Multilevel converter with reduced AC fault handling rating

A method of controlling fault handling in a phase leg of a multilevel converter converting between alternating current and direct current, the phase leg having a first and a second phase arm, said first phase arm being connected between a first pole providing a first potential and a first AC terminal and the second phase arm being connected between the first AC terminal and a second potential, said phase leg comprising cells of a first type having a unipolar voltage contribution and cells of a second type having a bipolar voltage contribution, where each cell comprises at least one energy storage element for providing said voltage contribution and switching units comprising cell switches with anti-parallel diodes for controlling the voltage contribution, wherein the cells of the second type in the second phase arm each comprise a bypass switch controllable to bypass the corresponding energy storage element, the method comprising:

• upon the detection of a phase fault on the AC side of the multilevel converter,
• blocking cell switches of the cells in the phase leg;
• and
• activating the bypass switches to bypass corresponding energy storage elements,
• wherein the activating is made with a delay in relation to the blocking;
• and
• detecting a DC fault on a pole connected to an AC terminal via a phase arm, blocking the cell switches of the cells of the phase leg until a DC fault current is cleared, deblocking, after clearing of a DC fault current, the cells of another phase arm connected between the AC terminal and ground and controlling the deblocked cells to synthesize an AC voltage waveform for injecting reactive power into the AC terminal.