CHAPTER 0NE 1.1 INRODUCTION Energy plays an important role in our daily activities. The degree of development and civilization of a country is measured by the amount of utilization of energy by human beings. Energy demand is increasing day by day due to increase in population, urbanization and industrialization. The world’s fossil fuel supply, the coal, petroleum and natural gas will thus be depleted in few hundred years. The rate of every day energy consumption increases, supply is depleting resulting in inflation and energy shortage. This is called the energy crisis. Hence alternative sources of energy have to be developed to meet the energy requirement for the future. The second chapter presents the literature review as well as a description of the components that make up the circuit, as this is a necessary background for understanding the discussion presented in subsequent chapters on the operation of the inverter. Chapter 3 presents details of the design, analysis and construction of the inverter. Chapter 4 explains the experimental tests and modifications introduced to the circuit also the economic analysis. Chapter 5 discusses the conclusion and recommendation. 1.2 AIM AND OBJECTIVE The aim of this project is to satisfy the need for automatic voltage supply due to ever increasing power failure, surges and fault occurrence on the power supply. The entire work from the engineering perspective deals with the design, construction and implementation of solar powered inverter. The need for backup power systems can never be over emphasized especially in this technological age. Almost everything used today uses electrical energy, only differing in 1
Transcript
CHAPTER 0NE
1.1 INRODUCTION
Energy plays an important role in our daily activities. The degree of development and
civilization of a country is measured by the amount of utilization of energy by human beings.
Energy demand is increasing day by day due to increase in population, urbanization and
industrialization. The world’s fossil fuel supply, the coal, petroleum and natural gas will thus be
depleted in few hundred years. The rate of every day energy consumption increases, supply is
depleting resulting in inflation and energy shortage. This is called the energy crisis. Hence
alternative sources of energy have to be developed to meet the energy requirement for the future.
The second chapter presents the literature review as well as a description of the
components that make up the circuit, as this is a necessary background for understanding the
discussion presented in subsequent chapters on the operation of the inverter. Chapter 3 presents
details of the design, analysis and construction of the inverter. Chapter 4 explains the
experimental tests and modifications introduced to the circuit also the economic analysis.
Chapter 5 discusses the conclusion and recommendation.
1.2 AIM AND OBJECTIVE
The aim of this project is to satisfy the need for automatic voltage supply due to ever
increasing power failure, surges and fault occurrence on the power supply. The entire work from
the engineering perspective deals with the design, construction and implementation of solar
powered inverter. The need for backup power systems can never be over emphasized especially
in this technological age. Almost everything used today uses electrical energy, only differing in
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terms of volume of consumption and sensitivity e.g. medical equipment are considered sensitive
because a power failure of whatever duration could have irreversible and undesirable
consequences, and also in the laboratory where research is being carried, hence the need for
backup systems. No country in the world no matter how advanced, can boast of 100% power
reliability, thus inverters and other auxiliary power sources are very important in the rural
communities where this is little or no source of electricity.
1.3 SCOPE AND LIMITATION
This project can be used as a standby power supply source to enhance power supply to homes
and is highly relevant for industrial applications. Typical applications for an inverter include:
washing machine, television, security light, video recorders, computers, life-support machines in
hospitals, air traffic control systems, military installation, broadcasting stations, power-tools and
monitoring communications equipment. Inverters make life simpler and save money on
appliances and light.
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CHAPTER TWO
2.1 LITERATURE REVIEW
With the push in the past ten years to improve methods of energy generation, there has been an
increasing need to optimize energy management. Our project was to create a DC to AC load-
balanced inverter. The function would be to take in a DC voltage input, something like a battery
or a solar cell, and output an AC voltage, resembling that which comes out of a wall outlet. The
crux of the design would be that it would be “load balanced” meaning that no matter how large
of a load (which could resemble anything from an array of light bulbs to a laptop) the output
voltage would not change.
An inverter is a circuit for converting direct current to alternating current. In the inverter
circuit, the direct current power is connected to a transformer through the center tap of the
primary winding. A switch control is switched back and forth to enable the current to flow back
to direct current source following two alternate paths at one end of the primary winding and then
the other on the other end. The alternation of the direction of current in the primary winding of
the transformer produces alternating current in the secondary circuit. This results in alternate
current-in, direct current-out. Inverting the connections to a converter and this result in direct
current-in and alternate current-out. In an advanced inverter designs various methods which are
used to improve the quality of the sine wave at the transformer input, rather than relying on the
transformer to smooth it. Capacitors and inductors can be used to filter the waveform at the
primary of the transformer. Also, it is possible to produce a more sinusoidal wave by having
split-rail direct current inputs at two voltages or positive inputs with a central ground. By
connecting the transformer input terminals in sequence between the positive rail and ground, the
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positive rail and the negative rail, the ground rail and the negative rail, then both to the ground
rail, a stepped sinusoid is generated at the transformer input and the current drain on the direct
current supply is less choppy. These methods result in an output that is called a modified-sine
wave. Modified sine inverters may cause some loads, such as motors, to operate in a low state.
A more expensive power inverter uses pulse width modulation (pwm) with a high frequency
carrier to more closely approximate a sine function. The quality of an inverter is described by its
pulse-rating.
An inverter used for backup power will use grid power or solar energy to keep the batteries
charged, and when grid power fails, it will switch to drawing power from the batteries and
supplying it to the building electrical system. For a business or home office, a reliable power
source is invaluable for preventing lost data on computer systems. Most modern inverters also
include over voltage protection, protecting sensitive equipment from dangerous power surges as
well. Three major waveforms are square-wave, modified sine-wave and pure sine-wave. The
output of this circuit is aimed to model that which flows on the grid, thereby able to be utilized
by everyday appliances.
