MPPT BASED GRID CONNECTED SYSTEM WITH P&O Aijrrest.org/issues/ijrrest_vol-4_issue-2_011.pdf · by...

IJRREST

INTERNATIONAL JOURNAL OF RESEARCH REVIEW IN ENGINEERING SCIENCE & TECHNOLOGY

(ISSN 2278–6643)

VOLUME-4, ISSUE-2, June - 2015

IJRREST, ijrrest.org 55 | P a g e

MPPT BASED GRID CONNECTED SYSTEM WITH P&O ALGORITHM 1Anuradha,

2Satish Kumar

1Scholar, RPIIT, Karnal, Haryana, India. Email: [email protected]

2Assistant Professor, RPIIT, Karnal, Haryana, India. Email: [email protected]

Abstract— The world is running out of conventional fuel resource with no alternate option other than to enhance the generation of clean and

resourceful energy from renewable energy sources. This led to increased attention towards solar system because it offers maximum

generation capacity among all renewable energy sources. Solar power has numerous advantages, but they do not show advantageous

efficiency. Solar cell efficiency depends up on various factors i.e. Temperature, Insolation, shadow, spectral characteristics of sunlight, dirt

etc. A rapidly changing irradiance on panels due to change in climate also reduces the result or output power of photovoltaic (PV) array. The

utilization factor of irradiants and capability of photovoltaic systems can be improved to a large extent by employing various control

techniques and algorithms. In this paper we present Perturb & Observe (P&O) to trace maximum power point with changing irradiations.

The simulation presents a 200 kilo-Watt PV array modeled and connected to a 25 kilo-Volt (kV) grid. The grid and the array are connected

by using two boost converters (DC-DC) and a single three-phase voltage source converter. This model allows using much larger time steps 50

μs, resulting in a much faster simulation.

Keyword— Solar Cell, grid, renewable energy.

1. INTRODUCTION

Total installed power capacity is the crucial factor which

decides the growth rate of any country. In order to

achieve and maintain the expected economic growth rate

(8-9%) India needs to generate more and more of electric

power. Renewable energy sources have major

contribution in electric power generation. These days

Renewable Energy resources and technologies are gaining

popularity and importance in the world. Solar energy,

geothermal energy, wind energy, etc. are among the

numerous renewable sources available for generation of

electric power. Solar energy is gaining popularity in the

field of electricity generation due to the ease it provides

by directly converting solar irradiance in to electricity or

electric energy. The easiness and the cleanliness make the

solar energy a good choice for electric power generation.

Silicon is commonly used in the fabrication of solar cell

and a series of these cells is called a PV solar module. The

current rating of these solar PV-modules depends on the

area of the each cell and increases as area of cell

increases.

A parallel and series combinations of PV modules results

in solar PV arrays, gives more power output. A typical

solar cell generally converts 30-40% of solar energy to

which it is exposed, into electrical energy.

Based on the type of semiconducting material that is used

in a module its efficiency varies. Several types of

semiconductor materials like C-SI, P-SI, A-SI, CIS etc.

are available. Cells that are connected in series satisfy

higher voltage requirements and that of parallel which

gives higher current requirements. The maximum amount

of power that can be extracted from a solar panel depends

on the solar irradiance, temperature, load so the input to

the solar cell is basically irradiance and temperature of

that particular area. Based on the isolation level and

temperature maximum power output from a panel varies.

A typical solar PV cell generates very low power and for

extracting maximum power output from a solar cell we

are using MPPT technique. MPPT techniques

automatically find the voltage or current at which a PV

module should operate to generate maximum power is

called maximum power point. A particular MPPT

technique is chosen based on the factors like, cost,

simplicity quick tracking under varying power output

locations, atmospheric conditions, small etc. Most MPPT

techniques would automatically respond to changes in

both temperature and irradiance.

This thesis analyzes Perturb & Observe technique through

results under normal and varying atmospheric conditions.

