1 Introduction
AS one kind of the basic insulating materials, polyimide has been widely used in turn to turn insulation and turn to ground insulation in inverter-fed motors because of its excellent dielectric properties. However, the pulse voltage from the converter often causes overvoltage and localized high electrical field in the insulation system, leading to the premature failure of the insulations, partial discharge and acceleration of insulation degradation [1], [2]. Many papers have investigated insulating properties of polyimide films and other materials under pulse voltage [3]–[5]. Serdyuk et al. investigated charging of polymer surfaces by means of impulse corona discharges and concluded that charge deposition was different in case of glow corona and burst corona [3]. Wu et al. studied the structure change of polyimide film aging under pulse voltage and found that the degradation of structure in polyimide propagates from surface to interior with continuous aging [4], [5]. Koltunowicz et al. studied the breakdown characteristics of the insulation of paper winding and found that the breakdown voltage increases as the rise time increases [12].