Patent ID: 8289068

Claim:
A method of uninterrupted changeover between winding taps of a tapped transformer with two load branches each connectable with a common load output line by a respective mechanical switch and a respective series circuit in series therewith and consisting of two oppositely connected IGBTs, a respective diode being connected in parallel with each IGBT, a respective varistor being provided in parallel with each IGBT and each of the two load branches being bridgeable by a mechanical latching main contact, the method comprising the following steps: closing the mechanical switches of the two branches, applying ignition voltage to gates of the IGBTs of the branch switching off and thus switching on those IGBTs, opening a latching main contact of the branch switching off, diverting load current to the IGBTs of the branch switching off, switching off the IGBTs of the branch switching off and switching on the IGBTs of the branch being switched on so as to hard switch off the IGBTs of the branch switching off, thereafter diverting the load current to the varistors of the branch switching off, thereafter diverting the load current to the IGBTs of the branch taking over, closing the latching main contact of the branch taking over, switching off the IGBTs of the branch taking over and opening the mechanical contacts of the two branches.