mppt enabled dc-dc converter-based bidirectional...

International Journal of Revolution in Electrical and Electronic Engineering (IJREEE) ISSN (Online): 2350 –0220, ISSN (Print): 2350 –0212

Volume 2 Issue 2 Jun 2015

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MPPT ENABLED DC-DC CONVERTER-BASED

BIDIRECTIONAL INVERTER FOR

RESIDENTIAL PHOTOVOLTAIC POWER SYSTEM

R.Thillaikarasi1, J.Mahadevan2

1PG Scholar, Department of Electrical and Electronics Engineering,

Dhanalakshmi Srinivasan College of Engineering and Technology, Chennai.

2Associate professor, Department of Electrical and Electronics Engineering,

Dhanalakshmi Srinivasan College of Engineering and Technology, Chennai.

Abstract— Photo-voltaic (PV) residential power system is an important application of renewable

energy. The parallel-connected configuration of PV modules rather than the series-connected

configuration become popular considering the safety requirements and making full use of the PV

generated power for the PV residential generation system. In this project, a novel soft- switching

voltage-fed full bridge front-end converter-based bidirectional inverter is proposed. The device

voltage is clamped naturally by secondary modulation without active clamping circuit or passive

snubber. Zero-current switching of primary devices and zero-voltage switching of secondary devices

is achieved. Soft-switching is inherent owing to proposed secondary modulation, load independent,

and is maintained during wide variation of input voltage and power transfer capacity, and thus is

suitable for PV applications. A laboratory prototype of the converter is built and tested to MPPT

control demonstrate the converter performance over wide variations in input voltage and output

power for PV applications. The validity of this topology is verified by computer simulation using

MATLAB/Simulink. The simulation results are presented.

Index Terms—Voltage-fed full bridge converter, Bi-direction inverter, MPPT controller, soft

switching, Residential photovoltaic (PV) power system.

I. INTRODUCTION

In this project, renewable energy has experienced impressive growth over the past decade due to the

fast depletion of fossil fuels, concern of energy security and green gas emission. Different renewable

energy resources like wind turbines, solar energy, etc., are integrated together with the energy storage

devices and relevant power conditioning, control and management systems to form a hybrid

distributed generation system to provide long-term sustainability.

Since distributed generation system is close to electricity users, it can overcome the inefficiency and

environmental issues from the centralized power plants. Among a variety of renewable energy

resources, solar photovoltaic (PV) has been proven to be very promising. Solar PV generation is

pretty flexible that is scalable from small-scale residential application to large-scale solar farms/power

plants.

However, PV modules have highly nonlinear voltage–current characteristics and the maximum power

point tracker (MPPT) varies dramatically with the ambient environmental factors such as solar

irradiance and temperature. For residential applications, the performance of PV inverter system is

easily to be affected by partial shadows and mismatch of electrical parameters. The configuration of

PV modules and corresponding power electronics design is crucial to draw maximum power from PV



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modules. In conventional centralized and string configuration of PV modules, a number of PV

modules are connected in series to obtain sufficient dc-link voltage for inversion operation. Building

integrated PV systems with several different power configurations have been evaluated considering

energy efficiency.AC module technology and PV dc-building-module (PVDCBM) based technology

are shown to be suitable for residential building integrated application for its excellent anti shading

and anti-mismatch performances.The high step-up dc/dc converter is used to boost the low output

voltage of the PV modules to a constant 200 or 400 V dc-link with MPPT operation. High

frequency (HF) transformer isolated dc/dc converter is preferred to obtain high step-up ratio and the

galvanic isolation between the PV modules and the utility. However, it is well known that the current-

fed converter suffers from high voltage spike across the switches at their turn-off.

II. OPERATION AND DESIGN CONSIDERATION

Fig 1 Block diagram

A.Proposed System Description

In general, photovoltaic power system consists of PV array, battery energy storage system,

high step-up dc-dc converter bidirectional inverter, dc loads and ac loads.

Isolated full bridge DC-DC converter with include a transformer, and thus can produce an

output of higher or lower voltage than the input by adjusting the turns ratio. Multiple

windings can be placed on the transformer to produce multiple output voltages.

Bidirectional inverter is used to make connection between dc and ac grid.

