Measuring the insulation of a transformer is one of the effective means to effectively detect whether the transformer is faulty. According to the relevant power preventive test procedures, the tests include chromatographic analysis, DC resistance detection, insulation resistance and absorption ratio, polarization index detection, insulation medium loss tangent detection, oil quality detection, partial discharge detection and insulation withstand voltage test.

Insulation resistance, absorption ratio and polarization index measurement

When measuring the winding insulation resistance, the insulation resistance value of each winding to the ground and other windings should be measured in turn, and the lead ends of the tested windings should be short-circuited. The other non-tested windings are all short-circuited to ground and the idle windings are grounded. The insulation state of the tested part to the grounded part and the different voltage parts can be measured.

In order to avoid the residual charge on the winding, the measured value is too large. Before measurement, short-circuit the tested winding and the fuel tank to ground. The discharge time should be no less than 2min.

When measuring the transformer just after stopping the operation, the transformer needs to stand for 30 minutes after being disconnected from the grid to make the oil temperature and the winding temperature tend to be the same. You can use an insulation resistance meter to measure insulation resistance.

For newly put-in or overhauled transformers, they should be filled with qualified oil and stand still for a period of time. After the bubbles are eliminated, the test can be carried out. Generally, larger power transformers of 8000kVA and above need to be allowed to stand for more than 20h, and for small-capacity power transformers of 3~10kVA, need to be left for more than 5h.