Noval Application of a PV Solar plant as STATCOM during Night
and Day in a Distribution Utility NetworkABSTRACTPv solar farms
produce power during the day are completely idle in the nights
.this paper presents utilization of a PvV solar plants as statcom
in the night for load reactive power compensation and voltage
control .this Statcom functionally wiil also be available to a
substantialdegree during daytime with inverter capacity remaining
after real power production .the local distribution company london
hydro implementingthis new technology ,for the first time in Canada
on 10Kw solar system to be installe don the roof top of their head
quarters building .The PV solar plant inverter controller will be
designed developed and tested in the university laband will be
designed ,developed and tested in the university lab and will be
designed ,developed and tested in the university lab and will then
be installed in fied by spring 2011
Chapter-1PHOTO VOLATIC CELLSPhotovoltaics(PV) is a method
ofgenerating electrical powerby convertingsolar radiationintodirect
currentelectricityusingsemiconductorsthat exhibit thephotovoltaic
effect. Photovoltaic power generation employssolar panelscomposed
of a number ofsolar cellscontaining a photovoltaic material.
Materials presently used for photovoltaics includemonocrystalline
silicon,polycrystalline silicon,amorphous silicon,cadmium
telluride, andcopper indium gallium selenide/sulfide.Due to the
growing demand forrenewable energysources, the manufacturing of
solar cells andphotovoltaic arrayshas advanced considerably in
recent years. Solar photovoltaics is growing rapidly, albeit from a
small base, to a total global capacity of 67,400megawatts(MW) at
the end of 2011, representing 0.5% of worldwide electricity demand.
The total power output of the worlds PV capacity run over a
calendar year is equal to some 80 billion kWh of electricity. This
is sufficient to cover the annual power supply needs of over 20
million households in the world.[5]More than 100 countries use
solar PV. Installations may be ground-mounted (and sometimes
integrated with farming and grazing)or built into the roof or walls
of a building (building-integrated photovoltaics).Driven by
advances in technology and increases in manufacturing scale and
sophistication, the cost of photovoltaics has declined steadily
since the first solar cells were manufactured[8]and the levelised
cost of electricity (LCOE) from PV is competitive with conventional
electricity sources in an expanding list of geographic regions.Net
meteringand financial incentives, such as preferentialfeed-in
tariffsfor solar-generated electricity, have supported solar PV
installations in many countries.[10]Solar CellsPhotovoltaics are
best known as a method for generating electric power by using solar
cells to convert energy from the sun into a flow of electrons.
Thephotovoltaic effectrefers to photons of light exciting electrons
into a higher state of energy, allowing them to act as charge
carriers for an electric current. The photovoltaic effect was first
observed byAlexandre-Edmond Becquerelin 1839. The term photovoltaic
denotes the unbiased operating mode of aphotodiodein which current
through the device is entirely due to the transduced light energy.
Virtually all photovoltaic devices are some type of
photodiode.Solar cells produce direct current electricity from sun
light, which can be used to power equipment or torecharge a
battery. The first practical application of photovoltaics was to
power orbiting satellites and otherspacecraft, but today the
majority ofphotovoltaic modulesare used for grid connected power
generation. In this case aninverteris required to convert the DC to
AC. There is a smaller market for off-grid power for remote
dwellings,boats,recreational vehicles, electric cars, roadside
emergency telephones,remote sensing, andcathodic
protectionofpipelines.Photovoltaic power generation employssolar
panelscomposed of a number ofsolar cellscontaining a photovoltaic
material. Materials presently used for photovoltaics
includemonocrystalline silicon,polycrystalline silicon,amorphous
silicon,cadmium telluride, andcopper indium gallium
selenide/sulfide. Due to the growing demand forrenewable
energysources, the manufacturing of solar cells andphotovoltaic
arrayshas advanced considerably in recent years. Cells require
protection from the environment and are usually packaged tightly
behind a glass sheet. When more power is required than a single
cell can deliver, cells are electrically connected together to form
photovoltaic modules, or solar panels. A single module is enough to
power an emergency telephone, but for a house or a power plant the
modules must be arranged in multiples asarrays.A significant market
has emerged in off-grid locations for solar-power-charged
storage-battery based solutions. These often provide the only
electricity available.[14]The first commercial installation of this
kind was in 1966 on Ogami Island in Japan to transitionOgami
Lighthousefrom gas torch to fully self-sufficient electrical
power.Due to the growing demand for renewable energy sources, the
manufacture of solar cells andphotovoltaic arrayshas advanced
dramatically in recent years. Solar photovoltaics is growing
rapidly, albeit from a small base, to a total global capacity of
67,400megawatts(MW) at the end of 2011, representing 0.5% of
worldwide electricity demand.[5]The total power output of the
worlds PV capacity run over a calendar year is equal to some 80
billion kWh of electricity. This is sufficient to cover the annual
power supply needs of over 20 million households in the
world.[5]More than 100 countries use solar PV.World solar PV
capacity (grid-connected) was 7.6 GW in 2007, 16 GW in 2008, 23 GW
in 2009, and 40 GW in 2010.More than 100 countries use solar
PV.Installations may be ground-mounted (and sometimes integrated
with farming and grazing)or built into the roof or walls of a
building (building-integrated photovoltaics).Photovoltaic power
capacity is measured as maximum power output under standardized
test conditions (STC) in "Wp" (Watts peak).The actual power output
at a particular point in time may be less than or greater than this
standardized, or "rated," value, depending on geographical
location, time of day, weather conditions, and other factors.Solar
photovoltaic arraycapacity factorsare typically under 25%, which is
lower than many other industrial sources of electricity. The
EPIA/GreenpeaceAdvanced Scenario shows that by the year 2030, PV
systems could be generating approximately 1.8TW of electricity
around the world. This means that, assuming a serious commitment is
made toenergy efficiency, enough solar power would be produced
globally in twenty-five years time to satisfy the electricity needs
of almost 14% of the worlds population.[21]Current
developments:Photovoltaic panels based oncrystalline siliconmodules
are encountering competition in the market by panels that
employthin-film solar cells(CdTe[22]CIGS,[23]amorphous
Si,[24]microcrystalline Si), which had been rapidly evolving and
are expected to account for 31 percent of the global installed
power by 2013.[25]However, precipitous drops in prices for
polysilicon and their panels in late 2011 have caused some
thin-film makers to exit the market and others to experience
severely squeezed profits.[26]Other developments
includecastingwafers instead of sawing,[27]concentrator
modules,'Sliver' cells, andcontinuous printingprocesses.TheSan
Jose-based company Sunpower produces cells that have an energy
conversion ratio of 19.5%, well above the market average of
1218%.[28]The most efficient solar cell so far is a multi-junction
concentrator solar cell with an efficiency of 43.5%[29]produced by
theNational Renewable Energy Laboratoryin April 2011. The highest
efficiencies achieved without concentration includeSharp
Corporationat 35.8% using a proprietary triple-junction
manufacturing technology in 2009,[30]and Boeing Spectrolab (40.7%
also using a triple-layer design). A March 2010 experimental
demonstration of a design by a Caltech group led byHarry
Atwaterwhich has an absorption efficiency of 85% in sunlight and
95% at certain wavelengths is claimed to have near perfect quantum
efficiency.[31]However, absorption efficiency should not be
confused with the sunlight-to-electricity conversion
efficiency.
For best performance, terrestrial PV systems aim to maximize the
time they face the sun.Solar trackersachieve this by moving PV
panels to follow the sun. The increase can be by as much as 20% in
winter and by as much as 50% in summer. Static mounted systems can
be optimized by analysis of thesun path. Panels are often set to
latitude tilt, an angle equal to the latitude, but performance can
be improved by adjusting the angle for summer or winter. Generally,
as with other semiconductor devices, temperatures above room
temperature reduce the performance of photovoltaics.[32]The 2011
European Photovoltaic Industry Association (EPIA) report predicted
that, "The future of the PV market remains bright in the EU and the
rest of the world," the report said. "Uncertain times are causing
governments everywhere to rethink the future of their energy mix,
creating new opportunities for a competitive, safe and reliable
electricity source such as PV. 2012 could see the installation of
2030GW of PV about the same as in 2011. Unfortunately, the
industry's capacity continues to expand, to perhaps as much as
38GW. The resulting glut of supply has crushed prices and
profits.[34]By 2015, 131196GW of photovoltaic systems could be
installed around the globe.[33]REVENUE MODEL OF PV SOLAR PLANT
Economics:Financial incentives for photovoltaics, such asfeed-in
tariffs, have often been offered to electricity consumers to
install and operate solar-electric generating systems. Government
has sometimes also offered incentives in order to encourage the PV
industry to achieve theeconomies of scaleneeded to compete where
the cost of PV-generated electricity is above the cost from the
existing grid. Such policies are implemented to promote national or
territorialenergy independence,high techjob creation and reduction
ofcarbon dioxide emissionswhich cause global warming. Due to
economies of scale solar panels get less costly as people use and
buy more as manufacturers increase production to meet demand, the
cost and price is expected to drop in the years to come.
NRELcompilation of best research solar cell efficiencies from
1976 to 2010According toShi Zhengrong, in 2012 unsubsidized PV
systems already produce electricity in some parts of the world,
more cheaply than coal and gas-fired power plants.[35][36]As PV
system prices decline it's inevitable that subsidies will end.
