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Resincap Journal of Science and Engineering
Volume 1, Issue 9 December 2017
ISSN: 2456-9976
243
Fuzzy Logic Based Quasi -Z- Source Inverter for Grid Connected
P-V System
Mr. Manoj S. Bone
M.E. (EPS), Student
Electrical Engineering Dept
SSGBCOET Bhusawal
manojsbone@gmail.com
Prof. Girish K. Mahajan
Associate Professor
Electrical Engineering Dept
SSGBCOET Bhusawal
girishmahajan_16@rediffmail.com
Prof. Rajesh.C. Patil
Assistant Professor
Electrical Engineering Dept
SSGBCOET Bhusawal
meet.rcpatil@gmail.com
ABSTRACT
Photovoltaic (PV) power generation helps in directly convert
the solar radiation into electric power without hampering the
environment. However, the stochastic fluctuation of solar
power is inconsistent with the desired stable power injected to
the grid, owing to the 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 plays a vital role in order to improve
stability of grid. The contributions have been made by the
cascade multilevel inverter. An effective control scheme
which includes system-level control and pulse width
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 balance
for all quasi-Z-source H-bridge inverter (qZS-HBI) modules.
Since, cascading reduce the functionality of the operation, an
increase in KVA rating of inverter to twice it rating with a PV
voltage range of 1:2; and the different PV panel output
voltages result in imbalanced dc-link voltages. Thus, here the
design process completely works to achieve faster response
and with good stability limits.
Keywords:
Space Vector Modulation (SVM), Maximum Power Point
Tracking (MPPT), Cascade Multilevel Inverter (CMI),
Quasi -Z- Source High Bridge Inverter, Photovoltaic (PV).
1. INTRODUCTION
Presently the World energy demand is increasing due to
population growth and Modern industrial society persuading a
lot of investments in alternative energy sources such as Solar,
Wind, bio-mass, fuel cells etc; Among the renewable energy
sources, Photovoltaic energy consistently shows its great
potential to serve as clean and inexhaustible energy source
and the concerns over greenhouse gas emission and the ever
rising fuel prices have stimulated urgent demands for
alternative energy. Government incentives and the soaring
cost of fossil fuels have significantly promoted the
development of renewable energies. Among them, solar
energy is one of the most important green energy resources
due to its environmental sustainability and inexhaustibility.
The use of photovoltaic (PV) energy as an alternative to
generate electricity has becomes significant in the recent
years. A PV inverter is widely used to convert the
photovoltaic energy into usable electrical energy as most of
the demands are in the AC voltage, either for local loads or
supplied into the grid. A recent upsurge in the study of
photovoltaic power generation emerges, since they directly
convert the solar radiation into electric power without
hampering the environment. 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.
Multilevel inverters are applying to photovoltaic power
systems. Three common multilevel inverter topologies are
Capacitor clamped, Diode clamped and Cascade Multilevel
inverter. Among these CMI is more widely used due to CMI
structure with separate PV arrays as input which yields high
voltage and high power grid tie without a transformer and
achieving distributed MPPT. Traditionally Voltage source
inverters or Current source inverters had been using for the
applications of Renewable energy sources. But these have
many disadvantages like limited output, no immunity towards
short circuits or open circuits and need dead time and overlap
in gate pulses to avoid short circuits and open circuits. Also a
DC-DC booster is used with PV which increases the size and
cost of the system. So, Proposed a new topology of inverter
called Z-source inverter. Z- Source inverter has the capability
to give output in any range i.e.; buck or boost because an
additional Shoot- through state is presented. But ZSI has a
discontinuous input current during the shoot through state due
to the blocking diode. So, QZS are newly added with the
feature of taking continuous current from input, with lower
switching stress and smaller component ratings in single stage
power conversion
The contributions have been made by the cascade multilevel
inverter. Nevertheless, the H-bridge inverter module lacks
boost function so that the inverter KVA rating requirement
Resincap Journal of Science and Engineering
Volume 1, Issue 9 December 2017
ISSN: 2456-9976
244
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. Since the I–V characteristic curve of
photovoltaic (PV) cells varies nonlinearly with the insolation
and temperature, it is crucial to operate PV system to a
specific point to extract maximum solar energy. This
technology is normally named as maximum power point
tracking (MPPT). Many MPPT methods have been developed
and implemented in previous studies, including perturb and
observe (P&O), incremental conductance (IncCon), fractional
open-circuit voltage, fractional short-circuit current, line
approximation, ripple correlation control (RCC) and fuzzy
logic control (FLC) approaches. These techniques have high
tracking accuracy under steady weather conditions, but still
exhibit some trade-offs between tracking speed and tracking
accuracy when insolation changes. 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.
