fpga based transformer less grid connected inverter … · converter for photo voltaic applications...

@IJMTER-2015, All rights Reserved 242

FPGA based Transformer less grid connected inverter using boost

converter for Photo voltaic applications

1M.Subashini,

2S.Divyaprasanna,

3V.Chithirai selvi,

4K.Devasena

1,2,3,4Assistant Professor, Department of Electrical and Electronics Engineering,

Shanmuganathan Engineering College, Arasampatti, Pudukkottai, India.

Abstract— This project proposes a novel transformer less grid connected power converter with

negative grounding for a photovoltaic generation system. The negative terminal of the solar cell

array can be directly connected to the ground in the proposed grid-connected power converter to

avoid the transparent conducting oxide corrosion that occurs in some types of thin-film solar cell

array. The proposed grid-connected power converter consists of a dc–dc power converter and a dc–

ac inverter. The salient features of the proposed power converter are that some power electronic

switches are simultaneously used in both the dc–dc power converter and dc–ac inverter, and only two

power electronic switches operate at high switching frequency at the same time (one is in the dc–dc

power converter and the other is in the dc–ac inverter). The leakage current of the photovoltaic

generation system is reduced because the negative terminal of the solar cell array is connected

directly to the ground. Finally, a prototype was developed to verify the performance of the proposed

grid-connected power converter. The experimental results show that the performance of the proposed

grid-connected power converter is as expected.

Keywords- Boost converter, FPGA, solar panel, dc link voltage control, voltage source inverter

I. INTRODUCTION

The salient features of the reduction of the transformer and proposed power converter are

only two power electronic switches of the power converter are operated at high switching frequency

simultaneously (one is a dc–dc power converter and the other is a dc–ac inverter), and the negative

terminal of the solar cell array is directly connected to the ground to solve the problems of TCO

corrosion and leakage current for some types of thin-film solar cell array. The experimental results

show that the proposed grid-connected power converter can trace the maximum power point of the

solar cell array, convert solar power to a high quality ac power to inject into the utility, and reduce

the leakage current of the solar cell array. Using an isolation transformer in the grid-connected

inverter can solve the problem of the leakage current caused by the earth parasitic capacitance in

solar modules. There are two types of grid-connected inverter with an isolation transformer.

a) Line frequency transformer

b) High-frequency transformer.

The solar modules can be grounded directly and there is no current path for leakage current

because the line frequency transformer is isolated. This system supplies no dc current to the grid and

has the advantage of a simple control circuit. However, the line frequency transformer’s

disadvantages are large volume, high weight, and high cost. The wide use of fossil fuels has resulted

in the emission of greenhouse gases and the cost of fossil-fuel energy has become higher and higher.

Climate change, caused by these greenhouse gases, has seriously damaged the environment. Because

of the problems associated with climate change, interest in renewable energy sources, such as solar

power and wind power, has increased Many materials can be used to manufacture solar cells, but

polycrystalline Si and mono crystalline Si is the most widely used. A thin-film solar cell can generate

power under conditions of low irradiation. Therefore, the thin-film solar cell has the potential to

International Journal of Modern Trends in Engineering and Research (IJMTER) Volume 02, Issue 11, [November – 2015] ISSN (Online):2349–9745 ; ISSN (Print):2393-8161


generate electrical power for a longer time than a crystalline Si solar cell. Since the thin-film cell can

be easily combined with glass, plastic, and metal, it can be incorporated in green architecture. The

use of thin-film solar cells has increased steadily and this trend is set to continue in the future.

In general, an earth parasitic capacitance will be generated between solar modules and their

ground. This parasitic capacitance is about 50–150 nF/kW for a glass-faced solar cell array.

However, this capacitance is increased to 1 μF/kW if the thin-film solar cell array is used. Serious

leakage current occurs if a high-frequency pulsating voltage is applied between the thin-film solar

modules and the ground. Corrosion damage in thin-film modules, caused by a so-called transparent

conductive oxide (TCO) corrosion of cadmium telluride (Cd-Te) or amorphous silicon (A-Si), is

observed if the voltage of the negative terminal of a solar module is lower than that of the ground.

The damage to the electrical conductivity of the inside of the glass cover cannot be repaired and

causes substantial power loss. Consequently, the life of thin-film solar modules is shortened.

However, TCO corrosion can be prevented by the negative grounding of solar modules.

II. TRANSFORMERLESS GRID CONNECTED INVERTER

By overcoming the disadvantages of existing with transformer power converter and reduction

in cost simultaneously is a very challenging task. Removing the transformer, removing the negative

current & avoiding the leakage current of the converter modification are considered in this project. In

below we discuss briefly the proposed project.

The proposed Novel Transformer less Grid-Connected Power Converter are show in figure.5,

it’s consist of Solar Cell, DC – DC power converter, DC –AC inverter and load. D.C voltage is

generated from the Solar panel. It will be given to DC – DC power converter. Boost converter is one

of the SMPS topologies. SMPS circuit consists of the power circuit and the control circuit. The

power stage performs the basic power conversion from the input voltage to the output voltage and

includes switches and the output filter. Then output of the dc to dc converter is given to Inverter then

output of the sine wave connected grid as we as load, FPGA is programmed to generate PWM.

