View of A STUDY ON PV FED MULTILEVEL CONVERTERS

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Vol.03, Issue 06, June 2018, Available Online:www.ajeee.co.in/index.php/AJEEE

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A STUDY ON PV FED MULTILEVEL CONVERTERS Madhur Ratan¹ And Hemant Kumar Verma²

1,2Department Of Electrical & Electronics Engineering, SSTC, SSGI-FET, Bhilai (C.G.) India

Abstract – This review paper is discussed about PV connected to the single phase grid. Also classical multilevel inverters are discussed. Normally the line commutated inverter has square shaped waveform of line current. So it has large amount of harmonics and produces excess of heat which causes damage to the winding of transformer. Multilevel inverter gives the output current waveform which is nearly sinusoidal in nature. This line current has fewer harmonics as compared to line commutated inverter. Basically multilevel inverter produces the output voltage by synthesis of several DC sources i.e. by increasing of DC voltages, the levels of the output increases. Multilevel inverter also used for electric drives applications for same reason i.e. harmonic reduction. The simulation can be done by MATLAB/SIMULINK. In simulation model we can use DC source instead of PV module.

Keywords – PV systems, DC/AC converter, PWM I. INTRODUCTION

As we know, due to environmental concern the solar PV based systems are mostly used in the field of power generation. Therefore there is lot of research is going on to increase the efficiency of photovoltaic power plant and decrease the system losses and cost. In 1975, the three level inverter is firstly utilized as a multilevel inverter [1-3].

Multilevel inverter is fundamentally a force electronic piece that gives the coveted voltage yield from various DC voltages as info i.e. by expanding the number of DC voltage origins, the converter yield voltage waveform methodologies a virtually a sinusoidal shape.

Multilevel converter is utilized for the lessening of THD in line current. In any case, the converters utilizing IGBT/MOSFET has certain hindrances like higher switching misfortunes, more level power taking care of limit and reliability. So inverters utilizing thyristors/IGBT/MOSFET as switching device [ 3-4]. Provincial line commutated inverters have square shaped wave of line current. In this manner, it has higher request sounds. Lines present with higher sounds generate EMI and great warming of heart of distribution/power transformer

[5-6]. So, grid connected inverter model using thyristors is useful.

II. REQUIREMENTS OF GRID, MODULE AND OPERATOR

 Requirements of grid

On investigating that the inverter is framework joined, certain norms given by utility group should be obeyed identified with crushing, power value and discovery of islanding operation. i. e. PV modules ought to be framework pounded and screened for ground blames when it achieves greatest esteem of yield voltage, the inverter might as well have the capacity to distinguish islanding scenario and take fitting cure for protection [2].

 Requirements of module

Nowadays, the mono-crystalline and multi-crystalline-silicon modules are mostly used for generation of maximum power from PV modules. Due to this each module of inverter must operate at MPP where maximum output energy can be obtaining [7].

 Requirements of operator

Finally, according to operator, the inverter must satisfy some conditions as follows:

 It should be cost effective.

 It should have high efficiency.

 It should have long operational life time.

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2 III. DEVELOPMENT OF PV MODULES

CENTRALISED INVERTER

STRING INVERTER

MULTISTRING INVERTER

AC MODULE

PV CELL INVERTER

Fig.1.Classification of PV cell inverter A. CENTRALISED INVERTER

In this, the strings of PV modules are connected in parallel with each other through string diodes for reaching high power levels [8]. Finally this arrangement is connected to the inverter. Centralized inverter has some limitations as follows:

i. High voltage DC links between the PV module and the inverter.

ii. Power misfortunes because of incorporated MPPT.

iii. Mismatch misfortune between PV modules.

iv. Losses in string diodes.

Grid connected inverter is generally line commutated using thyristors, which involves many harmonics in current and has poor power quality. Since it produces large harmonics, the need arises to invent new topology.

DC AC STRING

DIODE

THREE PHASE CONNECTION

PV PV

SINGLE PHASE CONNECTION

DC AC DC

AC

PV PV

Fig.2. Centralized inverter Fig.3.String inverter B. STRING INVERTER

This is developed technology of centralized inverter technology [9]. In this single string is connected to inverter. It has several advantages i.e.

i. No losses associated with string diodes and separate MPPT can be applied to each string.

ii. Overall efficiency increases.

iii. Reduces prize due to mass production

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3 C. MULTI-STRING INVERTER

DC AC DC

AC

PV PV

SINGLE PHASE or THREE PHASE CONNECTION

DC AC

SINGLE PHASE CONNECTION

DC AC DC

AC PV PV

Fig.4. Multi-string inverter Fig.5. AC module technology It's the further improvement in string

inverter. In this innovation some strings are associated to their particular DC converter and at last to the AC inverter.

Each string might be regulated separately.

This framework has heightened efficiency [2, 3, 4].

D. AC MODULE TECHNOLOGY

It is combination of inverter and PV module into one electrical circuit. It is seen as future technology for power generation. [10-11]

It has following advantages:

 It removes mismatch losses between PV modules.

 Due to their modular structure, it provides facilities to enlarge the system.

 It requires less manufacturing cost.

