The test results show that icing flashover voltages of 28-unit porcelain suspension insulators and composite insulators are, respectively, 20.5 and 18.7% lower than the rated operating voltage under the contamination level of rho(ESDD)/rho(NSDD) (equivalent salt deposit density/non-soluble deposit density) = 0.1/0.5 mg/cm(2) and severe icing conditions. When using alternate-diameter insulators in the same contamination level, the results are 21.2% higher than the rated operating voltage under moderate icing conditions, but 15.7% lower under severe icing conditions. For the same time period of icing, the maximum withstand voltages (the prospective value of the test voltage that the equipment is capable of withstanding when tested under specified conditions) of V-strings are 30% higher than the rated operating voltage under rho(ESDD)/rho(NSDD) = 0.1/0.5mg/cm(2), which have a much better icing flashover performance than the suspension insulator strings. Moreover, the discharge processes of standard insulator string and alternate-diameter insulator string were comparatively analyzed. A large number of air gaps present on the latter ones could be the reason for a better icing flashover performance than standard ones. But severe icing conditions can shorten the width of the air gaps and decrease the flashover voltage greatly. (C) Koninklijke Brill NV, Leiden, 2012