Modulation and Losses of Modular Multilevel Converters for HVDC Applications

The main advantage of a modular multilevel converter is the very low total harmonic distortion. Even a very high modulation frequency leads to only a moderate switching frequency per IGBT and therefore, compared to other converter topologies very low switching losses. However, this low switching frequency per submodule in combination with a high output modulation frequency leads to an uneven loss distribution between the submodules. With an average of only six to eight switching events per AC period the distribution of conduction and switching losses within a submodule can lead to unbalanced semiconductor temperatures and additional temperature cycles. Based on simulation results, this paper investigates the single module losses of a modular multilevel converter for HVDC applications with a high submodule count per phase and a low switching frequency per submodule.