Oil-immersed power transformers mainly use oil-paper barrier insulation, which is composed of electrical paper layers and insulating oil. Due to the complex structure of large transformers, the insulation is very uneven. When improper design causes internal defects such as excessive local field strength, poor technology or external factors, partial discharge will inevitably occur in the transformer, which will gradually develop, and finally cause damage to the transformer.

The internal partial discharge of power transformer mainly occurs in the following situations:

(1) Breakdown of the oil passage 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 close to the insulated wire and electrical paper (lead insulation, lap insulation, phase insulation);

(4) The longitudinal insulating oil passage between the coils (between the turns and the cake) is broken down;

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

(6) Creepage of other solid insulation;

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

Therefore, for the transformers that have been shipped from the factory, partial discharge tests must be carried out in the following situations:

(1) Perform a partial discharge test before the new transformer is put into operation, and check whether there is any insulation damage during the transportation and installation of the transformer after it leaves the factory.

(2) Carry out partial discharge test on the transformer after overhaul or modification to judge the insulation condition after repair.

(3) Make further qualitative diagnosis of transformers suspected of having insulation faults during operation, such as discharge faults in gas chromatographic analysis of 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.