Here, irradiance can be changed as per requirements. Grid

connected systems are common now-a-days. It is very

important to extract maximum power output from PV

modules, for which MPPT is employed. The benefits of

MPPT technique are that it automatically finds out the

maximum current and voltage from the PV panel such

that it operates under maximum power point. The most

common and useful MPPT algorithms are Perturb and

Observe algorithm, Fuzzy logic algorithm, Incremental

Conductance algorithm. For a grid connected system there

are two stages:

First stage: Boosting stage which boosts the output

from a solar PV module as per the requirements.

Second stage: It is a dc to ac conversion stage. When

a PV system is interconnected to the utility network

system the main demands to that type of system are

power quality and power system stability.

For a PV grid connected system there are certain

protection schemes that are implemented which

particularly provides protection against islanding, voltage

sag/swell, overvoltage and over current protection.

Through proper analysis and experiments the quality of

material and methods has been increased in due course of

time, making solar cell more efficient and productive. The

efficiency of collection process is based on various factors

including PV cell efficiency, energy storage process and

source radiation intensity. The efficiency of a PV-cell is

affected by materials used in fabrication. It is hard to

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make advances in the overall operation of the PV cell, and

hence the efficiency of overall process is limited.

Therefore, the other possible method is to increase the

intensity of solar radiation to improve the performance of

solar cell module. Three major approaches are there for

extraction of maximum power in solar systems. They are:

(i) Sun tracking

(ii) Hybrid (maximum power and sun tracking)

(iii) Maximum power point tracking (MPPT)

Solar energy has emerged as a major alternative over

conventional energy, but its low efficiency and high initial

cost are some constraints in its usage as a primary source

of power. Solar PV generating system has to operate at

the maximum power output point to utilize the maximum

available solar power of the array and to obtain the

highest energy conversion output from PV-array.

Since the maximum power point varies with radiation and

temperature, it is difficult to maintain optimum power

operation at all radiation levels. The MPPT methods

presented so far have been developed and implemented,

differ from each other in several aspects such as

complexity, required number of sensors, convergence

speed, cost, range of effectiveness, ease of hardware

implementation etc.

Although different methods have been developed by

different research groups, very little literature is available,

where different MPPT techniques/methods are compared

in terms of energy capture, conversion efficiency,

response time and reliability. This research work

comprises of the performance of Perturb & Observe

algorithm MPPT methods that is currently used in a solar

PV system and also advocates a new MPPT technique

which offers better performance than the existing ones.

The method used for analysis is as follows:

Initially, a MATLAB based solar PV array model is first

developed and validated; then, Maximum power point

tracking techniques based on Perturb & Observe

algorithm is employed on this PV array under varying

insolation conditions and temperature to study the

effectiveness of the particular Maximum power point

tracking technique under consideration.

The objective of this thesis is to design and simulation of

MPPT controlled solar system using Perturb & Observe

algorithm and track maximum power point from the

characteristic curve of PV array. The maximum power

depends up on various factors such as solar insolation,

temperature, voltage and current etc. So, with the change

in atmospheric condition, the power increases or

decreases as its relation varies with that factor

accordingly. So, it is essential to track maximum power

point to draw more efficiency from PV panel.

For this purpose, there are many MPPT algorithms are

available. Perturb and observe (P&O), incremental

conductance, constant voltage, constant current and

parasitic capacitance algorithm are various commonly

used algorithms. Here we used P&O algorithm to track

maximum power point as shown in results obtained from

proposed model.

2. MAXIMUM POWER POINT TRACKING

ALGORITHMS

A typical solar panel converts only 30 to 40 percent of the

incident solar irradiation into electrical energy. Maximum

power point tracking technique is used to improve the

efficiency of the solar panel. According to Maximum

Power Transfer theorem, the power output of a circuit is

maximum when the Thevenin impedance of the circuit

(source impedance) matches with the load impedance.

Hence our problem of tracking the maximum power point

reduces to an impedance matching problem.

In the source side we are using a boost convertor

connected to a solar panel in order to enhance the output

voltage so that it can be used for different applications

like motor load. By changing the duty cycle of the boost

converter appropriately we can match the source

impedance with that of the load impedance. There are

different methods used to track the maximum power point

(MPP). Some most popular techniques are:

(i) Perturb and observe (hill climbing method)

(ii) Neural networks

(iii) Fractional open circuit voltage

(iv) Fractional short circuit current

(v) Incremental Conductance method

(vi) Fuzzy logic

The choice of the algorithm depends on the time

complexity the algorithm takes to track the MPP

implementation cost and the ease of implementation.