Switching pulse given to the MOSFET it is generated from PWM Controller. The PWM pulses are

given to input of Optocoupler. Optocoupler is used to isolate between Control circuit and driver

circuit.

III. PROPOSED CONTROL STRATEGY FOR BLDCM

Fig. 1 shows the circuit configuration of the proposed photovoltaic generation system. As can

be seen, the grid-connected power converter is transformer less, and its negative terminal is

connected directly to the ground. Both the problems of TCO corrosion and leakage current in Cd-Te

or A-Si thin-film solar cell array can be avoided. The proposed transformer less grid-connected

power converter is composed of a dc–dc power converter and a dc–ac inverter. The dc–dc power

converter is a boost converter. The dc–dc power converter consists of three dc capacitors C1, C2, and

C3, an inductor L1, two diodes D1 and D2, and four power electronic switches G1, G2, G3, and G5.

The dc–dc power converter converts the dc voltage of the solar cell array to a stabilized dc voltage.

The dc–ac inverter consists of two dc capacitors C2 and C3, an AC inductor Lf, and four power

electronic switches G4, G5, G6, and G7. The dc–ac inverter further converts the output dc voltage of

the dc–dc power converter into ac power and injects into the grid. As seen in Fig. 1, the power

electronic switch G5 controls both the dc–dc power converter and the dc–ac inverter. Fig.3 wave

forms are shows the timing sequence for power electronic switches of the proposed power converter

for positive half cycle. This is the timing sequence for power electronic switches during the positive

half-cycle of the utility voltage. During the positive half cycle of the utility voltage, the dc–dc power

converter changes are noted.



NEGATIVE HALF CYCLE

Fig.2 wave forms are shows the timing sequence for power electronic switches of the

proposed power converter for negative half cycle. For the negative half-cycle of the utility voltage,

the dc–dc power converter charges the capacitor C2, so the duty cycle of G1 controls the voltage of

capacitor C2. (Du 2 = ton2/T2) Capacitor C3 supplies power to the dc–ac inverter, and G4 of the dc–

ac inverter is switched at high frequency to control the output current.

Figure 1. Circuit configuration of the proposed transformer less power converter for photovoltaic generation system

Figure 2. Timing sequence for power electronic switches of the proposed power converter. (a) Positive half-cycle. G2,

G3, G4, G5, and G7are

Low frequency, G1 and G6 are high frequency



Figure 3. Timing sequence for power electronic switches of the proposed power converter for Negative half-cycle G2,

G3, G5, G6 and G7 are low frequency and G1, G4 are high frequency

MODES OF OPERATIONS

The operation of power electronic switches during the each half-cycle of the utility voltage

can also be divided into three time intervals. The current paths during these time intervals of the each

half-cycle of the utility voltage are shown in below modes of operations.

MODE - I OPERATION [T01, T02]

Figure 4. Current path under time duration [t01, t02]

The current path during this time interval is shown in Fig.4 G1 of the dc–dc power converter

is switched on to energize the inductor L1. Because G3 and G5 are both ON, a diode D2 is series

connected to G3 to prevent C3 from short circuit. In the dc–ac inverter, G6 is switched ON and the

output voltage Vo is +Vdc2. The output current of the dc–ac inverter is supplied from C2.



MODE - II OPERATION [T02, T03]


The current path during this time interval is shown in Fig.5. In the dc–dc power converter, G1

and G2 are switched OFF. G3 and G5 are still ON. The energy stored in the inductor L1 is de-

energized via the path D2, G3 and the flying diode of G5 to charge the capacitor C3. G6 of the dc–ac

inverter is still ON, and the output voltage Vo is still +Vdc2. The filter inductor of the dc–ac inverter

is energized from C2.

MODE - III OPERATION [T03, T04]

The current path during this time interval is shown in below Fig.6 In the dc–dc power

converter, the energy stored in the inductor L1 is still de-energized via the path D2, G3 and the flying

diode of G5 to charge the capacitor C3. G6 of the dc–ac inverter is switched OFF, and the output

current is flowing through G5 and the flying diode of G7 to form a loop. Therefore, the voltage Vo is

0, and the filter inductor of the dc–ac inverter is de-energized.




MODE - IV OPERATION [T11, T12]


The current path during this time interval is shown in Fig.7. G1 of the dc–dc power converter

is switched on to energize the inductor L1. Because G4 and G7 are both ON, a capacitor C3 is series

connected to G4 to prevent G7 from short circuit. In the dc–ac inverter, G4 is switched ON and the

output voltage Vo is +Vdc3. The output current of the dc–ac inverter is supplied from C3.

