IRJET- A Review on Solar based Multilevel Inverter with Three Phase Grid Supply
Mini_Project
1. Modelling and Simulation of a
Solar Photovoltaic System With
P&O MPPT Control
Mini Project Report Submitted in partial fulfilment of the requirements for the award of the
degree
Bachelor of Technology
In
Electrical and Electronics Engineering
Under the guidance of
Dr. R. Kalpana
Assistant Professor
Department of Electrical and Electronics Engineering
National Institute of Technology Karnataka
Kunal Jha (13EE127)
Pratheek Rajan (13EE138)
S Harshavardhana Reddy (13EE142)
2. 2
CERTIFICATE
This is to certify that the Project Report titled “Modelling and Simulation of a
Solar Photovoltaic System with P&O MPPT control” is a bonafide work carried
out by:-
Kunal Jha (13EE127)
Pratheek Rajan (13EE138)
S Harshavardhana Reddy (13EE142)
, the Students of 5th semester B.Tech Electrical & Electronics Engineering,
National Institute of Technology Karnataka-Surathkal. This project has been
completed under the guidance of Dr. R Kalpana, Assistant Professor, Electrical &
Electronics Engineering, NITK-Surathkal during the academic year 2015-16 as a
design and development task in power electronics (EE348).
Signature of the Guide
Date:-
3. 3
Acknowledgement
It is our privilege to express our heartfelt gratitude for our guide Dr. R. Kalpana, Assistant
Professor, Department of Electrical and Electronics Engineering, National Institute of
Technology Karnataka, Surathkal for her invaluable guidance, inspiration and timely
suggestions and encouragement which facilitated the entire process of bringing out this report
on “Modelling and Simulation of a Solar Photovoltaic System with P&O MPPT control” .
We would also like to thank Mr Saravana Purushothaman for his continuous support and
help in the completion of this project.
Last but not the least, we thank our families and our friends for their motivation,
encouragement and moral support throughout the duration of the mini project.
4. 4
Table of Contents
Topics Page No.
1. Plan of work 5
2. Abstract
3. Introduction
6
7
4. Domain knowledge
4.1 PV Systems 8
4.2 MPPT Techniques 8
4.2.1 Classifications 9
4.2.2 Comparison of Methods 10
4.3 DC-DC Boost Converter 10
4.4 Three Phase Three Leg Inverter 11
4.5 Low Pass Filter 12
5. Design of Subsystems
5.1 Design of Solar PV Model 13
5.2 Design of MPPT Based Pulse Width Modulator 14
5.3 Design of Boost Converter 15
5.4 Design of Three Phase Three Leg Inverter 16
6. Block Diagram and Control Schemes
7. Modelling and Simulations
17
19
8. Results 28
9. Conclusion and future work 29
5. 5
1. Plan of Work
Sr. Task Name Period
Start Date End Date
1 Simulation of Solar Model July 23 July 28
2 Simulation of MPPT Subsystem July 28 August 18
3 Simulation of Boost Converter August 18 August 25
4 Simulation of 3-Phase 3-Leg Inverter August 25 September 15
5 Simulation of Auto Connected Transformer September 15 September 29
6 Harmonic Analysis of Waveforms September 29 October 13
7 Design of Low Pass Filter October 13 November 3
8 Result Compilation November 3 November 17
6. 6
2.Abstract
Photovoltaic (PV) systems are solar energy supply systems, which either supply power directly
to an electrical equipment or feed energy into the public electricity grid. In the photovoltaic
system, power electronic conversion is necessary to improve the efficiency of PV panels and
system stability. In these systems, the backstage power circuit consists of a high step-up DC
to DC converter and a three leg inverter to convert DC to AC, as the load voltage is AC in
nature. A 12 -pulse Auto-connected transformer is used to reduce harmonics in the output
waveform. Also a low pass filter is used which minimizes the Total Harmonic Distortion
(THD) caused by inverter so that the system is within its acceptable limits. A feedback control
circuit employing Maximum power point tracking technique (MPPT) is used in boost
converter, so as to regulate the converter output voltage. Control circuit is required to get
constant output voltage at load side as PV systems output voltage is continuously varying in
nature. This mini-project highlights the analysis of a three phase off-grid solar photovoltaic
system and grid connected solar photovoltaic system. The grid connection is implemented
using a VSC control topology inorder to adjust the waveform to meet the requirements of the
Utility Grid. System model is formulated and simulation is carried out.
