Space charge behavior of magnesium oxide filled epoxy nanocomposites at different temperatures and electric field strengths

Space charge accumulation is a phenomenon, which is typical for high voltage dc insulation. It is of special importance for polymers, since they do not possess self-healing properties. Thus, the accumulation of space charges, which is linked to ageing, proved to be a limiting factor for HVDC applications. The focus of this paper is the behavior of epoxy based nanocomposites with magnesium oxide filler material. Nanoscale magnesium oxide has already been shown to decrease the space charge density for high field strengths. Additionally, MgO-nanocomposites showed an increase in the short term dc breakdown strength for low filler concentrations. Base material for all samples is commercially available bisphenol-A epoxy resin. Transmission electron microscopy was performed to validate the particle size and dispersion and showed that MgO has an average particle size of 22 nm. Space Charge profiles were obtained with the PEA-method and compared to neat epoxy. The profiles were taken under dc field strengths between 10 and 18 kV/mm. To see the influence of temperature on the charge distribution, the measurement was performed both at room temperature and at 60°C. The field enhancement factor of both the neat epoxy and the nanocomposite for different field strengths and temperatures has been calculated. It turned out that MgO nanocomposites show overall better space charge behavior at higher field strengths and at higher temperatures, compared to the unfilled epoxy. Both the space charge accumulation and the field enhancement factor are reduced, when going to higher electric field strengths or temperatures. Possible explanations for the observed space charge behavior are given.