Extended direct power control of a three-level Neutral Point Clamped Voltage Source Inverter with unbalanced voltages

This paper presents the control of a three- level Neutral Point Clamped (NPC) Voltage Source Inverter under unbalanced grid voltages. The control method used is the Extended Direct Power Control (EDPC), which is a generic approach for Direct Power Control of multilevel inverters based on geometrical considerations. It defines, for each inverter vector, two geometrical loci of no power change (a circumference for active power and a straight line for reactive power). Each locus divides the plane into two regions. The relative position of grid voltage vector in these regions determines whether the application of a certain inverter voltage vector will increase or decrease power. Besides, dc link middle point voltage is controlled by taking into account the switching state of the inverter (those states which make the middle point voltage changing). Finally, a decision algorithm chooses the inverter vector to be applied among those which fulfil the three requirements. In case of imbalances in grid voltages, negative sequence components appear and system variables start oscillating at twice the fundamental frequency, affecting the performance of the inverter. In fact, if power is kept constant, output current becomes distorted, presenting mainly third and fifth harmonic components. In order to obtain sinusoidal balanced output currents and thus improve inverter performance under voltage imbalances, power reference is compensated with oscillating terms so that negative sequence current components disappear. The whole control system has been tested on a three-level NPC Voltage Source Inverter connected to the grid and results confirm the validity of the method.