"Rapid decline or outright disappearance has already been seen in
all the major solar markets except China and India".[36]As of 2011,
the price of PV modules per MW has fallen by 60 percent since the
summer of 2008, according to Bloomberg New Energy Finance
estimates, putting solar power for the first time on a competitive
footing with the retail price of electricity in a number of sunny
countries. There has been fierce competition in the supply chain,
and further improvements in the levelised cost of energy for solar
lie ahead, posing a growing threat to the dominance of fossil fuel
generation sources in the next few years.[37]As time progresses,
renewable energy technologies generally get cheaper,[38][39]while
fossil fuels generally get more expensive:The less solar power
costs, the more favorably it compares to conventional power, and
the more attractive it becomes to utilities and energy users around
the globe. Utility-scale solar power can now be delivered in
California at prices well below $100/MWh ($0.10/kWh) less than most
other peak generators, even those running on low-cost natural gas.
Lower solar module costs also stimulate demand from consumer
markets where the cost of solar compares very favorably to retail
electric rates.[40]As of 2011, the cost of PV has fallen well below
that of nuclear power and is set to fall further. The average
retail price of solar cells as monitored by the Solarbuzz group
fell from $3.50/watt to $2.43/watt over the course of 2011, and a
decline to prices below $2.00/watt seems inevitable:[41]For
large-scale installations, prices below $1.00/watt are common. In
some locations, PV has reached grid parity, the cost at which it is
competitive with coal or gas-fired generation. Photovoltaic power
is also generated during a time of day that is close to peak demand
(precedes it). More generally, it is now evident that, given a
carbon price of $50/ton, which would raise the price of coal-fired
power by 5c/kWh, solar PV will be cost-competitive in most
locations. The declining price of PV has been reflected in rapidly
growing installations, totaling about 23 GW in 2011. Although some
consolidation is likely in 2012, as firms try to restore
profitability, strong growth seems likely to continue for the rest
of the decade. Already, by one estimate, total investment in
renewables for 2011 exceeded investment in carbon-based electricity
generation.[41]
ApplicationsPower stationsMany solarphotovoltaic power
stationshave been built, mainly in Europe.[42]As of December 2011,
the largest photovoltaic (PV) power plants in the world are
theGolmud Solar Park(China, 200 MW),Sarnia Photovoltaic Power
Plant(Canada, 97 MW),Montalto di Castro Photovoltaic Power
Station(Italy, 84.2 MW),Finsterwalde Solar Park(Germany, 80.7
MW),Okhotnykovo Solar Park(Ukraine, 80MW),Lieberose Photovoltaic
Park(Germany, 71.8MW),Rovigo Photovoltaic Power Plant(Italy, 70
MW),Olmedilla Photovoltaic Park(Spain, 60MW), and theStrasskirchen
Solar Park(Germany, 54MW).[42]
There are also many large plants under construction. TheDesert
Sunlight Solar Farmunder construction inRiverside County,
CaliforniaandTopaz Solar Farmbeing built inSan Luis Obispo County,
Californiaare both 550MWsolar parksthat will use thin-film
solarphotovoltaicmodules made byFirst Solar. TheBlythe Solar Power
Projectis a 500 MW photovoltaic station under construction
inRiverside County, California. TheAgua Caliente Solar Projectis a
290 megawatt photovoltaic solar generating facility being built
inYuma County, Arizona. TheCalifornia Valley Solar Ranch(CVSR) is a
250megawatt(MW)solar photovoltaicpower plant, which is being built
bySunPowerin theCarrizo Plain, northeast ofCalifornia Valley.
The 230 MWAntelope Valley Solar Ranchis aFirst Solarphotovoltaic
project which is under construction in the Antelope Valley area of
the Western Mojave Desert, and due to be completed in
2013.TheMesquite Solar projectis a photovoltaic solar power plant
being built inArlington,Maricopa County,Arizona, owned bySempra
Generation. Phase1 will have anameplate capacityof 150megawatts.
Many of these plants are integrated with agriculture and some use
innovative tracking systems that follow the sun's daily path across
the sky to generate more electricity than conventional
fixed-mounted systems. There are no fuel costs or emissions during
operation of the power stations.
In buildings
Photovoltaic wall at MNACTEC Terrassa in SpainPhotovoltaic
arrays are often associated with buildings: either integrated into
them, mounted on them or mounted nearby on the ground.Arrays are
most often retrofitted into existing buildings, usually mounted on
top of the existing roof structure or on the existing walls.
Alternatively, an array can be located separately from the building
but connected by cable to supply power for the building. In 2010,
more than four-fifths of the 9,000 MW of solar PV operating in
Germany were installed on rooftops.[48]Building-integrated
photovoltaics (BIPV) are increasingly incorporated into new
domestic and industrial buildings as a principal or ancillary
source of electrical power.[49]Typically, an array is incorporated
into the roof or walls of a building. Roof tiles with integrated PV
cells are also common. A 2011 study using thermal imaging has shown
that solar panels, provided there is an open gap in which air can
circulate between them and the roof, provide a passive cooling
effect on buildings during the day and also keep accumulated heat
in at night. The power output of photovoltaic systems for
installation in buildings is usually described inkilowatt-peakunits
(kWp).In transportPV has traditionally been used for electric power
in space. PV is rarely used to provide motive power in transport
applications, but is being used increasingly to provide auxiliary
power in boats and cars. A self-containedsolar vehiclewould have
limited power and low utility, but asolar-charged vehiclewould
allow use of solar power for transportation. Solar-powered cars
have been demonstrated.[51]
Standalone devices
Solar parking paystation.Until a decade or so ago, PV was used
frequently to power calculators and novelty devices. Improvements
in integrated circuits and low powerliquid crystal displaysmake it
possible to power such devices for several years between battery
changes, making PV use less common. In contrast, solar powered
remote fixed devices have seen increasing use recently in locations
where significant connection cost makes grid power prohibitively
expensive. Such applications include water pumps,parking meters,
emergency telephones,trash compactors, temporary traffic signs, and
remote guard posts and signals.Rural electrificationUnlike the past
decade, which saw solar solutions purchased mainly by international
donors, it is now the locals who are increasingly opening their
wallets to make the switch from their traditional energy means.
That is because solar products prices in recent years have declined
to become cheaper than kerosene and batteries.In Cambodia, for
example, villagers can buy a solar lantern at US$25 and use it for
years without any extra costs, where their previous spending on
kerosene for lighting was about $2.5 per month, or $30 per year. In
Kenya a solar kit that provides bright light or powers a radio or
cell phone costs under $30 at retail stores. By switching to this
kit Kenyans can save $120 per year on kerosene lighting, radio
batteries and cell phone recharging fees.[57]Developing countries
where many villages are often more than five kilometers away from
grid power are increasingly using photovoltaics. In remote
locations in India a rural lighting program has been providing
solar powered LED lighting to replace kerosene lamps. The solar
powered lamps were sold at about the cost of a few months' supply
of kerosene.[58][59]Cuba is working to provide solar power for
areas that are off grid.[60]These are areas where the social costs
and benefits offer an excellent case for going solar though the
lack of profitability could relegate such endeavors to humanitarian
goals.
Solar roadways
The 104kW solar highway along the interchange ofInterstate
5andInterstate 205nearTualatin, Oregonin December 2008.In December
2008, the Oregon Department of Transportation placed in service the
nations first solar photovoltaic system in a U.S. highway
right-of-way. The 104-kilowatt (kW) array produces enough
electricity to offset approximately one-third of the electricity
needed to light the Interstate highway interchange where it is
located.[61]A 45mi (72km) section of roadway in Idaho is being used
to test the possibility of installing solar panels into the road
surface, as roads are generally unobstructed to the sun and
represent about the percentage of land area needed to replace other
energy sources with solar power.[62]Solar Power
satellitesSpace-based solar power (SBSP) is the concept of
collecting solar power inspacefor use onEarth. It has been in
research since the early 1970s. SBSP would differ from current
solar collection methods in that the means used to collect energy
would reside on anorbitingsatelliteinstead of on Earth's surface.
Some projected benefits of such a system are: higher collection
rate, longer collection period, and elimination ofweatherconcerns.