An electric battery is a device consisting of one or more electrochemical cells that

convert stored chemical energy into electrical energy. Each cell contains a positive

terminal, or cathode, and a negative terminal, or anode.

MOSFETs are used as a switching device in low voltage and high current applications.

Application requires high voltage, Insulated Gate Bi-polar Transistors (IGBT) are preferred

over BJTs, as the turn-on and turn-off times of IGBTs are lower than those of power

transistors (BJT), thus the frequency can be increased in the converters using them.

PWM is a technique used to generate analog output signal using digital signals. It is

commonly used to control average power delivered to a load, motor speed control, generating

analog voltage levels and for generating analog waveforms. All PWM modules in a PIC

Microcontroller uses Timer 2 for its operation, so you cannot set different frequencies for

different PWM modules.

http://en.wikipedia.org/w/index.php?title=Electrochemical_cell

http://en.wikipedia.org/w/index.php?title=Electrochemical_cell

http://en.wikipedia.org/w/index.php?title=Energy

http://en.wikipedia.org/w/index.php?title=Energy



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DC-BUS is technology for reliable and economical communication over noisy DC or AC

Power lines.The DC-BUS converts the digital input data into phase modulated signals,

protected against errors generated by noise over the power line. On the receiving side, the

received signal is demodulated into the original digital data.

B. Bidirection Dc-Dc converter

Fig 2 DC-DC Converter Operation circuit

In principle, bidirectional power transfer between two unipolar DC voltage sources may be

established with two unidirectional DC–DC converters C1 and C2. There, C1 is used to transfer

power from port 1 to port 2 (forward direction, forward operating mode) and C2 is needed to transfer

power in the opposite direction (backward direction, backward operating mode). In order to illustrate

an example of a practical converter realization including galvanic isolation, full bridge DC–DC

converters with high frequency (HF) transformers and output inductors are employed for C1and C2.

C. The Single-phase Dual Active Bridge (DAB) converter

It contains two voltage sourced full bridge circuits or half bridge circuits (or even push-pull circuits)

and a HF transformer. The reactive network simply consists of an inductor L connected in series to

the HF transformer; hence, the DAB directly utilizes the transformer stray inductance. Due to the

symmetric circuit structure, the DAB readily allows for bidirectional power transfer. The main

advantage of the DAB is the low number of passive components, the evenly shared currents in the

switches, and its soft switching properties. With the DAB converter topology, high power density is

feasible

Fig 3 Circuit Diagram of the single-phase DAB Converter



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D. Three-Phase DAB Converter

It uses three half bridges on theHV side and another 3 half bridges on the LV side. It requires 3

converterinductors and 3 HF transformers (which can be consolidated on a single threephaseHF

transformer. The three-phase DAB is operated with amodulation scheme similar to the conventional

modulation scheme employedfor the single-phase DAB (phase shift modulation. However, different

tothe single-phase DAB, further performance enhancements using alternativemodulation schemes are

not feasible for the three-phase DAB

Fig 4 Circuit Diagram of the Three-phase DAB Converter

E. The Bidirectional and Isolated Full Bridge DAB Converter

Converter contains a voltage sourced full bridge on the HV side and a current sourcedfull bridge on

the LV side (using the DC inductor LDC2). The power flow istypically controlled with the duty cycle

D. Additionally the LV side full bridge needs to be appropriately controlled in orderto allow for

bidirectional power flow. Thebidirectional and isolated full bridge converter facilitates ZVS operation

of theswitches of the HV side; the switches of the LV side switch at zero current.Thus, a high

switching frequency and a high power density are feasible atlow power levels, however, additional

circuitry is needed;Different to the DAB and the LLC converters, usable values for the transformer

turns ratio are limited.The isolated full bridge converter achieves a smooth current IcDC2 (t) and

therefore, compared to the DAB, a considerably lower capacitor RMS current IcDC2 results. A

disadvantage is the additional volume required for the DC inductor. Furthermore, the LV side

switches repeatedly connect the DC inductor LDC2 in series to the transformer stray inductance L and

thus, a snubber circuit is needed in order to avoid voltage spikes when switching. Instead of the LV

side full bridge converter, different AC–DC converter topologies can be used; typical substitutes are

the current doubler topology or the push-pull topology.

