Five-Level Virtual-Flux Direct Power Control for the Active Neutral-Point Clamped Multilevel Inverter

Although multilevel inverters present numerous advantages such as high quality waveform, low switching losses, high voltage capability and low electromagnetic compatibility concerns, some drawbacks are evident. They require a higher number of semiconductors and either multiple isolated dc sources or a bank of series connected capacitors. Consequently, the control complexity increases considerably, since more switching devices normally result in a higher number of possible combinations and the balance of the capacitors has to be guaranteed. But on the other hand, multilevel inverters create an extra degree of freedom due to existing redundant voltage vectors, which produce the same output phase voltage level but with diverse effect on the de-link and floating capacitors. Among the existing control techniques the Virtual-Flux Direct Power Control (VF-DPC) has showed to be very suitable for grid connected systems since it controls the active and reactive powers directly without any internal current control loop or PWM modulator. However, in order to adapt the VF-DPC for multilevel systems, specifically for the recently proposed five-level Active Neutral-Point Clamped converter, additional features must be included and/or modified in the inner main control loop. In order to allow the controller to select a higher number of available voltage vectors, the active and reactive power hysteresis strategies are modified. Additionally, a method to balance the dc-link and floating capacitor voltages by applying available redundant states is implemented, based on the actual condition of the voltage across the lower dc-link and floating capacitors, as well output phase currents direction. The proposed five-level VF-DPC has been implemented using a 6kW five-level prototype and has shown good static and dynamic performance.