The main contributions of this paper include: 1) a novel
multilevel space vector modulation (SVM) technique for the
single phase QZS-CMI is proposed, which is implemented
without additional 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 is employed in every QZS-HBI module to
balance the dc-link voltages;
Figure 1. System Block diagram
2. QZS-CMI-BASED GRID-TIE PV
POWER SYSTEM
Fig. 2 shows description Of Qzs-Cmi-Based Grid-Tie PV
Power System Fig. 1 shows the discussed qZS-CMI-based
grid-tie PV power system. The total output voltage of the
inverter is a series summation of qZS-HBI cell voltages. Each
cell is fed by an independent PV panel. The individual PV
power source is an array composed of identical PV panels in
parallel and series. A typical PV model in is performed by
considering both the solar irradiation and the PV panel
temperature.
For the qZS-CMI, the synthesized voltage is
Vdc=
Vp …….(1)
where is the output voltage of the kth PV array; vDCk is the dc-
link voltage of the kth qZS-HBI module; Dk and Bk represent
the shoot-through duty ratio and boost factor of the kth qZS-
HBI, respectively, is the output voltage of the kth module, and
is the switching function of the kth qZS-HBI.
Figure 2: Simulation Model of Qzs-CMI Based Grid-Tie
PV Power System
Resincap Journal of Science and Engineering
Volume 1, Issue 9 December 2017
ISSN: 2456-9976
245
Figure .3.Simuletion Model for generating control
3. QUASI –Z- SOURCE INVERTER
OPERATION
QZS inverter has nine switching states, including six active
states, shoot through zero state and two non-shoot through
zero states. The shoot through state can be made by the
switches of the phase in the inverter bridge are switched on
simultaneously for a very short duration. Figure 4 and 5 shows
the QZS inverter equivalent circuits operating in two modes.
Figure 4: Non Shoot-through state
Figure 5: Shoot-through state
4. CONTROL STRATERGY
The control objectives of the qZS-CMI based grid-tie PV
system are:1) The distributed MPPT to ensure the maximum
power extraction from each PV array; 2) The power injection
to the grid at unity power factor with low harmonic distortion;
3) The same dc-link peak voltage for all qZS-HBI modules.
The overall control scheme of Fig. 1 is proposed to ful fill
these purposes.
Total PV array voltage loop adjusts the sum of n PV array
voltages tracking the sum of n PV array voltage references by
using a proportional and integral (PI) regulator PIt . Each PV
array voltage reference is from its MPPT control
independently.Grid-tie current loop ensures a sinusoidal grid-
injected current in phase with the grid voltage. The total PV
array voltage loop outputs the desired amplitude of grid-
injected current. A Proportional + Resonant (PR) regulator
enforces the actual grid current to track the desired grid-
injected reference. The current loop output’s total modulation
signal subtracts the modulation signal sum of the second, third
and nth qZS-HBI modules to get the first qZS-HBI module’s
modulation signal.