SBSP also introduces several new hurdles, primarily the problem of
transmitting energy from orbit to Earth's surface for
use.AdvantagesThe 89PWof sunlight reaching the Earth's surface is
plentiful almost 6,000 times more than the 15TW equivalent of
average power consumed by humans.[63]Additionally, solar electric
generation has the highest power density (global mean of 170 W/m)
among renewable energies.[63]Solar power is pollution-free during
use. Production end-wastes and emissions are manageable using
existing pollution controls. End-of-use recycling technologies are
under development[64]and policies are being produced that encourage
recycling from producers.[65]PV installations can operate for many
years with little maintenance or intervention after their initial
set-up, so after the initialcapital costof building any solar power
plant,operating costsare extremely low compared to existing power
technologies.Grid-connected solar electricity can be used locally
thus reducing transmission/distribution losses (transmission losses
in the US were approximately 7.2% in 1995).[66]Compared to fossil
and nuclear energy sources, very little research money has been
invested in the development of solar cells, so there is
considerable room for improvement. Nevertheless, experimentalhigh
efficiency solar cellsalready have efficiencies of over 40% in case
of concentrating photovoltaic cells[67]and efficiencies are rapidly
rising while mass-production costs are rapidly
falling.[68]DisadvantagesPhotovoltaic panels are specifically
excluded in Europe from RoHS (Restriction on Hazardous Substances)
since 2003 and were again excluded in 2011. California has largely
adopted the RoHS standard throughEWRA. Therefore, PV panels may
legally in Europe and California contain lead, mercury and cadmium
which are forbidden or restricted in all other electronics.[69]In
some states of the United States of America, much of the investment
in a home-mounted system may be lost if the home-owner moves and
the buyer puts less value on the system than the seller. The city
of Berkeley developed an innovative financing method to remove this
limitation, by adding a tax assessment that is transferred with the
home to pay for the solar panels.[70]Now known asPACE, Property
Assessed Clean Energy, 28 U.S. states have duplicated this
solution
Chapter-2Solar cellsAsolar cell(also calledphotovoltaic
cellorphotoelectric cell) is asolid stateelectrical device that
converts the energy oflightdirectly intoelectricityby
thephotovoltaic effect.Assemblies of solar cells are used to
makesolar moduleswhich are used to capture energy fromsunlight.
When multiple modules are assembled together (such as prior to
installation on a pole-mounted tracker system), the resulting
integrated group of modules all oriented in one plane is referred
to in the solar industry as asolar panel. The electrical energy
generated from solar modules, referred to assolar power, is an
example ofsolar energy.Photovoltaicsis the field of technology and
research related to the practical application of photovoltaic cells
in producing electricity from light, though it is often used
specifically to refer to the generation of electricity from
sunlight.Cells are described asphotovoltaic cellswhen the light
source is not necessarily sunlight (lamplight, artificial light,
etc.). These are used for detecting light or otherelectromagnetic
radiationnear the visible range, for exampleinfrared detectors, or
measurement of light intensity.
ApplicationsSolar cells are often electrically connected and
encapsulated as amodule. Photovoltaic modules often have a sheet of
glass on the front (sun up) side, allowing light to pass while
protecting the semiconductorwafersfrom abrasion and impact due to
wind-driven debris,rain,hail, etc. Solar cells are also usually
connected inseriesin modules, creating an additivevoltage.
Connecting cells in parallel will yield a higher current; however,
very significant problems exist with parallel connections. For
example, shadow effects can shut down the weaker (less illuminated)
parallel string (a number of series connected cells) causing
substantial power loss and even damaging excessive reverse bias
applied to the shadowed cells by their illuminated partners. As far
as possible, strings of series cells should be handled
independently and not connected in parallel, save using special
paralleling circuits. Although modules can be interconnected to
create anarraywith the desired peak DC voltage and loading current
capacity, using independent MPPTs (maximum power point trackers)
provides a better solution. In the absence of paralleling circuits,
shunt diodes can be used to reduce the power loss due to shadowing
in arrays with series/parallel connected cells.To make practical
use of the solar-generated energy, the electricity is most often
fed into the electricity grid using inverters
(grid-connectedphotovoltaic systems); in stand-alone systems,
batteries are used to store the energy that is not needed
immediately. Solar panels can be used to power or recharge portable
devices.
TheoryThe solar cell works in three steps:1.
Photonsinsunlighthit the solar panel and are absorbed by
semiconducting materials, such as silicon.2. Electrons(negatively
charged) are knocked loose from their atoms, causing an electric
potential difference. Current starts flowing through the material
to cancel the potential and this electricity is captured. Due to
the special composition of solar cells, the electrons are only
allowed to move in a single direction.3. An array of solar cells
converts solar energy into a usable amount ofdirect current(DC)
electricity.EfficiencyThe efficiency of a solar cell may be broken
down into reflectance efficiency, thermodynamic efficiency, charge
carrier separation efficiency and conductive efficiency. The
overall efficiency is the product of each of these individual
efficiencies.Due to the difficulty in measuring these parameters
directly, other parameters are measured instead: thermodynamic
efficiency,quantum efficiency,integrated quantum efficiency,
VOCratio, and fill factor. Reflectance losses are a portion of the
quantum efficiency under "external quantum efficiency".
Recombination losses make up a portion of the quantum efficiency,
VOCratio, and fill factor. Resistive losses are predominantly
categorized under fill factor, but also make up minor portions of
the quantum efficiency, VOCratio.Thefill factoris defined as the
ratio of the actual maximum obtainablepower, to the product of the
open circuit voltage and short circuit current. This is a key
parameter in evaluating the performance of solar cells. Typical
commercial solar cells have a fill factor > 0.70. Grade B cells
have a fill factor usually between 0.4 to 0.7. The fill factor is,
besides efficiency, one of the most significant parameters for the
energy yield of aphotovoltaic cell.[14]Cells with a high fill
factor have a low equivalent series resistance and a high
equivalent shunt resistance, so less of the current produced by
light is dissipated in internal losses.Single p-n junction
crystalline silicon devices are now approaching the theoretical
limiting power efficiency of 33.7%, noted as theShockleyQueisser
limitin 1961. In the extreme, with an infinite number of layers,
the corresponding limit is 86% using concentrated
sunlight.[15]CostThe cost of a solar cell is given per unit of peak
electrical power. Manufacturing costs necessarily include the cost
of energy required for manufacture. Solar-specific feed in tariffs
vary worldwide, and even state by state within various
countries.[16]Such feed-in tariffs can be highly effective in
encouraging the development of solar power projects.
High-efficiency solar cells are of interest to decrease the cost
of solar energy. Many of the costs of a solar power plant are
proportional to the area of the plant; a higher efficiency cell may
reduce area and plant cost, even if the cells themselves are more
costly. Efficiencies of bare cells, to be useful in evaluating
solar power plant economics, must be evaluated under realistic
conditions. The basic parameters that need to be evaluated are the
short circuit current, open circuit voltage.[17]The chart below
illustrates the best laboratory efficiencies obtained for various
materials and technologies, generally this is done on very small,
i.e., one square cm, cells. Commercial efficiencies are
significantly lower.
Reported timeline of solar cell energy conversion efficiencies
(from National Renewable Energy Laboratory (USA))Grid parity, the
point at which photovoltaic electricity is equal to or cheaper
thangrid power, can be reached using low cost solar cells. It is
achieved first in areas with abundant sun and high costs for
electricity such as inCaliforniaandJapan.[18]Grid parity has been
reached inHawaiiand other islands that otherwise usediesel fuelto
produce electricity.George W. Bushhad set 2015 as the date for grid
parity in the USA.[19][20]Speaking at a conference in 2007,General
Electric's Chief Engineer predicted grid parity without subsidies
in sunny parts of the United States by around 2015.[21]The price of
solar panels fell steadily for 40 years, until 2004 when high
subsidies in Germany drastically increased demand there and greatly
increased the price of purified silicon (which is used in computer
chips as well as solar panels). Thegreat recessionof 2008 and the
onset of Chinese manufacturing caused prices to resume their
decline with vehemence. In the four years after January 2008 prices
for solar modules in Germany dropped from 3 to 1 per peak watt.
During that same times production capacity surged with an annual
growth of more than 50%. China increased market share from 8% in
2008 to over 55% in the last quarter of 2010.[22]Recently, since
the middle of 2010, the price has been dropped to $1.21.5/Wp
(crystalline modules).[citation needed]
MaterialsThe cost of a solar cell is given per unit of peak
electrical power. Manufacturing costs necessarily include the cost
of energy required for manufacture. Solar-specific feed in tariffs
vary worldwide, and even state by state within various
countries.[16]Such feed-in tariffs can be highly effective in
encouraging the development of solar power projects.High-efficiency
solar cells are of interest to decrease the cost of solar energy.
Many of the costs of a solar power plant are proportional to the
area of the plant; a higher efficiency cell may reduce area and
plant cost, even if the cells themselves are more costly.
Efficiencies of bare cells, to be useful in evaluating solar power
plant economics, must be evaluated under realistic conditions. The
basic parameters that need to be evaluated are the short circuit
current, open circuit voltage.[17]The chart below illustrates the
best laboratory efficiencies obtained for various materials and
technologies, generally this is done on very small, i.e., one
square cm, cells. Commercial efficiencies are significantly
lower.
Reported timeline of solar cell energy conversion efficiencies
(from National Renewable Energy Laboratory (USA))Grid parity, the
point at which photovoltaic electricity is equal to or cheaper
thangrid power, can be reached using low cost solar cells. It is
achieved first in areas with abundant sun and high costs for
electricity such as inCaliforniaandJapan.[18]Grid parity has been
reached inHawaiiand other islands that otherwise usediesel fuelto
produce electricity.George W. Bushhad set 2015 as the date for grid
parity in the USA.[19][20]Speaking at a conference in 2007,General
Electric's Chief Engineer predicted grid parity without subsidies
in sunny parts of the United States by around 2015.[21]The price of
solar panels fell steadily for 40 years, until 2004 when high
subsidies in Germany drastically increased demand there and greatly
increased the price of purified silicon (which is used in computer
chips as well as solar panels). Thegreat recessionof 2008 and the
onset of Chinese manufacturing caused prices to resume their
decline with vehemence. In the four years after January 2008 prices
for solar modules in Germany dropped from 3 to 1 per peak watt.
