Thermo-Mechanical and Electrical Stress Analyses of HVDC Cable Joint with Load Cycle

The paper presents a simulation model based on Finite Element Method (FEM) for estimating the pressure at the interface of cable joints and electrical stress associated with the load cycle in a HVDC cable joint. A 320 kV HVDC cable joint has been considered for the study. The cable and joint body interface can be prone to damage because insufficient interfacial pressure leads to premature joint failure. Cross-linked Polyethylene (XLPE) and Silicone Rubber (SiR) are commonly used as cable and joint bodies. As these two materials’ thermal and electrical properties vary, it must be addressed at the interface under operation. A temperature gradient is established across the joint body associated with the ohmic loss because of the current-carrying conductor. Moreover, mechanical force at the interface varies with temperature. A time-varying load cycle is imposed on the cable conductor to replicate the practical operating scenario. Radial and hoop stresses are computed across the joint at different instances of the load cycle. It is observed that during the thermal loading, both these stresses increase at the interface. However, these stresses were reduced substantially during the cooling period. The variation of these stresses over the period of the load cycle may result in void formation, leading to damage at the interface. Moreover, the electrical performance of the joint associated with the rated voltage of 320 kV over different instances of the load cycle has been estimated. As in the case of DC, the field is governed by the conductivity of the materials; with temperature, the field varies drastically. The field inversion phenomenon is observed during the heating period of the load cycle, where the interface of XLPE-SiR is highly stressed compared to other parts of the joint body. From both the thermo-mechanical and electrical stress analyses, it can be confirmed that XLPE-SiR interface is a point of concern in the cable body, as stress enhancement is observed across it.