06237218

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A New Grid-Connected PV System Based on Cascaded H-bridge Quasi-Z
Source Inverter
Article · May 2012
DOI: 10.1109/ISIE.2012.6237218
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A New Grid-Connected PV System Based on
Cascaded H-bridge Quasi-Z Source Inverter
Dongsen Sun 1, Baoming Ge 1,2, Fang Zheng Peng2, Abu Rub Haitham4, Daqiang Bi 3, Yushan Liu1,4
1 School of Electrical Engineering, Beijing Jiaotong University, Beijing, China
2 Department of Electrical and Computer Engineering, Michigan State University, East Lansing, Michigan, USA
3 State Key Lab of Power Systems, Dept. of Electrical Engineering, Tsinghua University, Beijing, China
4 Department of Electrical and Computer Engineering, Texas A&M University at Qatar, Doha, Qatar
E-mail: [e-mail]
Abstract- A new scheme for grid-connected photovoltaic (PV)
interface by combination of a quasi-Z source inverter (qZSI)
into cascaded H-bridge (CHB) is proposed in this paper. The
proposed scheme enables PV string voltage boost to a higher
level, and solves the imbalance problem of DC-link voltage in
traditional CHB inverters. A multilevel voltage waveform of
inverter output is generated by an improved phase shifted
sinusoidal pulse width modulation (PS-SPWM) algorithm, which
introduces shoot-through states into the conventional zero states
to control qZS-CHB module. The effective control schemes are
proposed to regulate the maximum power point tracking (MPPT)
of each string, and control the DC-link voltage of each H-bridge,
respectively. Grid injected power is controlled corresponding to
the proportionality factors of each PV string output power. A
1.5 kW system is built in MATLAB/SIMULINK, and the
simulation results verify the proposed novel multilevel PV
interface inverter and its control principles.
I. INTRODUCTION
As solar energy is one of the most promising renewable
energy, the photovoltaic (PV) systems are becoming more
and more popular. In recent years applying multilevel
inverters to PV power systems is getting more and more
attention due to the large power-scale demands. Three
common multilevel inverters topologies are as follows: 1)
diode clamped [1]-[3]; 2) capacitor clamped [4], [5]; and 3)
cascaded H-bridge (CHB) inverter [6]–[11]. Among these
topologies, the CHB inverter has unique advantages and is
more widely used in PV system.
Fig.1 shows a generic PV system using the CHB inverter.
The system offers some advantages such as the independent
maximum power point tracking (MPPT) of each string and
the modulatity by cascading more H-bridge modules. In
addition, the CHB output voltage can reaches medium
voltage and has a high number of levels (2n+1, n=1, 2, …),
which results in no step up transformer and much smaller
scale filter [6]-[11]. However, with the PV string connecting
to the inverter directly, a constant DC-link capacitor voltage
of each inverter module is impossible in this system, because
PV string voltage varies widely due to the changes of
temperature and solar irradiation, or some serious conditions
such as mismatch, partial shadows, etc. These cases will
cause an imbalance DC-link voltage among different H-
bridge modules [6]-[10]. Moreover, a variable DC-link
voltage leads to a higher KVA rating for H-bridge module in
practical applications. In [6], a control method is studied
under the equal DC-link voltages through neglecting the PV
string voltage differences. References [7]-[10] introduce a
factor to express the different voltage and power of each PV
string, but it cannot solve the DC-link imbalance problem. In
[11] an additional DC/DC converter is added to compensate
the imbalance of the DC-link voltage, but causing the whole
system complex and expensive.
Nowadays, the Z-source inverter (ZSI) and the quasi-Z
source inverter (qZSI) have been widely applied for
renewable energy power generation system due to some
unique features [12]-[16]. They can implement voltage boost
and power conversion simultaneously in a single stage, and
improve the reliability due to the shoot-through cases no
longer destroying the inverter [12]-[14]. However, PV system
based on qZS-CHB multilevel inverter has never been
presented by now. Introducing a quasi-Z source (qZS)
network into the CHB module, the system features several
advantages, such as PV string voltage boost, independent
tracking MPP of each PV string, and keeping an equal DC-
link voltage for each H-bridge inverter module.
This paper proposes a new PV system based on qZS- CHB
multilevel inverter. Its whole control scheme including
independent MPPT control, independent DC-link voltage
control, and the grid injected power control is studied in this
paper. Simulation results verify the proposed system and the
control scheme.
Module 1Module 2Module 3
Fig. 1. Generic CHB multilevel inverter PV system.
978-1-4673-0158-9/12/$31.00 ©2012 IEEE 951

II. OPERATING PRINCIPLE OF THE PROPOSED SYSTEM
A. Topology Description
The configuration of the PV system based on qZS-CHB
multilevel inverter is illustrated in Fig. 2. The system is
composed of three PV strings, three qZS H-bridge modules,
filtering inductance, and the distribution grid. Each PV
string’s output connects to a qZS H-bridge module.
