ijpam.eu A FUZZY CONTROLLED SEPIC FED SEVEN ......A FUZZY CONTROLLED SEPIC FED SEVEN LEVEL INVERTER FOR PHOTOVOLTAIC SYSTEM A.Bindu 1, M.Carolin Mabel 2, C.Bharatiraja 3 1CSI Institute

A FUZZY CONTROLLED SEPIC FED SEVEN LEVEL INVERTER FOR

PHOTOVOLTAIC SYSTEM

A.Bindu1, M.Carolin Mabel

2, C.Bharatiraja

3

1CSI Institute of Technology, Thovalai, India, [email protected]

2St.Xaviers Catholic College of Engineering, Chunkankadai,India, [email protected] 2Department of Electrical and Electronics, SRM university, India, [email protected]

Abstract

This paper bestows a single ended primary-inductor converter (SEPIC) whose duty ratio is adjusted

for tracking MPP using perturb and observe MPPT technique. The fuzzy controlled SEPIC MPPT

scheme preserves the voltage without variation and reduces current ripples, steady-state error and

overshoot. A seven-level inverter topology with minimum switching devices is used to decrease the

voltage stress in the switches. The scheme suggested ensures the use of PV module for maximum

efficiency and speed in changing load at the inverter outturn. The three level and seven level

topologies are compared and results are verified with MATLAB software. The result shows the

FLC controlled MPPT scheme for SEPIC produces improved efficiency in seven level inverter

compared to three level inverter. Simulation results presented proves the effectiveness of the

method.

Keywords - MLI, MPPT, FLC, THD, P&O

1. INTRODUCTION

The use of fossil fuel in large scale is causing green house gas emissions worldwide, which is

affecting the climate adversely. The use of solar energy leads to a new paradigm shift in the energy

scenario. The gap between demand and availability in the power sector of the country is increasing

day by day. This makes it convenient for tapping energy from the sun which is available free of

cost. Apart from the off-grid systems on-grid system would also be installed to satisfy the power

demand.

In PV system, inverters connected with grid portray a fast engendering area. A cost

effective PV system is possible with efficient power converter design. The output voltage from a

solar panel is converted into usable DC at its maximum power point using fuzzy logic controller

and then to AC by a seven level inverter. The maximum power point tracker maximizes the output

power from a PV system for a given set of conditions. This paper presents a SEPIC fed inverter

scheme to improve efficiency and minimize the overall system cost of PV array.

Depending upon change in atmospheric conditions, the grid current is tracked by the current

International Journal of Pure and Applied MathematicsVolume 114 No. 12 2017, 603-613ISSN: 1311-8080 (printed version); ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu

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controlled scheme and the stability of a PV system internal dynamics is analyzed by an energy

based Lyapunov function [5] due to the discontinuous input current power loss is more in buck and

buck-boost converters. The larger output voltage of boost converter than the input makes maximum

power tracking ability inflexible. CUK converter has an inverted output compared to SEPIC.

Artificial neural network determines the membership function and fuzzy rules in an adaptive neuro-

fuzzy controller which in turn determines the inverter output current. The performance using PI

controllers is slower and with more steady-state error compared to neuro-fizzy. Power factor

obtained is unity keeping the inverter current and line voltage in same phase and frequency [7].

The output of the inverter provides better THD due to the FLC-based MPPT scheme [1]. In

case of unbalanced DC-link voltage in each H-bridge module carrier based neutral voltage

modulation strategy produces maximum reachable output voltage [4]. Multicarrier PWM switching

technique can be used to generate low harmonic waveforms at the inverter output [3]. Although

many approaches have been suggested a multilevel space vector modulation can also be used for

single phase cascaded multilevel inverter to synthesize the staircase voltage waveform [2]. Some

new modifications in the inverter topology provide superior features such as reduced switching

devices in terms of number, cost, requirements in control and reliability, reduced PWM

complexities [6].

For PWM signal generation, three reference signals with same amplitude and frequency can

be produced and shifted with a difference equivalent to the triangular carrier signal amplitude.[9].

PWM signals are generated using dual reference modulation scheme also. The modulation index

can be varied but should not cross the maximum value otherwise over modulation occurs [11].

Multi channel carrier phase shifting sinusoidal PWM (CPS-SPWM) strategy is applied to generate

driving signals using FPGA chip [15].

