transformer. If the current is too high, the heat generated by the tested coil will increase the direct resistance. When testing a large transformer, 50 to 100A should be selected for testing. Current, reducing the test time, must wait until the data is stable before reading, so as not to enlarge the error due to reading problems.

3. After the measurement is completed, the wiring should be removed after the discharge is completed. It is forbidden to remove the wire after the discharge is completed. Otherwise, the transformer or the test instrument may be damaged due to the induced overvoltage.

Data analysis and results judgment: If the measured data is converted to the same temperature, if the deviation of the results is found to be too large, objective reasons should be eliminated first, such as poor contact of the test site, oxidation, and failure of the test instrument.

For large-capacity transformers (above 1.6MVA), the three-phase windings should have a direct resistance deviation of less than 2% of their average value. For transformers without a neutral conductor, the phase-to-phase resistance difference should not exceed 1%. For small-capacity transformers (below 1.6MVA), the phase-to-phase resistance difference is not greater than 4% of the three-phase average, and the difference between the lines is not greater than 2% of the three-phase average.

Compared with the data measured in the past, the change should not be too large.