2.2 ASSESSMENT OF NIGERIA AVAILABLE ENERGY RESOURCE AND
SOURCES
The availability of energy is vital for the economic and social development of any country, in
Nigeria, less than 40% of the country is connected to the national electric grid and less than 60%
of the energy demand by this group is generated and distributed. The Energy-induced
environmental degradation is already prevalent in the country.
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(1) This is characterized by deforestation as a result of falling of trees for fuel wood and
charcoal production, air pollution in urban areas arising from vehicular emission and burning of
traditional fuel for traditional cocking in household, noise pollution from use of small generators
to provide electricity due to inadequate supply from the national grid, land and water pollution
from oil spillage in the oil producing communities.
(2)This has led Nigeria and indeed the world to look for alternative power supply such as solar
energy among others. Unfortunately utilization and development of solar energy is rising in other
parts of the world but encountered with low pace of development and utilization in Nigeria. This
low pace of development is due to the associated problems such as purchasing power,
technology of installation and fabrications, awareness, governmental policy and politics, culture,
Nigerian factor, among many other variables.
(3) In Nigeria, more than 75% of Nigerian populations are rural dwellers.
(4) Less than 20% of Nigeria is connected to the National grid, and more than 70% of Nigeria’s
population of about 140 million lives in more than 80% of landmass of Nigeria which is not
connected to the national grid.
2.2.1 PETROLEUM
The existence of large and commercial fossil fuel producing fields in the Southern part of
Nigeria has always sustained Nigeria’s dream of finding commercial quantities of oil and gas.
Nigeria is the largest oil producing country in Africa and is one of the best oil in the world in
terms of quality because it is sulphur free and has low viscosity. It is dangerous to human, plant
and animal and need sophisticated technology to control.
2.2.2 HYDRO
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Hydro has been Nigeria’s most utilized renewable energy resource. Electricity generation
efficiency of hydro power plants are usually very high. However, hydro plants depend solely on
the weather which is unpredictable. And Nigeria major and oldest hydro power plant is the Kanji
dam in Niger state.
2.2.3 WIND
Wind resource has been used in many countries to produce large amounts of electricity.
Theoretically the maximum energy that can be tapped from the available wind for electricity
using today’s technology is about 500-600 Giga-watt hours every year. A Solar and Wind
Energy Resource Assessment (SWERA) project being run jointly by UNEP, Global Environment
Facility and the US National Renewable Energy Laboratory (NREL) in 2004 has identified some
spots within Nigeria particularly the coastline. Nigeria does not have any major wind energy
generation station.
2.2.4 SOLAR RESOURCE
Nigeria has an abundant amount of solar energy made up of about thirty percent diffused
radiation and seventy percent direct radiation. The theoretical energy available yearly in Nigeria
is about 500,000 GWh. The average duration of sunshine varies from a minimum of 5.3 hours
per day in the cloudy forest region to about 7.7 hours per day in the dry savannah region.
Nigeria’s average peak sun hours varies from 5.0 to 5.7 peak sun hours with Lagos having
average peak sun hours of 4.5. The major challenges with the utilization of Nigeria’s abundant
solar resource has been the high cost of installation and the lack of technical expertise on some
sectors like the grid connected sector of the solar industry.
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2.2.5 NUCLEAR RESOURCES
Uranium is the major fuel source for nuclear power which is generated through the fission heat
produced in nuclear power reactors. Based on the once-through cycle method, known uranium
reserves are expected to last for over 60 years.
2.3 SOLAR PANELS
A great amount of energy can be harnessed from the sun. The amount of energy reaching the
Earth’s surface every day from the Sun is far greater than the energy needs of man for the
foreseeable future, he key to using this vast source of energy is developing cost effective
methods for collecting and storing this energy, once this is done solar energy can contribute
significantly to satisfying man’s ever growing energy requirements.
This solar energy received has a range of frequencies and wavelengths from lower frequency
(longer wavelength) of infrared to the higher frequency radiation of ultraviolet. It is this higher
frequency radiation, visible light and ultraviolet light which can be harnessed to produce an
electrical current.
The basic method for using solar energy is to provide a system that can collect and store energy,
the collector is able to convert the solar radiation into electrical energy which can then be stored
as chemical energy in rechargeable batteries. Storage of this energy is necessary for times when
the sun is not shining and can also be delivered to the inverter systems.
The following diagram illustrates briefly how solar energy systems work. The type of collector is
determined by the type of energy conversion required. In our case our solar energy collector is a
photovoltaic cell and the energy is stored as chemical energy in a rechargeable 12V battery.
The most common conversion of solar energy into electricity is through the use of a
semiconductor light sensitive (photovoltaic) diode converter, commonly called a solar cell.
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Photovoltaic is the direct conversion of light into electricity at an atomic level. Some materials
exhibit a property known as the photoelectric effect that causes them to absorb photons of light
and release electrons. When these free electrons are captured, an electric current is the result that
can be used to power a load.
Figure 2.1 How solar energy system works
The material which is used in the solar panel that exhibits the photoelectric effect is a silicon
semi-conductor, the silicon on the n-type side has been doped with an impurity which allows
many free electrons on that side of the junction. The silicon on the other side of the junction has
been doped with an impurity which causes lack of electron, these voids in the electronic structure
are called holes. When the visible or ultraviolet light is incident on the n-type silicon the
electrons are excited and knocked loose from the atoms. If electronic conductors are attached to
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the positive and negative sides of the semiconductor junction, this forms an electrical circuit and
the free electrons can be captured in a circuit. The holes move in the opposite direction to the
Figure 2.2 Diagram of a photovoltaic cell showing layers of semiconducting material.
electrons and this is defined as the direction of the current flow. Due to the semiconductors
acting as diodes, the output current is direct current.
2.3.1 APPLICATIONS OF THIS SOLAR PANEL TECHNOLOGY
(i) Solar panels can be used to charge storage systems such as rechargeable batteries which then
supply power to electrical equipment and household electrical devices such as radios, televisions
and refrigerators. Many solar panels have been used for this purpose in many communities
across Nigeria without other access to power.
