Quasi-2/3-Level Modulation of Multilevel Converters for Medium-Voltage Applications

Silicon Carbide (SiC)-based power converters can reach higher power density than the Silicon (Si)-based. The voltage capability of SiC is higher than that of the Si devices. Hence, SiC-based converters can be used in MV applications. In contrast, the MV infrastructure has voltage levels up to 100 kV.  Whereas a simple 2-level SiC converter can be used for some grids, it is impossible to use this configuration for the grids with higher voltage. Multilevel (ML) converters do not solve the issue. Higher number of levels increases the number of required capacitors, operation and control complexity, communication burden and overall cost.

An alternative solution is to partially utilize available voltage levels, forcing ML converters to operate in 2-/3-Level manner. This approach is called Quasi-2-Level (Q2L) or Quasi-3-Level (Q3L) modulation, respectively. The main advantage of such modulations is that the intermediate capacitors are only used to ensure equal voltage distribution between the SiCs. Therefore, they contribute insignificantly to the total converter volume, allowing to increase the voltage levels without affecting the volume and operation complexity.

On the other hand, the limitations of Q2L and Q3L concepts need to be investigated. It is essential to determine how the number of series-connected SiC devices affects the performance of quasi-modulated converters. Furthermore, the capacitor selection needs to be done based on the voltage and current ripples. Due to very small capacitance, capacitor voltages must be constantly maintained around the reference points. In contrast, it is possible to take advantage of such a drawback. Intentionally changing the voltage across a single capacitor makes it possible to redistribute the voltage sharing between two adjacent semiconductors. This feature can be used to micro-adjust the power losses across all switching devices in the converter arm and to perform active thermal control.

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