3-point bending test AC breakdown voltage DC high voltage ECT Technology: general issues Weibull distribution activation energy ageing effect aramid paper arc lamps arc welding cellulose cellulose-aramid paper composite insulator deformation dielectric liquid mixtures dielectric materials dielectric polarization discrete Fourier transform drying electric arc electrical discharges electromagnetic radiation energy distribution frequency response analysis (FRA) gas insulation glass-reinforced epoxy core harmonic distortion insulating liquids insulation aging