Application of a Three-Level NPC Inverter

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Application of a Three-level NPC Inverter as a Three-Phase Four-Wire Power Quality Compensator by Generalized 3DSVM Ning-Yi Dai, Student Member, IEEE, Man-Chung Wong, Member, IEEE, and Ying-Duo Han, Senior Member, IEEE Abstract—A two-level four-leg inverter has been developed for the three-phase four-wire power quality compensators. When it is applied to medium and large capacity compensators, the voltage stress across each switch is so high that the corresponding causes large electromagnetic interference. The multilevel voltage source inverter topologies are good substitutes, since they can reduce voltage stress and improves output harmonic contents. The existing three-level neutral point clamped (NPC) inverter in threephase three-wire systems can be used in three-phase four-wire systems also, because the split dc capacitors provide a neutral connection. This paper presents a comparison study between the threelevel four-leg NPC inverter and the three-level NPC inverter. A fast and generalized applicable three-dimensional space vector modulation (3DSVM) is proposed for controlling a three-level NPC inverter in a three-phase four-wire system. The zero-sequence component of each vector is considered in order to implement the neutral current compensation. Both simulation and experimental results are given to show the effectiveness of the proposed 3DSVM control strategy. Comparisons with the - -0 hysteresis control strategy are also achieved. Index Terms—Three-dimensional space vector modulation (3DSVM), power quality, three-level neutral point clamped (NPC) inverter, three-phase four-wire system.



UE to the development of the “custom power” concept, three-phase four-wire systems will play a very important role in the distribution site. In past research, there are mainly two ways to provide neutral current compensation by two-level voltage-source inverters (VSIs): 1) using split dc-link capacitors and tying the neutral point to the mid-point of the dc-linked capacitors [1]–[3] and 2) using a four-leg inverter topology and tying the neutral point to the mid-point of the fourth neutral leg [4], [5]. For the medium and large capacity power quality compensators, the multilevel VSI topologies are good alternatives, among which the three-level inverter is the most promising one. The three-level structure not only reduces voltage stress across the switches but also provides more available vectors, which can improve harmonic contents of the VSI by selecting appropriate switching vectors [6], [7]. The decreasing voltage , which can stress leads to corresponding decrease of reduce the electromagnetic interference (EMI). Manuscript received September 27, 2004; revised August 29, 2005. Recommended by Associate Editor P. M. Barbosa. The authors are with the Department of Electrical and Electronic Engineering, University of Macau, Macau, China (e-mail: Digital Object Identifier 10.1109/TPEL.2005.869755

The three-level neutral-point-clamped (NPC) inverter, widely used in applications for a three-phase three-wire system, originally has the structure of split dc capacitors. So the existing dc neutral point can be directly utilized as the ground return. Actually, the three-level NPC inverter can be used in applications for a three-phase three-wire system and for a three-phase four-wire system. In this paper, comparisons between the threelevel four-leg NPC inverter and the three-level NPC inverter are given in Section II, and the three-level NPC inverter is chosen as a shunt power quality compensator for a three-phase four-wire system. When the VSI is applied to a shunt power quality compensator, it is the output current of the inverter that needs to be controlled. Various techniques of current control of pulse-width modulation (PWM) inverters have been studied and reported in literature [8], [9]. They can be classified into two...
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