Conditions of discharge-free operation of XLPE insulated power cable systems

A coherent methodology was developed for analysing the risk of partial discharge inception in the insulation of high voltage cables containing spherical voids located at the conductor screen interface and fissures located at the insulation screen interface. These two types of insulation defects present the two most important cases from the point of view of insulation integrity in high voltage cable systems. Our approach was based on the notion of sensitivity of dielectric stress to dimensional change of cable insulation and the notion of sensitivity of the size of discharge free defect to the change of electric field intensity. We investigated how modifications of insulation dimensions can affect the risk of discharge in typical defects. The obtained results revealed that so called slim design of HV cables requires that the size of voids is reduced by approximately a third to ensure discharge free operation. The size of fissures must be reduced approximately tenfold in comparison to the permissible size of similar fissures in a standard design to ensure discharge free operation at the interface with accessories. This poses a very high technological demand on new cable system designs of this type. Increasing the size of cable core creates an increased risk of partial discharge, mainly in fissures. Based on the dimensions of a typical 110 kV cable we showed that a reduction of fissure dimensions by approximately a quarter is required for a discharge free operation if the conductor size increases from 1000 mm² to 2000 mm². We have quantified this way the extent to which quality of the insulation extrusion process as well as precision of accessories assembly must improve, in order to assure discharge free operation of cable systems after typical design modifications.