MODE - V OPERATION [T12, T13]


The current path during this time interval is shown in Fig.8. In the dc–dc power converter, G1

is switched OFF. G4 and G7 are still ON. And the switch G2 is going to ON. The energy stored in

the inductor L1 is de-energized via the path D1, G2 and the charge the capacitor C3. G7 of the dc–ac

inverter is still ON, and the output voltage Vo is still +Vdc2. The filter inductor of the dc–ac inverter

is energized from C2.

MODE – VI OPERATION [T13, T14]

The current path during this time interval is shown in below Fig.9. In the dc–dc power

converter, the energy stored in the inductor L1 is still de-energized via the path D1, G2 and the flying

diode of G5 to charge the capacitor C3. G6 of the dc–ac inverter is switched OFF, and the output



current is flowing through G5 and the flying diode of G7 to form a loop. Therefore, the voltage Vo is

0, and the filter inductor of the dc–ac inverter is de-energized.


IV. SIMULATION AND EXPERIMENTAL RESULTS

To verify the feasibility of the proposed strategy, simulations and experiments are carried out.

Figure 10. Boost converter based transformer less grid connected systems Simulink diagram

Figure 11. Boost converter based transformer less grid connected systems PWM generation Simulink diagram



Figure 12. Boost converter based transformer less grid connected systems output voltage

Figure 13. Boost converter based transformer less grid connected systems PWM pulses to the inverter

Figure 14. Boost converter based transformer less grid connected systems hardware view.



V. CONCLUSIONS

The implementation of transformer less grid connected power converter for photovoltaic

generation system was analyzed in this thesis. The electric circuit parameters of the power converter

were empirically derived using a series of measurements collected from experimental test. Using

these parameters the current, voltage was designed for the transformer less grid connected power

converter for photovoltaic generation system. The transformer less grid connected power converter

for photovoltaic generation system was modeled in Mat lab’s Simulink. With the simulated

performance of the power converter ability to arrive at a desired voltage and current, testing was

conducted in the lab using the various parameter of the power converter. The laboratory model of the

power converter constructed to collect data to the transformer less grid connected power converter

for photovoltaic generation system of an accurate manner.

REFERENCES

[1] U. Boeke and H. van der Broeck, “Transformer-less converter concept for a grid-connection of thin-film

photovoltaic modules,” in Proc. IEEE Ind. Appl. Soc. Annu. Meet, Oct. 5–9, 2008, pp. 1–8.

[2] Chen, W. Wang, C. Du, and C. Zhang, “Single-phase hybrid clamped three-level inverter based photovoltaic

generation system,” in Proc. IEEE Int. Symp. Power Electron. Distrib. Generation Syst., Jun. 16–18, 2010, pp. 635–

638.

[3] Kerekes, M. Liserre, R. Teodorescu, C. Klumpner, and M. Sumner, “Evaluation of three-phase transformer less

photovoltaic inverter topologies,” IEEE Trans. Power Electron., vol. 24, no. 9, pp. 2202–2211, Sep. 2009.

[4] S. V. Araujo, P. Zacharias, and B. Sahan, “Novel grid-connected non-isolated converters for photovoltaic systems

with grounded generator,” in Proc. IEEE Power Electron. Spec. Conf., Jun. 15–19, 2008, pp. 58–65.

[5] L. Ma, T. Kerekes, R. Teodorescu, X. Jin, D. Floricau, and M. Liserre, “The high efficiency transformer-less PV

inverter topologies derived from NPC topology,” in Proc. Eur. Conf. Power Electron. Appl., Sep. 8–10, 2009, pp.

1–10.

M.Subashini received the B.E. Degree in Electrical and Electronics Engineering from

GANADIPATHY TULSI'S Engineering College, Vellore, Anna University, Chennai, India

in 2007 and Post graduation in Power Electronics & Drives in Shanmuganathan

Engineering College, Pudukkottai under Anna University, Chennai, India in 2015. She is

currently working as assistant professor in Shanmuganathan Engineering College,

Pudukkottai.

S.Divyaprasanna received B.E degree in Electrical and Electronics Engineering from

Government College of Engineering, Bargur in 2003 and M.E degree in Power Electronics

and Drives from Government College of Engineering, Tirunelveli in 2010.She is currently

working as assistant professor in Shanmuganathan Engineering College, Pudukkottai.

V.Chithirai selvi was born in 1984. She finished her graduation in 2007 at Alagappa

Chettiyar College of Engineering and Technology, Karaikudi in the field of Electrical and

Electronics Engineering. She finished her master degree in the field of Power electronics

and drives in the year of 2015at Shanmuganathan engineering college, Arasampatti. Since

2011 she has been an assistant professor at Shanmuganathan Engineering College,

Pudukkottai.

K.Devasena received B.E degree in Electrical and Electronics Engineering from PSNA

college of Engineering and Tech and M.Tech degree in Power systems from PRIST

UNIVERSITY Thanjavur in 2013.Now She is currently working as assistant professor in

Shanmuganathan Engineering College, Pudukkottai.

fpga based transformer less grid connected inverter … · converter for photo voltaic applications...

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