By hvhipot 
Various problems will be encountered during the use of the transformer DC resistance tester. For an experienced operator, he should learn to correctly handle various accidents in the use of the equipment. There are several points that we need to pay attention to. After the DC resistance test is completed, the transformer sometimes has residual magnetism, which may cause the situation that it cannot run immediately. After the test is completed, reverse the positive and negative poles of the test wire and perform a reverse charge to effectively avoid the above situation. The specific time is flexibly controlled according to the transformer capacity and the size of the test current. To save time, you can press the reset button during the test without waiting for the end of the discharge process, you can directly start the measurement again. When measuring the high-voltage side resistance of the on-load voltage-regulating transformer, start the measurement from the 1 or 17 resistance range. Try to choose a large current within the range to enhance the stable performance, and the current tolerance of the test product should also be considered.
The interpretation of the measurement results of the transformer DC resistance tester also requires certain skills, time experience, different capacity transformers, different measurement methods, different voltage levels, different core structures, etc. The measurement time is different. For a 2400MVA transformer, it will take about 8-10 minutes for each phase to measure the resistance of the low-voltage side of the low-voltage side if the magnetic method is used. It takes about 3-5 minutes for each phase to be measured by the 150MVA transformer assisted magnetic method. Without assisting magnetism, it takes about 30 minutes for each phase. For transformers with a voltage level of 35KV, the measurement time is greatly reduced. For three-core transformers below 120MVA, the magnetic method is not used, and the measurement can be performed directly within 20 minutes. For the star connection method, the measurement data is displayed as stable.
The law of data change. When measuring the direct resistance of the low-voltage side of a large transformer, the displayed data is far from the true value, and it remains stable for a period of time, and then gradually begins to change, becoming larger or smaller, and the speed of change gradually Speed up, and then gradually change the steamed bun to almost no change, and get the real resistance value. This change is generally a change in one direction, for example, it has been reduced to the true value. If large and small, erratic for a long time, or if the data remains unchanged or changes very little for a long time and is far from the true value, check the wiring for errors.
The resistance value experience, first understand the resistance value of each coil before the direct resistance measurement, in order to judge the authenticity of the data. For example, for a giant transformer of 240MVA or 360MVA, its low-voltage side resistance is generally about 1mΩ or 2mΩ. If the magnetic method is not used for measurement, the initial measurement data is about 4mΩ~10mΩ, and the data is very stable, and the data starts to decrease after about 10 minutes.