Temperature dependence of insulation resistance and current components for generator stator bars

Insulation resistance measurement is one of the widely used methods to diagnose insulation conditions of turbine generators. Since insulation resistance tends to decrease with the increasing temperature, it needs to be evaluated on exactly the same basis. Insulation resistance of the epoxy-mica based insulation system does have temperature dependence in the temperature range over 40°C, but the insulation resistance is nearly constant in the temperature range under 40°C. In order to study the mechanism of the temperature dependence, current components such as conduction current I
_G
, surface leakage current I
_L
and polarization current I
_P
were measured by applying a DC high voltage across the main insulation under the temperature range of 10°C to 80°C and the time range of 0 to 30 min. The dominant current component of 1 minute value is I
_P
, and it is nearly-constant less than 40°C and tends to increase over 40°C. Therefore the temperature dependence of the insulation resistance is determined by that of I
_P
. Since the insulation resistance is nearly-constant under 40°C, no temperature correction is required as long as it is measured under 40°C.