During that same times production capacity surged with an annual
growth of more than 50%. China increased market share from 8% in
2008 to over 55% in the last quarter of 2010.[22]Recently, since
the middle of 2010, the price has been dropped to $1.21.5/Wp
(crystalline modules).[citation needed]ManufactureBecause solar
cells are semiconductor devices, they share some of the same
processing and manufacturing techniques as other semiconductor
devices such ascomputerandmemorychips. However, the stringent
requirements for cleanliness and quality control of semiconductor
fabrication are more relaxed for solar cells. Most large-scale
commercial solar cell factories today make screen printed
poly-crystalline or single crystalline silicon solar
cells.Poly-crystalline silicon wafers are made by wire-sawing
block-cast silicon ingots into very thin (180 to 350 micrometer)
slices or wafers. The wafers are usually lightlyp-typedoped. To
make a solar cell from the wafer, a surface diffusion
ofn-typedopants is performed on the front side of the wafer. This
forms a p-n junction a few hundred nanometers below the
surface.Anti-reflection coatings, to increase the amount of light
coupled into the solar cell, are typically next applied. Silicon
nitride has gradually replaced titanium dioxide as the
anti-reflection coating, because of its excellent surface
passivation qualities. It prevents carrier recombination at the
surface of the solar cell. It is typically applied in a layer
several hundred nanometers thick using plasma-enhanced chemical
vapor deposition (PECVD). Some solar cells have textured front
surfaces that, like anti-reflection coatings, serve to increase the
amount of light coupled into the cell. Such surfaces can usually
only be formed on single-crystal silicon, though in recent years
methods of forming them on multicrystalline silicon have been
developed.The wafer then has a full area metal contact made on the
back surface, and a grid-like metal contact made up of fine
"fingers" and larger "bus bars" are screen-printed onto the front
surface using asilverpaste. The rear contact is also formed by
screen-printing a metal paste, typically aluminium. Usually this
contact covers the entire rear side of the cell, though in some
cell designs it is printed in a grid pattern. The paste is then
fired at several hundred degrees Celsius to form metal electrodes
inohmic contactwith the silicon. Some companies use an additional
electro-plating step to increase the cell efficiency. After the
metal contacts are made, the solar cells are interconnected by flat
wires or metal ribbons, and assembled intomodulesor "solar panels".
Solar panels have a sheet oftempered glasson the front, and
apolymerencapsulation on the back.
Life spanMost commercially available solar panels are capable of
producing electricity for at least twenty years.[citation
needed]The typical warranty given by panel manufacturers is over
90% of rated output for the first 10 years, and over 80% for the
second 10 years. Panels are expected to function for a period of 30
to 35 years.
Chapter-3Voltage-Source InverterA typical voltage-source PWM
converter performs the ac to ac conversion in two stages: ac to dc
and dc to variable frequency ac. The basic converter design is
shown in figure 1.1. The grid voltage is rectified by the line
rectifier usually consisting of a diode bridge. Presently,
attention paid to power quality and improved power factor has
shifted the interest to more supply friendly ac-to-dc converters,
e.g. PWM rectifier. This allows simultaneously active filtering of
the line current as well as regenerative motor braking schemes
transferring power back to the mains. The dc voltage is filtered
and smoothed by the capacitor C in the dc bus (figure 1.1). The
capacitor is of appreciable size (2-20 mF) and therefore a major
cost item [Bose 97]. Alternatively, the inverter can be supplied
from a fixed dc voltage. The filtered dc voltage is usually
measured for control purpose. Because of the nearly constant dc bus
voltage, a number of PWM inverters with their associated motor
drives can be supplied from one common diode bridge. The inductive
reactance L between rectifier and ac supply is used to reduce
commutation dips produced by the rectifier, to limit fault current
and to soften voltages spikes of the mains.
Neglecting the voltage drop of the inductances (current
depending) and diodes (Ud 1V if i > 0), the positive potential
of the dc bus voltage equals the highest potential of the three
phases and the negative potential equals the lowest potential of
the three phases. Since each phase owns one negative and one
positive maximum potential during one period of the net frequency,
the rectifier input voltage equals the maximum of the positive and
negative line voltages, respectively. Thus, the rectifier input
voltage traces six pulses as shown in figure 1.2 by the thick
line
Figure 1.2 presents typical voltage and current waveforms of a
B6-diode bridge supplied by a stiff grid. As indicated by the
dashed lines, the rectifier current iB6 increases, if the absolute
value of a line voltage is higher than the dc voltage.
Consequently, the dc voltage increases slightly. A dc voltage
higher than the current voltage supply causes a reduction of the
rectifier input current until the current equals zero and the diode
bridge blocks the supply voltage. The rectifier current iB6 is
identically reflected by the line currents. The sign of each line
current depends on the two non-blocking diodes each conducting the
positive and negative rectifier current, respectively. During the
conducting period, the difference of line and dc voltage is active
as voltage drop over the line inductances and resistances. The
higher the line inductances, the smaller the line current peaks.
However, the value of the line inductances is limited due to
economic and efficiency reasons. Furthermore, the average dc
voltage depends on the line inductances and the inverter output
power. The maximum dc voltage (no load) is equal to the maximum
amplitude of the line voltages. Due to voltage drops of line
inductances, resistances and rectifier diodes, the dc voltage
slightly decreases with increasing load. For more details
concerning the rectifier, see [Bose 97], [Dub 89] et al. According
to figure 1.1, the dc voltage is switched in a three-phase PWM
inverter by six semiconductor switches in order to obtain pulses,
forming three-phase ac voltage with the required frequency and
amplitude for motor supply. The switching devices must be capable
of being turned on as well as turned off. During the last years,
major progress has been made in the development of new power
semiconductor devices. The simpler requirement driving the power
switches and the higher maximum switching-frequency, enabling
higher operating frequencies (higher motor speed), provide
continually rising output power. The new generation of switching
devices is capable of conducting more current and blocking higher
voltages. The alternatives at present are gate turn-off thyristor
(GTO), MOS controlled thyristor (MCT), bipolar junction transistor
(BJT), MOS field effect transistor (MOSFET) and insulated gate
bipolar transistor (IGBT).
The IGBT is a combination of power MOSFET and bipolar transistor
technology and combines the advantages of both. In the same way as
a MOSFET, the gate of the IGBT is isolated and its driving power is
very low. However, the conducting voltage is similar to that of a
bipolar transistor. Presently, IGBTs dominate the medium-power
range of variable speed drives. Since the maximal current rating of
IGBT modules is around 1 kA and the voltage rating is approximately
3 kV, they will gradually replace GTOs at higher power levels [Vas
99]. Parallel to the power switches, reverse recovery diodes are
placed conducting the current depending on the switching states and
current sign. These diodes are required, since switching off an
inductive load current generates high voltage peaks probably
destroying the power switch. Exemplary for one inverter leg, figure
1.3 presents the basic configuration and the inverter output
voltage depending on the switching state and current sign. The
basic configuration of one inverter output phase consists of upper
and lower power devices T1 and T4, and reverse recovery diodes D1
and D4.
When transistor T1 is on, a voltage Udc is applied to the load.
Considering an inductive load, the current increases subsequently.
If the load draws positive current, it will flow through T1 and
supply energy to the load. To the contrary, if the load current ia
is negative, the current flows back through D1 and returns energy
to the dc source.
Figure 1.3: Basic configuration of a half-bridge inverter and
center-tapped inverter output voltage. Left: Switching states and
current direction. Right: Output voltage and line current.
Similarly if T4 is on, which is equal to T1 off, a voltage - Udc is
applied to the load and the current decreases. If iis positive, the
current flows through D4 returning energy to the dc source. A
negative current yields T4 conductinand supplying energy to the
load. According to figure 1.3, with T1 on and drawing positive load
current ia, the output voltage ua0 will be less than Udc by the
on-state voltage drop of T1. When the load current reverses, the
output voltage will be higher than Udc by the voltage drop across
D1. Similarly, the output voltage is slightly changed by the
voltage drop of the lower devices T4 and D4.
