Impact of humidity on dielectric response of rotating machines insulation system

Abstract

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.

PDC measurements to assess machine insulation

“…Since both charge and discharge currents are affected, this suggests moisture absorption into the bulk of the insulation system in addition to the moisture absorbed along the end windings. This assumption was confirmed when guarded measurements were conducted on the same coil [19]. Figure 4a shows the charge and discharge currents for an earlier epoxy-mica bar which has relatively low values of normalized resistances as compared with modern epoxymica insulation system.…”

“…For this bar, the exposure to humidity affected both the charge and the discharge currents. However, it did not lead to any significant change when guarded measurements were conducted indicating that the moisture absorption occurred only along the end winding portions of the bar [19]. For this bar, the stress grading system was an exposed ferrous oxide type paint, which normally has a low conductivity and does not lead to an interfacial polarization effect within the frame of observation [11].…”

“…More details on this investigation are given elsewhere [19]. The two epoxy-mica bars are from the same manufacturer but with different insulation technologies, the bar in Figure 4a being the oldest of the two.…”

“…Charge (filled symbols) and discharge currents (open symbols) for two epoxy-mica bars (a and c) and a polyester-mica coil (b) before (dry) and after (humid) one week under a humid atmosphere (25 o C, 80% RH)[19].…”

Influence of ambient and test conditions on insulation resistance/polarization index in hv electrical machines - a survey

“…The results of the electrical machine tests including online and offline may follow different behaviors in a same machine [11]. The results of these methods' measurement depend on different factors such as humidity [12], air pressure [13,14], mechanical pressure [15], type of fault [16], temperature [17], raw natural gas [18], power electronic arrangements [19], insulation class [20], high and low frequency pulses [21,22], corona discharges [23], and etc. Now the question is that why should a machine have different results in spite of similar tests while all tests are done through similar devices and procedures?…”