A cascaded flying capacitor multilevel converter for HVDC and FACTS

Over the last three decades, semiconductor devices like IGBT‘s (insulated gate bipolar transistors) and SIT‘s (static induction transistors) have been developed to reach high switching frequencies at considerable power levels. This opens up completely new research and development areas and fields of applications especially for power transmission and distribution systems such as high voltage direct current (HVDC) and Flexible AC Transmission systems (FACTS).
The limits concerning voltage and power can be pushed even further by placing these converter circuits in series. This paper presents a Cascade Flying Capacitor Multilevel Converter (CFCMC). The basic unit in this system is a 3-level full bridge flying capacitor converter. Each block switches between 0 to +Vdc and 0 to -Vdc at any point. One leg of a three phase CFCMC system can be formed as shown in Figure 1 by cascading these units. This circuit has some advantages when compared to the cascaded H-Bridge converter; the waveform performance is better due to the additional voltage level control, the stress upon switches is also lower. The presence of capacitors in each unit may seem to have cost implications, however each unit can handle higher power ratings therefore fewer units are required. The size of the capacitors required is also small when compared to those required for a standard flying capacitor multilevel converter. The particular merit of CFCMC also lies on its ability of supplying reactive power due to additional capacitors in each unit.
This will improve system stability margin especially when many large renewable sourced generators with low reactive power output are connected to the utility network.