A Multi-level Multi-Modular Flying Capacitor Voltage Source Converter for High Power Applications

Two vital and dynamically changing issues are arising in the electric grid - an increase in electrical power demand, and subsequent reduction in power quality. Power electronics based solutions such as the Static Synchronous Compensator are increasingly deployed to mitigate power quality issues while High Voltage DC Transmission converters are currently installed to support the existing grid transmission capacity. Both applications require high power and high voltage power converters using switching devices with limited voltage ratings. The advent of Modular Multilevel Converters (MMC) is one of the recent responses to this need. These use half or full H-bridge circuits stacked up to form a chain, and hence can withstand high voltages using lower-rated switching devices. This thesis introduces a new member into the MMC family, i.e the Modular Multi-level Flying Capacitor Converter (MMFCC). This uses a three-level flying capacitor full-bridge circuit as a sub-module and offers features of modularity, scalability and fault tolerance. The choice of FC topology in place of the simple H-bridge stems from the FC’s ability to offer two extra voltage levels in the sub-module output and hence more degrees of freedom per module in controlling the voltage waveform. A three-level full-bridge FC sub-module uses three capacitors - an outer one for supporting the sub-module voltage, and two inner floating ones with half of the outer one’s capacitance and voltage rating. This use of slightly more complex FC sub-modules gives the benefits of a modular structure but without using twice as many sub-modules with their associated capacitors for the same total voltage. The thesis presents the principles of this topology, switching states redundancies and a method for capacitor voltage balancing. Also discussed are: the configuration of MMCC including the MMFCC in Single-Star Bridge-Cell (SSBC) or Single-Delta Bridge-Cell (SDBC) for FACTS and Battery Energy Storage System (BESS) applications; and Double-Star Chopper-Cell (DSCC) or Double-Star Bridge-Cell (DSBC) for HVDC systems. A novel overlapping hexagon pulse width modulation scheme is introduced and discussed for switching control of the MMFCC. This uses multiple hexagons all centred on one point, the same in number as the cascaded FC sub-modules, which are phase displaced relative to each other. The approach simplifies the modulation algorithm and brings flexibility in shaping the output voltage waveforms for different applications. An MMFCC experimental rig was designed and built in-house to validate some of the simulation results obtained for the modulation of this new topology. Details of the rig as well as results captured are discussed.