the transformer with load voltage- regulating tap switches called on -load regulating transformer. Under the condition of not cutting load current, through tap switch, number of sub-jjoints will be led out from the transformer windings to change to change the sub-joint position and change its effective turn. that is ,no change of the output voltage through changing the vo;tage ratio of the transformer. in the process of change and sub-jointing , cycling current is absorbed and limited by transition resistance.