In a electric network of nongrounded neutral system electromagnetic potential transformer （PT） may bring about ferro-resonance overvoltage

and then a phenomenon of virtual grounding may appear in that network

A new controller is developed to solve this problem. The action of the controller is as follows. During the PT operating normally

the electronic switch of the controller is on turn off state

the resistance of resonace elimitation is out of acting. the virtual grounding occurring an order from the controller brings the electronic switch conducting to switch the resistance of resonance elimitation on. the resistance is used to consume resonance energy and to restrain the overvoltage occurred in this power network.