7. 7
3.Introduction
With the increasing concern about the non-renewable energy sources, constant increase in fossil fuel prices,
global warming, damage to environment and ecosystem, the renewable energy is becoming more popular and
is gaining more attention as an alternative to non-renewable energy sources. Among the renewable energy
sources, the energy through photovoltaic effect is being considered as the most essential and sustainable
energy resource as compared to other types of energy sources such as wind, tidal etc. Solar energy is a kind
of energy which converts solar radiation into electricity. The PV system connected to grid is called Grid
Connected PV System. Grid connected PV system have become more popular because of their applications
in distributed generation and for effectively using the PV array power.
Major system components
Solar PV system includes different components that should be selected according to your system type, site
location and applications. The major components for solar PV system are solar charge controller, inverter,
battery bank, auxiliary energy sources and loads (appliances).
PV module – converts sunlight into DC electricity.
Solar charge controller – regulates the voltage and current coming from the PV panels going to battery
and prevents battery overcharging and prolongs the battery life.
Inverter – converts DC output of PV panels or wind turbine into a clean AC current for AC appliances or
fed back into grid line.
Battery – stores energy for supplying to electrical appliances when there is a demand.
Load – is electrical appliances that connected to solar PV system such as lights, radio, TV, computer,
refrigerator, etc.
Auxiliary energy sources – is diesel generator or other renewable energy sources.
Advantages of solar power:
Solar energy is a clean and renewable energy source.
Once a solar panel is installed, solar energy can be produced free of charge.
Solar energy will last forever whereas it is estimated that the world’s oil reserves will last for 30 to 40
years.
Solar energy causes no pollution. Solar cells make absolutely no noise at all. On the other hand, the giant
machines utilized for pumping oil are extremely noisy and therefore very impractical.
Very little maintenance is needed to keep solar cells running. There are no moving parts in a solar cell
which makes it impossible to really damage them.
In the long term, there can be a high return on investment due to the amount of free energy a solar panel
can produce, it is estimated that the average household will see 50% of their energy coming in from solar
panels.
8. 8
4.Domain Knowledge
4.1 Working of a PV Cell
A typical silicon PV cell is composed of a thin wafer consisting of an ultra-thin layer of phosphorus-doped
(N-type) silicon on top of a thicker layer of boron-doped (P-type) silicon. An electrical field is created near
the top surface of the cell where these two materials are in contact, called the P-N junction. When sunlight
strikes the surface of a PV cell, this electrical field provides momentum and direction to light-stimulated
electrons, resulting in a flow of current when the solar cell is connected to an electrical load
Fig. 1 Working of PV Cell
Regardless of size, a typical silicon PV cell produces about 0.5 – 0.6 volt DC under open-circuit, no-load
conditions. The current (and power) output of a PV cell depends on its efficiency and size (surface area), and
is proportional to the intensity of sunlight striking the surface of the cell. For example, under peak sunlight
conditions, a typical commercial PV cell with a surface area of 160 cm^2 (~25 in^2) will produce about 2
watts peak power. If the sunlight intensity were 40 percent of peak, this cell would produce about 0.8 watts.
4.2 MPPT Techniques
Maximum power point tracking (MPPT) is a technique that charge controllers use for wind turbines and PV
solar systems to maximize power output. PV solar systems exist in several different configurations. The most
basic version sends power from collector panels directly to the DC-AC inverter, and from there directly to the
electrical grid.
Solar cells have a complex relationship between temperature and total resistance that produces a non-linear
output efficiency which can be analysed based on the I-V curve. It is the purpose of the MPPT system to
sample the output of the PV cells and apply the proper resistance (load) to obtain maximum power for any
given environmental conditions. MPPT devices are typically integrated into an electric power
converter system that provides voltage or current conversion, filtering, and regulation for driving various
loads, including power grids, batteries, or motors.
Solar inverters convert the DC power to AC power and may incorporate MPPT: such inverters sample the
output power (I-V curve) from the solar modules and apply the proper resistance (load) so as to obtain
maximum power.
MPP (Maximum power point) is the product of the MPP voltage (Vmpp) and MPP current (Impp).
9. 9
4.2.1 Classifications
Controllers usually follow one of three types of strategies to optimize the power output of an array. Maximum
power point trackers may implement different algorithms and switch between them based on the operating
conditions of the array.
