440 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 21, NO. 2, MARCH 2006
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 re-
duce voltage stress and improves output harmonic contents. The
existing three-level neutral point clamped (NPC) inverter in three-
phase three-wire systems can be used in three-phase four-wire sys-
tems also, because the split dc capacitors provide a neutral connec-
tion. This paper presents a comparison study between the three-
level four-leg NPC inverter and the three-level NPC inverter. A fast
and generalized applicable three-dimensional space vector modu-
lation (3DSVM) is proposed for controlling a three-level NPC in-
verter in a three-phase four-wire system. The zero-sequence com-
ponent of each vector is considered in order to implement the neu-
tral current compensation. Both simulation and experimental re-
sults are given to show the effectiveness of the proposed 3DSVM
control strategy. Comparisons between the 3DSVM and the 3-D
hysteresis control strategy are also achieved.
Index Terms—Power quality, three-dimensional space vector
modulation (3DSVM), three-level neutral point clamped (NPC)
inverter, three-phase four-wire system.
I. INTRODUCTION
D
UE to the development of the “custom power” concept,
three-phase four-wire systems will play a very important
role in the distribution site. Past research shows that 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 compen-
sators, 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
Manuscript received September 27, 2004; revised August 29, 2005. Recom-
mended by Associate Editor P. M. Barbosa.
N.-Y. Dai and M.-C. Wong are with the Department of Electrical and
Electronics Engineering, University of Macau, Macau, China (e-mail:
ya37404@umac.mo).
Y.-D. Han is with the Department of Electrical and Electronics Engineering,
University of Macau, Macau, China and also with the Department of Electrical
Engineering, Tsinghua University, Beijing, China.
Digital Object Identifier 10.1109/TPEL.2005.869755
appropriate switching vectors [6], [7]. The decreasing voltage
stress leads to corresponding decrease of
, which can
reduce the electromagnetic interference (EMI).
The three-level neutral-point-clamped (NPC) inverter, widely
used in applications for a three-phase three-wire system, orig-
inally 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 applica-
tions for a three-phase three-wire system and for a three-phase
four-wire system. In this paper, comparisons between the three-
level 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 compen-
sator, 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 large
classes: on–off control and predictive control. In on-off control
schemes, currents are compared to their reference using hys-
teresis comparators to determine the switching instants for the
inverter power switches. The on–off control is characterized
by a fast response but the current ripple is relatively large. In
addition, its switching frequency depends on system parameters
and operating conditions. On the other hand, in a predictive
control scheme the switching instants of power switches are
determined through calculating the required voltage to force the
output currents to follow the references. This control scheme
provides constant switching frequency and lower current
ripples. However, injecting circuit parameters and operating
conditions have to be known with sufficient accuracy in the
predictive control strategy.
Space vector modulation (SVM) techniques have been widely
used in predictive control for shunt power quality compensators
in a three-phase three-wire system, as this PWM technique can
reduce commutation losses and the harmonic contents of output
voltage, and can obtain higher amplitude modulation indexes
[5], [6]. Usually, SVM can increase 15% of dc voltage utiliza-
tion compared with sinusoidal PWM. Moreover, space vector
modulation techniques can be easily implemented in digital pro-
cessors.
Corresponding to the adoption of the inverter structure with
neutral-wire, three-dimensional (3-D) PWM techniques should
be developed for applications in a three-phase four-wire system.
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