Table 1 Specifications of the Experimental Prototype

Parameters

Values

Peak Output Power

Po = 250W

Maximum Open Circuit Voltage of PV

VOC,MAX = 50V

Voltage of PV Module at MPP

Vin = 22V to 41V

Dc Link Voltage

VDC = 200V



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Output Voltage

VO = 110 ( RMS value )

Output Waveform Frequency

FO = 50 Hz

Switching Frequency of DC-DC

Converter

FS = 100 KHz

Switching Frequency of Inverter

FSI = 20 KHz

F. Design of the converter

In this Section, design procedure is illustrated by a design example for the specifications given in

TableI. Design procedure is executed corresponding to Vin = 22 V delivering 250 W to the load. The

design equations are presented to determine or calculate the components‟ rating.

1. Average input current is

Iin = in

O

VP

(1)

Assuming an efficiency of nearly 95%,Iin =11.96 A.

2. Maximum voltage across the primary switches is

VP,SW = n

VDC (2)

where n is turns ratio of HF transformer.

3. Voltage conversion ratio or input and output voltages are related as

VDC = d

Vn in

1 (3)

where d is the duty cycle of the gating signal for the primary devices of front-end converter. This

equation is derived on the condition that antiparallel diode conduction time (e.g., interval 6) is quite

short and negligible. However, at light load condition of converter, and the antiparallel diode

conduction time is comparatively large, (3) is not valid any more. Due to the existence of longer

antiparallel diode conduction period, the output voltage is boosted to higher value than that of

nominal boost converter. Therefore, (3) is modified into

VDC = '1 dd

Vn in

(4)

where 'd is given b

'd = d - 0.5-

DC

sTlkin

V

fLIn _4 (5)

From (5), it can be observed that for a given value of TlkL _ and VDC,'d increases as the load is

decreased. At full load, 'd = 0 and (5) is converted into (3).



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4. Total leakage inductance of the transformer or series inductance TlkL _ is calculated using

TlkL _ =

sin

DC

fIn

dV

5.0 (6)

5. Variation of clamped voltage across primary switches Vclamp for various transformer turns-ratio n.

It is easy to find that with the increase of turns ratio, the clamped voltage will decrease, which

permits use of low-voltage devices with low on-state resistance. Thus, conduction losses in the

primary side semiconductor devicescan be significantly reduced. Variation of dc voltage gain M

(VDC/Vin) with respect to duty cycle d for various values of turns ratio. In order to boost low MPP

voltage range 22–41 V of the PV module to a constant 200 V dc-link voltage, dc voltage gain M

of the front-end converter should cover the range 4.8–9.1. For the merits of flexibility of voltage

regulation, duty cycle d is constrained to be within 0.55–0.85. Considering overall conduction

losses and control flexibility, turns ratio of n =1 .82, duty ratio d =0 .8 is chosen. From (6), series

inductance TlkL _ = 27.6 μH for the given values.

6. RMS current through the primary switches of the frontconverter is given by

IS1,rms = Iin

3

2 d

(7)

The calculated value is IS1,rms = 7.56 A.

7. Value of boost inductor is given by

L =

sin

in

fI

dV

5.0

(8)

where ΔIin is the boost inductor ripple current. For ΔIin = 1 A,L = 66 μH. Unequal design of series

inductors Llk1 and Llk2 is also permitted. For such cases, (8) is modified into following:

L =

sin

Tlk

lkDCDC

in

fI

dLn

LV

n

VV

5.0

2 _

min_

(9)

where Llk_min is the minimum value of Llk1 and Llk2.

8. RMS current through the secondary side switches is

IS3,rms = 6

45 d

n

I in (10)

The calculated value IS3,rms = 3.5 A.

9. Inverter switches : Average current through the full-bridge inverter switches is given by

IS5,avg = O

O

V

P

2

(11)

Peak current through the inverter switches is

IS5,peak = O

O

V

P2

(12)

Average and peak currents through the switch are obtained as IS5,avg = 1.02 A and IS5,peak = 3.2 A.

Switches have to be rated for dc-link voltage, i.e., 200 V.