Table I

Characteristics of Different MPPT Techniques

MPPT

Technique

Convergence

speed

Complexit

y

Tunin

g

Sensed

Parameter

s

Perturb &

Observe

Varies Low No Voltage

Incremental

Conductance

Varies Medium No Voltage,

Current

Fractional

Medium Low Yes Voltage

Fractional

Medium Medium Yes Current

Fuzzy Logic Fast High Yes Varies

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3. PROPOSED PERTURB & OBSERVE

ALGORITHM

The Perturb & Observe algorithm states that when the

operating voltage of the PV panel is perturbed by a small

increment, if the resulting change in power is positive,

then we are going in the direction of Maximum Power

Point (MPP) and we keep on perturbing in the same

direction.

If P is negative, we are going away from the direction of

MPP and the sign of perturbation supplied has to be

changed.

Figure. 1: Characteristics of Solar panel showing MPP

and operating points A and B

Fig.1 shows the plot of module voltage for a solar panel at

given irradiation versus module output power. The point

marked as MPP is the Maximum Power Point, from the

PV panel the maximum theoretical output can be

obtained.

Consider A and B are two operating points. The Point A is

on the left hand side of MPP as shown in the figure above.

Therefore, we can move towards the MPP by providing a

positive perturbation to the voltage. On the other hand,

point B is on the right hand side of the MPP.

When we give a positive perturbation, the value of P

becomes negative, thus it is imperative to change the

direction of perturbation to achieve MPP.

The flowchart for the P&O algorithm is shown in Fig.2.

Figure. 2: Flowchart of Perturb & Observe algorithm

4. SIMULINK MODEL OF GRID-

CONNECTED PV ARRAY

A Simulink model has been proposed of the solar system

connected PV modules to form a 200-kW array which is

connected to two DC-DC boost converters and a single

three-phase voltage source converter in order to feed a 25-

kV grid. The MPPT controller based on the “Perturb and

Observe” controller is used to trace Maximum Power

Point (MPP). The “Perturb and Observe” algorithm is

coded in C-language which can also be simulated in

MATLAB. The model contains:

Two PV arrays delivering each a rating of 100 kilo-

Watt at 1000 sun irradiance.

Two boost converter each for Panel-PV1 and Panel-

PV2 of rating 500 V DC. The two MPPT controllers

use the “Perturb and Observe” technique.

A Voltage Source Converter converting 500 V DC to

260 V AC at unity power factor.

A Capacitor bank (20-kvar) used to remove

harmonics introduced in the system by Voltage

source converter.

A three-phase coupling transformer (200-kVA) in

step up mode 260V/25kV.

A distribution feeder (25-kV) integrated with an

equivalent transmission systems (120 kV) to form a

complete utility grid.

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In the proposed Simulink-model the boost-converter and

voltage source converter are modeled with a voltage

sources producing the equivalent AC voltage averaged

over one cycle of the switching frequency. This model is

capable of doing faster simulation as it has much larger

steps time of 50 s. In order to obtain iterations and

accuracy, algebraic loops has been introduced in the PV

models.

4.1 PV array: The single module of PV-array (PV-1) is

having SPR (Sun-Power) of 305.2 Watt. These modules

are arranged in strings, each of having 66 modules. The

array of these strings consists of 5 series-connected

modules connected in parallel.

Similar calculation can be done for Kyocera-DD205GX-

LP used as solar panel-2 (PV-2).

Table II

Specifications for One Module

Module Specifications SunPower-

SPR305 (PV1)

Kyocera-

DD205GX-LP

(PV2)

Number of series-

connected cells

96 54

Open-circuit voltage: 64.2 V 33.1999 V

Short-circuit current: 5.96 A 8.35955 A

Voltage ( ) and

current( ) at maximum

power

54.7 V, 5.58 A 26.6 V, 7.70959

A

The I-V and P-V characteristics for one module and for

the whole array can be plot from the PV array block

menu. The characteristics of the SunPower-SPR305 array

are reproduced below.