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(ii) Solar panels can be used for household lighting if the current produced is converted to
alternating current using an inverter.
(iii) Solar panels can be used to provide heating for hot-water systems.
(iv) Solar panel plays an important role in providing energy for non-rotating satellites. The solar
cells are mounted on large flat panels that automatically adjust to a position to receive maximum
solar radiation and provide power to the satellite. The European Space Agency (ESA) is
researching the possibility of solar power satellites that would generate electricity in space and
beam them to Earth via laser or microwaves.
2.4 POTENTIAL OF SOLAR ENERGY
There is a huge potential of solar energy. It is so huge that the total energy needs of the whole
world can be fulfilled by the solar energy. The total energy consumption of the whole world in
the year 2008 was 474 exajoule (1EJ=1018 J) or approximately 15TW (1.504*1013 W). Almost
80%-90% of this energy came from fossil fuel. [12] From the sun earth receives 3,850,000 EJ of
energy. Which is equivalent to 174 peta-wattas (1 PW=1015 W). The earth does not hold all the
energy, a part of it reflects back. After reflection earth receives 89 PW of energy. Of this huge
amount only less than 0.02% is enough to replace the fossil fuel and nuclear power supply in the
whole world at present. By this we can easily understand the great potential of solar energy.
Considering green house effect, other environmental impact, cost, risk and availability solar
energy has the greatest potential among all the energy sources.
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2.5 WORKS ON SOLAR TECHNOLOGIES AROUND THE WORLD
There are huge works, research, thesis, implementation, design consideration and Improvement
on solar technologies is going on around the world as well as in our country. That is why we
have more company doing business, implementation and research on solar technologies.
University students around the globe are working with solar system, like a group of
students of Ahsanullah University of science and technology designed a solar system for their
university. A group of students of the Pennsylvania State University has designed and simulated
a Distributed photovoltaic system for their university as their thesis. Again Rajamangala
University of Technology Thanyaburi of Thailand installed PV system for their university to
promote solar energy project.
Scientist working on developing the solar panels, like scientist of Korea and California has
develop a new way of boosting the efficiency of plastic solar panels. By this they make it more
competitive to traditional solar panels. Commercial buildings, houses, offices, companies are
installing solar system for green energy. Such as the largest solar powered building in Dezhou,
Shangdong Province in northwest China.
Plate 2.1 The largest solar power building in northwest china
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The above picture is the largest solar powered building and it will be the venue of the 4 th world
solar city congress.
We can also see 100% solar powered buildings. Like the stadium for the world game 2009 in
Taiwan was 100% solar powered.
Plate 2.2 100% solar powered stadium in Taiwan.
The above shows that the 100% solar powered building in Taiwan. It has 8,840 solar panels in
the roof and can produce 1.14 million kWh/year. By this, it can prevent 660 tons of carbon
dioxide to release in the environment.
2.6 SITE AND LOAD BASED
Solar power is designed and supplied from a particular location to a particular consumers, such
as a house or apartment can use its rooftop, lawn, garden etc to implement their solar system to
get the desired power. Beside a solar power plant is designed for a particular amount of load,
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such as-Sarnia Photovoltaic Power Plant of Canada can deliver 80 MW of power, Olmedilla
Photovoltaic Park of Spain can deliver 60 MW of power.
2.7 TYPES OF SOLAR CELLS
There can be various types of solar cells design.
1. Mono-crystalline silicon
2. Poly-crystalline silicon
3. Thin film materials
2.7.1 MONO-CRYSTALLINE SILICON Mono-crystalline is the oldest, most efficient PV cells technology which is made from silicon
wafers after complex fabrication process. Mono-crystalline silicon cells are designed in many
shapes, round shapes, semi-round or square bars, with a thickness between 0.2mm to 0.3mm,
round cells are cheaper than semi-round or square cells since less material is wasted in the
production.
Figure 2.3 monocrystalline silicon module and cell layered structure
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The main properties of mono-crystalline silicon module are
Efficiency: 15% to 18% (Czochralski silicon).
Form: round, semi round or square shape.
Thickness: 0.2mm to 0.3mm.
Color: dark blue to black (with ARC), grey (Without ARC).
2.7.2 POLY-CRYSTALLINE SILICON
Poly-crystalline PV modules are cheaper per unit area than mono-crystalline; the module
structure is similar to the mono-crystalline. To increase the overall module efficiency, larger
square cells should be used, By using larger cells the module cost will be lower, because less
number of cells are used
Figure.2.4 Poly-crystalline silicon and module
The main properties of polycrystalline PV module are Efficiency: 13% to 16 %. Form: Square.
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Thickness: 0.24mm to 0.3mm. Color: blue (with ARC), silver, grey, brown, gold and green (without ARC).
2.7.3 FLEXIBLE AMORPHOUS THIN FILM
Figure 2.5 Flexible amorphous Thin-film cell
Flexible amorphous thin film technology represents the second PV generation; due to less
production materials and less energy consumption, it’s cheaper than crystalline technology.
Amorphous silicon, copper Indium Silinum (CIS) and Cadmium Telluride (CdTe) are used as
semiconductor materials. Because of the high light absorption of these materials, layer
thicknesses of less than 0.001mm are theoretically sufficient for converting incident irradiation.
2.8 SELECTION OF PV MODULE TO BE USED ( SOLAR PANEL)
In selection of a PV module for a solar systems the main limitation is the efficiency of the
module and the warranty of the product (since grid connected systems are designed to last for a
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long time. The BP solar 180Watt Photovoltaic module (Saturn technology) is chosen. The BP
7180 forms part of the high efficiency Saturn 7 series “real power” range of solar modules.