4.1. Independent DC-link voltage control
Figure 6: Dc-link peak voltage control block diagram
This control loop, adjust DC-link peak voltage using the
capacitor- voltage and the inductor-L2current for each QZS-
CMI module. Reference [16] presents the Kth QZS- CMI
module’s transfer function from the shoot-through duty ratio
to the DC-link peak voltage, GVdk(s) and from the shoot-
Resincap Journal of Science and Engineering
Volume 1, Issue 9 December 2017
ISSN: 2456-9976
246
through duty ratio to the inductor-L2current, GiLdk(s) as
follows
…(2)
5. FUZZY LOGIC CONTROLLER
In this paper, a Fuzzy logic based intelligent control technique
associated with an independent DC-link voltage control is
developed to reduce the transients and the system
demonstrates stable output voltage with reduced harmonic
distortion. The basic structure of Fuzzy Logic control used in
the control strategy is shown in fig 6.
Figure 6: Block dig. Basic fuzzy logic controller
The main elements in the control system are the Fuzzifier unit
at the input terminal, Defuzzifier at the output terminal,
Knowledge base and the inference engine. FLC system
requires input and output variables. Generally, an error and its
rate of change are chosen for input variables. The change of
current and voltage are selected to be the output variables. An
error in discrete time is the difference between the r(k)and the
process output variable y(k). The current sample of error e(k)
and the change of error Δe(k) are defined as
e(k) = r(k) – y(k) (3)
Δe(k) = e(k) – e(k –1) (4)
These variables are normalized to fit into the interval value
between -1 and +1and require seven membership functions.
When any input is not in this range, it is considered as too big
which generates large error signals.
For simplification, the triangular and trapezoidal membership
functions are utilized. By using these membership functions,
the controller manages to reduce the error signal in a faster
manner that increases the transient response. The membership
functions are labeled as NB for ―Negative Big‖, NS for
―Negative Small‖, NM for ―Negative medium‖, Z for ―Zero‖,
PB for ―positive Big‖ and PS for ―Positive Small‖. The input
variables are fuzzified through membership functions. Fuzzy
output is generated by the essence of the inference process and
with the aid of knowledge based rules. The main part of the
FLC is the Knowledge base elements; it consists of a list of
fuzzy rules. The inference process is to generate a fuzzy
output set based on if then rules.
Table 1 : Rules for fuzzy logic control
E
NB
NM
NS
Z
PS
PM
PB
NB PB PB PB PB PM PS Z
NM PB PB PB PM PS Z NS
NS PB PB PM PS Z NS NM
Z PB PM PS Z NS NM NB
PS PM PS Z NS NM NB NB
PM PS Z NS NM NB NB NB
PB Z NS NM NB NB NB NB
6. SPACE VECTOR MODULATION FOR
QZS-CMI
The space vector modulation for n-layer QZS-CML inverter is
shown in figure 2. The Space Vector Modulation Scheme can
be modified by inserting shoot-through states in place of non-
shoot-through zero states. This modified SVM can be used to
control the quasi ZSI. The boost can be controlled by
adjusting the time of shoot- through conduction. In order to
buck/boost dc-link peak voltage of QZS-H bridge inverter to
balance the voltage waveform separate pv panels, shoot-
through states need to be introduced into the upper and lower
switches of one bridge. The voltage vectors are composed of n
bridge vectors [3]. SVM is a method where the switching
states are viewed in voltage reference frame. Insert the shoot-
through into the cell, the switching times for each cell is
represented as T. consequently new group of switching times
are generated Ta, Tb, Tc. During each control cycle, the time
of shoot-through zero states Tsh is equally divided into four
parts and inserted into the bridges of the same cell. And are
the switching control signals for the upper switches and , are
those of lower switches respectively. The bridge vector of
same cell has a 180 degree phase difference. Additionally, the
voltage vectors between two adjacent layers have a phase
difference of 2 /nk in which k is the number of reference
voltages in each cycle.
