a series transformer having primary windings configured to be connected in series with corresponding phase lines of the AC network, said series transformer having secondary windings connected to each other in a delta connection and thus forming respective delta branches;

a series converter with an AC side to connect to the AC network via said series transformer; and

a bridging configuration connected between said series transformer and said series converter and configured to bridge said series converter, wherein said bridging configuration contains three bridging branches each having a switching unit with antiparallel thyristors and a resistance connected in series with said switching unit, each of said bridging branches is disposed in parallel to a respective delta branch of said respective delta branches.