Hysteresis algorithm based current control has the shortcomings of switching frequency

variation, heavy interference, harmonic distortion in multilevel inverter. Modulated hysteresis

control overcomes these undesirable factors [14]. Single phase bidirectional inverters for DC

distribution with buck/boost maximum power point trackers equally distribute PV array output. In

parallel operation the controllers online check the input configuration of MPPTs [12]. Cascade of

DC/DC booster and PWM inverter results in reduced energy utilization. The performance is

improved by the use of H-bridge power sharing algorithm. The speed of controller implementation

is increased by FPGA [10].

As the power allocation and the output voltage generation are coupled, wide range of

reactive power compensation can be achieved by a novel Discrete Fourier Transform (DFT) and

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phase locked loop (PLL) method [13]. The PWM switching and nonlinear load causes harmonics

in the output voltage of inverter. An L-C filter using fuzzy logic controller at the inverter output

reduces voltage waveform distortion is presented in [8].

2. MODELING OF SOLAR PANEL

Solar arrays are frequently used in shaggy and remote atmospheres.Hence modules must be

capable of prolonged maintenance free operations.A single solar cell is a two terminal device whose

operation is similar to a diode in the dark and when energized by the solar irradiation,it generates a

photo voltage and current.The generated dc photo voltage is of 0.5V to 1V and a photo current in

short circuit is of some tens of mA/cm2. The voltage is too low, eventhough there is reasonable

current for uttermost applications.The useful output is produced by connecting the cells together

and encapsulating into module.The module used consists of series and parallel combination of cells

to form arrays with increased current and voltage,according to the power requirement of the

application.

The solar array is designed and simulated using MATLAB software. The ratings of the

solar array used for simulation is maximum power (Pmax) :140Wp ± 3%, open circuit voltage (Voc):

22.46V, short circuit current (Isc):8.51A, voltage at maximum power (Vmax):17.89V, current at

maximum power (Imax):7.83A. 11pannels of same ratings are used to produce PV output of

approximately 221.25V.

Fig1.Proposed Solar Array

3. FUZZY CONTROLLED SEPIC CONVERTER

A. SEPIC converter

The PV array output is connected to SEPIC. Two inductors L1 & L2 with series resistance


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R1 & R2 are used for continuous current conduction and capacitors Cs & Co are used as series and

output capacitors respectively. The switching pulse is generated by FLC/PWM generator with

50KHz carrier signal. The input and output isolation is provided by the series capacitor. The current

ripples are suppressed using the output capacitor.

Fig.2 FLC based SEPIC

The duty cycle is given by

V OUT + VD

D = ………… (1)

V IN + V OUT + V D

The inductor value is calculated by

V IN (MIN)

L = × DMAX ………… (2)

∆ IL × fSW

fSW and DMAX represents the switching frequency & the duty cycle at the minimum input voltage.

In a SEPIC converter when the power switch Q1 is turned on the inductor starts charging,

and the output capacitor controls the ripple current.

The output capacitor

IOUT × D

COUT ≥ ………… (2)

VRIPPLE × 0.5 × fSW

B. Maximum power point tracking

The P&O algorithm is implemented to track the maximum power point. The P&O

algorithm compares the PV power with the previous perturbation. The direction of perturbation is

reversed if the power is decreasing otherwise the direction would be opposite. The P&O algorithm


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oscillates when the maximum power point is reached.

C. Fuzzy control scheme

The control scheme used for the SEPIC is shown in fig. Fuzzy logic method makes it

possible for the precise statement about the behavior of a complex system behavior. Fuzzy control

refers to the control of the duty cycle through fuzzy linguistic descriptions. Many fuzzy control

applications have been implemented successfully. The central core of a fuzzy controller is a

linguistic explanation about the approximate control action for a given condition. Fuzzy linguistic

descriptions involve associations of fuzzy variables and procedures for inferencing. Whereas

derivative controller, what is modeled is the physical system or process being controlled.

Fig3. Mamdani FIS

(a)

(b)

(c)

Fig.4. FLC membership functions for (a) e (tn), (b) ∆ e(tn) and (c) u (tn)

The input variables in fuzzy control are the error voltage e, which is the deviation of

measured variable y from the set point or reference r. At any time t = tn, the crisp error is defined as

e (tn) = r – y (tn) and the change in error ∆ e(tn), between two successive time steps. At time t = tn ,

the crisp change in error is the difference between the present error and the error in the previous

step t = tn-1 namely e(tn) – e(tn-1). The output variable is the control voltage u, at time t = tn , that

is the change in the action (∆ u) . Hence, if the defuzzified output at time ’n’ is ∆u*( tn ), the overall


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crisp output of the controller will be

u (tn) = u (tn-1 ) + ∆u*( tn) ……….(4)

Since it requires fewer data points in the output universe of discourse in order for the

controller to operate with reasonable accuracy.

