This document summarizes a seminar report on a proposed transformer-less, two-stage, grid-connected photovoltaic system. The system consists of solar panels, a boost converter-based MPPT charge controller in the first stage to regulate the variable output of the solar panels. A neutral point clamped inverter is used in the second stage to convert the DC output to AC for connection to the grid. The neutral point clamped inverter offers improved output waveforms with lower harmonic distortion compared to conventional inverters, allowing for a smaller filter size. The proposed topology aims to improve efficiency and reliability while eliminating the need for an isolation transformer.

1. TRANSFORMER LESS FPGA CONTROLLED 2-STAGE
ISOLATED GRID CONNECTED PV SYSTEM
A seminar done in partial fulfillment of the requirements for the award of
MASTER OF TECHNOLOGY
IN
POWER SYSTEM AND CONTROL - ELECTRICAL ENGINEERING
OF
UNIVERSITY OF KERALA
BY
ANOOP S
(14401005)
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
GOVERNMENT ENGINEERING COLLEGE
BARTON HILL

2. GOVERNMENT ENGINEERING COLLEGE, BARTON HILL,
THIRUVANANTHAPURAM-695035
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
2014-2016
CERTIFICATE
This is to certify that the report entitled “Transformer Less FPGA Controlled 2-
Stage Isolated Grid Connected PV System” is a bonafide record of seminar
presented by ANOOP S (14401005) towards partial fulfillment of the requirements
for the award of the Master of Technology in Power System and Control –
Electrical Engineering of the University of Kerala.
Dr.K N Pavithran
Adjunct Professor
in EEE
Prof. K L Sreekumar
Assistant Professor
in EEE
Prof.Sheela S
Professor & Head of Dept.
EEE

3. ACKNOWLEDGMENT
First of all I thank the Almighty God for the blessings showered upon me during
the presentation.
I would like to express my sincere gratitude to Prof. Sheela S, Professor and Head
of the Dept. Electrical and Electronics Engineering for her continuous support in
accomplishing the presentation.
I express my heartfelt thanks to Professors K L Seekumar , Dr. K N Pavithran ,
Dr. J S Savier ,Dept. of EEE for their valuable suggestions ,advice and guidance
throughout the preparation of seminar .
Last but not the least, I place on record my gratefulness to my parents, friends and
classmates for their suggestions, criticisms and assistance towards the
improvement and successful completion of the report.
Anoop S

4. ABSTRACT
The grid-connected photovoltaic systems are an important part of renewable
energy sources and their integration is getting more and more widespread. In order
to improve the efficiency, practicality and reliability of the PV systems, many
kinds of new inverter topologies have been proposed to avoid using a grid
isolation transformer. The neutral point clamped inverter topology is discussed. In
this topology no common mode voltage is generated, thus changes in the behavior
of the inverter in terms of high efficiency and ensures that no DC will be injected
into the load. Constant voltage MPPT charge controller is designed based on small
signal analysis of converter. After this charge controller output is fed to multilevel
inverter for the conversion of dc to ac. Proposed neutral point clamped inverter is
offering very low line voltage THDs compared with conventional inverter; offering
less size and cost of the filter.

5. CONTENTS
1. INTRODUCTION....................................................................................................1
2. PROPOSED PV SYSTEM TOPOLOGY ................................................................2
3. PV MODULE...........................................................................................................3
4. BOOST CONVERTER BASED MPPT CHARGE CONTROLLER .....................5
4.1 MODES OF OPERATION....................................................................................5
4.1.1 Charging Mode................................................................................................5
4.1.2 Discharging Mode...........................................................................................6
4.2 MAXIMUM POWER POINT TRACKING .........................................................6
4.2.1 Methods for MPPT..........................................................................................6
5. NEUTRAL POINT CLAMPED INVERTER..........................................................7
5.1 TOPOLOGY..........................................................................................................8
5.2 OPERATION.........................................................................................................9
5.3 FIRING SCHEME...............................................................................................11
6. CONCLUSION ......................................................................................................12
7. REFERENCES.......................................................................................................13

6. LIST OF FIGURES
Figure 1: Block diagram of proposed system 2
Figure 2: Equivalent circuit of solar cell 4
Figure 3: P-V I-V curve of a solar cell at given temperature and solar irradiation 4
Figure 4: Boost converter 5
Figure 5: Topology of neutral point clamped inverter 8
Figure 6: Line voltage waveforms 10
Figure 7: Gate pulses generated from FPGA 11