As the qZS network is embedded to the HBI module, the
SVMfor each qZS-HBI can be achieved bymodifying the
SVM technique for the traditional single-phase inverter. Using
the first qZS-HBI module of Fig. 1 as an example, the voltage
vector reference is created through the two vectors and, by
(5)
Resincap Journal of Science and Engineering
Volume 1, Issue 9 December 2017
ISSN: 2456-9976
247
Where and is the carrier frequency; the time interval is the
duration of active vectors, and is the duration of traditional
zero voltage space vectors. Thus, the switching times for the
left and right bridge legs in traditional HBI are However, the
shoot-through states are required for the independent qZS-
HBI module.
For this purpose, a delay of the switching times for upper
switches or a lead of the switching times for lower switches
are employed at the transition moments, as Fig. 5(a) shows.
During each control cycle, the total time of shoot-through zero
state is equally divided into four parts. The time intervals of
and remain unchanged; and are the modified times to generate
the shoot-through states; and are the switching control signals
for the upper switches, and are that for the lower switches. In
this way, the shoot-through states are distributed into the qZS-
HBI module without additional switching actions, losses, and
resources. To generate the step-like ac output voltage
waveform from the qZS-CMI,a phase difference, in which is
the number of reference voltage vectors in each cycle, is
employed between any two adjacent voltage vectors, as Fig.
5(b) shows. The total voltage Vector is composed of reference
vectors from the qZS-HBI modules.
Figure.7.Proposed multilevel SVMfor the single-phase
qZS-CMI. Switching pattern of one qZS-HBI module
For a three-phase-leg two level VSI, both continuous
switching (e.g., centered SVM) and discontinuous switching
(e.g., 60 – discontinuous PWM) are possible with each having
its own unique null placement at the start and end of a
switching cycle and characteristic harmonic spectrum. The
same strategies with proper insertion of shoot through modes
could be applied to the three-phase-leg z–source inverter with
each having the same characteristic spectrum as its
conventional counterpart.
There are fifteen switching states of a three-phase-leg z-
source inverter. In addition to the six active and two null states
associated with a conventional VSI, the z- source inverter has
seven shoot-through states representing the short-circuiting of
a phase-leg (E1), two phase-legs (E2) or all three phase-
legs(E3). These shoot- through states again boost the dc link
capacitor voltages and can partially supplement the null states
within a fixed switching cycle without altering the normalized
volt–sec average, since both states similarly short-circuit the
inverter three-phase output terminals, producing zero voltage
across the ac load. Shoot-through states can therefore be
inserted to existing PWM state patterns of a conventional VSI
to derive different modulation strategies for controlling a
three-phase-leg z-source inverter.
7. MATLAB SIMULATION RESULTS A Seven-level Quasi Z source cascade H-bridge inverter for
grid connected PV power system is simulated using
MATLAB the results are shown below.
Figure.8.Simuletion Result at the grid-tie case
Figure.9.qZS-CMI output Voltage
Resincap Journal of Science and Engineering
Volume 1, Issue 9 December 2017
ISSN: 2456-9976
248
F i g u r e 9 : q Z S - C M I o u t p u t
v o l t a g e
Figure.10. Grid voltage, and current
Figure.11.For PV Voltages at MPPT
Figure 8 shows the simulation results, where the second
module’s dc-link peak voltage is boosted to the same voltage
value when compared with other modules, but with a longer
shoot-through time interval. Also, the qZS-CMI outputs the
seven-level voltage with equal voltage step from one level to
another level.
8. CONCLUSION
This paper shows grid-injected power was fulfilled at unity
power factor, all qZS-HBI modules separately achieved their
own maximum power points tracking even if some modules’
PV panels had different conditions. Moreover, the
independent dc-link voltage closed-loop control ensured all
qZS-HBI modules have the balanced voltage, which provided
the high quality output voltage waveform to the grid. The
control parameters were well designed to ensure system
stability and fast response. A multilevel SVM integrating with
shoot through states was proposed to synthesize the staircase
voltage waveform of the single-phase qZS-CMI.
