The influence of mechanical stresses an the local dielectric strength of HV extruded cables

Mechanical stresses from manufacturing always exist in extruded HV cables. These stresses are generated by the temperature gradients created by the temperature decrease after extrusion or by the difference between thermal expansion coefficients of two different materials in contact (for example metal-polymer interface), These stresses are not uniformly distributed in the cable insulation bulk material. Five different HV cables were analyzed with respect to residual mechanical stresses and dielectric breakdown strength. Photoelastic measurements have been carried out and show that residual stresses range from 2.2 to 5.1 MPa and are located near the conductor shield. Breakdown strength measurements with respect to mechanical stresses have also been performed up to the maximum stresses observed