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To solve the contamination flashover problem for transmission lines in high-altitudes area, an analysis of the {lashover performance of contaminated ± 800 kV ultra high voltage (UHV) DC composite insulators of various suspension modes at a high altitude (2 100 m) is presented. The 50% withstand voltage (U60%) was determined by the up-and-down method. Experimental results show that suspension modes play an important role in the flashover performance since DC arcs detach from the insulator surfaces more easily at high altitudes than at sea level. Flashover voltages of the long string composite insulators gradually decreased with salt deposit density (SDD). The flashover voltage of type II string is by 4.6% lower than that of type I string. For type V string, the ionized air could more easily diffuse away from the insulator surface, while producing a higher utilization ratio of the leakage distance. Therefore the flashover voltage of type V string is by 8.7 %-14.4% higher than that of type I string. Type Y string configuration appears to lead to much lower flashover voltages than type I and V string configurations.