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