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In this paper, the dc pollution flashover performance of various types of porcelain, glass, and composite insulators is investigated. It also presents analysis of the dc flashover process of polluted insulator string at high altitude using insight from high-speed photography. The research results indicate that the relationship between the dc pollution flashover voltage and the string length of insulators is basically linear, the characteristic exponents describing the influence degree of air pressure on pollution flashover voltage vary between 0.35 and 0.77 and are related to the insulator types and pollution degree, etc., the characteristic exponents describing the influence degree of pollution on flashover voltage vary between 0.24 and 0.36 and are related to the insulator types and air pressure, etc. Based on the flashover phenomena using the insight from high-speed photography, a new physical model explaining the flashover mechanism for a polluted insulator string at high altitude is introduced, which can be expressed as an electrical circuit consisting of a surface arc of length x1 and air-gap arc of length x2 in series with a resistance representing the wet pollution layer. In addition, the exponent n describing the influence degree of air pressure on flashover voltage for the polluted insulator is discussed.