WSL 口 Series de-energized tap-changer is used for oil-immersed transformer with no-load tap-changer. It is usually of cage type structure. The switch transition type is variable with its inner structure. It has linear regulating, single-bridge stridden over connection, double-bridge stridden over connection, Y一△ transition, series -parallel connected transition, positive or reverse regulation ate. This tap-changer is classified into two type, one called tank head type, the other called bell covered type. The operation method can be classified into two type, one is handled on the tank head or to the grounding, the other is driven with motor. They are used for power transformer, rectification transformer or furnace transformer in following specification range, rated frequency with 50Hz or 60Hz, highest voltage for equipment with 252kV and below, rated current with 250A~3000A. This type of tap-changer can only be operated in transformer under non-energized condition.