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
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: [email protected]
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 Hbridge 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
978-1-4673-0158-9/12/$31.00 ©2012 IEEE
Module 1
I.
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 DClink 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 2
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.
Module 3
2
Fig. 1. Generic CHB multilevel inverter PV system.
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.
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 v̂PN1 can be obtained as
1− D
⎧
⎪vC1 = 1 − 2 D vpv1
⎪
D
⎪
vpv1
.
⎨vC2 =
1 − 2D
⎪
1
⎪
⎪ vˆPN1 = vC1 + vC2 = 1 − 2 D vpv1
⎩
where vpv1 is the PV string output voltage.
Two inductor currents iL1, iL2 can be calculated as
B. Quasi-Z Source Inverter
iL2 = iL1 = P vpv1 .
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.
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.
(b)
Fig. 3. Equivalent circuit of the qZSI. (a) Non-shoot-through state.
(b) Shoot-through state.
(2)
where P is the PV string output power.
Fig. 2. Configuration of the proposed system.
(a)
(1)
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 Hbridge 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.
B. Independent DC-link Voltage Control
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.
Ppv (W)
600
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 v̂PN1 of DC-link will be controlled in
a constant value. As the expression shown in (1), vˆPNn is
made of the sum of vC1 and vC2. So vC1 and vC2 are measured
and added together to produce vˆPNn . A proportional and
integral (PI) controller is employed to control vˆPNn by
adjusting the output power reference P*n. Although the
reference of vˆPNn 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
P*total = ∑ P*n .
n =1
*
* = 2 P total .
iˆgrid
vˆgrid
PV1
PV2
PV3
200
20
40
60
vpv (V)
80
100
(a)
10
5
0
0
40
60
vpv (V)
80
an =
100
P*n
P*total
.
(5)
Using SPWM modulation method, the modulation index mn
for each module can be calculated by
a v
mn = n total .
vˆPNn
(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.
(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]
PV1
PV2
PV3
20
(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
400
0
0
ipv (A)
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.
(6)
With the modulation index mn and the shoot-through duty
ratio Dn, the gate signals for three qZS H-bridge modules are
953
produced in PS-SPWM controller. The block diagram that
performs the system power control is shown in Fig. 7.
vˆPNn
vˆPNn
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 PSSPWM 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. 6. Control scheme of qZS H-bridge module.
vpv (V)
80
2
v̂grid
2.1
2.2
2.3
2.4
2.5
600
vˆPNn
1
Ppv (W)
∑ Pn
n =1
60
50
2
3
PV1
PV2
PV3
70
PV1
400
2.2
Ptotal
PV2
500
PV3
2.25
2.3
2.35
t (s)
2.4
2.45
2.5
Fig. 7. Power control of the whole system.
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
Fig. 8. PV voltages and output powers of three modules.
vC1 and vC2 (V) vC1 and vC2 (V) vC1 and vC2 (V)
IV.
150
vC1
100
vC2
50
0
2.2
150
2.25
2.3
2.35
2.25
2.3
2.35
2.4
vC1
100
2.5
Module 2
2.45
2.5
Module 3
vC2
50
2.25
2.3
2.35
t (s)
2.4
Fig. 9. Capacitor voltages of three qZS H-bridge modules.
954
2.45
vC2
50
0
2.2
2.4
vC1
100
0
2.2
150
Module 1
2.45
2.5
10
2.25
2.3
2.35
2.4
2.45
2.5
2.25
2.3
2.35
2.4
2.45
2.3
2.35
2.4
2.45
2.5
2.25
2.3
2.35
t (s)
2.4
2.45
2.5
0
-20
2.2
2.5
Module 3
Fig. 13. Grid voltage and current of qZS-CBH multilevel Inverter.
10
0
2.2
2.25
20
Module 2
10
0
2.2
20
400
200
0
-200
-400
2.2
igrid (A)
0
2.2
20
vgrid (V)
Module 1
2.25
2.3
2.35
t (s)
2.4
2.45
1
2.5
m1
iL1 and iL2 (A)
iL1 and iL2 (A)
iL1 and iL2 (A)
20
0
Fig. 10. Inductor currents of three qZS H-bridge modules.
-1
2.2
1
vPN2 (V)
0
2.2
200
2.25
2.3
2.35
2.4
2.45
2.3
2.35
2.4
2.45
2.35
2.4
2.45
2.5
2.25
2.3
2.35
2.4
2.45
2.5
2.25
2.3
2.35
t (s)
2.4
2.45
2.5
0
2.5
-1
2.2
100
0
2.2
Fig. 14. Modulation index of each module.
2.25
2.3
2.35
t (s)
2.4
2.45
2.5
Fig. 11. DC-link voltages of three modules.
500
vH (V)
2.3
0
-1
2.2
1
2.25
2.25
2.5
100
0
2.2
200
vPN3 (V)
m2
100
m3
vPN1 (V)
200
0
-500
2.2
2.25
2.3
2.35
t (s)
2.4
2.45
Fig. 12. Output voltage of qZS-CBH multilevel Inverter.
2.5
V.
CONCLUSION
A new multilevel inverter for PV power generation
systems is proposed. It is a combination of qZSI and CHB
multilevel topology, and has both advantages of them. This
enables independent MPPT control and independent DC-link
voltage control for each PV generation module. Moreover,
the proposed system effectively overcomes the DC-link
voltage imbalance problem.
The control methods of the proposed system including
independent MPPT control, independent DC-link voltage
control, and the output power control are presented in detail.
A 1.5 kW simulation model is built to test the performance of
the system. Simulation results verify the operating principle
and control strategies of the proposed system.
955
ACKNOWLEDGEMENTS
Parts of this work, specifically Sections III and IV were
supported by Education Development Program of Delta
Environmental & Educational Foundation under grant No.
DREG2010001 and the State Key Laboratory of Control and
Simulation of Power System and Generation Equipments
under grant No. SKLD11KM01, Tsinghua University,
Beijing 100084, China. In addition, we also acknowledge
support from NPRP grant No. 09-233-2-096 from the Qatar
National Research Fund (a member of Qatar Foundation) for
Section II, and from the Beijing Jiaotong University
Foundation under grant No. 2009JBM093 for Section IV. The
statements made herein are solely the responsibility of the
authors.
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