Comparing to the conventional H-bridge module, an
inductor-capacitor impedance network is introduced. The
output voltage of the qZS-CHB multilevel inverter is made of
the sum of all module output voltages, and it has seven levels.
The number of levels increases when increasing the series
number of the qZS H-bridge modules.
B. Quasi-Z Source Inverter
Take one module in the proposed system as an example to
analyze the qZSI operating principle. The qZSI can be
operated in two states, i.e., the non-shoot-through state and
the shoot-through state [12]-[15]. Fig. 3 shows the qZSI
equivalent circuits operating in the two states and defines the
polarities of all voltages and currents.
Fig. 2. Configuration of the proposed system.
(a) (b)
Fig. 3. Equivalent circuit of the qZSI. (a) Non-shoot-through state.
(b) Shoot-through state.
The details of operating principle have been illustrated in
many literatures [12]-[16]. Assuming T is one switching cycle,
T0 is the interval of the shoot-through state, and T1 is the
interval of non-shoot-through states. Their relationship is
T0+T1=T, and the shoot-through duty ratio is D=T0/T. In
steady state the following derivations can be obtained.
Two capacitor voltages vC1 and vC2, and the peak voltage of
DC-link PN1
ˆ
v can be obtained as
C1 pv1
C2 pv1
PN1 C1 C2 pv1
1
12
.
12
1
ˆ
12
D
vv
D
D
vv
D
vvv v
D
−
⎧=
⎪−
⎪
⎪=
⎨−
⎪
⎪
=+ =
⎪−
⎩
(1)
where vpv1 is the PV string output voltage.
Two inductor currents iL1, iL2 can be calculated as
L2 L1 pv1.iiPv== (2)
where P is the PV string output power.
C. Improved PS-SPWM Algorithm
The modulation method used in this proposed system is an
improved phase shifted sinusoidal pulse width modulation
(PS-SPWM) algorithm shown in Fig. 4. As an qZS network
introduced into the conventional H-bridge, two shoot-through
modulation reference, which are denoted as 1-Dn and Dn-1,
are introduced into the conventional zero states. If the
triangular carrier signal is bigger than Dn or smaller than 1-Dn,
two switches of one leg in H-bridge module are turned on
simultaneously. But the PWM output voltage is still kept at
zero, the same as the conventional zero states.
Fig. 4. Modulation scheme for the proposed system.
952

With the PWM shown in Fig. 4, each H-bridge module is a
3-level inverter. We take the H-bridge module 1 as an
example, S1 and S2 (dashed line) are carriers for two legs
(left leg and right leg of H-bridge), respectively, which have a
phase shift in 180° each other. The output voltage va1 of H-
bridge module 1 is a 3-level PWM signal. The carriers of
different H-bridge modules are shifted in 60° each other to
produce the multilevel stepped voltage waveform. By
applying this method, 7-level PWM signal are generated as
shown in the Fig. 4.
III. CONTROL SCHEME
Each qZS H-bridge module presents two independent
control freedoms: shoot-through duty ratio Dn and modulation
index mn. Dn is independently used to control the each PV
string voltage and adjust the voltage to a desired reference
given by the MPPT algorithm. While mn is used to control
each DC-link voltage and the grid injected power. The
following subsections describe the independent MPPT
control, the independent DC-link voltage control, and the grid
injected power control.
A. Independent MPPT Control
In this paper Perturb and Observe (P&O) method is used to
fulfill the MPPT [16], [17]. The PV string in each module is
composed by the series connection of four KD135GH-2P PV
panels. Fig. 5 shows the P-V and I-V characteristics of the PV
strings, which are corresponding to different environmental
conditions, i.e., PV1: S=1.0 kW/m2, T=15 °C; PV2: S=1.0
kW/m2, T=35 °C; PV3: S=1.0 kW/m2, T=55 °C. From Fig. 5
we can see that the MPP voltage decreases with the
environment temperature increasing.
(a)
(b)
Fig. 5. Characteristics of PV strings in different temperature. (a) P-V
characteristics of three PV strings. (b) I-V characteristics of three PV strings.
The independent MPPT control of each module is shown in
Fig. 6. The output voltage and current of each PV string are
detected and inputted to the MPPT controller. The controller
outputs a reference to the PV voltage close-loop regulator.
The PV voltage reference is refreshed every 0.0002 s by
using MPPT algorithm. By regulating the shoot-through duty
ratio Dn the output voltage of each PV string will be
controlled at different values according to the environment
conditions.
B. Independent DC-link Voltage Control
The DC-link voltage control scheme is illustrated in Fig. 6.
As the system has three independent DC-links, each one
should be controlled independently. The DC-link voltage vPN
of qZS H-bridge changes between the peak value and zero.
