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.
DC testing is probably the most commonly used maintenance and diagnostic tests periodically conducted on machine stator insulation systems. With the commercial availability of more sophisticated equipment it is now possible to continuously monitor both charge and discharge current during a step voltage test, also known as polarization/depolarization current (PDC) test. This test is related to the dielectric response of the insulation system. This paper presents theoretical considerations on the dielectric response of the various types of machine winding insulation systems encountered in the field and the usefulness of using both the charge and discharge currents to assess the condition of the insulation system.
Index Terms-Machine insulation, dielectric response, polarization and depolarization currents.
Dielectric characteristics of insulation system of large rotating machines can be affected by operational and environmental conditions. Temperature, humidity and contamination affect dielectric parameters such as insulation resistance, polarization index and dissipation factor in certain degrees. In order to investigate temperature effect on characteristics of different insulation materials, time and frequency domain dielectric spectroscopy were conducted on five pieces of different bars and polarization and depolarization currents and dissipation factor were studied at different temperatures. Also temperature effect on insulation resistance of the samples is studied and accuracy of different temperature correction coefficients is investigated.
Dielectric characteristics of insulation system of large rotating machines can be affected by operational and environmental conditions. Humidity, temperature and contamination affect dielectric parameters such as insulation resistance, polarization index and dissipation factor in certain degrees. In order to investigate the effect of humidity on characteristics of modern insulation systems, some bars with different insulation materials or different manufacturing processes were subjected to high humidity for a certain period and its effect on time and frequency domain dielectric responses were studied. Also to study the effect of water contamination of the insulation system, some bars were partially immersed in water and the variation of parameters such as insulation resistance, polarization index, loss factor, etc. were investigated. AC and DC hipot tests were conducted to verify the degree of effect of humidity/water absorption on dielectric strength of the insulation materials.
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