Core design stress levels

The methods for designing the cable insulation thickness of EHV cables are based on the mean or the maximum stress. It mainly consists in determining the design field strengths of the insulation: the AC field strength and the lightning impulse field strength are generally considered. The most complete approach is based on the statistical analysis of the reference strength. Besides the design methods based on fixed values for either maximum or mean stress, the statistical method takes fully into account the volume effect for the insulation breakdown. For a global reliability of the design, constraints or possible limiting factors such as the following are also to be considered: external stress allowed by accessories (field strength at the interface); acceptable AC breakdown risk level in service; and AC voltage level for the routine test. The paper summarizes these methods on the basis of examples taken from published papers and on the report of CIGRE WG 21.04 who had been given the task of assessing the design criteria for HV extruded cables. Current practice for conductor and external stress on EHV cables is also considered