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Numerical analysis, on-site measurements, and laboratory experiments are used in this paper to analyze and solve the flashover problems of 330 kV AC high altitudes transmission line middle phase glass suspension insulator strings. A sub-model approach based on a finite element method (FEM) applied in calculating the potential and electric field distribution along the insulator strings under clean and dry conditions. Using this approach, three dimensional electrostatic models (taking into account the grading rings, subconductors, tower framework, and yoke plates) are set up and investigated. On-site and laboratory measurements were also carried out to make a comparison. A relatively good agreement was obtained among the calculated, the on-site, and the laboratory measured results, which demonstrated that insulator disk nearest to the high-voltage end is required to withstand relatively high voltage according to the standard in China; in addition, the insufficient dry arc distance of the insulator strings is also proposed. Subsequently, the long insulator-string flashover tests under dry, wet, and artificial contaminated (ESDD=0.05 mg/cm/2, NSDD=0.3 mg/cm/2) conditions, considering the middle (flashover) and side phase (never flashover), respectively, are carried out to validate the proposed insufficient dry arc distance and discuss the measures for solving the flashovers. From the effective, convenient and economical standpoint for the reconstruction of the flashover lines, lowering the grading rings' position and adding a unit of insulator are considered to lengthen the dry arc distance of the middle-phase insulator string in the tests. The experimental results are compared and discussed, and it has been concluded that adding a unit of insulator is an effective and economical way to solve the flashover problems. The methods for analyzing the flashover problems are effective and can be used for other voltage-level transmission lines.