1. A control apparatus for a voltage source converter, the voltage source converter comprising an arm connecting an alternating current terminal to a direct current terminal, the arm comprising a valve and a first reactor, the valve comprising a plurality of sub-modules, a plurality of energy storage devices, and a plurality of switching elements, each of the plurality of sub-modules comprising one of the plurality of energy storage devices and at least one of the plurality of switching elements, the control apparatus comprising: a valve voltage control configured to control the plurality of switching elements to selectively connect the plurality of energy storage devices into the arm; a model module configured to generate a modelled value of a voltage of at least one of the plurality of energy storage devices, wherein the modelled value of the voltage of the at least one of the plurality of energy storage devices is based on: a measured output current of the valve; the control of the plurality of switching elements by the valve voltage control; and a capacitance value of at least one of the plurality of energy storage devices; and wherein the valve voltage control is configured to control the plurality of switching elements based on the modelled value of the voltage of the at least one of the plurality of energy storage devices.