Water absorption in a glass-mica-epoxy composite [II: field distribution and diagnostic in a stator bar geometry]

In this paper, the water concentration profile within the insulation of a stator bar in the presence of a water leak is calculated by resolving the Fick's equation. The temperature strongly activates diffusion processes, but the shapes of concentration profiles remain similar. From the measured dielectric properties versus water content, an electrical model is developed in order to calculate the field and potential distribution within the insulation. The field is reduced in wet zones due to the high local permittivity and conductivity compared to dry zones. This effect is favorable since the breakdown field is lowered in wet zones. On the other hand, the field in dry zones is reinforced, but the value attained remains lower than the short-term breakdown strength. This explains why stator bars in the presence of waters leaks may survive for long times, in spite of high local water concentrations in excess of 1%. The high sensitivity of dielectric properties at low frequency to the presence of water suggests non-destructive methods to detect wet bars in a real power generator.