Material properties of solid HVDC insulation systems

The numbers of HVDC systems, already installed and under consideration, are increasing throughout the world. From an insulation materials’ viewpoint, there are some significant differences in operation under AC and DC conditions. The purpose of this brochure is to highlight these differences and to provide a short overview of the material parameters that are essential in designing and evaluating solid HVDC (high voltage, direct current) insulation systems. Thus, the report is intended to be useful to those working in HVDC insulation design and diagnostics, who need to know the key HVDC material parameters. The brochure concentrates primarily on solid dielectrics; however, most of the properties are also of importance for liquid and gaseous dielectrics. The design of solid insulation for DC high voltage apparatus involves a number of formidable challenges that are absent or less frequently encountered in the design of AC apparatus [1], [2]. One fundamental challenge is the considerably higher electrical stresses involved, which come about from the compact nature of DC equipment/systems. In general it can be said that for AC insulation the electrical design is practically independent from the mechanical and thermal designs. However, in DC equipment, the electrical design cannot be separated from the mechanical and, in particular, the thermal design. A clear example is the strong effect of temperature on the conductivity of the insulation and its charging dynamics