Being the largest, most powerful module manufactured by BP Solar, the BP 7180 is ideal for this
installation since high power is required in a very limited area. Efficiency of solar cell depends
on the technology used. Silicon solar cell has the highest efficiency. Thin film has low
efficiency, but they can be ideal for some applications. Another important consideration is
temperature, module efficiency decreases as the module temperature increases. When modules
operating on roof top, it heats up substantially, cell inner temperature reaches to 50-70 degree
Celsius. In high temperature areas, it is better to choose a panel with low temperature co-
efficient.
2.9 PERFORMANCE The module has a rated power of 180W and a module efficiency of 14.35 and nominal voltage of
24 Volts.
Figure2.6 Module picture
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2.10 TYPICAL ELECTRICAL CHARACTERISTICS
The typical electrical characteristics of the PV module BP 7180S measured under Standard test
conditions (irradiance of 1000W/m2 ,Air mass of 1.5G solar spectrum and a effective cell
temperature of 250C) are as follows;
Warranted minimum power - 180W
Voltage at Pmax (Vmp) - 36.0V
Current at Pmax (Imp) - 5.0A
Short circuit current - 5.3 A
Open circuit current - 44. 2V
Temperature derating factor -0.05%/ 0C
Maximum system voltage - 1000VDC
Maximum series fuse rating - 600V DC
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Figure 2.7 Solar panel layout
2.11 MECHANICAL CHARACTERISTICS
The module is made up of 72 cells connected in series with a weight of 15.4kg and dimensions
of 1593 x 790 x 50 and its frame is clear anodized aluminum alloy type.
2.12 SOLAR CHARGE CONTROLLER
When battery is included in a system, the necessity of charge controller comes forward. A charge
controller controls the excess voltage build up. In a bright sunny day the solar cells produce more
voltage that can damage battery.The primary function of the charge controller is to maintain
battery health by preventing battery overcharge by the solar panels and full discharge by the
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electrical loads. Either condition will lead to severely reduced battery lifespan. Charge
controllers come in all sizes, and protection and monitoring features. The selection depends on
the size of installed solar panel(s) and the complexity of loads and future expansion possibility.
Different charging and maintenance algorithms are employed depending on the state and the type
of the battery. There are many electrical protection features in a suitably designed charge
controller that are beneficial in SHS type of applications. Protection features such as reverse
polarity, short circuiting, over-current, low-voltage-disconnect.
Plate2.3 Solar charge controller
2.13 BATTERY
To store charges for the solar or the inverter charger batteries are used. There are many types of
batteries available in the market. But all of them are not suitable for inverter. Mostly used
batteries are nickel/cadmium batteries. There are some other types of high energy density
batteries such as- sodium/sulphur, zinc/bromine flow batteries. But for the medium term batteries
nickel/metal hydride battery has the best cycling performance. For the long term option
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iron/chromium redox and zinc/manganese batteries are best. Absorbed Glass Mat (AGM)
batteries are also one of the best available types for running an inverter use.
2.13.1 WET CELL
The wet batteries have a limited range in the amount that can be discharged; the higher the daily
discharge, the lower the number of recharging cycles the battery will have in its lifetime, so it is
not suitable to be use as inverter battery. And wet cell batteries are mostly use in vehicle be it has
the ability to start the engine and goes back to its normal state while the vehicle continue to
charge the battery, it does not have much ability to withstand continuous discharge.
2.13.2 DEEP CYCLE (GEL OR AGM)
Deep cycle batteries have a lower efficiency of 85%, and are more expensive than lead acid
types, but have a wider temperature range and are less susceptible to over-charging. The military,
large industrial plants and the space program use deep cycle batteries also known as nickel
cadmium, due to its high durability and higher economic rate of return on large projects. They
are newer form of battery that is being developed to have a higher energy density and longer life
span. This is the reason why deep cycle battery is use for this project
2.13.3 BATTERY SIZING AND SPECIFYING
The main goal of this research is to provide the suitability of the implementation of solar system
as an alternative source of energy to solve the regular power failure problem; the battery bank is
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sized to cater for supply to load during power failure. Therefore the system will work as a
backup power system during the grid failure.
The final battery capacity will depend on the following;
1. The total energy that the battery bank must supply during power failure.
2. Maximum power demand
3. Maximum depth of discharge
4. System voltage
5. Charge current and recharge time.
2.14 INVERTER
The main purpose of an inverter is to convert DC power to AC and to monitor the load current to
guard against power surges. The inverter system for this project was designed to handle the input
voltage of 12V stored in the battery. The power of the load was the second factor that went into
consideration in determining the type of inverter to be built. A 12V Power Bright inverter
matched all criteria for the project and was capable of supporting 1000W of output. The output
voltage was 220V AC, with a maximum current output of 13.5A. The inverter input voltage
operated between 12V and 14.5V DC.
Various inverters may have different features making them better suited different specific
applications. Very small inverters are available that connect to a car cigarette lighter, with a
single three-prong AC outlet as the output. Large inverters are generally designed to be
hardwired into a building electrical system.
Inverters are designed to produce mains (220v AC) power from a low voltage supply 12v.
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Generally an inverter consists of an inverter circuit, charger circuit and a battery. The charger
circuit keeps the battery charged when the mains power supply is available and when the mains
AC supply fails, the inverter circuit takes the DC power stored in the battery and converts it into
240v/50hz AC supply which can be used to power any common electrical/electronic equipment.
2.14.1 SIZING AND SPECIFYING INVERTER
The inverter is the main determinant of the system and can be designed first. The inverter is the
junction between the PV system (solar panel), the grid system and the loads. The choice of an
inverter in a PV connected system (solar panel) with battery backup is dependent on the input
and output voltages, the phase type ( 3-phase or single phase), output KVA (power), full load
efficiency, operation type and the presence of utility fault protection features.
