On load regulating transformer is equipped with on-load tap-changer. The SZ transformers are leaded out several taps from the winding and connected with the on load tap changer; promise not to cutoff the load current circumstance to alternate the tap position and change the effective turns. That is to change the transformer voltage ratio to reach the output voltage invariable. The energy in the tapping period will be absorbed by transition resistor to limit the circulation current.