Parametric sensitivity analysis of the innovative enlargement law for extruded HVDC cables

The paper studies the effect of the Weibull shape parameter β and of conductor temperature on the reduction of breakdown voltage with cable length in the enlargement from test (small size) HVDC cables to power (full-size) HVDC cables for some typical extruded dielectrics, each characterized by the relevant temperature and field coefficients of electrical resistivity. A “reference enlargement” is considered – by selecting a typical set of values of conductor cross-section and insulation wall thickness for the test cable and the power cable – and various insulation compounds are compared. The behaviors are quite different for the treated compounds, thereby showing that beside the Weibull shape parameter, also the electrical resistivity of the insulation has a major effect in the “reference enlargement”. Secondly, the behavior of the function H appearing in the enlargement formula is analyzed; for HVAC cables such a function is mainly close to 1 in practice, whereas for HVDC cables it has a much more cumbersome expression, whose behavior is not so easy to be assessed a priori. For this reason, the Monte Carlo method is used by generating a huge number of random values of the above-mentioned parameters falling within the typical range of variation for HVDC cables commercially available. By plotting the relevant sampling distribution of the function H against each parameter, the sensitivity of the enlargement process to the various parameters is evaluated comparatively and the parameters that have a major effect on the function H are singled out. Finally, on the basis of the Monte Carlo analysis, simplified – although approximate – expressions are proposed for the function H. These simplified expressions help in a fast preliminary screening of the enlargement effects, becoming a fundamental tool for insulation design and coordination.