The proposed FLC keeps the output voltage of SEPIC constant. Vdc and Vdc ref are

compared and the error is fed to the fuzzy logic controller. The error is reduced by using

knowledge based rules and the voltage Vdc is maintained constant.

4. SEVEN LEVEL INVERTER

A seven level inverter with a conventional H-bridge and two bidirectional switches and

capacitor acts as a voltage divider is proposed to generate output AC voltage from the PV array.

The topology is shown fig.5. The PV array output is connected to the inverter through a SEPIC.

The SEPIC maintains the output voltage from the PV array reducing current ripples. By providing

proper switching DC output voltage of SEPIC can be converted to seven level output voltage. The

generated AC voltage from the inverter is delivered to the load.

Fig.5 .Diagram of seven level inverter

The output voltage produced by the inverter can be expressed as ∞

V0(Ө) = A0+ ∑ An cos nӨ + Bn sin nӨ ……….(5) n=1 Due to quarter wave symmetry A0 and Bn are zero. The above equation can be written as

∞

V0(Ө) = ∑ An cos nӨ ……….(6)

n=1,3,..

4Vdc p m


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where An= ∑ [(-1) sin (nαm)] .……….(7)

nπ m =1

Therefore,

∞ 4Vdc p m

V0(Ө) = ∑ ∑ [(-1) sin(nαm)] cos nӨ …..(8)

n=1,3,.. nπ m =1

TABLE 1

SEVEN LEVEL OUTPUT SWITCHING STATES

In this work a multi reference PWM scheme is used. A carrier signal of high frequency and

three reference signals of same amplitude and frequency but phase shifted with an offset equal to

the carrier signal amplitude are compared to produce switching pulses. Switching pattern is

produced when Vref1 and Vref2 are greater than the peak amplitude of the Vcarrier , then Vref3 is

checked for zero crossing. After that Vref2 is compared for zero crossing and then again Vref1 is

compared with Vcarrier . The carrier signal frequency is selected as 10 KHz in this work.

5. RESULTS AND DISCUSSION

The suggested PV system is simulated using MATLAB SIMULINK. The PV output is

tracked using MPPT and presented as the input signal for SEPIC. The output voltage, current and

power waveforms are given in fig.6. The FLC is used in the SEPIC to produce control signal

depending up on the error signal. The output voltage of SEPIC shown in fig.7. The three

reference signals and a triangular carrier signal used to generate switching pulses are shown in.8.(a).

The signals given to the six switches used in the inverter are shown in fig.8. The seven level

inverters output voltage and current wave forms are shown in fig.9. The PV output voltage is

221.25V DC with an output current of 4A. The power output is 884.9W. The switching frequency

of SEPIC is 50 KHz and it produces an output voltage of approximately 221.25V.

S1 S2 S3 S4 S5 S6 V0

1 0 0 1 0 0 VDC

0 0 0 1 1 0 2VDC/3

0 0 0 1 0 1 VDC/3

0 0 1 1 0 0 0

0 1 0 0 1 0 -VDC/3

0 1 0 0 0 1 -2VDC/3

0 1 1 0 0 0 -VDC


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(a)

(b)

(c)

Fig.6.PV output waveforms(a)voltage,(b)current(c)power

Fig.7.SEPIC output waveform

(a)


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Fig.8.(a)Multicarrier PWM, (b)switching signals

(a)

(b)

Fig.9.Seven level inverter output waveforms (a)voltage (b)current

This paper proposed SEPIC fed single phase seven level inverter for a PV system. The

performance of the inverter is optimized by a fuzzy controlled SEPIC. The proposed FLC scheme

gives better performance in terms of overshoot and speed than PI controlled converters. A seven

level inverter topology with reduced switching devices is compared with three level inverter. The

simulation results show that the seven level inverter has improved output and the MPPT and FLC

scheme gives a better solution for the PV system.


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ijpam.eu A FUZZY CONTROLLED SEPIC FED SEVEN ......A FUZZY CONTROLLED SEPIC FED SEVEN LEVEL INVERTER FOR PHOTOVOLTAIC SYSTEM A.Bindu 1, M.Carolin Mabel 2, C.Bharatiraja 3 1CSI Institute