EMF modeling HVDC HVDC outdoor insulators History of engineering & technology SF6 Technology: general issues VFTO anti-icing arc propagation artificial clean fog test atmospheric icing characteristics extraction coatings composite crossarm core diameter de-icing discharges double exponential function dry bands dry-band arcing dynamic pollution model elastomer electrical properties energy-controlled switch entropy threshold segmentation epoxy resin equivalent salt deposit density (ESDD) erosion performance erosion resistance filtered surge arrester