Electricity Print

7/31/2019 Electricity Print

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What Is Electricity?

Electricity is a form of energy that starts with atoms. Atoms

are too small to see, but they make up everything around us.

An atom has three tiny

parts: protons, neutrons, and electrons. The center of theatom has at least one proton and one neutron. At least one

electron travels around the center of the atom at great speed.

Electricity can be created by forcing electrons to flow from

atom to atom.

How Electricity Is Generated

Most electricity used in the United States is produced atpower plants. Various energy

sources are used to turn turbines. The spinning turbine shafts turn electromagnets thatare surrounded by heavy coils of copper wire inside generators. This creates a

magnetic field, which causes the electrons in the copper wire to move from atom to

atom.

How Electricity Travels

Electricity leaves the power plant and is sent over high-

power transmission lines on tall towers. The very strong

electric current from a power plant must travel long

distances to get where it is needed. Electricity loses someof its strength (voltage) as it travels, so it must be helped

along by transformers, which boost or step upits power.

When electricity gets closer to where it will be used, its voltage must be decreased.

Different kinds of transformers at utility substations do this job, stepping down

electricitys power. Electricity then travels on overhead or underground distributionwires to neighborhoods. When the distribution wires reach a home or business,

another transformer reduces the electricity down to just the right voltage to be used in

appliances, lights, and other things that run on electricity.


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A cable carries the electricity from the distribution

wires to the house through a meter box. The meter

measures how much electricity the people in the house

use. From the meter box, wires run through the walls

to outlets and lights. The electricity is always waiting

in the wires to be used.

Electricity travels in a circuit. When you switch on an

appliance, you complete the circuit. Electricity flows along power lines to the outlet,

through the power cord into the appliance, then back through the cord to the outlet and

out to the power lines again.

Electricity travels fast (186,000 miles per second). If you traveled that fast, you could

travel around the world eight times in the time it takes to turn on a light! And if you

had a lamp on the moon wired to a switch in your bedroom, it would take only 1.26

seconds after you flipped the switch for electricity to light the lamp 238,857 milesaway!

How Electricity Is Measured

Volts, amps, and watts measure electricity. Volts

measure the pressure under which electricity flows.Amps measure the amount of electric current. Watts

measure the amount of work done by a certain amount

of current at a certain pressure or voltage.

To understand how they are related, think of water in a

hose. Turning on the faucet supplies the force, which is

like the voltage. The amount of water moving through the hose is like the amperage.

You would use lots of water that comes out really hard (like a lot of watts) to wash off

a muddy car. You would use less water that comes out more slowly (like less watts) to

fill a glass.

1 watt = 1 amp multiplied by 1 volt

1 amp = 1 watt divided by 1 volt


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Most electricity in the United States is generated using coal, oil, natural gas, nuclear

energy, or hydropower. Some production is done with alternative fuels like

geothermal energy, wind power, biomass, solar energy, or fuel cells.

The electricity you buy may be generated using one or more of these methods. No

matter what fuels produce the electricity you use, your lights shine, your radio plays,and your computer runs in the same way.

Hydropower

Hydroelectric plants use the power of falling water to turn the turbines that help

generate electricity. Water stored behind a dam is released and directed through

special tubes to flow against the blades of turbines and make them turn. Hydropower

provides about 10 percent of the electricity generated in the United States. The most

famous hydroelectric facility in the country is Hoover Dam.


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Fossil Fuels

The majority of electricity used in the United States is generated from power plants

that burn fossil fuels (coal, oil, and natural gas) to heat water and make steam. The

highly pressurized steam is directed at the blades of turbines to make them spin.

Coal, oil, and natural gas are known as fossil fuels because

they were formed from the fossilized remains of animals or

plants that lived long ago. Even before the dinosaurs, these

plants and animals died and settled to the bottom of lakes

and oceans to be covered over by sand and mud. Over

millions of years, the earths pressure and heat convertedtheir remains into coal, oil, and natural gas.