Normally, the on-state voltage and diode drops (1 V) are ignored
and the center-tapped inverter is represented as generating the
voltage Udc and - Udc, respectively. Neglecting additionally the
dead-time interval dead, the behavior of the power devices together
with the reverse recovery diode is equally described by ideal
two-position switche
Chapter-4POWER FACTOR CORRECTION.Capacitive Power Factor
correction is applied to circuits which include induction motors as
a means of reducing the inductive component of the current and
thereby reduce the losses in the supply. There should be no effect
on the operation of the motor itself. An induction motor draws
current from the supply, that is made up of resistive components
and inductive components. The resistive components are:1) Load
current.2) Loss current. and the inductive components are:3)
Leakage reactance.4) Magnetizing current.The current due to the
leakage reactance is dependant on the total current drawn by the
motor, but the magnetizing current is independent of the load on
the motor. The magnetizing current will typically be between 20%
and 60% of the rated full load current of the motor. The
magnetizing current is the current that establishes the flux in the
iron and is very necessary if the motor is going to operate. The
magnetizing current does not actually contribute to the actual work
output of the motor. It is the catalyst that allows the motor to
work properly. The magnetizing current and the leakage reactance
can be considered passenger components of current that will not
affect the power drawn by the motor, but will contribute to the
power dissipated in the supply and distribution system. Take for
example a motor with a current draw of 100 Amps and a power factor
of 0.75 The resistive component of the current is 75 Amps and this
is what the KWh meter measures. The higher current will result in
an increase in the distribution losses of (100 x 100) /(75 x 75) =
1.777 or a 78% increase in the supply losses.In the interest of
reducing the losses in the distribution system, power factor
correction is added to neutralize a portion of the magnetizing
current of the motor. Typically, the corrected power factor will be
0.92 - 0.95 Some power retailers offer incentives for operating
with a power factor of better than 0.9, while others penalize
consumers with a poor power factor. There are many ways that this
is metered, but the net result is that in order to reduce wasted
energy in the distribution system, the consumer will be encouraged
to apply power factor correction. Power factor correction is
achieved by the addition of capacitors in parallel with the
connected motor circuits and can be applied at the starter, or
applied at the switchboard or distribution panel. The resulting
capacitive current is leading current and is used to cancel the
lagging inductive current flowing from the supply.Capacitors
connected at each starter and controlled by each starter is known
as "Static Power Factor Correction" while capacitors connected at a
distribution board and controlled independently from the individual
starters is known as "Bulk Correction".
3.1 BULK CORRECTION:The Power factor of the total current
supplied to the distribution board is monitored by a controller
which then switches capacitor banks In a fashion to maintain a
power factor better than a preset limit. (Typically 0.95) Ideally,
the power factor should be as close to unity as possible. There is
no problem with bulk correction operating at unity.3.2 Static
Correction:As a large proportion of the inductive or lagging
current on the supply is due to the magnetizing current of
induction motors, it is easy to correct each individual motor by
connecting the correction capacitors to the motor starters. With
static correction, it is important that the capacitive current is
less than the inductive magnetizing current of the induction motor.
In many installations employing static power factor correction, the
correction capacitors are connected directly in parallel with the
motor windings. When the motor is Off Line, the capacitors are also
Off Line. When the motor is connected to the supply, the capacitors
are also connected providing correction at all times that the motor
is connected to the supply. This removes the requirement for any
expensive power factor monitoring and control equipment. In this
situation, the capacitors remain connected to the motor terminals
as the motor slows down. An induction motor, while connected to the
supply, is driven by a rotating magnetic field in the stator which
induces current into the rotor. When the motor is disconnected from
the supply, there is for a period of time, a magnetic field
associated with the rotor. As the motor decelerates, it generates
voltage out its terminals at a frequency which is related to it's
speed. The capacitors connected across the motor terminals, form a
resonant circuit with the motor inductance. If the motor is
critically corrected, (corrected to a power factor of 1.0) the
inductive reactance equals the capacitive reactance at the line
frequency and therefore the resonant frequency is equal to the line
frequency. If the motor is over corrected, the resonant frequency
will be below the line frequency. If the frequency of the voltage
generated by the decelerating motor passes through the resonant
frequency of the corrected motor, there will be high currents and
voltages around the motor/capacitor circuit. This can result in
severe damage to the capacitors and motor. It is imperative that
motors are never over corrected or critically corrected when static
correction is employed.Static power factor correction should
provide capacitive current equal to 80% of the magnetizing current,
which is essentially the open shaft current of the motor.The
magnetizing current for induction motors can vary considerably.
Typically, magnetizing currents for large two pole machines can be
as low as 20% of the rated current of the motor while smaller low
speed motors can have a magnetizing current as high as 60% of the
rated full load current of the motor. It is not practical to use a
"Standard table" for the correction of induction motors giving
optimum correction on all motors. Tables result in under correction
on most motors but can result in over correction in some cases.
Where the open shaft current can not be measured, and the
magnetizing current is not quoted, an approximate level for the
maximum correction that can be applied can be calculated from the
half load characteristics of the motor. It is dangerous to base
correction on the full load characteristics of the motor as in some
cases, motors can exhibit a high leakage reactance and correction
to 0.95 at full load will result in over correction under no load,
or disconnected conditions.Static correction is commonly applied by
using on e contactor to control both the motor and the capacitors.
It is better practice to use two contactors, one for the motor and
one for the capacitors. Where one contactor is employed, it should
be up sized for the capacitive load. The use of a second contactor
eliminates the problems of resonance between the motor and the
capacitors.3.3 INVERTER:Static Power factor correction must not be
used when the motor is controlled by a variable speed drive or
inverter. The connection of capacitors to the output of an inverter
can cause serious damage to the inverter and the capacitors due to
the high frequency switched voltage on the output of the
inverters.The current drawn from the inverter has a poor power
factor, particularly at low load, but the motor current is isolated
from the supply by the inverter. The phase angle of the current
drawn by the inverter from the supply is close to zero resulting in
very low inductive current irrespective of what the motor is doing.
The inverter does not however, operate with a good power factor.
Many inverter manufacturers quote a cos of better than 0.95 and
this is generally true, however the current is non sinusoidal and
the resultant harmonics cause a power factor (KW/KVA) of closer to
0.7 depending on the input design of the inverter. Inverters with
input reactors and DC bus reactors will exhibit a higher true power
factor than those without.The connection of capacitors close to the
input of the inverter can also result in damage to the inverter.
The capacitors tend to cause transients to be amplified, resulting
in higher voltage impulses applied to the input circuits of the
inverter, and the energy behind the impulses is much greater due to
the energy storage of the capacitors. It is recommended that
capacitors should be at least 75 Meters away from inverter inputs
to elevate the impedance between the inverter and capacitors and
reduce the potential damage caused. Switching capacitors, Automatic
bank correction etc, will cause voltage transients and these
transients can damage the input circuits of inverters. The energy
is proportional to the amount of capacitance being switched. It is
better to switch lots of small amounts of capacitance than few
large amounts.3.4 SOLID STATE SOFT STARTER:Static Power Factor
correction capacitors must not be connected to the output of a
solid state soft starter. When a solid state soft starter is used,
the capacitors must be controlled by a separate contactor, and
switched in when the soft starter output voltage has reached line
voltage. Many soft starters provide a "top of ramp" or "bypass
contactor control" which can be used to control the power factor
correction capacitors.The connection of capacitors close to the
input of the soft starter can also result in damage to the soft
starter if an isolation contactor is not used.
The capacitors tend to cause transients to be amplified,
resulting in higher voltage impulses applied to the SCRs of the
Soft Starter, and the energy behind the impulses is much greater
due to the energy storage of the capacitors. It is recommended that
capacitors should be at least 50 Meters away from Soft starters to
elevate the impedance between the inverter and capacitors and
reduce the potential damage caused.Switching capacitors, Automatic
bank correction etc, will cause voltage transients and these
transients can damage the SCRs of Soft Starters if they are in the
Off state without an input contactor. The energy is proportional to
the amount of capacitance being switched. It is better to switch
lots of small amounts of capacitance than few large amounts.3.5
CAPACITOR SELECTION:Static Power factor correction must neutralize
no more than 80% of the magnetizing current of the motor. If the
correction is too high, there is a high probability of over
correction which can result in equipment failure with severe damage
to the motor and capacitors. Unfortunately, the magnetizing current
of induction motors varies considerably between different motor
designs. The magnetizing current is almost always higher than 20%
of the rated full load current of the motor, but can be as high as
60% of the rated current of the motor. Most power factor correction
is too light due to the selection based on tables which have been
published by a number of sources. These tables assume the lowest
magnetizing current and quote capacitors for this current. In
practice, this can mean that the correction is often less than half
the value that it should be, and the consumer is unnecessarily
penalized. Power factor correction must be correctly selected based
on the actual motor being corrected. The Electrical Calculations
software provides two methods of calculating the correct value of
KVAR correction to apply to a motor. The first method requires the
magnetizing current of the motor. Where this figure is available,
then this is the preferred method. Where the magnetizing current is
not available, the second method is employed and is based on the
half load power factor and efficiency of that motor. These figures
are available from the motor data sheets.
FOR EXAMPLE:Motor A is a 200 KW 6 pole motor with a magnetizing
current of 124A. From tables, the correction applied would be
37KVAR. From the calculations, this would require a correction of
68.7 KVAR Motor B is a 375KW 2 pole motor with a half load
efficiency of 93.9% and a half load power factor of 0.805, the
correction recommended by the tables is 44 KVAR while the
calculations reveal that the correction should be 81.3KVAR
Electrical Calculations is a shareware program which means that you
can try it before you buy it. You can freely distribute copies to
anyone you please, but if you find it to be useful, as I'm sure you
will, then you must purchase it at $NZ35.00 Registered copies of
Busbar will be eligible for continued updates, and registered users
will be advised of all major upgrades as they become available.
Static Power factor correction can be calculated from known
motor characteristics for any given motor, either the magnetizing
current and supply voltage (method 1) or half load efficiency and
half load power factor(method 2), or, as a last resort, table
values can be used. These will almost always result in under
correction.Bulk power factor correction can be calculated from
known existing power factor, required new powerfactor, line voltage
and line current.3.6 SUPPLY HARMONICS:Harmonics on the supply cause
a higher current to flow in the capacitors. This is because the
impedance of the capacitors goes down as the frequency goes up.
