Novel 3-DSVM scheme for three-phase four-wire tri-level APFs

This paper main focus on the 3-D modulation scheme and neutral-point unbalance problem of 3-level 3-leg converter for active power filters. The neutral point unbalance problem is a key issue for this topology based APFs. To date there hasn´t any perfect 3-D modulation solutions to tackle with neutral point unbalance problem well. In this paper, a detailed discussion on this issue is carried out. The relationship between the neutral point potential and the relative voltages and currents is analyzed in this paper. Then a new concept named factor of neutral-point balance (Fn) is proposed and the calculation formula is derived. Basing on Fn, a novel 3-D modulation scheme is proposed in a-b-c coordinates. By measuring the output currents and DC voltages, Fn is calculated in every sampling period and the most fit space vectors are selected to approximate the reference vector. In addition, a concept called neutral point control grade (NPCG) and another concept named average-NPCF (A-NPCG) are utilized in this paper to evaluate the control capability of neutral point potential control scheme. The simulation and experiment results verified the correctness and validity of this novel 3DSVM scheme.