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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