Most of the dry-type transformer windings are cast by resin, and the conductor material is sealed in it, so the decrease of its insulation resistance can be divided into the following categories:

I. The high and low-voltage windings are all made of epoxy resin casting. If the insulation resistance decreases, it may be because there is a large amount of water vapor or a large amount of dust accumulated on the surface of the winding;

II. If the high-voltage winding is poured with epoxy resin but the low-voltage winding is not poured, the epoxy board used to fasten the low-voltage winding is easy to absorb moisture, resulting in a lower insulation level;

III. When installing the transformer, the gaps inside the iron core column and the low-voltage winding have sundries, and the presence of sundries will reduce the insulation level;

IV. When using a megohmmeter to measure the insulation of the iron core to the ground, the problem of insulation degradation will also occur. There are three main considerations: Whether there is a multi-point grounding phenomenon; whether the insulation of the iron core covering paint itself has dropped; whether there is moisture absorption during the application of the insulating board used for the iron core.

  For these phenomena, the following treatments can be taken:

I. When cleaning the winding surface, if there is a large amount of water vapor condensing on the surface, it should be wiped dry with a dry cloth, and then ventilated to make it dry again;

II. It can be dried with a heater or ventilated with a fan;

III. Disconnect the three-phase connecting copper bars, check and confirm the problem with a megohmmeter, and then carefully complete the corresponding search work.