Fault handling

A method of fault handling for a power distribution network, the method comprising:

• during normal mode of operation, operating a voltage source converter to exchange electrical power with an AC system having multiple electrical phases (A, B, C) via an interface apparatus, the interface apparatus comprising a transformer comprising a set of primary windings connected to the electrical phases of the AC system and a set of secondary windings connected to the voltage source converter, wherein the multiple electrical phases comprises at least a first phase and one or more other phases with different phase offsets from the first phase;
• wherein the interface apparatus further comprises a fault module comprising an energy storage element connected in parallel with a passive resistive element providing a path for any DC currents entering the neutral point between a reference voltage and a neutral point of one of the set of primary windings or the set of secondary windings;
• the method further comprising monitoring, by a fault controller, for a fault and, upon detecting a fault:
• simultaneously issuing an open command to each of a plurality of the AC circuit breakers, one for each of the multiple electrical phases, wherein a first AC circuit breaker is associated with the first phase and one or more other circuit breakers are associated with the one or more other phases;
• and
• once the first AC circuit breaker is opened:
• causing a non-zero voltage to be developed across the energy storage element and the passive resistive element;
• inducing a current to flow in the one or more other electrical phases of the one or more other AC circuit breakers that have not yet opened that modulates current profile(s) of the one or more other phases;
• and
• opening one or more of the other AC circuit breakers after the one of the AC circuit breakers has opened and at a time when the one or more other AC circuit breakers experience zero-crossings in current.