Fuzzy Logic Based Quasi -Z- Source Inverter for Grid ... Bhusawal SSGBCOET Bhusawal [email protected] Associate Professor [email protected] Prof. Rajesh.C. Patil Assistant

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

[email protected]

Prof. Girish K. Mahajan

Associate Professor


SSGBCOET Bhusawal

[email protected]

Prof. Rajesh.C. Patil

Assistant Professor


SSGBCOET Bhusawal

[email protected]

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 inconsistent 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



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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



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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-



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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)



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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



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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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[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.

Documents

Fuzzy Logic Based Quasi -Z- Source Inverter for Grid ... Bhusawal SSGBCOET Bhusawal [email protected] Associate Professor [email protected] Prof. Rajesh.C. Patil Assistant