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1. INTRODUCTION
A grid-connected photovoltaic power system or grid-connected PV system is electricity
generating solar PV system that is connected to the utility grid. A grid-connected PV
system consists of solar panels, one or several inverters, a power conditioning unit and
grid connection equipment. They range from small residential and commercial rooftop
systems to large utility-scale solar power stations. Unlike off-grid systems, a grid-
connected system rarely includes an integrated battery solution, as they are still very
expensive. When conditions are right, the grid-connected PV system supplies the excess
power, beyond consumption by the connected load, to the utility grid.
Two main topologies have been stated in the photovoltaic system i.e. with and without
the galvanic isolation. The main aim of the galvanic isolation is to offer safety for the
user, but this decreases the overall efficiency of the system. In the case of the transformer
less system the efficiency of the system raises up. The most important advantage of the
Transformer less system is that it offers higher efficiency, smaller in size and petite in
weight as compared to system with transformer.
PV inverter, which is the heart of a PV system, is used to convert dc power obtained from
PV modules into ac power to be fed into the grid. Improving the output waveform of the
inverter reduces its respective harmonic content and, hence, the size of the filter used and
the level of Electromagnetic Interference (EMI) generated by switching operation of the
inverter. In recent years, multilevel inverters mainly neutral point clamped inverter is
being used. They offer improved output waveforms, smaller filter size and lower EMI,
lower Total Harmonic Distortion (THD). The use of multi-level inverters also eliminates
the use of grid isolation transformers that are usually used for providing personal
protection and avoiding leakage currents between the PV system and the ground.

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2. PROPOSED PV SYSTEM TOPOLOGY
Figure shows the block diagram of the proposed transformer less system which consists
of two stages. In this system a solar array has been constructed using solar cells by
combining it into series and parallel combinations. The output of solar cell is variable so
we have deployed a DC-DC converter which converts variable DC into fixed DC, this is
done in first stage. In second stage DC is converted into AC which will be utilized by the
appliances.
Figure 1: Block diagram of proposed system

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3. PV MODULE
The basic building block of a photovoltaic module is the photovoltaic cell; these convert
solar energy into electricity. The power output will depend on the amount of energy
incident on the surface of the cell and the operating temperature of the photovoltaic cell.
The power output of a single cell can supply small loads like calculators or watches, but
in order to be useful for high energy demand projects these cells must be arranged in
series and parallel connections. A photovoltaic module is an array of photovoltaic cells
pre-arranged in a single mounting mold. The type of module is therefore determined by
the cells that compose the module itself. There are three dominating cell technologies:
 Monocrystalline: As the name implies, these are cells that are grown from a single
crystal. The production methods are difficult and expensive. These tend to be more
efficient (more power in less area) and more expensive.
 Multicrystalline: The production process allows multiple crystalline structures to
develop within the cell. It is easier to implement in a production line. It is
relatively cheaper than monocrystalline at the expense of lower efficiency.
 Thin-film: Uses less silicon to develop the cell allowing for cheaper production
costs. It tends to be less expensive but also lower efficiency
An ideal solar cell may be modelled by a current source in parallel with a diode; in
practice no solar cell is ideal, so a shunt resistance and a series resistance component are
added to the model.

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Figure 2: Equivalent circuit of solar cell
Figure 3: P-V I-V curve of a solar cell at given temperature and solar irradiation

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4. BOOST CONVERTER BASED MPPT CHARGE CONTROLLER
DC-DC Boost converter is used to magnify the voltage from PV to a suitable form of
energy accepted by the load. Boost converter is a second order system consists of an
inductor, a capacitor, a diode, and with the load resistance connected in parallel with the
capacitor. As the output from PV is not constant due to the ambient temperature and
environmental condition, the modeling of such converter is crucial.
Figure 4: Boost converter
4.1 MODES OF OPERATION
There are two modes of operation of a boost converter. Those are based on the closing
and opening of the switch. The first mode is when the switch is closed; this is known as
the charging mode of operation. The second mode is when the switch is open; this is
known as the discharging mode of operation.
4.1.1 Charging Mode
In this mode of operation; the switch is closed and the inductor is charged by the source
through the switch. The charging current is exponential in nature but for simplicity is
assumed to be linearly varying. The diode restricts the flow of current from the source to
the load and the demand of the load is met by the discharging of the capacitor.

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4.1.2 Discharging Mode
In this mode of operation; the switch is open and the diode is forward biased . The inductor
now discharges and together with the source charges the capacitor and meets the load
demands. The load current variation is very small and in many cases is assumed constant
throughout the operation.
4.2 MAXIMUM POWER POINT TRACKING
The efficiency of a solar cell is very low. In order to increase the efficiency, methods are to
be undertaken to match the source and load properly. One such method is the Maximum
Power Point Tracking (MPPT). This is a technique used to obtain the maximum possible
power from a varying source. In photovoltaic systems the I-V curve is non-linear, thereby
making it difficult to be used to power a certain load. This is done by utilizing a boost
converter whose duty cycle is varied by using a mppt algorithm. Few of the many
algorithms are listed below.
4.2.1 Methods for MPPT
There are many methods used for maximum power point tracking a few are listed below:
• Perturb and Observe method
• Incremental Conductance method
• Parasitic Capacitance method
• Constant Voltage method
• Constant Current method

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5. NEUTRAL POINT CLAMPED INVERTER
Inverters are used to transform dc current to ac currents. The voltage and current
waveforms produced by inverters are never perfect sinusoids, therefore some
harmonic currents are expected during normal operation. Total harmonic distortion
(THD) is a measure of the harmonic content in current and voltage waveform in
order to improve the output voltage waveform and to eliminate the use of isolation
transformers multilevel inverter topologies are used. The most commonly used
multilevel topology is the diode clamped inverter, in which the diode is used as the
clamped device to grip the dc bus voltage so as to attain steps in the output voltage.
A neutral point clamped (NPC) inverter system has a DC power and an NPC inverter
having a neutral point connected to the positive and negative poles of the DC power
source to convert the DC voltage into AC voltage characterized in that first and
second branch means having a switching element provided between the positive
and negative poles sides of the DC power source and the neutral point of the NPC
inverter, and control means for turning the switching elements of the second and
first branch on when short-circuit current of the NPC inverter flow through the
neutral point of the NPC inverter.

