The Generation of Electricity

Sources of Electricity in the USA

Electric Power Generation

• Most of the electricity in the United States is produced in steam turbines. 

• A turbine converts the kinetic energy of a moving fluid (liquid or gas) to mechanical energy. 

• Steam turbines have a series of blades mounted on a shaft against which steam is forced, thus rotating the shaft connected to the generator which produces electricity. 

Coal Power PlantIn a fossil-fueled steam turbine, the fuel (mainly coal) is burned in a furnace to heat water in a boiler to produce steam.

Electric Power Generation

• In power plants, steam is pushed through the turbine which rotates a shaft

• The rotating shaft drives a generator by turning a coil of wire in a magnetic field.

• 60 Hz AC Electricity is generated at 25,000 volts

Electricity and Magnetism

• The science of electricity has its roots in observation, known in 600 BC that a rubbed piece of amber will attract a bit of straw

• Study of magnetism goes back to the observation that certain naturally occurring stones attract iron

• The two sciences were separate until 1820 when Hans Christian Oersted saw the connection between them…an electric current in a wire will affect a compass needle

Magnets• Iron and several other

metals can act as magnets

• A magnet always has two poles: a north magnetic pole and a south magnetic pole

• Like electric charges, opposite poles attract and like poles repel (e.g. north poles are attracted to south poles)

Magnetic Fields

• Magnets produce a magnetic force field , similar to an electrical force field.

• A magnetic field is described by invisible lines along which other magnets (like iron filings) will align.

Charged Particles in Magnetic Fields

• Electrons (like any other charged particle) are deflected by magnetic fields

• The deflection depends on the direction of the particles motion and the magnetic field

Electricity and Magnetism

• A coil or wire with a current passing through it act as a magnet, similar to a bar magnet.

• This is called an Electromagnet.

Bar Magnet Electromagnet

Electric Currents Create Magnetic Fields

• When electric current flows through a wire, a magnetic field is produced

• The arrows indicate the magnetic field lines as a result of the current in the wire.

• Moving electric charges can create a magnetic field

• Likewise, a moving charged particle can experience force in a magnetic field.

Electric Motors• In an electric motor, a

coil of wire with current flowing through it will tend to line up with a pair of electromagnets

• If the direction of the current is changed at just the right time, the motor will continue to turn

Electric Motors

• Current reversal is possible with a communicator – two connecting rings on the shaft that make contact alternately with the positive and negative terminals of the battery.

• If the wire carries alternating current AC), then a communicator is not necessary.

Electromagnetic Induction

• Production of a current in a wire when there is relative motion between the wire and the magnetic field.

• The motion produces a potential difference between the ends of the wire.

• In the diagram, the faster the magnetic is moved, the greater the induced voltage.

Electric Generators

• An electric generator is based on the principle of electromagnetic induction: – spinning coils of wire within magnetic

fields– property of electromagnetism that a

changing magnetic field through a loop of wire produces a voltage along the loop

– this voltage can drive a current and provide energy to an external circuit

Electric Generators

• An electric generator is just an electric motor working in reverse, or visa versa

AC and DC

• In a dc (Direct Current) electrical system, the movement (or flow) of electric charge is only in one direction and the voltage remains constant

• In an ac (Alternating Current) the movement (or flow) of electric charge periodically reverses direction. An electric charge would for instance move forward, then backward, then forward, then backward, over and over again. The voltage fluctuates rapidly from positive to negative values

• In a household electrical outlet (which is AC), the voltage fluctuates from -120 V to +120 V sixty times every second (60 Hz)

Why Two Types of Current?• dc is better when:

– an appliance needs to run on batteries [because batteries produce dc currents]

– an appliance uses integrated circuits (e.g. computer chips) [because most chips are designed to run with 5 V or 12 V dc]

• ac is better when:– high voltages are required [because the voltage can

be changed using a simple device called a transformer]

– the appliance needs to run a motor [because ac motors are easier to build]

A Brief History of Electricity

• Electricity was first studied in detail in the 18th century

• Many practical uses for electricity (e.g. the light bulb) were invented by Thomas Edison in the late 19th century

• Edison (an inventor) co-founded Edison Electric (now General Electric), which used dc

• Nikola Tesla (a physicist) developed ac because ac can be efficiently transferred over large distances compared to dc

• To prove that ac is dangerous, Edison built an electric chair (which was used)

• Eventually, ac became the industry standard

How to Transfer Electrical Power

• Wires used to transfer electrical power have a small resistance R

• Power loss (P = I2R) is greater when the current I is large

• High voltage is better because more power can be transferred with less current

Example 1

• A power plant want to transfer 1 MW through a line with a resistance of 1

• Case 1: The voltage is 1200 V, what is the power loss?

• Answer: First find the current using P = IV

which is the same as I = P/V

so I = (1,000,000 W)/(1200 V) = 833 A

the power loss is P = I2R

so P = (833 A)2(1 ) = 693,000 W !!!!