This increase in current flow through the capacitor will result in
additional heating of the capacitor and reduce it's life. The
harmonics are caused bu many non linear loads, the most common in
the industrial market today, are the variable speed controllers and
switchmode power supplies. Harmonic voltages can be reduced by the
use of a harmonic compensator, which is essentially a large
inverter that cancells out the harmonics. This is an expensive
option. Passive harmonic filters comprising resistors, inductors
and capacitors can also be used to reduce harmonic voltages. This
is also an expensive exersize.In order to reduce the damage caused
to the capacitors by the harmonic currents, it is becomming common
today to install detuning reactors in series with the power factor
correction capacitors. These reactors are designed to make the
correction circuit inductive to the higher frequency harmonics.
Typically, a reactor would be designed to create a resonant circuit
with the capacitors above the third harmonic, but sometimes it is
below. (Never tuned to a harmonic frequency!!) Adding the
inductance in series with the cpacitors will reduce their effective
capacitance at the supply frequency. Reducing the resonant or tuned
frequency will reduce the the effective capacitance further. The
object is to make the circuit look as inductive as possible at the
5th harmonic and higher, but as capacitive as possible at the
fundemental frequency. Detuning reactors will also reduce the
chance of the tuned circuit formed by the capacitors and the
inductive supply being resonant on a supply harmonic frequency,
thereby reducing damage due to supply resonances amplifying
harmonic voltages caused by non linear loads.3.7 DETUNING
REACTORS:Detuning reactors are connected in series with power
factor correction capacitors to reduce harmonic currents and to
ensure that the series resonant frequency does not occur at a
harmonic of the supply frequency. The reactors are usually chosen
and rated as either 5% or 7% reactors. This means that at the line
frequency, the capacitive reactance is reduced by 5% or 7%.Using
detuning reactors results in a lower KVAR, so the capacitance will
need to be increased for the same level of correction. When
detuning reactors are used in installations with high harmonic
voltages, there can be a high resultant voltage across the
capacitors. This necessitates the use of capacitors that are
designed to operate at a high sustained voltage. Capacitors
designed for use at line voltage only, should not be used with
detuning reactors. Check the suitability of the capacitors for use
with line reactors before installation. The detuning reactors can
dissipate a lot of heat. The enclosure must be well ventillated,
typically forced air cooled. The detuning reactor must be specified
to match the KVAR of the capacitance selected. The reactor would
typically be rated as 12.5KVAR 5% meaning that it is a 5% reactor
to connect to a 12.5KVAR capacitor.
3.8 SUPPLY RESONANCE:Capacitive Power factor correction
connected to a supply causes resonance between the supply and the
capacitors. If the fault current of the supply is very high, the
effect of the resonance will be minimal, however in a rural
installation where the supply is very inductive and can be a high
impedance, the resonances can be very severe resulting in major
damage to plant and equipment. Voltage surges and transients of
several times the supply voltage are not uncommon in rural areas
with weak supplies, especially when the load on the supply is low.
As with any resonant system, a transient or sudden change in
current will result in the resonant circuit ringing, generating a
high voltage. The magnitude of the voltage is dependant on the 'Q'
of the circuit which in turn is a function of the circuit loading.
One of the problems with supply resonance is that the 'reaction' is
often well removed from the 'stimulus' unlike a pure voltage drop
problem due to an overloaded supply. This makes fault finding very
difficult and often damaging surges and transients on the supply
are treated as 'just one of those things'. To minimize supply
resonance problems, there are a few steps that can be taken, but
they do need to be taken by all on the particular supply. 1)
Minimize the amount of power factor correction, particularly when
the load is light. The power factor correction minimizes losses in
the supply. When the supply is lightly loaded, this is not such a
problem.2) Minimize switching transients. Eliminate open transition
switching - usually associated with generator plants and
alternative supply switching, and with some electromechanical
starters such as the star/delta starter.3) Switch capacitors on to
the supply in lots of small steps rather than a few large steps.4)
Switch capacitors on o the supply after the load has been applied
and switch off the supply before or with the load removal.Harmonic
Power Factor correction is not applied to circuits that draw either
discontinuous or distorted current waveforms.Most electronic
equipment includes a means of creating a DC supply. This involves
rectifying the AC voltage, causing harmonic currents. In some
cases, these harmonic currents are insignificant relative to the
total load current drawn, but in many installations, a large
proportion of the current drawn is rich in harmonics. If the total
harmonic current is large enough, there will be a resultant
distortion of the supply waveform which can interfere with the
correct operation of other equipment. The addition of harmonic
currents results in increased losses in the supply. Power factor
correction for distorted supplies can not be achieved by the
addition of capacitors. The harmonics can be reduced by designing
the equipment using active rectifiers, by the addition of passive
filters (LCR) or by the addition of electronic power factor
correction inverters which restore the waveform back to its
undistorted state. This is a specialist area requiring either major
design changes, or specialized equipment to be used.3.9 Benefits of
Power Factor CorrectionMost people associate electricity and energy
with kilowatts (kW). In fact, kW only makes up a part of the
overall energy usage in a home, commercial building or an
industrial manufacturing plant. In the world of AC power, there are
actually three types of power: Apparent Power (measured in
Volt-Amps) Real Power (measured in Watts) Reactive Power (measured
in VARs)The relationship between Apparent Power and the other two
is influencedby what is called Power Factor (PF). The PF can be
thought of as a measure of electrical efficiency ina power
system.Numerous benefits can be derived by providing power factor
correction to a facility. Benefit: Reduced Utility BillsUtilities
have several different rate structures that may be used for
billing. kVA Billing straight charges for all apparent power
consumed kVAr Billing additional charges for reactive power Power
Factor Penalty charges based on the customers actual power factor
Adjusted kW Demand the real power demand is adjusted by a formula
and is based on the customers actual power factor In all cases, the
power factor of a customer will become a direct or indirect factor
in the utility bill.Power bills may be reduced by introducing
capacitors to the facility, which can reduce the need for kVAr
required from the utility. Capacitors have the added effect of
reducing line losses which can reduce the amountof kW hours
required by a facility. Additional benefits of reducing linelosses
will be discussed later in this document.Benefit: Electrical System
Capacity Capacitors in a facility produce reactive energy that
motors require to produce magnetizing current for induction motors
and transformers. This reduces the overall current needed from the
power supply. This translates into reduced loads on both
transformers and feeder circuits.Capacitors Provide Reactive
Power
Reduced loads on transformers can have a variety of positive
impacts that include but are not limited to: less maintenance,
reduced breaker trips, and higher full-load capacity.Benefit:
Improved Voltage LevelsLow voltage may be caused by a lack of
reactive energy. Additionally, voltage drops are often caused by
dynamic load changes. In both cases, the effects can be harmful. In
facilities with motors, low voltage reduces motor efficiency and
can cause overheating. Interference may be introduced by low
voltage in lighting and other electrical instruments (i.e.
Computers). Welding plants in particular, may suffer from voltage
drops. The quality of a weld is directly proportional to the
voltage. These voltage drops can cause bad welds which translate
into scrap or possible product recalls if allowed to persist.
Real-time capacitor systems ( , the STATCOM supplies reactive power
to the ac system. If V V i T< , the STATCOM absorbs reactive
power. State of the art for STATCOM is by the use of IGBT
(Insulated Gate Bipolar Transistors). By use of high frequency
Pulse Width Modulation (PWM), it has become possible to use a
single converter connected to a standard power transformer via
air-core phase reactors. The core parts of the plant are located
inside a prefabricated building. The outdoor equipment is limited
to heat exchangers, phase reactors and the power transformer. For
extended range of operation, additional fixed capacitors, thyristor
switched capacitors or an assembly of more than one converter may
be used.
The semiconductor valves in a STATCOM respond almost
instantaneously to a switching order. Therefore the limiting factor
for the comple plant speed of response is determined by the time
needed for voltage measurements and the control system data
processing. A high gain controller can be used and a response time
shorter than a quarter of a cycle is obtained. The high switching
frequency used in the IGBT based STATCOM concept results in an
inherent capability to produce voltages at frequencies well above
the fundamental one. This property can be used for active filtering
of harmonics already present in the network. The STATCOM then
injects harmonic currents into the network with proper phase and
amplitude to counteract the harmonic voltages. By adding storage
capacity to the DC side of STATCOM, it becomes possible not only to
control reactive power, but also active power. As storage facility,
various kinds of battery cells can be used, depending on the
requirements on the storage facility. The result, STATCOM with
energy storage (Fig. 7), is expected to come into use in years to
come as dynamic storage facility particularly of renewable energy
(wind, solar).
Impact of FACTS in interconnected networks The benefits of power
system interconnection are well established. It enables the
participating parties to share the benefits of large power systems,
such as optimization of power generation, utilization of
differences in load profiles and pooling of reserve capacity. From
this follows not only technical and economical benefits, but also
environmental, when for example surplus of clean hydro resources
from one region can help to replace polluting fossil-fuelled
generation in another. For interconnections to serve their purpose,
however, available transmission links must be powerful enough to
safely transmit the amounts of power intended. If this is not the
case, from a purely technical point of view it can always be
remedied by building additional lines in parallel with the
existing, or by uprating the existing system(s) to a higher
voltage.