Perturb and observe
In this method the controller adjusts the voltage by a small amount from the array and measures power; if the
power increases, further adjustments in that direction are tried until power no longer increases. This is called
the Perturb & Observe method and is most common, although this method can result in oscillations of power
output. It is referred to as a hill climbing method, because it depends on the rise of the curve of power against
voltage below the maximum power point, and the fall above that point. Perturb and observe is the most
commonly used MPPT method due to its ease of implementation. Perturb and observe method may result in
top-level efficiency, provided that a proper predictive and adaptive hill climbing strategy is adopted.
Incremental conductance
In the incremental conductance method, the controller measures incremental changes in PV array current and
voltage to predict the effect of a voltage change. This method requires more computation in the controller, but
can track changing conditions more rapidly than the Perturb & Observe method (P&O). Like the P&O
algorithm, it can produce oscillations in power output. This method utilizes the incremental conductance
(dI/dV) of the photovoltaic array to compute the sign of the change in power with respect to voltage (dP/dV).
The incremental conductance method computes the maximum power point by comparison of the incremental
conductance (IΔ / VΔ) to the array conductance (I / V). When these two are the same (I / V = IΔ / VΔ), the
output voltage is the MPP voltage. The controller maintains this voltage until the irradiation changes and the
process is repeated.
Current Sweep
The current sweep method uses a sweep waveform for the PV array current such that the I-V characteristic of
the PV array is obtained and updated at fixed time intervals. The maximum power point voltage can then be
computed from the characteristic curve at the same intervals.
Constant voltage
The term "constant voltage" in MPP tracking is used to describe different techniques by different authors, one
in which the output voltage is regulated to a constant value under all conditions and one in which the output
voltage is regulated based on a constant ratio to the measured open circuit voltage (VOC). The latter technique
is referred to in contrast as the "open voltage" method by some authors. If the output voltage is held constant,
there is no attempt to track the maximum power point, so it is not a maximum power point tracking technique
in a strict sense, though it does have some advantages in cases when the MPP tracking tends to fail, and thus
it is sometimes used to supplement an MPPT method in those cases.
10. 10
In the "constant voltage" MPPT method (also known as the "open voltage method"), the power delivered to
the load is momentarily interrupted and the open-circuit voltage with zero current is measured. The controller
then resumes operation with the voltage controlled at a fixed ratio, such as 0.76, of the open-circuit voltage
VOC. This is usually a value which has been determined to be the maximum power point, either empirically or
based on modelling, for expected operating conditions. The operating point of the PV array is thus kept near
the MPP by regulating the array voltage and matching it to the fixed reference voltage Vref = k×VOC. The value
of Vref may be also chosen to give optimal performance relative to other factors as well as the MPP, but the
central idea in this technique is that Vref is determined as a ratio to VOC.
One of the inherent approximations to the "constant voltage" ratio method is that the ratio of the MPP voltage
to VOC is only approximately constant, so it leaves room for further possible optimization.
4.2.2 Comparison of methods
Both perturb and observe, and incremental conductance, are examples of "hill climbing" methods that can find
the local maximum of the power curve for the operating condition of the PV array, and so provide a true
maximum power point.
P&O can produce oscillations of power output around the maximum power point even under steady state
irradiance.
The incremental conductance method has the advantage over the P&O method that it can determine the
maximum power point without oscillating around this value. It can perform maximum power point tracking
under rapidly varying irradiation conditions with higher accuracy than the P&O. However, the incremental
conductance method can produce oscillations and can perform erratically under rapidly changing atmospheric
conditions. The computational time is increased due to slowing down of the sampling frequency resulting
from the higher complexity of the algorithm compared to the P&O method.
In the constant voltage ratio (or "open voltage") method, the current from the photovoltaic array must be set
to zero momentarily to measure the open circuit voltage and then afterwards set to a predetermined percentage
of the measured voltage, usually around 76%. Energy may be wasted during the time the current is set to
zero. The approximation of 76% as the MPP/VOC ratio is not necessarily accurate though. Although simple
and low-cost to implement, the interruptions reduce array efficiency and do not ensure finding the actual
maximum power point. However, efficiencies of some systems may reach above 95%.
4.3 DC-DC Boost Converter
The key principle that drives the boost converter is the tendency of an inductor to resist changes in current by
creating and destroying a magnetic field. In a boost converter, the output voltage is always higher than the
input voltage.
(a) When the switch is closed, electrons flow through the inductor in clockwise direction and the inductor
stores some energy by generating a magnetic field. Polarity of the left side of the inductor is positive.
