Influence of mixed-frequency medium-voltage and environmental stress on the aging of epoxy

Abstract

Recent developments in power electronic technologies lead to new challenges for insulation systems. This contribution aims to clarify the influence of a broad range of mixed-frequency medium-voltage (MF-MV) and environmental stress parameters on the aging of epoxy insulation. For this purpose, test samples are stressed with an AC (50 Hz) or a DC voltage, superimposed with a pulse-width modulated (PWM) voltage (kHz range). An analysis of the samples’ health state is carried out after the aging by evaluation of potential aging markers (AC breakdown strength, dielectric permittivity, glass transition temperature, FTIR spectra). Although the main focus of this work is on aging below the inception of partial discharges (PDs), it was first confirmed that PD-related aging depends mainly on the peak voltage stress. In contrast, the results obtained by aging below PD inception suggests a dependence on the root-mean-square (RMS) of the applied voltage stress, and consequently on the energy dissipation. Aging in the PD-free regime was only observed at alternating electric field stress and high relative humidity (RH) or elevated temperatures. No influences of space charge and of the slew rate of the PWM voltage were observed. Remarkably, higher PWM frequencies lead to less insulation aging. This might be attributed to the increasing hindrance of polymer side chain movement at higher frequencies, as observed by dielectric spectroscopy. In addition, it is indicated that the aging mechanisms under mixed-frequency voltage stress result from superimposed single-frequency aging mechanisms and that aging is activated after a latency period. Of the investigated potential aging markers, only the residual breakdown strength revealed aging effects, which correlates with lifetime observations in the PD-free voltage stress regime. It is hypothesized that the aging mechanism is associated with a
rearrangement of the free volume in the polymer, followed by a localized breaking of van der Waals bonds.

Influence of the stoichiometric ratio on the material properties and the aging behavior of anhydride-cured epoxy systems

“…Comparatively little is known about aging mechanisms of polymeric insulation in the absence of PDs and dielectric heating. Previous studies on this topic [13,14] revealed a strong decrease in the residual breakdown strength of anhydridecured epoxy samples, if, in addition to electric field stress (with the confirmed absence of PDs and dielectric heating), a high air humidity (leading to higher amount of absorbed water) was present.…”

“…The results are displayed in figure 4 and show that the highest breakdown strength is achieved at SR = 95%, i.e. for the base epoxy material used for the post-curing measurements in this work and for the aging studies presented in [13,14].…”

“…The short term AC breakdown strength was measured (as described in detail in [13,14]) according to IEC 60 243-1 and ASTM D149 with a voltage ramp of 2 kV s −1 to reach the breakdown usually in 10 . .…”

“…Small variations of the measured thickness d meas ∈ [190 µm, 210 µm] were considered by relating d meas to the nominal thickness d n = 200 µm according to [24], which was experimentally verified in [14].…”

“…For this purpose, anhydride-cured epoxy samples of different resin-to-hardener ratios (in the following labeled as different SRs) were manufactured and afterwards hygroelectrically stressed under similar conditions as applied in earlier works [13,14]. The assessment of the material properties before and after different aging sequences is similar to the analysis in [13], but only the Fourier-transform infrared (FTIR) spectra and the short-term AC breakdown strength (representing the material's residual breakdown strength) are used in this work.…”