Different capacity transformers, different measurement methods, different voltage grades, different core structures and so on, their measurement time is different. If the single phase measurement is directly on the low pressure side, it takes 40~50 minutes to measure a phase . For a 150 MVA transformer magnetometer, it takes approximately 3~5 minutes to measure each phase. And if without saturable magnetic circuit, it takes about 30 minutes for each phase. For the transformer with a voltage rating of 35 KV, the measurement time of the transformer is greatly reduced and it's not necessary to use the saturable magnetic circuit method. For the three core transformers below 120 MVA, the saturable magnetic circuit method is not used, too. The measurement can be carried out directly within 20 minutes generally. For star connection, measurement data shows that it is stable.
Resistance experience: check the resistance of the coils to determine the veracity of the data before DC resistance test. For example, for a giant transformer of 240 MVA or 360 MVA, its low voltage side resistance is generally about 1 M Omega or 2M Omega. If without saturable magnetic circuit method, the initial measurement data is about 4 m Ω ~ 10 m Ω and very stable. After about 10 minutes the date begins to get small.
The change law of data: in the case of the direct resistance measurement of the low-voltage side of large transformers, the first data is far away from the real value, and it is stable for a period of time. Then the date gradually starts to change, bigger or smaller, and the change gets fast. And then the date gradually changes slowly to achieve almost no change finally and get the real resistance. This change tends in one direction, for example, getting smaller and smaller until the real value. If the date is erratic or remains (almost) unchanged for a long time and it's far away from the real value, we should check whether the wiring is wrong.