Coal is extracted from the ground at large mines. Coal is

used to generate about half of the electricity used in theUnited States.

Natural gas and oil are obtained through wells drilled deep in the earth. Natural gas is

used to generate about 10 percent of the electricity used in the United States, and oil is

used to generate about 2 percent of electricity used in the United States

Nuclear Power

Nuclear power plants use the heat from splitting

atoms to convert water into the steam that turnsturbines. These plants rely on uranium, a type of

metal that must be mined from the ground and

specially processed. Fuel rods containing uranium

are placed next to each other in a machine called a

nuclear reactor. The reactor causes the uranium

atoms to split and in so doing, they release a

tremendous amount of heat.


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Geothermal Energy

Steam (or hot water converted to steam) from under the

ground is used to turn turbines.

Wind Power

The force of the wind is used to spin

many small turbines. Most wind

power is produced from wind farms

large groups of turbines located inconsistently windy locations.

Biomass

Biomass is organic matter, such as agricultural wastes and wood chips and bark left

over when lumber is produced. Biomass can be burned in an incinerator to heat water

to make steam, which turns a turbine to make electricity. It can also be converted into

a gas, which can be burned to do the same thing.

Solar Energy

Solar energy is generated without a turbine or

electromagnet. Special panels of photovoltaic cells

capture light from the sun and convert it directly into

electricity. The electricity is stored in a battery.

Fuel Cells

Fuel cells produce electricity through a chemical reaction.


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high-voltage transmission lines

High-voltage transmission lines are used to transmit electric power over relatively long distances, usuallyfrom a central generating station to main substations. They are also used for electric power transmissionfrom one central station to another for load sharing. High voltage (HV) transmission lines are made ofhigh voltage (between 138 and 765 kilovolts) overhead and underground conducting lines of either

copper or aluminum.

One of the key concerns in transmission of electricity is power loss in transmission lines (called line lossor transmission loss), dissipated as heat due to the resistance of the conductors. The smaller the surfacearea of the conductors, the smaller the loss to heat dissipation. High voltages require less surface area,resulting in reduced line loss. With high-voltage lines, the voltage can be stepped up at the generatingstation, transmitted through the transmission grid to a load center, and there stepped down to the lower

transmission line, transmission system, distribution lineTransmission line can refer either to a singlecable carrying electricity over a transmission system, or to the physical path travelled by electricity on itsway from producer to consumer. Transmission line and distribution line are sometimes usedinterchangeably, but a distribution line is normally considered to be a line that carries less than 69,000volts and is used to distribute power drawn from high-voltage transmission systems to end-use

customers. Transmission lines may occasionally low voltage, notably lines used in networks that servesparsely-populated areas.

distribution

Refers to the process of transporting energy from transmission systems to end-use customers.Transmission systems are somewhat like the interstate highway systems of the energy industry,conducting large amounts of energy along high-volume routes that intersect at strategic locations.Distribution systems are the off-ramps, feeder routes and sideroads. They carry electricity from high-voltage transmission networks to end-use customers.

In some contexts, distribution is considered to be any transmission of energy on lines carrying less than69,000 volts.

Prior to deregulation, distribution was typically provided as part of a full package of services ranging fromgeneration through to billing. Distribution is considered a separate service under deregulation, and distinctdistribution companies, referred to as DISCOs, will eventually emerge as a competitive sector of thisindustry. Distribution companies will maintain everything from the feeders that tap high-voltage lines tosubstation transformers that convert this voltage to commercial or household voltage to the service dropsthat carry energy from power lines to residences and commercial sites.

transmission voltage

Refers to a high line voltage used on transmission systems. Transmission voltage varies depending onthe system. While 69,000 volts is used as a standard figure, actual transmission voltages on a given

system or subsystem can range upward of that figure to as high as 750,000 volts.

network

A cross-connected, multiple-access web of transmission and distribution lines, usually used in urbanareas, which provides power to large numbers of customers and includes sufficient interconnection pointsto allow rapid rerouting of energy when demand or emergency conditions require it. Network is the termusually used to refer to regional or municipal distribution infrastructure; grid is more commonly applied tohigh-voltage transmission systems that feed these distribution systems.


