Capacitor Voltage Balance Control of Five-Level Modular Composited Converter With Hybrid Space Vector Modulation

Abstract

Multilevel converters are widely employed in the 30–50 kW medium-voltage grid-connected photovoltaic (PV) generation systems due to their higher power quality and lower switching losses compared with the two-level converters. Dual T-type three-level converters and one three-level diode neutral point clamped circuit can be integrated to derive a simplified five-level modular composited converter (5L-MCC) for the medium-voltage PV grid-connected systems. However, realizing the capacitor voltage balance is a challenge to enhance its reliability. First, the model of dc capacitor voltage variation of the 5L-MCC is discussed to expose a limitation once the nearest three-vector synthesis method is employed with conventional space vector modulation (SVM). In order to extend the operational range, a hybrid space vector modulation (HSVM) strategy is proposed, which integrates the optimized SVM switching sequence for the low-modulation ( $M\, ) and a simplified vector synthesis method for the high-modulation region ( $M\geq 0.5$ ). The proposed hybrid modulation maintains the dc capacitor voltage balance over the full modulation range and power factor. Finally, the effectiveness of the proposed HSVM strategy is verified with the simulation and experimental results.