1: The stator winding is wet. For generators that have been stopped for a long time or overhauled for a long time, the insulation resistance should be measured before being put into operation. Those who fail to pass the test shall not be put into operation. The damp generator should be dried.

2: The winding itself is defective or the repair process is improper, causing winding insulation breakdown or mechanical damage. Insulation materials should be selected according to the specified insulation level, and embedded windings and dip paint drying should be strictly performed according to process requirements.

3: The winding is overheated. After the insulation is overheated, the insulation performance will be reduced, and sometimes it will cause insulation breakdown at high temperatures. Daily inspections should be strengthened to prevent overheating of all parts of the generator and damage to the winding insulation.

4: Aging of insulation. Generally the generator runs for more than 15-20 years, the winding insulation is aging, the electrical performance changes, and even the insulation breakdown. The maintenance and preventive test of the generator should be done well. If the insulation is found to be unqualified, the defective winding insulation or the winding should be replaced in time to extend the service life of the generator.