Monitoring of Moisture on Stator Winding Insulation during Drying Process

This paper presents the condition assessment of generator stator winding due to moisture ingress in their asphalt, polyester and epoxy mica insulations. The work has two parts i.e. laboratory and field investigations. In the laboratory investigations, two naturally wetted asphalt mica and two dry (polyester and epoxy mica one each) spare stator windings were tested using, dc ramp test, time variation of insulation resistance (IR) and frequency dielectric spectroscopy (FDS) measurements. The two wet samples were dried inside an oven whereas the dry samples were wetted inside a water bath. The dc ramp and IR tests repeated on them and finally FDS measurements were also taken. The results were compared among the different test methods and de ramp test was used for field investigation due to its simplicity and cost effectiveness. In the field investigations, seven selected hydro and one thermal generators belonging to Ceylon Electricity Board was tested. It was found that dc ramp test can be used effectively to investigate the moisture effect on generator insulation.