Voltage source converter and control thereof

Abstract

Methods and apparatus for hard switching of Alternate-Arm-Converter voltage source converters. Such voltage source converters have a phase limb with a high and low side converter arm connecting an AC terminal to a high and low side DC terminal, respectively, including a chain-link circuit in series with a director switch. Each chain-link circuit includes series connected cells that can be switched to generate a controlled voltage across the chain-link circuit. In embodiments, a controller turns-off the director switch of a converter arm that is conducting current in response to a hard-switching request for a first phase limb. In response to a hard-switching request, the controller controls the chain-link circuits of the first phase limb at any point in a phase cycle to control: a DC voltage across the director switch and/or the current flowing through the director switch to a predetermined level before turning the director switch off.