Charge buildup in lossy dielectrics with induced inhomogeneities

In addition to their normal inhomogeneities, insulation may also become inhomogeneous under stress, for instance if their temperature is not uniform. Then, a charge density builds up wherever material properties such as conductivity and permittivity are not uniform. This is the case of HVDC cables, in which the core is heated by the Joule effect, while the sheath remains at near ambient temperature, so that an average temperature gradient of equal to or less than 50 K/cm may occur in the insulation, under a field which can reach 100 kV/cm. If the conductivity of the insulation is assumed to depend only on temperature, with an activation energy of 1 eV, the steady charge density predicted by electrostatics may reach levels easily measurable by the techniques now available. This charge density is somehow lowered in non-ohmic insulation. An early analysis of the transient charge build-up is discussed here, and extended to the case of a non-ohmic insulation. This allows calculation of the transient current generated in the external circuit by the accumulating charge, in the cases of ohmic and non-ohmic insulation. Finally, other manifestations of the space charges in question are mentioned. Among these, unexpected results on electroconvection in slightly ionic liquids in the presence of a temperature gradient are analyzed in terms of these charges, and suggest practical applications.