Observer-based Direct Power Control for three-level NPC rectifiers

In this paper we propose a direct power controller (DPC) for three phase three-level neutral point clamped (NPC) rectifiers. This type of converter can operate as rectifier or inverter; as rectifier, two capacitors are placed at the dc output in order to produce smooth voltage, while reducing the size of the capacitors themselves. The control objective becomes twofold. From the power management perspective, reactive power must be driven to zero and active power to a pre-specified set-point, while at voltage level, the charge must be equally balanced between both capacitors, taking their voltage difference as a control variable to be stabilized at zero. The power control algorithm introduces a complex nonlinear term in the capacitors voltage dynamics. In this paper, it is shown that this term can be approximated by a third-order harmonic signal that can be estimated by a Luenberger state observer. The estimation is used to cancel the nonlinear term and control the capacitors voltages.