A model for space charge injection in dielectrics

Space charges are known to play a very important role in dielectric ageing. The difference between impulse and dc electric strength, the influence of prestressing and impulse rise time on breakdown voltages and the polarity dependence of the breakdown voltage in the needle plate geometry all have been explained by space charge build up (1). In addition, the action of highly energetic charge carriers are often held responsible for the initiation of electrical trees (2). In a recent publication (3) it has been shown that growth of an electrical tree is possible if the mechanical stress at the tip of a discharge channel exceeds the local yield stress of the dielectric. Both quantities are strongly influenced by space charge injection because the mechanical stress is directly dependent on the space charge distribution ρ(x) through the force field f(x) = ρ(x) · F(x), and the local yield stress may be reduced by the injection of charge through local heating or plasma chemical damage. The latter effects will be particularly important under ac conditions.