Low-switching-frequency active damping methods of medium-voltage multilevel inverters

High power converters adopted in oil and gas applications or in wind turbine applications can benefit from the introduction of medium voltage multilevel inverters, in terms of volume and efficiency. The low switching frequency lead to currents characterized by an high harmonic content despite the use of multilevel technology, hence to high filtering needs. The use of LCL-filter is the solution to limit the size and cost of inductors. However to avoid possible resonance, passive damping is adopted leading to a decrease in the efficiency. Alternative solutions consist in the use of selective resonant damping or in active damping. These solutions are reviewed in this paper. Particularly the latter are difficult to implement due to the limited sampling frequency, typically double of the switching frequency. All the solutions are compared in terms of harmonic content and losses.