This, however, is expensive, time-consuming, and calls for
elaborate procedures for gaining the necessary permits. Also, in
many cases, environmental considerations, popular opinion or other
impediments will render the building of new lines as well as
uprating to ultra-high system voltages impossible in practice. This
is where FACTS is coming in. Examples of successful implementation
of FACTS for power system interconnection can be found among others
between the Nordic Countries and between Canada and the United
States. In such cases, FACTS helps to enable mutually beneficial
trade of electric energy between the countries. Other regions in
the world where FACTS is emerging as a means for AC bulk power
interchange between regions can be found in South Asia as well as
in Africa and Latin America. In fact, AC power corridors equipped
with SVC and/or SC transmitting bulk power over distances of more
than 1.000 km are a reality today.
Chapter-7PV SOLAR PLANT CONTROL CIRCUIT
Chapter-8INTRODUCTION TO MATLAB
MATLAB is a software package for computation in engineering,
science, and applied mathematics.
It offers a powerful programming language, excellent graphics,
and a wide range of expert knowledge. MATLAB is published by and a
trademark of The MathWorks, Inc.
The focus in MATLAB is on computation, not mathematics: Symbolic
expressions and manipulations are not possible (except through the
optional Symbolic Toolbox, a clever interface to maple). All
results are not only numerical but inexact, thanks to the rounding
errors inherent in computer arithmetic. The limitation to numerical
computation can be seen as a drawback, but its a source of strength
too: MATLAB is much preferred to Maple, Mathematical, and the like
when it comes to numerics.
On the other hand, compared to other numerically oriented
languages like C++ and FORTRAN, MATLAB is much easier to use and
comes with a huge standard library.1 the unfavorable comparison
here is a gap in execution speed. This gap is not always as
dramatic as popular lore has it, and it can often be narrowed or
closed with good MATLAB programming. Moreover, one can link other
codes into MATLAB, or vice versa, and MATLAB now optionally
supports parallel computing. Still, MATLAB is usually not the tool
of choice for maximum-performance Computing.The MATLAB niche is
numerical computation on workstations for non-experts in
computation.
This is a huge nicheone way to tell is to look at the number of
MATLAB-related books on mathworks.com. Even for supercomputer
users, MATLAB can be a valuable environment in which to explore and
fine-tune algorithms before more laborious coding in another
language.Most successful computing languages and environments
acquire a distinctive character or culture.In MATLAB, that culture
contains several elements: an experimental and graphical bias,
resulting from the interactive environment and compression of the
write-compile-link-execute analyze cycle; an emphasis on syntax
that is compact and friendly to the interactive mode, rather than
tightly constrained and verbose; a kitchen-sink mentality for
providing functionality; and a high degree of openness and
transparency (though not to the extent of being open source
software).
The fifty-cent tourWhen you start MATLAB, you get a multipaneled
desktop. The layout and behavior of the desktop and its components
are highly customizable (and may in fact already be customized for
your site). The component that is the heart of MATLAB is called the
Command Window, located on the 1Here and elsewhere I am thinking of
the old FORTRAN, FORTRAN 77. This is not a commentary on the
usefulness of FORTRAN 90 but on my ignorance of it.
INTRODUCTION
Right by default. Here you can give MATLAB commands typed at the
prompt, >>. Unlike FORTRAN and other compiled computer
languages, MATLAB is an interpreted environmentyou give a command,
and MATLAB tries to execute it right away before asking for
another.
At the top left you can see the Current Directory. In general
MATLAB is aware only of files in the current directory (folder) and
on its path, which can be customized. Commands for working with the
directory and path include cd, what, addpath, and editpath (or you
can choose File/Set path. . . from the menus). You can add files to
a directory on the path and thereby add commands to MATLAB; we will
return to this subject in section 3.Next to the Current Directory
tab is the Workspace tab. The workspace shows you what variable
names are currently defined and some information about their
contents. (At start-up it is, naturally, empty.) This represents
another break from compiled environments: variables created in the
workspace persist for you to examine and modify, even after code
execution stops. Below the CommandWindow/Workspace window is the
Command History window. As you enter commands, they are recorded
here. This record persists across different MATLAB sessions, and
commands or blocks of commands can be copied from here or saved to
files.
As you explore MATLAB, you will soon encounter some toolboxes.
These are individually packaged sets of capabilities that provide
in-depth expertise on particular subject areas. There is no need to
load them explicitlyonce installed, they are always available
transparently. You may also encounter Simulink, which is a
semi-independent graphical control-engineering package not covered
in this document.Graphical versus command-line usage
MATLAB was originally entirely a command-line environment, and
it retains that orientation.But it is now possible to access a
great deal of the functionality from graphical interfacesmenus,
buttons, and so on. These interfaces are especially useful to
beginners, because they lay out the available choices clearly.2 As
a rule, graphical interfaces can be more natural for certain types
of interactive work, such as annotating a graph or debugging a
program, whereas typed commands remain better for complex, precise,
repeated, or reproducible tasks. One does not always need to make a
choice, though; for instance, it is possible to save a figures
styles as a template that can be used with different data by
pointing and clicking. Moreover, you can package code you want to
distribute with your own graphical interface, one that itself may
be designed with a combination of graphical and command-oriented
tools. In the end, an advanced MATLAB user should be able to
exploit both modes of work to be productive.
That said, the focus of this document is on typed commands. In
many (most?) cases these have graphical interface equivalents, even
if I dont explicitly point them out.In particular, feel free to
right-click (on Control-click on a Mac) on various objects to see
what you might be able todo to them.WHAT IS SIMULINK
Simulink (Simulation and Link) is an extension of MATLAB by Math
works Inc. It works with MATLAB to offer modeling, simulating, and
analyzing of dynamical systems under a graphical user interface
(GUI) environment. The construction of a model is simplified with
click-and-drag mouse operations. Simulink includes a comprehensive
block library of toolboxes for both linear and nonlinear analyses.
Models are hierarchical, which allow using both top-down and
bottom-up approaches. As Simulink is an integral part of MATLAB, it
is easy to switch back and forth during the analysis process and
thus, the user may take full advantage of features offered in both
environments. This tutorial presents the basic features of Simulink
and is focused on control systems as it has been written for
students in my control systems .
Getting StartedTo start a Simulink session, you'd need to bring
up Matlab program first. From Matlab command window, enter:>>
simulinkAlternately, you may click on the Simulink icon located on
the toolbar as shown
To see the content of the blockset, click on the "+" sign at the
beginning of each toolbox.To start a model click on the NEW FILE
ICON as shown in the screenshot above.Alternately, you may use
keystrokes CTRL+N.A new window will appear on the screen. You will
be constructing your model in this window. Also in this window the
constructed model is simulated. A screenshot of a typical working
(model) window that looks like one shown below:
To become familiarized with the structure and the environment of
Simulink, you are encouraged to explore the toolboxes and scan
their contents.
You may not know what they are all about but perhaps you could
catch on the organization of these toolboxes according to the
category. For instant, you may see Control System Toolbox to
consist of the Linear Time Invariant (LTI) system library and the
MATLAB functions can be found under Function and Tables of the
Simulink main toolbox. A good way to learn Simulink (or any
computer program in general) is to practice and explore. Making
mistakes is a part of the learning curve. So, fear not, you should
be.
A simple model is used here to introduce some basic features of
Simulink. Please follow the steps below to construct a simple
model.
STEP 1: CREATING BLOCKS.
From BLOCK SET CATEGORIES section of the SIMULINK LIBRARY
BROWSER window, click on the "+" sign next to the Simulink group to
expand the tree and select (click on) Sources.
A set of blocks will appear in the BLOCKSET group. Click on the
Sine Wave blockand drag it to the workspace window (also known as
model window)
A set of blocks will appear in the BLOCKSET group. Click on the
Sine Wave blockand drag it to the workspace window (also known as
model window)
I am going to save this model under the filename: "simexample1".
To save a model, you may click on the floppy diskette icon. Or from
FILE menu, select Save or CTRL+S. All Simulink model file will have
an extension ".mdl". Simulink recognizes file with .mdl extension
as a simulation model (similar to how MATLAB recognizes files with
the extension .m as an MFile).
Continue to build your model by adding more components (or
blocks) to your model window. We'll continue to add a Scope from
Sinks library, an Integrator block from Continuous library, and a
Mux block from Signal Routing library.
NOTE: If you wish to locate a block knowing its name, you may
enter the name in the SEARCH WINDOW (at Find prompt) and Simulink
will bring up the specified block.
To move the blocks around, simply click on it and drag it to a
desired location.Once all the blocks are dragged over to the work
space should consist of the following components:
You may remove (delete) a block by simply clicking on it once to
turn on the "select mode" (with four corner boxes) and use the DEL
key or keys combination CTRL-X.
STEP 2: MAKING CONNECTIONSTo establish connections between the
blocks, move the cursor to the output port represented by ">"
sign on the block. Once placed at a port, the cursor will turn into
a cross "+" enabling you to make connection between blocks.To make
a connection: left-click while holding down the control key (on
your keyboard) and drag from source port to a destination port.
The connected model is shown below.