IV. DIFFERENT TYPES OF MULTILEVEL INVERTER TOPLOGY Nowadays, multilevel inverters are widely used in power industries. It starts from three level inverter and five level and so on. Voltage unbalance problem is one of the major issues in working of multilevel inverter. So to reduce it, there are main three types of multilevel inverter are used.

These are as follows:

1. DIODE-CLAMPED MULTILEVEL INVERTER

The diode-clamped multilevel inverter is shown in fig.6. It requires (m-1) capacitors on dc bus to produce m levels of voltage [5].

Fig.6. Diode-clamped MLI [5] Fig.7. Flying Capacitor Based MLI [6]

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4 It has different features like, high-voltage rating required for blocking diodes, unequal device rating and capacitor voltage unbalance.

Advantages:

a. High efficiency.

b. No need of filters to reduce harmonics.

c. Reactive power flow can be controlled.

Disadvantages:

a. For high levels, more number of diodes are required.

b. Real power flow control for individual converter is difficult.

2. FLYING CAPACITORS MULTILEVEL INVERTER

Figure 7 shows the flying capacitors based multilevel inverter topology. It requires (m-1) capacitors on dc bus form level converter.

Advantages:

a. Extra ride through capability during power outage.

b. No need of filters to reduce harmonics.

c. It gives proper switching combination to balance different voltage levels.

d. Real and reactive power flow can be controlled.

Disadvantages:

a. High numbers of capacitors are required for high level.

b. For real power transmission switching frequency and losses are high [5-6].

3. CASCADE H-BRIDGE MULTILEVEL INVERTER WITH SEPARATE DC SOURCES

Figure 8 demonstrates the course multilevel inverter with isolated DC sources. This inverter can maintain a strategic distance from additional bracing diodes or voltage adjusting capacitors-[5- 6].

Advantages:

a. Less number of components is needed for getting same number of voltage level.

b. No need of extra diodes and capacitors.

c. Because of same structure it allows the scalable, modularized circuit layout and packaging.

Disadvantages:

a. Separate DC sources are required for the real power conversion.

Vdc

S1a S1b

Vdc

S1a^, S1b^,

S2a S2b

S2a^, S2b^,

Va1

Va2 Vo

Fig.8. Cascaded H-Bridge Multilevel Inverter [5].

V. MODULATION METHODS FOR INVERTERS

According to switching frequency, following types of modulation techniques are used.

According to fundamental switching frequency:-

a. Space vector control.

b. Selective harmonic elimination technique.

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5 According to high switching frequency

PWM a. Space vector PWM.

b. Sinusoidal PWM.[7]

VI. GRID CONNECTED MULTILEVEL INVERTER FOR SOLAR PV BASED SYSTEM

As of late, renewable force causes are utilized for cycle of force. It happens because of the different ecological issues. The force produced by renewable life causes is given to the network. Be that as it may for supplying power to the lattice, it may as well be AC force. If there should be an occurrence of sun oriented PV based framework, the produced power is DC. Thusly the multilevel inverter is utilized for the change of this DC power into AC. Fig. 9 Indicates the essential square chart of matrix joined multilevel inverter topology [8].

PV

Source _Converter^Boost Multilevel GRID

Converter

Fig.9. Block Diagram of Grid Connected MLI 1. GRID CONNECTED MULTILEVEL

INVERTER USING MULTI-WINDING TRANSFORMER.

This sort of inverter is essentially easy recurrence DC/AC inverter. DC voltage of the root is changed over into the easy recurrence square wave AC voltage. This is done by H-connect. This square wave include voltage is having an association with the essential of multi-winding transformer. Then transformer changes over this info into different synchronous level recurrence yield with some adequacy. Beyond any doubt, by deciding on respectable switching plan we can get the coveted yield waveform. This inverter takes on the synthesis of info AC voltages [9-11]. As the number of data stages builds, the THD [total symphonists distortion] abatement's.

VII. CONCLUSION

This paper focuses on the various factors on which the working of PV inverter depends. Also it discussed the development of PV based inverter and classical multilevel inverter. This review paper explained about the various modulation techniques used to trigger the semiconductor devices. But PWM methods have some disadvantages like high switching frequency, low power handling capability and reliability. For reason behind, this paper suggests the topology i. e. multilevel inverter using multi-winding transformer.

REFERENCES

1. Adil Sarwar, Mohammad S. J. Asghar,

“Simulation and Analysis of A Multilevel Converter Topology for Solar PV Based Grid Connected Inverter. Smart Grid and Renewable Energy, 2011. 2, pp. 56-62.

2. Soeren Baekhoej Kjaer, Member, IEEE, John K. Pedersen, Senior Member, IEEE, and Frede Blaabjerg, Fellow, IEEE,” A Review of Single-Phase Grid-Connected Inverters for Photovoltaic Modules.” IEEE Transactions on Industry Applications, vol.

41, no. 5, October 2005.

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IEEE Transactions on Industrial Electronics, vol. 49, no. 4, august 2002.

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“multi-winding transformer based diode clamped multilevel inverter.” IEEE symposium on industrial electronic and applications October 2010 Penang Malaysia.

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11. Abu Tariq Mohammed Aslam Husain Mohammad Ahmad Mohd. Tariq,

“Simulation and study of a grid connected Multilevel Converter with varying DC input”

IEEE conference 2011.