In coming years there will be a need for ever more high voltage and extra high voltage cable connections and among the most important applications will be submarine cables to off shore wind parks. Moreover, long distance land transmission by cable and use of EHV cables to supply major urban centres will also become major factors in power system development. For short distances, such as when supplying a megalopolis, AC cable is the preferred solution. Here, cable design has moved decisively away from paper insulated and toward polymeric insulated types. By contrast, for longer distances and for numerous special applications there is now strong incentive to use HVDC cable systems. However, the decision between different insulation alternatives for these types of cable is more complex than for AC. For example, material selection will depend on the applicable converter technology (LCC or VSC), maximum voltage, cable weight, transmission capacity of the system and on the increasing influence of protecting the environment.
A review of different HVDC cable insulation materials highlights some of their relative advantages and disadvantages and also the main design requirements for their accessories:
The oldest technology – paper insulated, oil-filled cable – has had long and mostly positive service experience in AC as well as DC and is suitable for both converter types. Voltage level is up to 800 kV and cross-sections go to 3000 mm². When applied to HVDC, this insulation system has high power transmission capacity but problems include limitation in system length to about 80 km due to need for pumping stations while, in some cases, the lead sheath and oil in the cable are seen as drawbacks.
Mass impregnated, non-draining compound (MIND) insulation is another cable technology (see Fig. 1) that also has had more than 60 years of service experience but has no limitations when it comes to length. Voltage level can go up to 550 kV and cross-section diameter to 2500 mm², enabling high power transmission capacity. Disadvantages include the lead sheath, the impregnation compound and a maximum operating temperature of 55°C in the case of paper insulation. Switching from paper to paper/polypropylene laminate (PPL) allows increasing conductor temperature up to 85°C.
XLPE cable Insulation (see Fig. 2) has had relatively short service experience but is suitable for voltage source converters. Voltage level can go as high as 525 kV and cross-sections up to 2500 mm² with an operating temperature up to 90°C, thereby enabling high power transmission capacity. The main advantages are no lead sheath, no oil or mass-impregnating compound and no limitation in cable length.
High performance thermoplastic elastomer (HPTE) is a new polypropylene insulation that has already been used for years at medium voltages and also in 110 kV AC applications. The material has also been qualified since 2015 for DC cables up to 320 kV and cross-section diameters up to 2500 mm². Operating temperature can be as high as 90°C. The main advantages include no oil or mass impregnating compound, no lead sheath, no limitation in length, easier production without need for cross-linking and, unlike XLPE, no gas generated during vulcanization.
Despite only relatively short service history, extruded polymeric cable insulation represents the future not only for AC but also for HVDC systems. These types also offer the possibility to use pre-molded components for joints and terminations but the space charge phenomenon requires careful selection of materials.
For AC joints, silicone is the most widely used material at high voltages. But this same construction can cause problems in the case of DC joints. One solution is using EPDM, which, due to lower volume resistivity, reduces presence of space charges in HVDC joints, especially at 500 kV. Another solution, applied in 320 kV HVDC joints, is treatment of the silicone with nano-scale fillers. Design of terminations for HVDC polymeric cables can use existing material knowledge in joints when it comes to dimensioning their pre-molded stress control devices. Furthermore, termination design in general, especially for 500 kV XLPE cable, will increasingly shift from oil-filled porcelain insulators toward gas-filled composite housings.
Professor Klaus-Dieter Haim