2.14.2 OPERATION OF AN INVERTER
At the start of each day, the solar array charges the battery bank through the charge controller
(sometime fitted in the inverter). When the battery voltage rises above float voltage, the inverter
will convert the excess solar power into AC power to be supplied to the load circuit. The inverter
will also ensure that the batteries are held at this voltage, and this will be directly from the solar
power during the day. On the AC side, the inverter is both connected to the grid and the loads. If
the excess solar power supplied to the load circuit is not enough for its performance, the grid will
supplement it. If on the other hand, the solar power output is more than that required by the
loads, the remaining AC power is fed into the grid. When the grid power fails, the standard
protection devices within the inverter will disconnect the inverter from the grid. The system then
becomes like a stand-alone power system and the batteries supply power to the load circuits.
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When the grid power returns, the inverter will act as a battery charger and it, along with the solar
array (if it is during sunshine time) will charge the batteries up to the equalization voltage. After
the equalization charge has occurred, the batteries will then be held at float voltage. The system
will then return to the standard operation, that is, if the battery voltage is raised above the float
voltage due to the solar array, then the excess power will be exported to the AC grid connection
side of the inverter.
2.14.3 MODIFIED SINE WAVE INVERTERS
The least expensive type of modern inverter produces modified sine wave power. The waveform
looks like a stair-step where the power rises straight from zero to upper peak voltage straight
back to zero, and straight to lower peak voltage, resting at each point for a moment. Modified
sine wave inverters will run many household appliances such as television, radios and
microwaves with occasional minor electrical “noise” present. Sensitive equipment like battery
chargers, tools with variable speed motors, laser printers and certain heating controllers will run
erratically or not all with modified sine wave power.
2.14.4 PURE SINE WAVE INVERTERS
The power supplied by utility companies and engine generators is in pure sine wave form. This is
the most reliable waveform for household use. The pure sine wave power passes from the upper
and lower peak voltages in a smooth curve wave, rather than the stair-step of the modified sine
wave.
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MSW inverters usually cost far less than PSW units; however the power they produce is not as
clean and can cause interference problems with some appliances. Pure sine wave inverters
produce very high quality power, often with fewer spikes and surges than grid supplied power.
All appliances and electronic equipment will run as intended when using sine wave inverters will
produce AC power as good as or better than utility power, ensuring that even the most sensitive
equipment will run properly.
Figure 2.8 Switching mosfet gate
While pure sine wave inverters are more expensive than modified sine wave models, the quality
of their waveform is a good advantage.
For office building considering a backup power inverter, a pure sine wave model will allow
proper function of all electronic office equipment and fluorescent lighting. For residential power,
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anyone using battery chargers, electric drills, digital clock radios or other sensitive electronics
should consider a pure sine wave inverter to ensure proper function of all household appliances.
2.15 COMPONENT OF THE INVERTER
This are the parts which are networked together to build the inverter, and they are of high quality
material because there is no chance for try and error so that any inferior material might risk into
fire outbreak that will destroy the whole inverter in a matter of seconds.
2.15.1 MOSFET
The mosfet is a metal-oxide-semiconductor, this structure is obtained by depositing a layer of
silicon dioxide (SiO2) and a layer of metal (polycrystalline silicon is actually used instead of
metal) on top of a semiconductor die. As the silicon dioxide is a dielectric material its structure is
equivalent to a plane capacitor with one of the electrodes replaced by a semiconductor. When a
voltage is applied across a mosfet structure, it modifies the distribution of charges in the
semiconductor.
The mosfet simplify circuits since they are voltage controlled devices which operate with a very
low instantaneous input current. The output from the circuit is alternating which is useful when
power fails. They are very robust due to absence of the secondary breakdown phenomenon
peculiar to the bipolar transistor. Mosfets are much faster than bipolar transistor of comparable
dimensions as they are not subjected to delays and storage times due to minority carrier. The
mosfet has a positive temperature coefficient and protects itself by imposing a uniform
distribution of currents in the silicon chip. In a bipolar transistor, especially when the collector
emitter voltage is high, there is a current concentration in the base, giving rise to hot spots.
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These hot spots tend to constitute pits which swallow up the current, causing instant destruction
of the chip.
2.15.2 SWITCHES AND SOCKET
The power On/off switches and reset switches are commonly used in inverters. There set switch
is used to cut-off an over load circuit and restarts the supply.
2.15.3 LIGHT INDICATOR (LED)
Light emitting diodes are small semiconductors devices which emit light when a small forward
current is applied to them.
2.15.4 TRANSISTORS
Transistors are used in the inverter circuit to generate oscillation signals, amplification of signals
and to switch various circuits on/off.
2.15.5 REGULATOR
The regulator is a component with an internal standard voltage with which it continually
compares the actual output of the supply and makes appropriate adjustments by electronically
controlling the resistor as connected in the circuit. The regulator is useful because many
electronic circuits impose a varying load on their power supplies which will not function
properly when the supply voltage alters.
The regulator IC provides internal thermal overload protection and internal short circuit current
limiting. It has 3 terminals: input, ground and output. The voltage is specified by the last two
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digits of the IC part number. The regulator used in this inverter is the IC 7812 and it supplies
12Vdc.
We take for granted the fact that our mains power is very well regulated. So you can plug almost
any appliance into a standard point outlet, and it will operate correctly. That’s because the
electricity supplier has enormous generating plants, with automatic regulation systems to keep
the mains voltage and frequency very close to constant, despite load variations of many
megawatts. Inverter, connected to a modest battery or solar panel as the energy source. However
most modern inverters can provide reasonably good regulation for loads up to their rated
capacity (given in watts) assuming of course that they’re running from a well-charged battery. In
this type of inverter it isn’t feasible to control the peak-to-peak output, because this is largely
fixed by the battery voltage and the transformer step-up ratio. So in most cases the regulation is
achieved in a different way: by varying the width of the rectangular pulses, to control the form
factor and hence the RMS value of the output voltage. This is called pulse width modulation
(PWM), and is usually done by having a feedback system which senses the inverters output
voltage (or load current). When this feedback senses that the load on the inverters output has
increased, the inverters control circuitry acts to increase the width of the pulses which turn on the
MOSFETs. So the MOSFETs turn on for longer each half-cycle, automatically correcting the
RMS value of the output to compensate for any droop in peak-to-peak output.
