An Effective Control Method for Quasi-Z-Source Cascade Multilevel Inverter-Based Grid-Tie Single-Phase Photovoltaic Power System

1. www.projectsatbangalore.com 09591912372
IEEE TRANSACTIONS ON INDUSTRIAL INFORMATICS, VOL. 10, NO. 1, FEBRUARY 2014 399
An Effective Control Method for Quasi-Z-Source
Cascade Multilevel Inverter-Based Grid-Tie
Single-Phase Photovoltaic Power System
Yushan Liu, Student Member, IEEE, Baoming Ge, Member, IEEE, Haitham Abu-Rub, Senior Member, IEEE,
and Fang Z. Peng, Fellow, IEEE
Abstract—An effective control method, including system-level
control and pulsewidth modulation for quasi-Z-source cascade
multilevel inverter (qZS-CMI) based grid-tie photovoltaic (PV)
power system is proposed. The system-level control achieves the
grid-tie current injection, independent maximum power point
tracking (MPPT) for separate PV panels, and dc-link voltage bal-
ance for all quasi-Z-source H-bridge inverter (qZS-HBI) modules.
The complete design process is disclosed. A multilevel space vector
modulation (SVM) for the single-phase qZS-CMI is proposed
to fulfill the synthetization of the step-like voltage waveforms.
Simulation and experiment based on a seven-level prototype are
carried out to validate the proposed methods.
Index Terms—Cascade multilevel inverter (CMI), photovoltaic
(PV) power system, quasi-Z-source inverter, space vector modula-
tion (SVM).
I. INTRODUCTION
Arecent upsurge in the study of photovoltaic (PV) power
generation emerges, since they directly convert the solar
radiation into electric power without hampering the environ-
ment. However, the stochastic fluctuation of solar power is in-
consistent with the desired stable power injected to the grid,
owing to variations of solar irradiation and temperature. To fully
exploit the solar energy, extracting the PV panels’ maximum
power and feeding them into grids at unity power factor be-
come the most important. The contributions have been made by
the cascade multilevel inverter (CMI) [1], [2]. Nevertheless, the
H-bridge inverter (HBI) module lacks boost function so that the
Manuscript received October 17, 2012; revised January 31, 2013; accepted
August 20, 2013. Date of publication August 29, 2013; date of current version
December 12, 2014. This work was supported by the Qatar National Research
Fund, a member of Qatar Foundation, under Grant NPRP-EP X-033-2-007.
Paper no. TII-12-0726.
Y. Liu is with the School of Electrical Engineering, Beijing Jiaotong Uni-
versity, Beijing 100044, China, and also with the Department of Electrical and
Computer Engineering, Texas A&M University at Qatar, Doha 23874, Qatar
(e-mail: yushan.liu@qatar.tamu.edu).
B. Ge is with the School of Electrical Engineering, Beijing Jiaotong Uni-
versity, Beijing 100044, China, and also with the Department of Electrical and
Computer Engineering, Michigan State University, East Lansing, MI 48824
USA (e-mail: gebaoming@tsinghua.org.cn; bm-ge@263.net).
H. Abu-Rub is with the Department of Electrical and Computer En-
gineering, Texas A&M University at Qatar, Doha 23874, Qatar (e-mail:
haitham.abu-rub@qatar.tamu.edu).
F. Z. Peng is with the Department of Electrical and Computer Engineering,
Michigan State University, East Lansing, MI 48824 USA (e-mail: fzpeng@egr.
msu.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TII.2013.2280083
inverter KVA rating requirement has to be increased twice with
a PV voltage range of 1:2; and the different PV panel output
voltages result in imbalanced dc-link voltages. The extra dc–dc
boost converters were coupled to PV panel and HBI of the CMI
to implement separate maximum power point tracking (MPPT)
and dc-link voltage balance [3], [4]. However, each HBI module
is a two-stage inverter, and many extra dc–dc converters not
only increase the complexity of the power circuit and control
and the system cost, but also decrease the efficiency.
Recently, the Z-source/quasi-Z-source cascade multilevel
inverter (ZS/qZS-CMI)-based PV systems were proposed in
[5]–[8]. They possess the advantages of both traditional CMI
and Z-source topologies. For example, the ZS/qZS-CMI: 1)
has high-quality staircase output voltage waveforms with
lower harmonic distortions, and reduces/eliminates output filter
requirements for the compliance of grid harmonic standards;
2) requires power semiconductors with a lower rating, and
greatly saves the costs; 3) shows modular topology that each
inverter has the same circuit topology, control structure and
modulation [1], [2]; 4) most important of all, has indepen-
dent dc-link voltage compensation with the special voltage
step-up/down function in a single-stage power conversion of
Z-source/quasi-Z-source network, which allows an independent
control of the power delivery with high reliability [9]–[11]; and
5) can fulfill the distributed MPPT [6], [8].
In order to properly operate the ZS/qZS-CMI, the power
injection, independent control of dc-link voltages, and the
pulsewidth modulation (PWM) are necessary. The work in
[5] and [7] focused on the parameter design of the ZS/qZS
networks and the analysis of efficiency. The work in [8] pre-
sented the whole control algorithm, i.e., the MPPT control of
separate quasi-Z-source H-bridge inverter (qZS-HBI) module,
and the grid-injected power control, whereas the phase-shifted
sinewave PWM (PS-SPWM) is the only existing PWM tech-
nique for the single-phase ZS/qZS-CMI. The PS-SPWM
consumes more resources to achieve the shoot-through states
because two more references are compared with the carrier
waveform. Additionally, the ZS/qZS-CMI based grid-tie
PV system has never been modeled in detail to design the
controllers.
The main contributions of this paper include: 1) a novel mul-
tilevel space vector modulation (SVM) technique for the single-
phase qZS-CMI is proposed, which is implemented without ad-
ditional resources; 2) a grid-connected control for the qZS-CMI
based PV system is proposed, where the all PV panel voltage
references from their independent MPPTs are used to control
the grid-tie current; the dual-loop dc-link peak voltage control
1551-3203 © 2013 IEEE