Effect of Humidity on Charge and Discharge Current of Large Rotating Machines Bar Insulation

Abstract

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.

Impact of humidity on dielectric response of rotating machines insulation system

“…A similar study showed that dielectric response of modern thermoset insulating materials experiences no or minor changes while they are immersed in water [6]. On the other hand, the results of this study showed that the end-winding portion and particularly the stress-grading paint/tape plays a major role when the winding is subjected to high humidity.…”

“…This indicates that the straight portions of these modern bars resist moisture penetration and this supports the conclusion that it is the end-winding that is the most vulnerable part of the winding when the machine is subjected to high humidity. Further details have been published in [4,6]. Figures 21 and 22 show the charge currents of the polyester coil in dry condition and during the first humidification period for the unguarded (complete) and the guarded configuration respectively.…”

“…Measurement of polarization and depolarization currents recently has attracted more attention and many papers have been published reporting PDC in combination with frequency domain measurements as a diagnostic tool for detecting various problems for rotating machine insulation systems [2][3][4][5][6][7][8][9][10][11][12][13][14]. The dielectric response of modern epoxy bonded insulating materials has been completely characterized over very low frequency range for aged and unaged bars in [2].…”