2.15.6 TRANSFORMER
The transformer is one of the most important components of an inverter. It converts the battery
supply into 240v supply. The primary winding of the transformer is 230v and secondary winding
is 24v (a step-up transformer 12 0 12).
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2.15.7 RELAY
A relay is an electromagnetic switch. The switching on/off of relay is based on flow of current
through its coil. A relay is used for switching on/off various high voltage circuits. In inverters,
relays are used in various cut-off circuits and to switch the output between AC mains supply and
inverter generated power supply.
2.15.8 VOLTAGE SPIKES
Inverters are that appliances and tools with fairly inductive load impedance and can develop
fairly high voltage spikes due to inductive EMF. These spikes can be transformed back into the
primary of the inverter’s transformer, where they have the potential to damage the MOSFETs
and their driving circuitry. The risk of damage is fairly small during the actual power pulses of
each cycle, because at these times one end of the primary is effectively earthed. Transformer
action thus prevents the other end from rising higher than about twice the battery voltage.
However as you can see from Fig.2, there are times during every cycle of operation when neither
of the switching MOSFETs is conducting: the flats between the rectangular pulses. It’s at these
times that the spikes can produce excessive voltage across the MOSFETs, and potentially cause
damage. Another approach is to have high-power standard diodes connected from each end of
the primary to a large electrolytic capacitor, which becomes charged up to twice the battery
voltage. When the ends of the primary attempt to rise higher than this voltage, the diodes conduct
and allow the capacitor to absorb the spike energy. Quite apart from the generation of voltage
spikes, heavily inductive loads tend to demand current which is strongly shifted in phase relative
28
to the inverter’s output voltage pulses. This makes it hard for the inverter to cope, because the
only energy available to the load between the pulses is that stored in the transformer.
CHAPTER THREE
DESIGN AND CONSTRUCTION OF THE INVERTER
3.1 BASIC WORKING PRINCIPLE OF THE INVERTER
When the AC mains is available, the AC mains supply goes to the mains sensor informs the relay
about availability signal from the AC mains sensor, it passes the AC mains signals directly to the
inverter output socket. The battery charging section converts this AC mains supply into DC
supply, the DC supply is then regulated to provide required voltage and current to charge the
inverter battery. Same thing goes to the solar system, when there is no AC available and the sun
is out, it send a signal to the charge controller so as to allow the energy generated from the sun to
charge the battery.
Plate 3.1 Inverter connection and supply
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When the AC mains supply is not available, an oscillator section inside the inverter generates a
50Hz frequency MOS drive signal, this MOS drive signal is amplified by a driver section and
sent to the output section, the output section uses MOSFET devices for switching operation.
These MOSFETs are connected to the primary winding of the inverter transformer, whenever
these MOSFETS receive the MOS drive signal from the driver section, they start to switch on
and off at the speed of 50Hz. This switching on/off of MOSFET starts an alternating current with
50Hz frequency at the primary winding of the inverter transformer. This result in a 240V AC
supply (with 50Hz frequency) at the secondary winding is sent to the output socket of the
inverter through a changeover relay.
3.2 CIRCUIT DESCRIPTION OF PWM BASED 24V MOSFET INVERTER
PWM or Pulse Width Modulation is used to keep the AC supply output of the inverter to a
constant 240V. In an ordinary inverter, the inverter output changes with any change in the value
of the load connected at the inverter output. The PWM based inverter corrects the output value
based on the value of the load connected at the inverter output socket. This is done by changing
the width of the switching frequency generated by the oscillator.
In a PWM based inverter, the AC supply at the inverter output depends on the width of the
oscillator frequency generated by the oscillator section. In this inverter, a small part of the
inverter output is given as reference voltage to the PWM controller IC. Based on this reference
voltage, the PWM section will increase or reduce the width of the oscillation pulse generated by
the oscillator section. This change in the width will compensate any change in the inverter
30
output, and the inverter output will always stay constant, even if there is any change in the load
at the inverter output.
3.3 OBTAINING 50Hz FREQUENCY BY THE OSCILLATION SECTION
The oscillation section of this inverter uses a PWM controller IC1 (SG3525). This IC is used to
generate the 50Hz frequency required to generate AC supply by the inverter. To start this
process, battery supply is applied to the pin-15 of IC1, through the inverter on/off switch and
diodes D17 and D18. Pin-8 of the IC1 is connected to the negative terminal of the battery. A
voltage regulator s used to regulate the 12V supply from the battery. Pin-6 and pin-7 are
oscillation section pins. The frequency produced by the IC1 depends on the value of the
capacitor and resistance at these pins. The capacitors determine the 50Hz frequency output by
IC1. Pin-6 is the timing resistance pin. A resistance at this point keeps the oscillator frequency
constant. The signal generated by the oscillator section of IC1, reaches the flip-flop section of
IC1. This section converts the incoming signal into a signal with changing polarity. In two
signals with changing polarity, when the first signal is positive, the second signal will be
negative and when the second signal becomes positive, the first signal will be negative. This is
repeated 50 times per second, i.e. an alternating signal with 50Hz frequency is generated inside
the flip-flop section of the IC1. This alternating signal is known as the MOS Drive Signal.
