Dielectric properties of epoxy/mica insulation

Discusses the assessment of the condition of high-voltage bar insulation of large machines by the use of nondestructive methods such as the measurement of insulation resistance, polarization index, time constant, charging and discharging current, and capacitance and loss factor. Such measurements were carried out on a composite dielectric of special interest, epoxy/mica/glass, to a sufficient extent to allow conclusions to be drawn concerning processes taking place in the insulation in the given conditions. Specimens of both the epoxy/mica insulation and the resin alone were prepared for measurement. The components of the complex relative permittivity of the specimens were determined in the frequency range from 10^-4 Hz to 10⁵ Hz at temperatures of 23, 40, 85, 110, 150 and 180°C and at field strengths below 1 kV/mm, at which a linear response can be expected. This was verified by measurement of the DC current five minutes after the application of voltage when it was successively increased from 30 V up to 1000 V, as well as of the dissipation factor at 50 Hz when the voltage was increased in steps from 50 V to 1000 V