Research on the control algorithm of DC voltage for active filter with a three-level NPC inverter

Harmonic pollution is regarded as being one of the major problems that degrade electric power quality. Control of harmonic perturbations by active power filters (APF) has become a hot topic in the power engineering field. But due to the limitation of the voltage capability of the power devices, it is very difficult to handle the nonlinear loads for the traditional active power filter with two-level inverter in high voltage grid. In recent years, multilevel technology has become an effective and practical solution for high-voltage high-power application field. As described in many literatures, using multilevel technology, the voltage stress on switches will be reduced, the shape of output waveform will be improved and the rate of voltage and power can be increased too. Diode-clamped three-level inverter is the most widely used and investigated topology at present. This paper adopts an active power filter with a three-level NPC inverter. This APF is very suitable for high voltage system. The paper describes the theoretic bases of the three-level APF. This paper presents a close loop control algorithm of DC voltage for the three-level APF. The mathematic model of the three-level APF is set up. The PI controller regulates DC voltage in the outer-loop controller. In the current-loop controller, this paper researches the issues of neutral-point voltage control from the space vector PWM(SVPWM) point of view in detail. The conditions that the vectors affect the neutral-point voltage balance are summarized. Then this paper proposes a simple neutral-point voltage control method which requires the knowledge of the current direction in each phase and the neutral-point voltage ripple's direction. Based on that information, the small vectors that will move the neutral-point voltage in the direction opposite from the direction of unbalance can be selected. The simulation results illustrate that the performance of the proposed approach is satisfactory.