Effect of Humidity on Lifetime and Dielectric Properties of PET Film

Due to recent developments in semiconductor switching technology, the associated insulation systems are exposed to higher pulse voltage amplitudes and frequencies as well as shorter rise times, which might accelerate material degradation. Often, the insulation is also subjected to humidity changes which affect the dielectric properties and possibly the insulation lifetime. This motivates the study of humidity effects under DC-biased pulse-width modulated (PWM) voltage stress. The aim of this contribution is to provide an experimental basis as well as explanations for humidity-induced variations in the dielectric properties and changed lifetime behavior of PET film. Throughout all experiments, the relative humidity (RH) of air is varied. The lifetime is evaluated as time-to-failure (TTF) by applying DC voltages with and without superimposed PWM voltages below and above the partial discharge inception voltage (PDIV). In order to quantify changes in the dielectric properties, the complex dielectric permittivity is evaluated by means of broadband dielectric spectroscopy (BDS). The water absorption is measured gravimetrically. Although the observed relative water absorption of the PET specimens is always below 0.37 %, the lifetime is drastically decreased by more humid conditions under DC stress (below PDIV). Above PDIV, PD erosion dominates at low RH, whereas DC breakdown processes become more significant at high RH. The BDS results reveal humidity-induced changes in the relaxation and conduction behavior. Finally, the breakdown mechanisms, which are altered by water absorption, are discussed in terms of thermal and space-chargedriven breakdown.