(b) When the switch is opened, current will be reduced as the impedance is higher. The magnetic field
previously created will be destroyed to maintain the current towards the load. Thus the polarity will be reversed
(means left side of inductor will be negative now). As a result two sources will be in series causing a higher
voltage to charge the capacitor through the diode D.
11. 11
If the switch is cycled fast enough, the inductor will not discharge fully in between charging stages, and the
load will always see a voltage greater than that of the input source alone when the switch is opened. Also
while the switch is opened, the capacitor in parallel with the load is charged to this combined voltage. When
the switch is then closed and the right hand side is shorted out from the left hand side, the capacitor is therefore
able to provide the voltage and energy to the load. During this time, the blocking diode prevents the capacitor
from discharging through the switch. The switch must of course be opened again fast enough to prevent the
capacitor from discharging too much.
The basic principle of a Boost converter consists of 2 distinct states:
In the On-state, the switch S is closed, resulting in an increase in the inductor current;
In the Off-state, the switch is open and the only path offered to inductor current is through the flyback
diode D, the capacitor C and the load R. This results in transferring the energy accumulated during the On-
state into the capacitor.
The input current is the same as the inductor current as can be seen in. So it is not discontinuous as in
the buck converter and the requirements on the input filter are relaxed compared to a buck converter.
Fig. 2 Boost Converter
4.4 Three Phase Three Leg Inverter
Three phase inverter Three-phase inverters are used for variable-frequency drive applications and for high
power applications such as HVDC power transmission. A basic three-phase inverter consists of three single-
phase inverter switches each connected to one of the three load terminals. For the most basic control scheme,
the operation of the three switches is coordinated so that one switch operates at each 60 degree point of the
fundamental output waveform. This creates a line-to-line output waveform that has six steps. The six-step
waveform has a zero-voltage step between the positive and negative sections of the square-wave such that the
harmonics that are multiples of three are eliminated as described above. When carrier-based PWM techniques
are applied to six-step waveforms, the basic overall shape, or envelope, of the waveform is retained so that
the 3rd harmonic and its multiples are cancelled.
To construct inverters with higher power ratings, two six-step three-phase inverters can be connected in
parallel for a higher current rating or in series for a higher voltage rating. In either case, the output waveforms
12. 12
are phase shifted to obtain a 12-step waveform. If additional inverters are combined, an 18-step inverter is
obtained with three inverters etc. Although inverters are usually combined for the purpose of achieving
increased voltage or current ratings, the quality of the waveform is improved as well.
Fig. 3 Three Phase Three Leg Inverter
4.5 Low Pass Filter
Low pass filters are used in a wide number of applications. Particularly in radio frequency applications, low
pass filters are made in their LC form using inductors and capacitors. Typically they may be used to filter out
unwanted signals that may be present in a band above the wanted pass band. In this way, this form of filter
only accepts signals below the cut-off frequency.
Low pass filters using LC components, i.e. inductors and capacitors are arranged in ether a pi or T network.
For the pi section filter, each section has one series component and either side a component to ground. The T
network low pass filter has one component to ground and either side there is a series in line component. In the
case of a low pass filter the series component or components are inductors whereas the components to ground
are capacitors.
Fig. 4 LC Filter
13. 13
5. Design of Subsystems
5.1 Design of Solar PV Model
The complete behaviour of PV cells as described by five model parameters (IPV, N, I0, RS and RP) which
represent the physical behaviours of PV cell/module.
