Power transformers mainly use oil-paper barrier insulation, which is composed of electrical paper layers and insulating oil interlaced. Due to the complex structure of large transformers, the insulation is very uneven. When improper design, resulting in high local field strength, poor workmanship or external reasons, etc., cause internal defects, partial discharge will inevitably occur in the transformer, and gradually develop, and finally cause transformer damage. Partial discharge inside power transformer

Mainly in the following situations:

(1) Breakdown of the oil channel in the oil-paper barrier insulation in the middle of the winding;

(2) Breakdown of the oil passage at the end of the winding;

(3) Breakdown of the oil gap next to the insulated wire and electrical paper (lead insulation, lap insulation, interphase insulation);

(4) The breakdown of the longitudinal insulating oil passage between the coils of the power transformer (between turns and cakes);

(5) Branch discharge of insulating cardboard enclosures, etc.;

(6) Creepage of other solid insulation;

(7) Discharge of other metal foreign objects infiltrated in the insulation, etc.

Therefore, for the transformer that has been shipped from the factory, the partial discharge test must be carried out in the following situations:

(1) Before the new transformer is put into operation, perform a partial discharge test to check whether the transformer has insulation damage during transportation and installation after leaving the factory.

(2) For power transformers, perform partial discharge tests on the overhauled or transformed transformers to judge the insulation condition after repairs.

(3) Make further qualitative diagnosis for transformers suspected of having insulation faults during operation, such as discharge faults in gas chromatographic analysis in oil, and other abnormal conditions involving insulation.

(4) As a preventive test item or online inspection content, monitor the insulation condition of the transformer during operation.