Observer based monitoring and control of submodules in modular multilevel converter

This application relates to control methods and apparatus for a voltage source converter (100) comprising a plurality of energy storage devices (202) which can be selectively connected into an arm (301) of the voltage source converter. The apparatus comprises a model module (304) configured to generate a modelled value related to a first electrical parameter of the voltage source converter based on one or more model parameters and the operation of the voltage source converter. The apparatus further includes a model adaptation module (305) configured to receive the modelled value (VS) and also a measured value (VCap) related to the first electrical parameter, to determine an error between said modelled value and said measured value, and to generate a model control signal (Mcon) for adapting the model based on said error so that the modelled value substantially corresponds to the measured value. Control circuitry (303) is responsive to said model module and/or said model adaptation module to control operation of the VSC.