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mobile substation

A portable structure or collection of substation equipment designed for use in situations when apermanent substation is out of service, for temporary installation at locations such as large constructionsites, or for emergency backup.

feederAn electrical line that feeds energy to the next level of distribution. A distribution feeder is an underground

or overhead line connected to a transmission system which carries power into the distribution network

where it is delivered to end-use customers.

1.1 Introduction to Electric Power Supply SystemsElectric power supply system in a country comprises of generating units that produce electricity;

high voltage transmission lines that transport electricity over long distances; distributionlines that

deliver the electricity to consumers; substations that connect the pieces to each other;and energycontrol centers to coordinate the operation of the components.The Figure 1.1 shows a simple

electric supply system with transmission and distributionnetwork and linkages from electricity

sources to end-user.Figure 1.1

Figure 1.1 Typical Electric Power Supply Systems

Power Generation Plant

The fossil fuels such as coal, oil and natural gas, nuclear energy, and falling water (hydel) arecommonly used energy sources in the power generating plant. A wide and growing variety ofunconventional generation technologies and fuels have also been developed, including

cogeneration, solar energy, wind generators, and waste materials.

About 70 % of power generating capacity in India is from coal based thermal power plants.Theprinciple of coal-fired power generation plant is shown in Figure 1.2. Energy stored in thecoal is

converted in to electricity in thermal power plant. Coal is pulverized to the consistencyof talcum

powder. Then powdered coal is blown into the water wall boiler where it is burned at


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temperature higher than 1300C. The heat in the combustion gas is transferred into steam. This

high-pressure steam is used to run the steam turbine to spin. Finally turbine rotates the generator

to produce electricity.

Figure 1.2 Principle of Thermal Power GenerationIn India, for the coal based power plants, the overall efficiency ranges from 28% to 35%

depending upon the size, operational practices and capacity utilization. Where fuels are thesource of generation, a common term used is the HEAT RATE which reflects the efficiency

of generation. HEAT RATE is the heat input in kilo Calories or kilo Joules, for generating

one kilo Watt-hour of electrical output. One kilo Watt hour of electrical energy being

equivalentto 860 kilo Calories of thermal energy or 3600 kilo Joules of thermal energy. The

HEATRATE expresses in inverse the efficiency of power generation.

Transmission and Distribution LinesThe power plants typically produce 50 cycle/second(Hertz), alternating-current (AC) electricity

with voltages between 11kV and 33kV. At the power plant site,the 3-phase voltage is stepped up

to a higher voltage fortransmission on cables strung on cross-country towers.High voltage (HV)and extra high voltage (EHV)transmission is the next stage from power plant totransport A.C.

power over long distances at voltageslike; 220 kV & 400 kV. Where transmission is over

1000 kM, high voltage direct current transmission isalso favoured to minimize the losses.Sub-transmission network at 132 kV, 110 kV, 66 kVor 33 kV constitutes the next link towards

the end user.Distribution at 11 kV / 6.6 kV / 3.3 kV constitutes thelast link to the consumer, who

is connected directly orthrough transformers depending upon the drawl level of service. The

transmission and distribution network include sub-stations, lines and distributiontransformers.

High voltage transmission is used so that smaller, more economical wire sizes canbe employedto carry the lower current and to reduce losses. Sub-stations, containing step-downtransformers,

reduce the voltage for distribution to industrial users. The voltage is furtherreduced for

commercial facilities. Electricity must be generated, as and when it is needed sinceelectricitycannot be stored virtually in the system.There is no difference between a transmission line and a

distribution line except for the voltagelevel and power handling capability. Transmission lines

are usually capable of transmittinglarge quantities of electric energy over great distances.