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III.MPPT CONTROLLER

Various algorithms may perform MPPT. Important factors to consider when choosing a technique to

perform MPPT are the ability of an algorithm to detect multiple maxima, costs, and convergence

speed.The irradiance levels at different points on a solar panel's surface tend to vary. This variation

leads to multiple local maxima power points in one system. The efficiency and complexity of an

algorithm determine if the true maximum power point or a local maximum power point is calculated.

In the latter case, the maximum electrical power is not extracted from the solar panel.

The type of hardware used to monitor and control the MPPT system affect the cost of implementing

it. The type of algorithm used largely determines the resources required to build an MPPT system.For

a high-performance MPPT system, the time taken to converge to the required operating voltage or

current should be low. Depending on how fast this convergence needs to occur and your tracking

system requirements, the system requires an algorithm (and hardware) of suitable capability.

A. Mppt algorithm

Start the program

Find out the voltage and current values

Calculate the current power values

power output of system is checked by varying the supplied voltage

If on increasing the voltage, power is also increases then further „δ‟ is increased otherwise

start decreasing the „δ'

While decreasing voltage if power increases the duty cycle is decreased.

This step till maximum power point is reached. The corresponding voltage at which MPP is

reached is known as reference point (Vref).

B. Perturb and Observe

The concept behind the "perturb and observe" (P&O) method is to modify the operating voltage or

current of the photovoltaic panel until you obtain maximum power from it. For example, if increasing

the voltage to a panel increases the power output of the panel, the system continues increasing the

operating voltage until the power output begins to decrease. Once this happens, the voltage is

decreased to get back towards the maximum power point. This perturb lance continues indefinitely.

Thus, the power output value oscillates around a maximum power point and never stabilizes.

Fig 5 Flowchart of P&O MPPT



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IV. SIMULATION RESULTS

Fig 6 Input Voltage

Thus the Fig.6 shows the Input voltage waveform. The input voltage is given into the converter. The

voltage is increased after some time it will get constant.

Fig 7 shows the Transform primary output voltage waveform

Fig 8 Transformer secondary Output Voltage



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Thus the Fig.8. shows the transformer secondary output voltage waveform. The transform AC output

voltage is constant. It does not depend upon the time.

Fig 9 Quadrupler Output Voltage

Thus the Fig.9. shows the Quadrupler output voltage waveform. In the quadrupler the output voltage

getting increased.

Fig 10 AC Output Voltage

Thus the Figure.10. shows the AC output voltage waveform.

Fig 11 Battery Output Voltage

Thus the Fig.11.shows the Battery output voltage waveform. In the battery the ouput voltage is

linearly increased.



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IV.CONCLUSION

This project proposes a two-stage single-phase inverter consisting of novel high step-up current-fed

push–pull front-end converter followed by full-bridge inverter for the PV residential power system.

Push–pull topology with voltage double configuration reduces the number of the switches. Maximum

solar efficiency would be achieved with the help of MPPT controller. The voltage quadruple

configuration reduces the number of the transformer windings. Voltage stress and no of turns would

be reduced by 1/4th. The Proposed system solves the basic problem of device turn-off in current-fed

converter and is absolutely new and innovative. Proposed modulation achieves zero current

commutation and natural voltage clamping of the devices without snubber or any auxiliary circuit. It

relieves the need of extra reactive snubber or active clamping circuit making it novel and snubber less.

Shoot through effect will be reduced drastically. Switching losses are reduced significantly owing to

ZCS of primary switches and ZVS of secondary switches. Soft switching permits high switching

frequency operation leading to design compact, low cost, light weight system and high efficiency over

wide range of load and input voltage.

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Up DC/DC Converters in Renewable Energy Applications” IEEE Transactions,2009.

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Transactions on Power Electronics Vol. 59, No. 4, 2011.

3.Doronshmilovitz,Yoash Levron,“Distributed Maximum Power Point Tracking in Photovoltaic

Systems–Emerging Architectures and Control Methods” Online ISSN 1848-3380, Print ISSN 0005-

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9.Adam Tomaszuk, Adam Krupa “ Step-up DC/DC converters for photovoltaic applications – theory

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mppt enabled dc-dc converter-based bidirectional...

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