Equivalent circuit of a PV panel consists of a light generated current source, a diode representing the nonlinear
impedance of the p-n junction, RS, the series resistance representing the internal electrical losses, and, RP, the
shunt resistance exists mainly due to the leakage current of the p–n junction. Applying Kirchhoff’s current
law, to above circuit, the terminal current of a PV panel is,
𝑰 = 𝑰 𝒑𝒗 − 𝑰 𝑫 −
𝑽 + 𝑰𝑹 𝑺
𝑹 𝑷
Where IPV is the current generated by PV panel, also known as photovoltaic current, Id is the diode current, I
is terminal current of a PV panel and V is terminal voltage of a PV panel. The current generated by PV cell is
given as,
𝑰 𝒑𝒗 = (𝑰 𝒑𝒗𝒏 + 𝑲 𝑰)
𝑮
𝑮 𝒏
Where, Ipvn is the light-generated current at the nominal condition (usually 25°C and 1000 W/m2
), KI is short-
circuit current/temperature co-efficient, ΔT = T- Tn (where T and Tn being the actual and nominal temperatures
in Kelvin, respectively), G (W/m2
) is the solar irradiation on the device surface, and Gn is the nominal
irradiation. Diode current Id is given as,
𝑰 𝑫 = 𝑰 𝑶 [𝒆
(
𝑽+𝑹 𝑺 𝑰
𝑽 𝑻 𝒂
)
− 𝟏]
Where, I0 is the reverse saturation current of a diode, ‘a’ is diode ideality factor and Vt = Ns kT/q, is the thermal
voltage of the array with Ns as number of cells connected in series, k is Boltzmann constant (1.3806503×10-
23
J/K), q is the electron charge (1.60217646×10-19
C). The reverse saturation current of a diode is given by,
𝑰 𝑶 =
𝑰 𝑺𝑪𝑵 + 𝑲 𝑰∆𝑻
𝒆
(
𝑽 𝑶𝑪𝑵+𝑲 𝑽∆𝑻
𝑽 𝑻 𝒂
)
− 𝟏
14. 14
Where, ISCN is the short circuit current at nominal condition. VOCN is the open circuit voltage at nominal
condition. KV is open circuit voltage/temperature co-efficient.
5.2 Design of MPPT based Pulse Width Modulator
The P & O algorithm operated by the periodically perturbing (increasing or decreasing) the terminal voltage
or current and then compare with the output power by the previous perturbation cycle. If the power increases
then one continues increasing the voltage or current in the same direction. If power decreases then continue
vary the voltage or current in the reverse direction.
Fig. 5 P&O MPPT Flowchart
15. 15
5.3 Design of Boost Converter
Fig. 6 Boost Converter
When the switch is turned closed, the voltage across the inductor is given by
𝑽𝒍 = 𝑳
𝒅𝒊
𝒅𝒕
The peak to peak ripple current in the inductor is given by
∆𝑰 =
𝑽 𝑺
𝑳
𝒕 𝟏
The average output voltage is
𝑽 𝟎 = 𝑽 𝑺 + 𝑳
∆𝑰
𝒕 𝟐
= 𝑽 𝑺 (𝟏 +
𝒕 𝟏
𝒕 𝟐
) =
𝑽 𝑺
𝟏 − 𝑫
From the above equations we can observe that if we keep changing the duty ratio of the pulse applied to the
gate of the MOSFET, we can control the output voltage of the boost converter. This is achieved by MPPT
based PWM generator which controls the duty ratio to get the maximum power point voltage for varying input
voltages.
16. 16
5.4 Design of Three Phase Three Leg Inverter
Fig. 7 Phase Voltages- 180° Mode of Conduction
Load Phase Voltages:
17. 17
6. Block Diagram and Control Schemes
System Block Diagram:
Vsc Control Block Diagram:
28. 28
8. Results
THD of the Output waveform connected to grid:
THD of the Output waveform of the off-grid system:
29. 29
9. Conclusion and Future Work
In this project, a solar PV system consisting of a MPPT control block, DC-DC boost converter, three-phase
three leg inverter. Auto-connected transformer, LC Low Pass filter and VSC Control, connected to the utility
grid. A simulation in MATLAB was performed and the harmonic analysis has been performed on the resulting
output waveforms.
The pulse width for the MOSFET in the Boost Converter is varied in accordance to the P&O MPPT Algorithm.
In the Perturb and Observe Method, the voltage, current and power of the previous time period are measured
and compared with the present values, if there is an increase in the power the duty cycle is further increased
otherwise decreased. The main purpose of this is to keep the output voltage of the solar panel equal to the
maximum power voltage of the panel.
The output of the boost converter is given to the three leg inverter which produces a three phase alternating
voltage at its output. An auto-connected transformer is used to reduce the amount of harmonics present in the
voltages. The LC Low pass filter is designed to convert this waveform into sine wave.
The entire system is connected to local loads and the 25kV utility grid, The PWM Pulses for the inverter are
controlled in accordance to the grid requirements by sensing the voltage and current values at the grid end.
In the future we plan to implement the MPPT control with a Microcontroller which senses the voltage and
current using sensors and drives the MOSFET accordingly.
30. 30
DECLARATION
We, students of 5th semester B.Tech Electrical and Electronics Engineering,
hereby declare that we have successfully completed the project titled:-
Modelling and Simulation of a Solar Photovoltaic System with P&O MPPT
control. This project has been made during the academic year 2015-16 and
contains information that is true to the best of our knowledge.
Date:-
Signature of members
Place:-
Signature of Guide