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They operate at high voltages.Distribution lines carry limited quantities of power over shorter

distances.Voltage drops in line are in relation to the resistance and reactance of line, length andthecurrent drawn. For the same quantity of power handled, lower the voltage, higher the

currentdrawn and higher the voltage drop. The current drawn is inversely proportional to the

voltagelevel for the same quantity of power handled.

The power loss in line is proportional to resistance and square of current. (i.e. PLOSS=I2R).Higher voltage transmission and distribution thus would help to minimize line voltage drop in

the ratio of voltages, and the line power loss in the ratio of square of voltages. For instance, if

distribution of power is raised from 11 kV to 33 kV, the voltage drop would be lower by a factor1/3 and the line loss would be lower by a factor (1/3)2 i.e., 1/9. Lower voltage transmission

and distribution also calls for bigger size conductor on account of current handling capacity

needed.

Cascade EfficiencyThe primary function of transmission and distribution equipment is to transfer power

economicallyand reliably from one location to another.

Conductors in the form of wires and cables strung on towers and poles carry the high-voltage,AC electric current. A large number of copper or aluminum conductors are used to form

the transmission path. The resistance of the long-distance transmission conductors is to be

minimized.Energy loss in transmission lines is wasted in the form of I2R losses.Capacitors are used to correct power factor by causing the current to lead the voltage. When

the AC currents are kept in phase with the voltage, operating efficiency of the system is

maintainedat a high level.Circuit-interrupting devices are switches, relays, circuit breakers, and fuses. Each of these

devices is designed to carry and interrupt certain levels of current. Making and breaking the

current


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carrying conductors in the transmission path with a minimum of arcing is one of the most

important characteristics of this device. Relays sense abnormal voltages, currents, and frequencyand operate to protect the system.

Transformers are placed at strategic locations throughout the system to minimize power

losses in the T&D system. They are used to change the voltage level from low-to-high in stepup

transformers and from high-to-low in step-down units.The power source to end user energy efficiency link is a key factor, which influences the

energy input at the source of supply. If we consider the electricity flow from generation to the

user in terms of cascade energy efficiency, typical cascade efficiency profile from generation to

1133 kV user industry will be as below:


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The cascade

efficiency in the T&D system from output of the power plant to the end use is

87% (i.e. 0.995 x 0.99 x 0.975 x 0.96 x 0.995 x 0.95 = 87%)


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Industrial End UserAt the industrial end user premises, again the plant network elements like

transformers at

receiving sub-station, switchgear, lines and cables, load-break switches, capacitors

cause losses,which affect the input-received energy. However the losses in such systems are

meager and

unavoidable.

A typical plant single line diagram of electrical distribution system is shown in

Figure .


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ONE Unit saved = TWO Units GeneratedAfter power generation at the plant it is transmitted and distributed over a wide

network.The standard technical losses are around 17 % in India (Efficiency = 83%). But the

figures for

many of the states show T & D losses ranging from 1750 %. All these may not

constitute

technical losses, since un-metered and pilferage are also accounted in this loss.

When the power reaches the industry, it meets the transformer. The energy

efficiency of the

transformer is generally very high. Next, it goes to the motor through internal plant

distributionnetwork. A typical distribution network efficiency including transformer is 95%

and motor efficiency

is about 90%. Another 30 % (Efficiency =70%)is lost in the mechanical system

which

includes coupling/ drive train, a driven equipment such as pump and flow control

valves/throttling

etc. Thus the overall energy efficiency becomes 50%. (0.83 x 0.95x 0.9 x 0.70 =

0.50, i.e.

50% efficiency)

Hence one unit saved in the end user is equivalent to two units generated in thepower plant.

(1Unit / 0.5Eff = 2 Units)

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