3.4 OUTPUT SECTION
The 50Hz alternating MOS drive signal reaches each MOSFET channel separately. This results
in the MOSFET channels being alternatively on and off. This on/off switching process is
repeated 50 times per second. The drains (middle pint) of all the MOSFETs of one channel are
connected together to the heat sink which absorbs and dissipates the heat in the circuit. The gates
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of the MOSFETs of one channel are connected via resistors to the input signal; the resistors limit
the pulses going into the gates. The sources of the MOSFETs are connected together to form the
ground. Because the polarity of the 50Hz MOS drive signal at pin-11 and pin-14 are different,
only one channel from the output remains on at a time, the other channel stays off. When the first
MOSFET channel is on, the current flows through the first half of the bifilar winding of the
inverter transformer. When the second MOSFET channel turns on, the current flows through the
second half of the inverter transformer’s winding. This switching on and off of the MOSFET
channels will start an alternating current in the bifilar winding of the inverter transformer. This
AC current in the winding will induce a 50Hz AC current in the 240V tapping of the transformer.
3.5 PULSE WIDTH MODULATION (PWM) SECTION
PWM is used to keep the inverter output to a constant 240V AC. To make it work, the PWM IC1
must always receive a feedback of AC supply generated by the inverter circuit. If the IC1 does
not receive feedback, the value of the load connected to the inverter output socket changes with
the pulse width output from pin-11 and pin-14. This results in the fluctuation of the inverter
output supply.
To provide feedback to the PWM controller IC1, the AC supply from the inverter output s
converted into DC voltage by a bridge rectifier circuit. DC voltage from the bridge rectifier is
sent to pin-1 and pin-2 of the opto-coupler IC2 (814/817). When these two pins receive supply,
an LED inside the IC starts to glow. The light from this LED falls on the base of a
phototransistor inside the IC2. This will conduct the photo-transistor. The collector of the photo-
transistor is connected to pin-5 of IC2 while the emitter is connected to pin-4. Pin-5 receives 12V
supply from the battery. When the photo-transistor conducts, the supply at the collector of the
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phototransistor conducts, the supply at the collector of the photo-transistor is output as feedback
at its emitter (-4) the feedback at its emitter (pin-4). The feedback signal at pin-4 is fed to pin-1
of IC1 through a potential divider circuit and PWM adjust. Pin-1 and pin-2 are input pins and
pin-9 is an output pin of IC1. They are three pins of an op-amp. When the value of the load
connected to at the inverter output changes, the voltage at pin-14 of IC2 also changes. This will
result in variations in the feedback voltage reaching pin-1 of IC1 which will result in change in
output from pin-9 of IC1 is internally connected to the section which controls the width of the
oscillating frequency. Change in signal in oin-9 will result in a change in the width of the output
frequency and this will in turn result in a change in the 50Hz frequency output at pin-11 and pin-
14. This change will return the inverter output to its original 230V.
3.6 DRIVER SECTION
The MOS Drive signal from pin-11 and 14 of IC1 are given to the base of the MOS driver T1
and T2. This results in the MOS drive signal getting separated into two different channels.
Transistors T1 and T2 amplify the 50Hz MOS drive signal at their base to a sufficient level and
outputs them from the emitter. The 50Hz signal from the emitter of T1 is given to the gate G of
each MOSFET in the first MOSFET channel through a resistance. Likewise, the 50Hz signal
from the emitter of T2 is given to the gate G of each MOSFET in the second MOSFET channel
via resistance.
But, NS/NP = VS/VP = IP/IS = n
Where n = transformer turns ratio
If VP = 24V; VS = 230V; NS = 600 turns
Therefore, VS/VP = 230/24/10
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And n = NS/NP
NP = 600/10 = 60 turns
The transformer is a centre tap at its primary, with the number of turns (NP)
= 60 in each winding.
3.7 CONSTRUCTION
After testing the transformers and the output of the ICs, the components of the control, MOSFET
and charging circuits were soldered onto vero-boards.
Plate 3.1 Soldering of the inverter components
The component assembly was done in stages to allow for signal test at the output of individual
stages. All the circuits, transformers and switching panel were bolted into the metal casing with
other necessary accessories such as the power switch, a circuit breaker, output socket fuse and
voltmeter. Aluminium heat sinks were attached to the MOSFETs to quicken heat radiation. Extra
care was taken to ensure that the casing is strong enough to carry the heavy components and
34
provide ventilation and there is no partial contact or short circuit between the components of the
circuit.
Figure 3.2 Basic circuit scheme used
35
Plate 3.2 The oscillator
Figure 3.3 The driver circuit
36
3.8 DESIGN ANALYSIS OF THE DRIVER CIRCUIT
V I N−VBE = I B R B
hfe= I C
I B
R B=V I N − V BE
I B
Where,
IC = collector current
IB = base current
VIN = input voltage
VBE= Base-Emitter voltage
Hfe= current gain
From the data sheet,
I C=100 mA
37
Hfe=700
V BE = 0.7v
Therefore,
I B =100 mA
700
I B= 0.1429mA
V IN = 7.5v,
R B = 7.5 - 0.7
0.1429×10
= 47,585.72Ω
= 47.59kΩ
3.9 PRINCIPLE OF OPERATION
During the first (positive) half-cycle of the oscillator operation, the signal turns on transistor A
because its base is driven positive and it draws collector current upwards from the battery while
transistor B remains off because its base has a negative voltage and therefore has no collector
current. Hence the signal output is that of a positive half-cycle. During the negative half-cycle,
transistor B conducts while transistor A remains off because transistor B has a base driven
38
negative. Hence the output signal appears as that of a negative half-cycle. A combination of
these two signals gives the desired output waveform in square or sinusoidal forms.
3.10 THE POWER CIRCUIT
The N-channel enhancement Hi-speed MOSFET transistor IRPF 150N was made use of in
designing the power amplifier stage and this requires a centre-tapped transformer in the
MOSFET push-pull arrangement of switching as shown.
