Extended direct power control for multilevel inverters including DC link middle point voltage control

Abstract

A novel extended direct power control (EDPC) algorithm for multilevel inverters is presented. Based on the well known direct power control (DPC) of two-level inverters, DPC has been extended to multilevel inverters using geometrical considerations of inverter voltage vectors based on active and reactive power at the output. Voltage control at the middle point of DC link is based on the same considerations for direct control. Thus, three hysteresis controllers are used to control active power, reactive power and voltage at the middle point of the DC link. The algorithm inputs are power and voltage references and also line voltages and currents. On account of the possibility of redundancy of inverter voltage vectors in fulfilling control requirements, a decision algorithm has also been developed. Several optimisation criteria can be implemented, depending on different control priorities, for example, active and reactive power control, middle point voltage control, minimisation of switchings, and so on. The algorithm has been applied to a three-level neutral point clamped voltage source inverter (VSI) connected to the grid and results show the good performance of the proposed control method in terms of improving the dynamic response of multilevel VSI.