(a)

(b)

Figure. 3 : I-V and P-V characteristics of PV array (a)

For a Single Cell (b) For the Array

Red dots on blue curves indicate module manufacturer

specifications (Voc, Isc, Vmp, Imp) under standard test

conditions (25 degrees Celsius, 1000 W/m2). Any of the

ten various array types can be selected from the “Module

type” menu for the simulation. Here we used two 100

kilo-watt PV arrays. PV1 uses SunPower-SPR305

modules and PV2 uses Kyocera-DD205GX-LP modules.

4.2 Boost converter: In the detailed model, the boost

converter boosts DC voltage from 273.5 V to 500V. This

converter uses a MPPT system which automatically varies

the duty cycle in order to generate the required voltage to

extract maximum power. Look under the mask of the

“Boost Converter Control” block to see how the MPPT

algorithm is implemented.

4.3 VSC Converter: The three-level Voltage Source

Converter (VSC) regulates DC bus voltage at 500 V and

keeps power factor to unity. The control system uses two

control loops:1) Internal control loop 2) External control

loop. An Internal control loop regulates the grid currents

Iq and Id (reactive and active currents components) and

an external control loop regulates DC link voltage to +/-

250 V. The output of the DC voltage external controller is

reference Id current. Iq current reference is set to zero in

order to maintain power factor unity. Vq and Vd are the

outputs voltage of the current controller are converted to

three modulating signals abc used by the Pulse Width

Modulator (PWM) three-level pulse generator.

The control system uses a sample time of 100 µs for

voltage and current controllers as well as for the PLL

synchronization unit. In the detailed model, pulse

generators of Boost and VSC converters use a fast sample

time of 1µs in order to get an appropriate resolution of

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PWM waveforms. Observe the performance of the two

Perturb and Observe MPPTs under various irradiance

changes. It can be seen that this type of MPPT controller

tracks maximum power only while irradiance stays

constant.

Diode characteristic

where:

Id = diode current (A)

Vd = diode voltage (V)

Isat = diode saturation current (A)

T = cell temperature (K),

k = Boltzman constant = 1.3806e-23 J.K^-1

q = electron charge = 1.6022e-19 C

Qd = diode quality factor

Ncell= number of series-connected cells per module

Nser = number of series-connected modules per string

Figure. 4: Simulink model of PV array

5. RESULTS OF THE PROPOSED MODEL

Figure. 5: Solar Irradiance Panel 1 & Panel 2

Figure. 6: Reference Voltage (Vref) & Average Voltage

(Vmean)

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Figure. 7: Modulation Index

Figure. 8: Power on Bus B1 (kw)

Figure. 9: Grid Volatge(Va) and Grid Current(Ia)

Figure. 10: Pmean1 (Panel1) and Pmean2 (Panel2)

Figure. 11: Voltage variation of Panel1 & Panel2

according to the solar Irradiance

Figure. 12: Duty cycle of the boost converters (D1 & D2)

All the above results outlined the variation of current,

voltage and power at the grid/bus and the solar panels in

accordance with change in irradiations in order to get

maximum output using P&O integrated MPPT technique.

6. CONCLUSION & SCOPE

The investigation of results performance has been

successfully demonstrated in Simulink. The developed

model provides the optimal performance of the designed

control algorithm for obtaining the maximum power

under solar irradiations variation. The PV array has been

mathematically modeled and its performance is tested on

Simulink platform while varying the temperature and

irradiations. The result waveforms obtained clearly

presents the effect of temperature and irradiance on output

voltage and power from the PV system. Finally the

experimental study was carried out for extracting the

maximum power from PV array using Simulink.

The Perturb & Observe algorithm developed in Simulink

model can be verified with the real time environment. The

manual control of the MPPT in reference to the analysis

using MATLAB-Simulink can replaced with a well-

developed control algorithm so as to give a better

performance. The experimental setup that was build can

be used for studying the effects of partial shading of PV

array.

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