Inside the main transformer, the lead wire of the high-voltage side contact leads to an open-circuit tap changer. The movable contact has two pairs of parallel contacts, which are tightly pressed on the fixed point by the spring; The connecting rod is matched with the external istismar rod through gear coupling, and the operating rod is connected to the lower operating box of the transformer. Operate the operating handle in the tap control box, through the cooperation of the bevel gears of each component, to reach the position of operating the moving contact of the open-circuit tap changer inside the main transformer.