A Hybrid Modular Multilevel Voltage Source Converter for HVDC Power Transmission

HVDC transmission systems are becoming increasingly popular when compared to conventional ac transmission. HVDC voltage source converters (VSCs) can offer advantages over traditional HVDC current source converter topologies, and as such, it is expected that HVDC VSCs will be further exploited with the growth of HVDC transmission. This paper presents a novel modular multilevel converter hybrid VSC intended for the HVDC market. The concept of the converter operation is described based on steady-state ac-dc power balance. Techniques for dynamic voltage control, enabling the active and reactive powers exchanged with the grid to be controlled, are introduced. Simulation results further illustrate the theory of operation of the converter and confirm the viability of the proposed control approaches. Detailed predictions of the semiconductor losses confirm the potential to achieve very high efficiencies with this topology. Experimental results are provided to validate the presented converter operation.