DC Capacitor Voltage Equalization in Neutral Clamped Inverters for DSTATCOM Application

Multilevel neutral point clamped (NPC) inverter systems are increasingly used in load compensation applications. However, the most significant problem associated with these compensators is the capacitor voltage imbalances and drift due to dc components in the zero sequence current, resulting in degradation of tracking performance of the voltage source inverter. This paper proposes a carrier-based pulsewidth modulation control for an inverter-chopper circuit in order to regulate the capacitor voltages to their reference values. To demonstrate the simplicity and effectiveness of the aforementioned control scheme, a three-phase four-wire three-level NPC compensator system is taken as an example. Detailed simulation has been carried out in PSCAD 4.2.1. Experiments are conducted to validate the proposed control scheme.