Design aspects on HVDC cable joints

The insulation of HVDC extruded cable system joints. Part 1: Review of materials, design and testing procedures

“…However, this is quite difficult, since the pre-molded joint body has to be a rubber to apply the proper surface pressure to the cable and avoid PDs and breakdown. This effect is shown in Figure 4 where the tangential electric field distributions in a simplified 150kV HVDC joint is simulated [22]: the joint with the material 2, with temperature dependence of conductivity more similar to XLPE than material 1, has a better tangential electric field distribution along the interface compared to material 1. Hence, a matching of the dielectrics and a reduction of the temperature dependence of the conductivity leads to a better design.…”

“…Field joints only ( Figure 2 [16,22]) are found in land cable interties, as cables longer than 1-1.5 km would require excessive weight and size of the drums needed to carry them on both roads and railroads [1]. Conversely, field joints are few in submarine cable links, where they are also called "repair joints" (Figure 3 [1]) because the same type of joint and installation procedure is commonly set up both for field and repair joints.…”

“…Factory joints, made in the factory in tightly-controlled conditions to connect the various cable core lengths, are much less prone to contamination and much more reliable than field Figure 2. Typical designs for field joints of the prefabricated type for HVDC land extruded cable systems; (a) design with geometric field grading (after [22]), (b) design with field grading material (FGM) layer (after [16]). joints.…”

“…The body of HVDC pre-moulded joints is made of three insulating or semiconducting layers, i.e. (Figure 2 [15,22]): 1) conducting inner deflector, made of semiconducting rubber, acting as live electrode and electric screen of the connector; 2) main insulation, made of insulating rubber; 3) conducting outer deflector/ screen, made of semiconducting rubber, acting as earth electrode. SiR and EPDM are the two most-common materials for the main insulation of pre-moulded joints.…”

“…Thus, SiR is mostly used for HVAC joints. For HVDC joints, EPDM is mainly used, since -due to its lower volume resistivity related to its more complex and compounded terpolymer structure -it reduces space charge accumulation in HVDC joints especially at higher voltage levels and at dielectric interfaces [22,24].…”