While, the peak voltage PN1
ˆ
v of DC-link will be controlled in
a constant value. As the expression shown in (1), PN
ˆn
v is
made of the sum of vC1 and vC2. So vC1 and vC2 are measured
and added together to produce PN
ˆn
v. A proportional and
integral (PI) controller is employed to control PN
ˆn
v by
adjusting the output power reference P*n. Although the
reference of PN
ˆn
v is set at the same value, the reference P*n
will be different according to each PV string output power.
C. Grid Injected Power Control
The total power injected to the grid is the sum of each PV
string output power. So the total power reference is
3
**
total 1
.
n
n
P
P
=
=∑ (3)
As the three qZS H-bridge modules are in the series
connection, the grid-injected current drawing from each
module is the same. So the peak value of the grid current is
*
*total
grid grid
2
ˆ.
ˆ
P
iv = (4)
To ensure unity power factor operation, the phase locked
loop (PLL) is performed to measure the phase of grid voltage.
The grid current is measured and fed back to the current
control loop regulator. The qZS-CHB multilevel inverter
output voltage vtotal is given by the current loop to produce the
modulation index mn for each module.
As each qZS H-bridge module provides different power,
the proportionality factors can be calculated as follows [6],
[11]
*
*total
.
n
n
P
a
P
= (5)
Using SPWM modulation method, the modulation index mn
for each module can be calculated by
total
PN
.
ˆ
n
nn
av
mv = (6)
With the modulation index mn and the shoot-through duty
ratio Dn, the gate signals for three qZS H-bridge modules are
020 40 60 80 100
0
200
400
600
v
pv
(V)
P
pv
(W)
PV1
PV2
PV3
020 40 60 80 100
0
5
10
v
pv
(V)
i
pv
(A)
PV3
PV2
PV1
953

produced in PS-SPWM controller. The block diagram that
performs the system power control is shown in Fig. 7.
PN
ˆ
n
v
PN
ˆ
n
v
Fig. 6. Control scheme of qZS H-bridge module.
3
1n n
P
=
∑
grid
2
ˆ
v
total
1
P
PN
ˆn
v
Fig. 7. Power control of the whole system.
IV. SIMULATION RESULTS
In order to verify the proposed system and its control
schemes, the simulations are performed by using MATLAB
/SIMULINK. A 1.5 kW single-phase 7-level qZS-CHB
multilevel inverter system, as shown in Fig. 2, is built for
simulation. The simulation parameters are as follows: carrier
frequency fc=2 kHz, L1=L2=3 mH, C1=C2=2 mF, C3=1 mF,
Lf=1 mH, and the resistances of L1 and L2 are r1=r2=0.1 Ω.
The grid voltage has a constant phase voltage of 220 V rms,
with a frequency of 50 Hz. The PV string is modeled and
simulated based on KD135GH-2P. To verify the proposed
control scheme in the case of PV strings outputting different
voltage and different power, we assume the three PV strings
operating in different temperature conditions. The
characteristics of the PV strings in three modules are shown
in Fig. 5. The peak value of DC-link voltage of all three
modules is controlled at 145 V.
Fig. 8 shows the voltage and power of PV string in three
modules. All of the three PV strings operate at their MPPs,
and the voltage and power of PV 1, PV 2, and PV 3 are 71.5
V /545 W, 66 V/500 W, 60.5 V/455 W, respectively. And the
total power injected into the grid is 1500 W, which
corresponds to the grid-injected current with a peak value of
9.6 A. Fig. 9 and Fig. 10 show the capacitor voltages and
inductor currents of three qZS H-bridge modules, which
illustrate the operating principle in (1) and (2).
The DC-link voltage of each module is shown in Fig. 11.
With the independent DC-link voltage control, all their peak
values are kept at the reference 145 V. The output voltage of
qZS-CHB multilevel inverter is shown in Fig. 12. The 7-level
voltage verifies the correct operation of the improved PS-
SPWM techniques. Fig. 13 shows that the grid voltage vgrid
and the grid current igrid are in phase, which ensures unity
power factor operation. The modulation index of each module
is shown in Fig. 14. Module 1 has a higher modulation index
because it provides the most power. While module 2 and
module 3 have a lower modulation index, corresponding to
their lower output power.
Fig. 8. PV voltages and output powers of three modules.
Fig. 9. Capacitor voltages of three qZS H-bridge modules.
22.1 2.2 2.3 2.4 2.5
50
60
70
80
vpv
(V)
2.2 2.25 2.3 2.35 2.4 2.45 2.5
400
500
600
t (s)
Ppv
(W)
PV3
PV3
PV1
PV2
PV2
PV1
2.2 2.25 2.3 2.35 2.4 2.45 2.5
0
50
100
150
v
C1
and v
C2
(V)
2.2 2.25 2.3 2.35 2.4 2.45 2.5
0
50
100
150
v
C1
and v
C2
(V)
2.2 2.25 2.3 2.35 2.4 2.45 2.5
0
50
100
150
t (s)
v
C1
and v
C2
(V)
v
C1
v
C1
v
C1
v
C2
v
C2
v
C2
Module 1
Module 2
Module 3
954