Example 2

• A power plant want to transfer 1 MW through a line with a resistance of 1

• Case 2: The voltage is 120,000 V, what is the power loss?

• Answer: First find the current using P = IV

which is the same as I = P/V

so I = (1,000,000 W)/(120,000 V) = 8.33 A

the power loss is P = I2R

so P = (833 A)2(0.1 ) = 69.4 W !!!!

Transformers• A transformer is an electrical device that takes

electricity of one voltage and changes it into another voltage.

• Transformers only work for ac circuits• Basically, a transformer changes electricity

from high to low voltage using two properties of electricity:1. Electromagnets: In an electric circuit, there is

magnetism around it2. Electromagnetic Induction: Whenever a magnetic

field changes (by moving or by changing strength) a voltage is made.

Transformers

• Transformers are constructed by simply winding wire around two ends of a rectangular piece of metal or the “core”

• Electricity moving through the primary coil creates a magnetic field around the core.

• This magnetic field induces an electric field and moves electrons in the secondary coil, thus producing an electric current.

Voltage In

Voltage Out

Transformers

• The input line connects to the 'primary' coil, while the output lines connect to 'secondary' coil.

• The alternating current in the primary coil induces an alternating magnetic field that 'flows' around the metal core, changing direction during each electrical cycle.

• The alternating field in the core in turn induces an alternating current on the secondary coil.

• The voltage of the secondary coil is directly related to the primary voltage by the turns ratio, or the number of turns in the primary coil divided by the number turns in the secondary coil.

Transformers

Types of Transformers• Step-up transformers convert low voltages to high

voltages. Note that there are more turns on the secondary coil.

• Step-down transformers convert high voltages to low voltages. Note that there are less turns on the secondary coil.

Step-up Step-down

Example

• For example, if you wanted to increase your house voltage from 110 volts (110V) to 220V in order to power your electric stove, you could use a transformer with twice the turns in the secondary coil as in the primary coil.

• The relationship is written as:input volts / input turns =

output volts / output turns

• 110V / 5 turns = 22 = 220V / 10 turns

The “Grid”

• Electrical power travels from the power plant to your house through a system called the power distribution grid.

The Continental U.S. power transmission grid consists of about 300,000 km of lines operated by

approximately 500 companies.

The Power Plant

A power plant

generates 25,000V

A power station steps up the voltage to

345,000V

so it can be transferred through power line

Why not Transfer Electricity at a Billion Volts?

• For one thing, air is not a perfect insulator

• At very high voltages, arcs of “lightning” would shoot between power lines and the ground

• Safety is also an issue (Note: birds can land on a bare wire because they do not create a closed circuit, i.e. a path to the ground)

Area Substation

A substation steps the

voltage down to 100,000 V

Distributing Substation

A distributing substation steps the voltage down to 20,000 V

Residential Distribution Transformer

A step down transformer outside your house steps the voltage down to 120 V and 240 V (240 V is for large appliances like your washer and dryer)

Problems with U.S. Grid• Current hardware is old and needs to be replaced.

– Large blackouts and service disruptions are

likely in the future.• 65% of electricity is lost between generation and

distribution. – Low efficiency increases both the need for

fossil fuels and the amount of pollution and greenhouse gases emitted.

• Grid cannot easily accommodate renewable sources such as wind and solar power.

What is the “Smart Grid?”

What is the “Smart Grid?”• According to research sponsored by the U.S.

Government, improving the efficiency of the national electricity grid by 5 percent would be the equivalent of eliminating the fuel use and carbon emissions of 53 million cars.

• This could be accomplished by implementing a "smart grid" that links all aspects of the electric grid together (from generator to consumer) with sensors and smart devices, in order to provide enhanced operational capabilities that: eliminate waste, incorporate alternative power sources (solar and wind), and improve reliability.

What is the “Smart Grid?”• Smart grid technology will also allow customers

to sell electricity they generate from rooftop solar panels and other renewable sources back into the system, which will be able to locally store energy and incorporate intermittent sources like wind and solar.

• Using smart grid technology, a home would be as likely to be powered by electricity from a neighbor's roof-top solar panel, or a windmill on the edge of town, as from a traditional power plant 50 or 100 miles away.

What is the “Smart Grid?”• Under the current power grid

system, utilities don't learn about power outages until their customers call. Smart grids will be able to diagnose themselves, as well as monitor electricity demands in real time.

• Using smart meters, customers will be able to monitor their electricity use online – and get suggestions for minimizing it, either by turning certain appliances off or using them at different times.

Smart Grid Obstacles

• But the Electric Power Research Institute "has estimated the cost of building a smart grid at a staggering $165 billion -- about $8 billion a year for two decades.

• On top of the staggering costs, there are also many different competing interests involved with smart grid technology. Upgrading the energy grid will require staid utility companies, new upstart energy firms, local and state governments and federal authorities to works together. Trying to get all of these entities to partner up may be even harder than raising funds