A sensitivity analysis of cable parameters and their influence on design choices for minimum sheath voltage limiter specification in underground cable systems

Recent papers have emphasised the importance of including the earth return impedance in calculations for specifying the minimum required sheath voltage limiter (SVL) rating. SVLs are a vital component of cable protection in any underground network and if sized incorrectly can have catastrophic results. This paper examines the effect of several parameters integral to the earth return impedance and their impact on the voltages subjected to SVLs. Parameters examined include cable geometry, soil resistivity, sheath inner radius and sheath resistivity. A sensitivity analysis on these parameters demonstrates that, depending on how the systems are modelled, SVL ratings can be significantly underestimated. This paper seeks to illustrate the conditions under which the theoretical models in EMTP-RV can be considered to give conservative results. Extensive field injection testing has been undertaken on two underground cable configurations in order to verify the results and findings of the modelling.