Dielectric response of rotating machine stator insulation system

Abstract

In order to assess the state of the rotating machine insulation winding system, time-domain dielectric measurements are frequently conducted. Time-domain measurements are commonly used in the field in the form of polarization/depolarization test (PDC), high voltage ramped test or other type dc tests. Such measurements, which are related to the lowfrequency dielectric response of the insulation system, depend on the insulating materials, the way the components of the insulation system are laid out, the aging of the materials and on external factors such as temperature, moisture absorption or contamination. In the paper, measurements conducted in the time domain are presented for modern epoxy mica winding insulation technology. The influence of the temperature is investigated for epoxy-mica insulation systems for stator bars and a correction procedure for insulation resistance (IR) for epoxy mica insulation is suggested.

PDC measurements to assess machine insulation

“…The above equation holds for the various machine insulation technologies in the 1 to 1000 s time range (or 10 -4 to 10 -1 Hz frequency range) at room temperature. At higher temperature, a relaxation peak related to the alpha relaxation of the resin will start to appear resulting in a change of the dielectric response [9] and equation (10) is no longer valid. Accordingly, the dielectric response given by equation (10) at room temperature can be interpreted as the high frequency tail of the alpha relaxation peak related to the glass transition temperature.…”

“…With the commercial availability of modern equipment, it is now possible to monitor both the charge and the discharge currents, or the polarization/depolarization currents (PDC) which allows a better evaluation of the insulation condition. Many papers have been published recently reporting machine insulation assessment with PDC measurements [3][4][5][6][7][8][9][10][11][12][13][14][15] but the advantage of monitoring the discharge current in addition to the charge current have not been precisely pointed out. This paper presents the theoretical background relating the results of dc testing to the dielectric properties of the machine insulation and thus providing the dielectric characterization of the various machine insulation systems.…”

Temperature Effect on Dielectric Characteristics of Large Rotating Machines Insulation

“…The rule was derived from the measurement on some insulation system during the late 1950s and is not necessarily correct for modern thermoset insulation systems. Some efforts have been reported that investigated the validity of the correction factor proposed by IEEE standard and proposed an alternative correction factor for modern insulation systems [2][3][4]. Equations 6 to 8 are representing the correction factors which are proposed by [1], [9] and [4] respectively.…”

“…This rule is based on testing of some of the insulation systems of the late 1950s and is not strictly true for all insulation systems. Recently some efforts has been made to find a proper relation between temperature and dielectric characteristics of modern insulation material of rotating machines and to propose an accurate temperature correction coefficient for insulation resistance [2][3][4].…”

Impact of humidity on dielectric response of rotating machines insulation system

“…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].…”