Impact of humidity on dielectric response of rotating machines insulation system

The study of dielectric response is a significant method to characterize the electrical properties of insulating materials and a useful tool for condition assessment of the insulation systems of rotating machines. Time and frequency domain measurements can be used to determine the dielectric response. Polarization and depolarization current measurements are usually used as a time domain measurement while in frequency domain the capacitance and dissipation factor of the samples can be measured over a broad range of frequencies. The dielectric response of machine insulation is affected by variety of parameters such as degree of aging, contamination and the environmental conditions such as temperature and humidity. In this work, time and frequency methods are used to determine the impact of humidity on the dielectric response of different insulation technologies used for stator insulation of large rotating machines. The measurements were conducted on bars and coils in laboratory conditions as a comparison base and then the dielectric response of the samples was monitored in an environmental chamber during two different humidification periods. The samples were selected to cover the most common materials and technologies used in operating rotating machines including asphalt; epoxy and polyester based insulating systems. The results showed considerable change in the dielectric response when the samples were exposed to moisture. Further studies demonstrated that the change in dielectric response is mostly attributable to the end-winding and stress-grading layer.