A Modulation Scheme Based on Virtual Voltage Levels for Capacitor Voltage Balancing of the Four-Level Diode Clamped Converter

This article proposes a modulation scheme based on the concept of virtual voltage levels for regulating the dc-link capacitor voltages of a four-level diode clamped converter under restricted operating conditions. The proposed virtual-level space vector modulation (SVM) scheme sets the average neutral point currents to zero, which results in no net change in dc-link capacitor voltages and thereby voltage balancing. In addition, a closed-loop algorithm is developed to compensate for any voltage drift due to unideal practical conditions. Consequently, the modulation of the reference voltages results in the desired output voltages while regulating the dc-link capacitor voltages at various operating points. In the proposed modulation scheme, switching frequencies are not significantly increased, and they are lower than those in the classic virtual-vector SVM method. The simulation results demonstrate the effectiveness of the proposed modulation scheme and highlight its main benefits and features. The simulation results are verified through a laboratory-type experimental setup.