Figure 3.4 Power circuit diagram
The power circuit as shown above consists of a centre-tapped (12-0-12) step-up transformer
which increases the circuit voltage to match the actual A.C requirements of devices. The chosen
transformer must be able to handle the inverter wattage output. Since the design is a 2 KVA
inverter, a 2 KVA rated transformer would be appropriate, hence the choice of a 2 KVA rated
centre-tapped transformer. The MOSFETS are connected in parallel to increase the power within
39
the circuit. The primary side of the transformer is rated 2 KVA, 24 V, hence the current rating of
the transformer can be determined, and hence the required current rating of the MOSFETS can
be calculated.
3.11 DESIGN ANALYSIS OF THE POWER CIRCUIT
Current Rating = 2000 12
= 83.33A
A centre-tapped transformer is being used; hence the current rating per side can be calculated i.e.
Per side current rating = 83.33 2
= 41.67A
Having 3 identical MOSFETS, the required current rating for each of the MOSFETS, for each
side of the centre-tapped transformer can also be determined.
Required current rating for each MOSFETS = 41.67 3 = 13.8A
To prevent the MOSFETS from burning out due to excessive current, it is advised to use a
MOSFETS of three times the required MOSFETS current rating.
Therefore: 13.89×3= 41.67A
Hence 6 MOSFETS of 40A were used, with 3 on each side of the centre tapped transformer for
protection of the MOSFETS.
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3.12 THE TRANSFORMER
This is a static equipment or device through which electric power in one circuit is transformed
into electric power in another circuit with no change in frequency. It can raise or lower voltage in
a circuit but with a correspondent decrease or increase in current. The working principle of a
transformer is based on mutual induction between two circuits linked by a common magnetic
flux i.e. two coils which are electrically separated but magnetically linked through a path of low
reluctance. For the inverter to be useful in handling appliances that make use of alternating
current of power rating higher than that available within the circuit, there is a need for a device
such as the transformer.
3.13 CASTING OF THE CASE
A well-ventilated metal casing was constructed for the inverter circuit with the components on
the vero board taken into consideration. The inverter transformer which is large and heavy was
specially considered when constructing the casing. A fan was included to cool the heat sinks
attached to the MOSFET circuit. Other necessary accessories included in the casing are the
power switch, 15A output socket, fuse, an LED and the voltmeter.
3.14 SOLAR ARRAY
The solar array should be
* Mounted facing true south + or – 5 degrees.
*Mounted at a tilt angle of between 100 to 150 to the horizontal.
*Sited to minimize shading by trees and buildings.
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*Mounted to meet wind loading standards
Plate 3.3 Mounting it on clear environment
42
CHAPTER FOUR
TEST AND RESULT
4.1 IMPLEMENTATION
The implementation of this project was done on a breadboard. Stage by stage testing was done
according to the block representation on the breadboard before soldering of the circuit
commenced on the vero board.
4.2 ARRANGEMENT OF THE COMPONENTS
All the components used were arranged on vero boards using the circuit diagrams shown
previously. Each circuit was tested before being added to the overall inverter circuit.
4.3 OUTPUT VOLTAGE
The exact value of the output voltage was determined by inserting the terminals of a digital
multimeter into the socket. This way the output voltage was 240V
4.4 NO-LOAD TEST
One 12-V sealed rechargeable battery was connected to the inverter circuit. The positive terminal
of the battery was connected to the centre tap of the inverter transformer while the negative
terminal of the battery was connected to the overall ground of the inverter circuit.
When the inverter was switched on, a loud humming noise from the transformer was observed
which indicated that the output voltage was too high. A screwdriver was used to adjust the pulse
43
width output by turning the variable resistor in the control circuit figure 6. The exact value of the
output voltage was determined by inserting the terminals of a digital multimeter into the socket,
this way the output voltage was reduced to 240V
4.5 LOAD TEST
A television and three light bulbs with a combined wattage of about 500 watts were run on the
inverter using the rechargeable batteries and no unusual events were recorded. The humming of
the transformer was tolerable.
4.6 WIRING AND CURRENT CARRYING CAPACITY
It is vital to have a clear understanding of the wiring connections required in the system. Some
components do not allow for trial and error since they burn out immediately when not connected
properly. As much as the design/sizing of the grid connected PV system is important, the
accurate selection of system wiring cables is very essential in order that the system is safe.
The wiring must not reduce the performance of any of the components in the system, the wire in
the inverter system must be sized correctly to reduce the voltage drops in the cable and to make
sure that the safe current handling capacity of the cable is not exceeded.
In addition it is necessary to note that cables have a rated current carrying capacity and when
purchasing cables for this installation, all cables must have a current carrying capacity higher
than the current rating of the protection device connected to it so that the over current protection
devices clears before any of the current carrying components in any over current condition.
44
4.7 ECONOMICS ANALYSIS
The generator generates a lot of noise during operation and most of them do not have automatic
start/stop in the event of power failure /restoration. They require highly inflammable products
such as fuel which emit smoke and a bad odour which could be harmful to people around it.
Also, the generator has many mechanical parts which require constant maintenance.
On the other hand, the inverter works noiselessly, provides completely automatic switching
operations during power failure, does not produce any harmful emissions and does not require
any special maintenance apart from the battery used which requires routine service once in 3
months.
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CHAPTER FIVE
CONCLUSION AND RECOMMENDATION
5.1 CONCLUSION
The basic objectives of this project that were realized include the guarantee of supply during the
absence of power from PHCN, the elimination of power surges and energy management and
reduction of bills amassed from fuel costs and generator maintenance. The inverter is more
environmentally friendly, easier to operate and requires very little maintenance compared to a
generator.
5.2 RECOMENDATION
This project can be used as an alternative power supply source for home and medical appliances
and the design and construction of three-phase inverters should be encouraged for industrial
applications. It is highly recommended for use in rural areas where there is no electricity; solar
energy can be utilized in this case. Future designs of the inverter should include more efficient
charging circuits with better overload, over current protection, the use of digital displays and
faster, switching devices.
46
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