Abstract: A phase-shifting interconnection apparatus for interconnecting respective phase lines of first and second synchronous polyphase AC networks, includes for each pair of respective phase lines subjected to an interconnection a pair of circuit branches in parallel, having first and second common branch points provided with connecting means for connecting respectively the branch points to the respective phase lines, the circuit branches including respectively inductive and capacitive susceptances; and series transformer for phase-shifting a voltage applied to at least one of said susceptances, the series transformer including at least one primary winding having at least a portion connected in series with the at least one of the susceptances, and at least one secondary winding having at least a portion adapted to receive an excitation voltage derived from the AC networks, the excitation voltage having a phase angle with respect to a phase voltage provided by one of the respective phase lines, whereby, in operation,