FAULT HANDLING

This application relates to methods and apparatus for handling a fault associated with a voltage source converter (VSC) for exchanging electrical power between an AC system (101, 102) and a DC system (106-1, 106-2). The VSC (104) is connected to the AC system via an interface apparatus, comprising a transformer (107) with a set of primary windings (202) for coupling to a plurality of AC phases (A, B, C) of the AC system. In embodiments of the disclosure the set of primary windings having a neutral point (N) and the interface apparatus includes a fault module (301) having an energy storage element (302) connected in parallel with a resistive element (303) between the neutral point of the set of primary windings and a reference voltage, such as ground. The fault module does not interfere with normal operation but in the event of a phase-to-ground fault on the secondary side of the transformer, can induce earlier zero-crossing in the AC phases than otherwise would be the case, thus allowing AC breaker (108) to open with a reduced arcing time.