Deadbeat control system with novel neutral-point balancing scheme for Active Power Filters

In this paper a deadbeat control system with a novel 3-dimension (3-D) space vector modulation scheme for 3-phase 4-wire tri-level three-leg Active Power Filter (APF) is proposed. To improve the compensation effect a repetitive harmonic predictor is utilized to predict the harmonic current and a state observer is used to estimate the output currents of the APF at the sampling instants in the deadbeat control system. For a 3-phase 4-wire tri-level three-leg APF the direct current (DC) bus neutral-point unbalancing is an inherent problem. To date there has not been an effective method to solve this problem. In this paper, a detailed discussion of neutral-point unbalancing problem is presented and a conception named Neutral-Point Control Grade (NPCG) is proposed for the first time, to describe the affected extent of the neutral-point voltage by the space vectors. Then a neutral-point controllable 3-D space vector modulation scheme, which is based on the NPCGs, is proposed in alpha-beta-0 coordinates. The simulation result verifies the correctness and effectiveness of the proposed control strategy. A comparison focusing on the neutral-point voltage between the proposed NPCGs based 3-D modulation scheme and another advanced 3-D modulation scheme is also presented in this paper.