Technology & analysis of transient overvoltage measurements for a 420 kV sectionalized cross-bonded underground cable system in a Siphon circuit of the Dutch EHV transmission grid

In January 2023, one 380kV outdoor cable termination failed at the transition point between the underground cable section and the overhead line section of one Siphon circuits in the Dutch transmission grid. The root-cause analysis concluded that the cable termination failed mechanically and that the failure mechanism was not related to dielectric stresses. However, following the repair works, a HVDC test was conducted to check the integrity of the cable sheath. During the test, an abnormally low resistance was detected at the sheath interruption of one of the cross-bonding joints of the underground cable system. The laboratory investigations indicated traces of severe electrical activity on the sheath interruption ring. Based on these findings, transient overvoltages were considered as a possible root-cause of the cable joint failure. In April 2025, on-field transient measurements were performed, aiming to capture the overvoltages during circuit switching on the cable main insulation and on one of the link boxes respectively. The analysis of the measurement data was further used to a) confirm the initially formed hypothesis on the failure root-cause and b) validate and further enhance the complex