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The failure of polymeric solid insulation under transient voltages is closely associated with space charge. In this study, DC and AC field dependent conduction characteristics of epoxy resin were investigated using the space charge limited current and high voltage broadband dielectric spectrum methods. The breakdown voltages (BDVs) were examined over a wide range of DC voltage ramp rates. The results reveal a threshold effect of AC conduction loss below 600 Hz with respect to the electric field. By analyzing charge migration laws, a mobility distribution was deduced, with an upper limit of 10 − 9 m 2 V − 1 s − 1 , which is much higher than the measured DC mobility of 10 − 14 m 2 V − 1 s − 1 . A multi-mobility bipolar charge transport model was proposed by introducing the obtained mobility distribution. Compared to the single-mobility model, high-mobility carriers induce larger unblocked space charge currents for high dv/dt voltages, leading to lower values of BDV, which is consistent with experiment results. As the voltage ramp rate decreases, both simulation and experiment demonstrate higher BDVs due to the field shielding effect of homocharges. [ABSTRACT FROM AUTHOR]

The carbon fiber (T300-6K) with a three-dimensional four-way braiding structure was selected as the reinforcement, and the 3D-Cf/6061Al composites at different braiding angles (10°, 20° and 30°) were prepared by the pressure infiltration method, and the effects of different braiding angles on the mechanical properties of the composites were discussed. It is found that the pressure infiltration avoids the weak bonding force between the carbon fiber and the aluminum matrix to a certain extent, so that a better infiltration effect can be obtained. The bending performance of the composites become worse with the increase of braiding angle. With the increase of braiding angle, the strain force of the composites also increases, and the greater the elongation is, the better the toughness is. When the braiding angle was larger, the transverse component force of the fiber was larger, the unevenness of the fiber fracture surface increases, the pulling phenomenon was obvious.

Power electronic conversion systems introduce different types of voltage stresses on high-voltage epoxy cast insulation in medium-frequency transformers (MFTs), which challenges the reliability of MFTs. The endurance of epoxy insulation at different voltages (AC, DC and pulse) and environmental (thermal and humidity) stresses was studied using encapsulated electrodes in the absence of partial discharge. To assess aging processes and select aging state indicators, various tests including AC breakdown strength (BDE), broadband dielectric spectroscopy, small-angle x-ray scattering (SAXS), Fourier-transform infrared spectroscopy and differential scanning calorimetry (DSC) were first conducted on aged flat samples. The increase of mesoscopic free volume from the SAXS result and local densification (physical aging) of epoxy network from the DSC result were both found. BDE is proved to be sensitive to various aging conditions and thus determined as a test method for encapsulated samples. It is found that the physical aging effect under thermal stress at early aging stages can cause a reversible increase in BDE compared to non-aged samples. This effect can be erased by coupled medium-low electrical stress during aging. In contrast, a sufficiently high electric field will deepen the physical aging extent. Occasional sample failures were observed simultaneously within this process. According to the SAXS and DSC results on flat samples, the failure of encapsulated samples is possibly attributed to the chain fracture around increased mesoscopic free volume during electron bombardment and microcracks generated by interfacial stress release during physical aging. At longer aging periods, the volumetric absorption and diffusion of chemical reactants (oxygen and water) in epoxy networks at high temperatures determines the eventual decrease in BDE, which can be accelerated by the electric field. Ages with different voltage types were gauged by the reduction of BDE compared to pure environmental stress. Bipolar pulses bring heavier aging effects than ACs at the same RMS value. However, the DC component has opposite effects on the BDE with and without thermal stress. [ABSTRACT FROM AUTHOR]