A sine signal is generated by the Sine Wave block (a source) and
is displayed by the scope. The integrated sine signal is sent to
scope for display along with the original signal from the source
via the Mux, whose function is to multiplex signals in form of
scalar, vector, or matrix into a bus.
STEP 3: RUNNING SIMULATION
You now can run the simulation of the simple system above by
clicking on the play button (alternatively, you may use key
sequence CTRL+T, or choose Start submenu under Simulation
menu).
Double click on the Scope block to display of the scope.
INTRODUCTION
SimPowerSystems and other products of the Physical Modeling
product family work together with Simulink to model electrical,
mechanical, and control systems.
SimPowerSystems operates in the Simulink environment. Therefore,
before starting this users guide, you should be familiar with
Simulink. For help with Simulink, see the Simulink documentation.
Or, if you apply Simulink to signal processing and communications
tasks (as opposed to control system design tasks), see the Signal
Processing Block set documentation.
The Role of Simulation in Design
Electrical power systems are combinations of electrical circuits
and electromechanical devices like motors and generators. Engineers
working in this discipline are constantly improving the performance
of the systems.Requirements for drastically increased efficiency
have forced power system designers to use power electronic devices
and sophisticated control system concepts that tax traditional
analysis tools and techniques. Further complicating the analysts
role is the fact that the system is often so nonlinear that the
only way to understand it is through simulation.
Land-based power generation from hydroelectric, steam, or other
devices is not the only use of power systems. A common attribute of
these systems is their use of power electronics and control systems
to achieve their performance objectives.
What Is SimPowerSystemsSimPowerSystems is a modern design tool
that allows scientists and engineers to rapidly and easily build
models that simulate power systems.SimPowerSystems uses the
Simulink environment, allowing you to build a model using simple
click and drag procedures. Not only can you draw the circuit
topology rapidly, but your analysis of the circuit can include its
interactions with mechanical, thermal, control, and other
disciplines. This is possible because all the electrical parts of
the simulation interact with the extensive Simulink modeling
library. Since Simulink uses MATLAB as its computational engine,
designers can also use MATLAB toolboxes and Simulink block sets.
SimPowerSystems and Sim Mechanics share a specialPhysical Modeling
block and connection line interface.
SimPowerSystems Libraries
You can rapidly put SimPowerSystems to work. The libraries
contain models of typical power equipment such as transformers,
lines, machines, and power electronics. These models are proven
ones coming from textbooks, and their validity is based on the
experience of the Power Systems Testing and Simulation Laboratory
of Hydro-Qubec, a large North American utility located in Canada,
and also on the experience of cole de Technologie Suprieure and
Universit Laval. The capabilities of SimPowerSystems for modeling a
typical electrical system are illustrated in demonstration files.
And for users who want to refresh their knowledge of power system
theory, there are also self-learning case studies.The
SimPowerSystems main library, power lib, organizes its blocks into
libraries according to their behavior. The power lib library window
displays the block library icons and names. Double-click a library
icon to open the library and access the blocks. The main
SimPowerSystems power lib library window also contains the Powergui
block that opens a graphical user interface for the steady-state
analysis of electrical circuits.
Nonlinear Simulink Blocks for SimPowerSystems ModelsThe
nonlinear Simulink blocks of the power lib library are stored in a
special\block library named powerlib_models. These masked Simulink
models are used by SimPowerSystems to build the equivalent Simulink
model of your circuit. See Chapter 3, Improving Simulation
Performance for a description of the powerlib_models library
You must have the following products installed to use
SimPowerSystems: MATLAB Simulink
3.4. PULSE WIDTH MODULATION TECHNIQUE:
The advent of the transformerless multilevel inverter topology
has brought forth various pulse width modulation (PWM) schemes as a
means to control the switching of the active devices in each of the
multiple voltage levels in the inverter. The most efficient method
of controlling the output voltage is to incorporate pulse width
modulation control (PWM control) within the inverters. In this
method, a fixed d.c. input voltage is supplied to the inverter and
a controlled a.c. output voltage is obtained by adjusting the on
andoff periods of the inverter devices. Voltage-type PWM inverters
have been applied widely to such fields as power supplies and motor
drivers. This is because: (1) such inverters are well adapted to
high-speed self turn-off switching devices that, as solid-state
power converters, are provided with recently developed advanced
circuits; and (2) they are operated stably and can be controlled
well.
The PWM control has the following advantages:
(i) The output voltage control can be obtained without any
additional components.(ii) With this type of control, lower order
harmonics can be eliminated or minimized along with its output
voltage control. The filtering requirements are minimized as higher
order harmonics can be filtered easily.The commonly used PWM
control techniques are:(a) Sinusoidal pulse width modulation (sin
PWM)(b) Space vector PWM
The performance of each of these control methods is usually
judged based on the following parameters: a) Total harmonic
distortion (THD) of the voltage and current at the output of the
inverter, b) Switching losses within the inverter, c) Peak-to-peak
ripple in the load current, and d) Maximum inverter output voltage
for a given DC rail voltage.
From the above all mentioned PWM control methods, the Sinusoidal
pulse width modulation (sinPWM) is applied in the proposed inverter
since it has various advantages over other techniques. Sinusoidal
PWM inverters provide an easy way to control amplitude, frequency
and harmonics contents of the output voltage.
Sinusoidal Pulse Width ModulationIn the Sinusoidal pulse width
modulation scheme, as the switch is turned on and off several times
during each half-cycle, the width of the pulses is varied to change
the output voltage. Lower order harmonics can be eliminated or
reduced by selecting the type of modulation for the pulse widths
and the number of pulses per half-cycle. Higher order harmonics may
increase, but these are of concern because they can be eliminated
easily by filters. The SPWM aims at generating a sinusoidal
inverter output voltage without low-order harmonics. This is
possible if the sampling frequency is high compared to the
fundamental output frequency of the inverter. Sinusoidal pulse
width modulation is one of the primitive techniques, which are used
to suppress harmonics presented in the quasi-square wave. SAMPLING
TECHNIQUEIn this method of modulation, several pulses per
half-cycle are used. Instead of maintaining the width of all
pulses, the width of each pulse is varied proportional to the
amplitude of a sin-wave evaluated at the centre of the same pulse.
By comparing a sinusoidal reference signal with a triangular
carrier wave, the gating signals are generated. The frequency of
reference signal determine the inverter output frequency and its
peak amplitude, controls the modulation index, M, and then in turn
the RMS output voltage. Fig.3.2 shows the more common carrier
technique, the conventional sinusoidal pulse width modulation
(SPWM) technique, which is based on the principle of comparing a
triangular carrier signal with a sinusoidal reference waveform
(natural sampling).
The figure below gives the sinusoidal pulse width
modulation.
(a)
(b)
(c) (a) Modulating/reference and carrier waveform . (b)
Line-to-neutral switching pattern. (c) Line-to-line output
waveform.Graph. 3.1 (a), (b), (c) Sinusoidal Pulse Width Modulation
(SPWM).
By varying the modulation index M, the RMS output voltage can be
varied. It can be observed that the area of each pulse corresponds
approximately to the area under the sine-wave between the adjacent
midpoints of off periods on the gating signals. The phase voltage
can be described by the following expressions:
where wm is the angular frequency of modulating or sinusoidal
signal.wc is the angular frequency of the carrier signal.M is
modulation index
.
E is the dc supply voltage. f is the displacement angle between
modulating and carrier signals.and Jo and Jn are Bessel functions
of the first kind.The amplitude of the fundamental frequency
components of the output is directly proportional to the modulation
depth. The second term of the equation gives the amplitude of the
component of the carrier frequency and the harmonics of the carrier
frequency. The magnitude of this term decreases with increased
modulation depth. Because of the presence of sin(m /2), even
harmonics of the carrier are eliminated. Term 3 gives the amplitude
of the harmonics in the sidebands around each multiple of the
carrier frequency. The presence of sin((m+n) / 2) indicated that,
for odd harmonics of the carrier, only even-order sidebands exist,
and for even harmonics of the carrier only oddorder sidebands
exist. In addition, increasing carrier or switching frequency does
not decrease the amplitude of the harmonics, but the high amplitude
harmonic at the carrier frequency is shifted to higher frequency.
Consequently, requirements of the output filter can be improved.
However, it is not possible to improve the total harmonic
distortion without using output filter circuits. In multilevel
case, SPWM techniques with three different disposed triangular
carriers were proposed as follows:
1. All the carriers are alternatively in opposition (APO
disposition)2. All the carriers above the zero value reference are
in phase among them, but in opposition with those below (PO
disposition)3. All the carriers are in phase (PH disposition)4.
Multi carrier modulation techniqueIn the proposed inverter circuit
the multi carrier modulation technique is employed.
PROPOSED MODULATION TECHNIQUE:3.5.1 MULTICARIER MODULATIONThis
technique involves the carrier based PWM3.5.2 CARRIER BASED
PWMThese are the classical and most widely used methods of pulse
width modulation. They have as common characteristic subcycles of
constant time duration, a subcycle being defined as the total
duration Ts during which an active inverter leg assumes two
consecutive switching states of opposite voltage polarity.
Operation at subcycles of constant duration is reflected in the
harmonic spectrum by two salient sidebands, centered around the
carrier frequency, and additional frequency bands around integral
multiples of the carrier. The multi carrier modulation technique is
very suitable for a multilevel in