This paper proposed a new control method for QZS- Cascade
Seven-level inverter based single phase grid connected PV
system. The proposed system enables grid injected current
was fulfilled at unity power factor, independent dc-link
voltage control enforced all QZS-HBI modules have the
balanced voltage. A SVM technique integrating with the
shoot-through states, to synthesize the stair case voltage
waveform of the single phase QZS- CMI. A fuzzy logic
controller is introduced for Quasi-Z- source cascade
multilevel inverter Grid connected PV system. Fuzzy
controlled is used for PV output voltage to achieve closed
loop control which can balance the DC- link voltage and
minimize the grid voltages impact on grid current. As
compared to the conventional method, results indicated that
the proposed FLC scheme reduces the total harmonic
distortion and can provide faster response and fewer
oscillations around the steady state.
REFERENCES
[1] ―Yushan Liu, Student Member, IEEE, Baoming Ge,
Member, IEEE, Haitham Abu-Rub, Senior Member, IEEE and
Fang Z. Peng, Fellow, IEEE‖,―An Effective Control Method
for Quasi-Z-Source Cascade Multilevel Inverter-Based Grid-
Tie Single-Phase Photovoltaic Power System‖, 1551-3203 ©
2013 IEEE
[2] Buticchi, G.; Barater, D.; Lorenzani, E.; Concari, C.;
Franceschini, G. A nine-level grid-connected converter
topology for single-phase transformerless PV systems. IEEE
Trans. Ind. Electron. 2014, 61 , 3951–3960.
[3] Z. Zhao, M. Xu,Q. Chen, J. S. Jason Lai, and Y. H. Cho,
―Derivation, analysis,and implementation of a boost– buck
converter-based high-efficiency pv inverter,‖IEEE Trans.
Power Electron., vol. 27, no. 3, pp. 1304– 1313,
[4] K. Hasegawa and H. Akagi, ―Low-modulation-index
operation of a five level diode-clamped pwm inverter with a
dc-voltage-balancing circuit for a motor drive,‖ IEEE Trans.
Power Electron., vol. 27, no. 8, pp. 3495–3505,Aug. 2012
[5] N. A. Rahim, K. Chaniago, and J. Selvaraj, ―Single-phase
seven-level grid-connected inverter for photovoltaic system,‖
IEEE Trans. Ind. Electr. vol. 58, no. 6, pp. 2435– 2443, Jun.
2011.
[6] Jinn-Chang Wu, Member, IEEE, and Chia-Wei Chou,
―Solar Power Generation system with a Seven-Level
Inverter,‖ IEEE Transactions on Power Electronics, vol. 29,
no. 7, July 2014
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Volume 1, Issue 9 December 2017
ISSN: 2456-9976
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[7] Arango, E.; Ramos-Paja, C.A.; Calvente, J.; Giral, R.;
Serna, S. Asymmetrical interleaved DC/DC switching
converters for photovoltaic and fuel cell applications— Part1:
Circuit generation, analysis and design. Energies 2012, 5,
4590–4623.
[8] Walker, G.R.; Sernia, P.C. Cascaded DC–DC converter
connection of photovoltaic modules. In Proceedings of the
33rd Annual Power Electronics Specialists Conference,Cairns,
Queensland, Australia, 22–27 June 2002; pp. 24–29.
ABOUT AUTHOR Manoj S. Bone received the B.E. degree from K.C.E. & IT,
Jalgaon in 2014, the M.E. pursing from SSGBCOET,
Bhusawal, Jalgaon.
G. K. Mahajan received the B.E. degree from Shri Sant
Gajanan Maharaj College of Engineering, Shegaon in 1999
and M.E. degree from Government College of Engineering,
Aurangabad in 2012. Total teaching experience of 16 years.
R. C. Patil received the B.E. degree from J.T.Mahajan
College of Engineering,Fejpur and M.E. degree from Govt.
College of Engineering, Aurangabad. Total teaching
experience of 10 years.
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