Decoupled capacitor voltage control of modular multilevel converters

Modular multilevel converters have several attractive features such as high voltage capability and scalability, the excellent quality of the output voltage waveforms, a modular structure and the utilisation of standard components. Despite of all these benefits, the main drawback of this converter is the control system complexity due to the multiple external and internal control objectives. One of these objectives is the control of the capacitor voltages which is usually divided into average voltage control and balancing voltage control. This control scheme cannot manage independent voltage references, as required for example in photovoltaic applications, where each arm capacitor voltage must be controlled independently to achieve maximum power point tracking. This paper proposes a voltage control strategy of a MMC capable to manage the average capacitor voltage of each arm independently. Results validating the performance of the proposed control scheme are shown.