Reliability of Interfaces for HVDC Cable Accessories

HVDC cable systems are widely used in modern transmission networks. This project investigates the dielectric withstand properties of the interfaces existing in the accessories (joints, termination). Interfaces are a weak point as cavities exist at the contact between the cable and the accessory body. Interfaces have been studied as the mating of two elastic surfaces using elastic-plastic mechanics. Their withstand properties are greatly improved by using rubbery material (e.g., silicon rubber) and oiling the surfaces as the number and size of the cavities is reduced. However, thermal aging of the surfaces (with loss of elasticity) and the migration of the lubricant might lead to conditions where the interfaces are again the weak link of the system. The goal of this thesis is to understand breakdown mechanisms of XLPE/XLPE and XLPE/LDPE interfaces through the analysis of partial discharges, space charge accumulation and leakage current measurement. Space charge is measured through the Thermal Step and the Pulsed Electro-Acoustic methods (TSM and PEA). TSM highlights that there exists a critical voltage at which the space charge shifts from homo- to hetero-charge. The conductivity characteristics of the XLPE insulation of HVDC cable are investigated under different applied mechanical stress and the breakdown voltage which could be affected by crosslinking byproducts within the insulation of the material. Partial Discharge is a key tool to ensure the reliability and life extension of High Voltage electrical equipment. For cable interfaces, it is generally assumed that surface partial discharges create carbonized tracks at the contact between the cable insulation and the accessory body leading to breakdown. In this thesis, it was not possible to reproduce this condition. Breakdown occurred through a mechanism that remains to be unraveled. A possible connection with the shifts mentioned above, from homo to hetero charge at the interface, seems to exist.