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5.1 TOPOLOGY
Figure 5: Topology of neutral point clamped inverter
Figure shows the main circuit of the NPC-PWM inverter. (A1,A4), (B1, B4), (C1, C4) are
main transistors operating as switches for PWM; and (A2, A3), (B2, B3), (C2, C3) are
auxiliary transistors to clamp the output terminal potentials to the neutral point potential,
together with diodes. To this inverter, all conventional PWM techniques can be applied.
Auxiliary transistors (A 3, A 2) are driven complementary to the main transistors (A1 , A4),
respectively. With such control, each output terminal potential is clamped to the neutral
potential in the off-periods of the PWM control.
The phase outputs are the center point of a series connection of four IGBTs , and the DC
bus input is connected to the top and bottom row of devices, Al, Bl, CI, and A4, B4, C4,
R
Y
B

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respectively. The center point of the DC bus is shown by a ground symbol and is
connected between a pair of series connected diodes in each phase. These six clamping
diodes connected to the neutral bus control the voltage distribution among the four
IGBTs in each phase leg. A conventional inverter requires the switches to sustain the full
voltage drop between the positive and negative DC buses. However, the voltage drop
(stress) across each switch of the NPCI is one half of the voltage between the positive and
negative bus since the switches on either side of the neutral bus are in series, and an
actual neutral point exists. Each IGBT has an individual gate signal that must be
referenced between the respective IGBT gate and emitter terminal. The diode shown
between the collector and emitter of each IGBT is an internal "body diode" inherent to
the IGBT device structure.
5.2 OPERATION
This specific NPCI topology uses 3-level switching instead of 2-level switching used in
conventional 3-phase inverters. The three levels correspond to the positive, negative, and
neutral buses. Taking leg A of Figure 4 as an example, the phase output A is connected to
the positive bus by turning on switches Al and A2. Turning switches A3 and A4 on
connect the phase A output to the negative bus, and turning switches A2 and A3 on
connects the phase A output to the neutral bus. The other two phases operate in the same
manner, but with phase shifted results with respect to phase A. The phase and line
voltages are given by
Pole voltages VRO = 0, ± , VYO = 0, ± , VBO = 0, ±
Line voltage VRY = 0, ± , ± Vdc
Phase voltage VRN = 0, ± , ± , ± , ±

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Figure 6: Line voltage waveforms

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5.3 FIRING SCHEME
The output terminal voltages of the neutral point clamped inverter based on the on off
position of the switches of one leg are given below
From the table it can be observed that the switches A1 and A3 are complimentary and A2
and A4 are also complimentary. Hence we need to produce only to gating pulses i.e for
A1 and A4. Gate signals for A2 and A3 can be obtained by complimenting firing pulses
of A1 and A4 respectively
Figure 7: Gate pulses generated from FPGA
A1 A2 A3 A4 VRO
1 1 0 0 +𝑉𝑑𝑐
2
0 1 1 0 0
0 0 1 1 −𝑉𝑑𝑐
2

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6. CONCLUSION
Transformer less inverters offers a better efficiency, compared to those inverters that have a
galvanic isolation. In the proposed topology no common mode voltage is generated, thus
changes in the behavior of the inverter in terms of high efficiency and insures that no DC
will be injected into the load. After this charge controller output is fed to multi-level
inverter for the conversion of dc to ac. The proposed multilevel inverter using neutral point
clamped inverter is offering very low line voltage THDs compared with conventional
inverter; offered less size and cost of the filter. Leakage current will be eliminated since
midpoint of capacitors is connected to ground. It also ensures better waveform quality and
harmonic elimination. The power devices and the DC-link capacitors have to stand only one
half of the DC-link voltage and hence the switching losses will also be less.

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7. REFERENCES
1. Dogga Raveendhra, Saad Faruqui and Parvesh Saini,“Transformer Less FPGA
Controlled 2-Stage Isolated Grid Connected PV System”, 2014 Power and
Energy Systems: Towards Sustainable Energy (PESTSE 2014)
2. Y. Xue, K. e. Divya, G. Griepentrog, M. Liviu, S. Suresh, and M. Manjrekar,
"Towards next generation photovoltaic inverters" ,in Froc. iEEE Energy
Converso Congr. Expo., 2011, pp. 2467-2474
3. Raveendhra, D.; Pathak, M.K.; Panda, A, "Power conditioning system for solar
power applications: Closed loop DC-DC convertor fed FPGA controlled diode
clamped multilevel inverter," Electrical, Electronics and Computer Science
(SCEECS), 2012 IEEE Students' Conference on, vol., no., pp.l,4, 1-2 March 2012