1. A control apparatus for a voltage source converter, the voltage source converter comprising a plurality of energy storage devices which can be selectively connected into an arm of the voltage source converter, the control apparatus comprising: a model module configured to generate a modelled value of the voltage of at least one of the energy storage devices of the voltage source converter based on one or more model parameters and the operation of the voltage source converter; a model adaptation module configured to receive the modelled value and also a measured value of the voltage of the at least one energy storage device, to determine an error between said modelled value and said measured value, and to generate a model control signal for adapting the model based on said error so that the modelled value substantially corresponds to the measured value; and control circuitry responsive to said model module and/or said model adaptation module.