Dual-Stage Control Strategy for a Three-Level Neutral Point Clamped Converter with Selective Harmonic Mitigation PWM

Grid-connected converters must meet the requirements imposed by grid codes, such as harmonic emission limits and grid voltage support during voltage dips. Selective harmonic mitigation pulsewidth modulation (SHMPWM) is a very interesting technique for high power converters to meet the maximum harmonic emission levels, while keeping a low switching frequency. However, the combination of this modulation with a proportional integral (PI) controller requires slow dynamics, which makes it difficult to comply with the dynamic response requirements of grid codes. As an alternative, model predictive control (MPC) offers a very fast dynamic response, but a wide spread harmonic spectrum in steady state. Thus, the combination of MPC with a PI controller with SHMPWM is advantageous. In this work, a dual-stage control strategy is implemented. During transients, finite control set MPC (FCS-MPC) is activated to rapidly drive the current to the desired reference, while in steady state, the PI controller with SHMPWM is used. Therefore, the dual-stage control strategy allows to comply with the two requirements of grid codes, becoming a suitable strategy for grid-connected converters.