Hybrid Modulation Technique of MV Grid-Connected Cascaded H-Bridge-Based Power Electronic Transformer Technology

Due to advancements in semiconductor devices, power electronic transformer (PET) technology is becoming the most attractive solution for direct integration of MVac grid with LVac/LVdc grid in battery charging stations, wind and solar power generation firms, etc. The MV grid-connected converters introduce harmonics in grid current (poor current THD) due to the effect of dead time and harmonics present in grid voltages especially at lower load currents. This article proposes an easy and effective closed-loop control architecture with a hybrid modulation technique for MV stage of the cascaded H-bridge (CHB)-based three-stage (MV/dc-dc/LV) PET to improve the current profile of the MV side converter. In the proposed technique, one H-bridge cell in each phase is operated using hysteresis current control, whereas the remaining cells are operated in vector control mode and switched using phase-shifted SPWM. In this way, the proposed strategy maintains sinusoidal MV grid current even under light load conditions, resulting in improved current THD. It also reduces the magnitude of no-load current, resulting in improved no-load losses. A PWM shuffling logic is also introduced among CHB cells to ensure balanced switching losses. Experimental validation at different light loads, on 1.2 kV (MVac)/400 V (LVdc)/200 V (LVac), 20 kW CHB-based PET prototype confirms the effectiveness of the proposed technique.