Low-frequency dielectric response of asphalt bonded insulation

PDC measurements to assess machine insulation

“…Similarly, the normalized resistances for the complete winding were also significantly lower than for the spare or the neutral end coil. When guarded measurements were conducted on the coils, the normalized resistances in charge and discharge were still found to be lower in the case of the field aged coils (middle and line end positions) than for the unaged coils [5]. This is a clear indication that "field aging", at least in the case of asphalt-mica technology, leads to an increase of the dielectric loss of the ground wall insulation that can be easily detected by PDC measurement.…”

“…deterioration of the stress grading coating, embrittlement of the bonding resin, delamination, … This will normally lead to a change in both charge and discharge currents, usually an increase. Figure 7 and Table 4 show the results calculated from the PDC measurements conducted on a spare, unaged asphalt-mica coil, coils with the same design removed from the stator after 52 years in service [5] as well as on the complete winding, in the field, before removal of the coils (the machine was scheduled for rewinding). The measurements were conducted for voltage steps of 1, 5 and 10 kV corresponding to an electric field in the straight portions of the coil of 0.2, 1, 2 kV/mm (the insulation thickness was about 5 mm).…”

“…Accordingly, it must be corrected to a common temperature with an appropriate correction factor. When temperature is taken into account, the RC' parameter can be used to trend the condition of the insulation system [3,5,10,13,18]. An abnormally low value of this parameter, which translates a high value of absorption current, normally reflects a lack of curing, thermal aging or moisture absorption into the bulk.…”

“…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.…”

Dielectric response of rotating machine stator insulation system

“…Indeed, methods based on the broadband dielectric response of insulation systems are being used more and more frequently to assess the condition of various electrical equipments [1]. For rotating machinery winding insulation, time domain tests, such as step voltage and ramped voltage test have been indeed used with a certain degree of success [2][3][4][5][6][7][8][9][10]. Findings drawn from the measurement of the dielectric response, either in the time or the in frequency domain, become most useful when the dielectric response of the unaged insulation system is known.…”