Operating region extension for multilevel converters in HVDC applications by optimisation methods

Modular multilevel converters, based on cascading of halfbridge converter cells, can combine low switching frequency with low harmonic interference. They can be designed for high operating voltages without direct series connection of semiconductor elements. This has led to a rapid adoption within high-power applications such as high voltage direct current transmission. However, the overall hardware expenditure is generally greater than for conventional converters. Improving the operating range in the PQ-plane of the converter is thus desirable. In this work it is shown that optimisation of the internal variables of the converter can increase the power handling capability significantly for a given set of rating constraints. In this process optimal values for the circulating currents, the zero-sequence ac-side voltages and the average stored energy are sought. Also, a control algorithm is demonstrated by which the converter can be operated in the optimised operating point. Simulation results clearly indicate the feasibility of the proposed methodology.