Diagnostics and accelerated life endurance testing of polymeric materials for hvdc application - Cigré - WG D1.23

There is a renewed and growing interest in high voltage direct current (HVDC) transmission and particularly in the use of polymeric materials as insulation in HVDC equipment. However, many polymers show a non-linear behaviour when they are stressed with DC voltages, a phenomenon which is further amplified by the temperature dependence of the resistivity. Space charges may accumulate that can increase the local electric field and may cause premature failure of the insulation. Thus, space charge formation and dynamics play an important role in the ageing of polymers under HVDC. Partial discharges may occur at dc stress as well and they will be influenced by the space charge formation. Evidently, testing and diagnostics of polymeric materials considered for use in HVDC applications need to take into account the above mentioned phenomena. There are however few standards or technical brochures currently available that tackle this topic. Therefore, the main aim of this document is to describe the techniques used for quality control of materials and combinations of materials for HVDC applications in the factory and on-site, from models of insulation systems to full-size equipment and suggest new techniques where they are deemed to be important. The approach taken by the WG is first to examine the fundamental processes of electrical ageing of polymeric insulation under dc stress. Then, an overview of diagnostic properties is given to probe the effect of these ageing processes and to determine insulation quality. In Chapter 3, the document focuses on space charge and, more in particular, on the relation between chemical composition and stability and the presence of space charge. The interaction between space charge and polymer ageing is discussed in detail. Chapter 4 deals with HVDC testing, focusing on the tests used for HVDC cable systems from the design phase, via prequalification and type testing to routine testing and diagnostic measurements in service. Although many measurement techniques are now available, diagnostic tests that can be used online are virtually non-existent. In Chapter 5, stressing the need for further development in this area, a number of possible future testing methods is presented. Finally, some conclusions are drawn in Chapter 6. In Annexes 1 and 2, currently used practices are described for the measurement of space charge on full-size cables, respectively the test conditions used in Japan for the qualification of polymeric materials for HVDC applications.