Control of a hybrid modular multilevel converter during grid voltage unbalance

The recently proposed parallel hybrid modular multilevel converter is considered to be a low loss, low component count converter with soft switching capability of the `main´ bridge. The converter has similar advantages to other emerging modular multilevel converter circuits being considered for HVDC power transmission. However, during ac network unbalance the individual `chain-links´ exchange unequal amounts of power with the grid which requires appropriate remedial action. This paper presents research into the performance of the converter and proposes a suitable control method that enables the converter to operate during grid voltage unbalance. The proposed control concept involves the use of asymmetric third harmonic voltage generation in the `chain-links´ of the converter to redistribute the power exchanged between the individual `chain-links´ and the grid. Mathematical analysis and simulation modelling with results are presented to support the work described.