What is the purpose of partial discharge test for transformers

Power transformers mainly use oil paper barrier insulation, which is composed of alternating layers of electrical paper and insulating oil. Due to the complex structure and uneven insulation of large transformers. When improper design leads to high local field strength, poor craftsmanship, or external factors that cause internal defects, partial discharge will inevitably occur inside the transformer and gradually develop, ultimately causing damage to the transformer. Partial discharge inside power transformers mainly occurs in the following situations:
(1) Oil channel breakdown in the oil paper barrier insulation in the middle of the winding;
(2) Oil channel breakdown at the end of the winding;
(3) Oil gap breakdown in close proximity to insulated wires and electrical paper (lead insulation, lap insulation, phase to phase insulation);
(4) Longitudinal insulation oil channel breakdown between coils (turns, cakes);
(5) Branch discharge of insulation cardboard screens and other materials;
(6) Creepage of other solid insulation materials;
(7) Discharge of other metal foreign objects infiltrating into the insulation.
Therefore, for transformers that have already been manufactured, there are several situations that require partial discharge testing:
(1) Before the new transformer is put into operation, a partial discharge test should be conducted to check for insulation damage during transportation and installation after leaving the factory.
(2) Conduct partial discharge tests on transformers that have undergone major repairs or modifications to assess their insulation condition after repair.
(3) Further qualitative diagnosis should be conducted on transformers suspected of having insulation faults during operation, such as discharge faults in gas chromatography analysis of oil, as well as other abnormal insulation situations.

(4) As a preventive testing project or online inspection content, monitor the insulation condition of transformers during operation.