3.5generation & transmission of electricity

7/28/2019 3.5generation & Transmission of Electricity

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GENERATION & TRANSMISSION

OF ELECTRICITY


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Electricity is generated in power stations,

transmitted (sent) through long-distance cables,

and then distributed to consumers.


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Electrical energy can be generated by many ways.

In Malaysia, the sources of energy like hydro,

diesel, and natural gas used to generate electric.

Source of energy can be classified to two that is:

Renewable energy sources : an energy resource that is continually replaced and will not run

out.

Non-renewable sources:

an energy resource that cannot be replaced once it has beenused such as fossil fuels (oil, diesel, natural gas, coal).


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Renewable energy sources are the energy sources

which can be continuously replaced.

Examples of renewable energy are:

Hydro energy

Solar energy

Wind energy

Geothermal energy Biomass

Tidal energy

o

Advantages of Using Renewable Energy1. Clean and do not pollute the environment.

2. Easily obtain.

3. Can be replenished once it has been used.

4. Can prevent energy crisis.


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ELECTRICITY TRANSMISSION

The electricity you use at home may be transmitted from a power station several

hundred kilometres away from your house.

We need step up transformers are used to increase the voltage at the power plant so

that low current produced.

Step down transformers are used to decrease the voltage before being delivered to the

consumers either at school/house/office (240 V) , light industry (11kV), or commuter

train (25kV).

In the transmission, long thick cables used as the transmission line are made ofcopper

or aluminium because they have low resistance. Thus less energy loss.


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WHYWEAREUSINGHIGHVOLTAGETRANSMISSIONRATHERTHANLOWVOLTAGETRANSMISSION?

To understand why, look at the following example:

In a model of a power station, electrical power istransmitted to consumers using a power transmissioncable with a total resistance of 1 Ohm. The powerstation generates 1200 W of electricity.

Model A: Low voltage transmission If the power is transmitted at 12 V, by using P=IV, the

transmitted current is 100 A.

Power wasted due to the resistance of transmission cable, P

is given by

power wasted, P=IV= I2R = 1002 (1) = 10 000 W


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Model B: High voltage transmission

If the output voltage is stepped up 20 times and transmitted at

240 V (20x12V), by using P=IV, the transmitted current is 5A.

Power wasted due to the resistance of the transmission cable,

P is given by

power wasted, P=IV= I2R = 52 (1) = 25 W

Only 25 W loss in high voltage transmissioncompared to 10 000 W in low voltagetransmission!!!!

So, that is why we used high voltagetransmission. Because less energy loss.


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In conclusion,

The greater the transmission voltage, the

smaller the current in the power lines.

The smaller the current, the smaller the powerloss during transmission.


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HIGHVOLTAGETRANSMISSION

Electricity generated in power stations is raised to a very high voltage

for transmission. This is to reduce the current flow to the

transmission cables.

Ploss = I2R

The power loss is proportional to the square of the current, thus a

small current greatly reduces heat loss. As seen from the equation

above, a small current can be achieved by using a high voltage. For

example, if we double (2) the transmission voltage, the currentwould be halved (1/2), and the power loss would be reduced to a

quarter, (1/2)2 = 1/4, i.e. 25% of the original value.


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Low Resistance Transmission Wire

We see from the equation above that the power

loss in the transmission wire Ploss is directly

proportional to the resistance Rof the wire. Thelower the resistance, the lower will be the power

loss.

Copper and aluminium are the most commonly

used metals in transmission wires. They are verygood conductors, cheap, resistant to corrosion, and

strong.

The resistance of the transmission wire is lowered

by making the wire thicker. Thicker wires havelarger cross-sectional areas and therefore lower

resistance.


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Electrical Transmission by Overhead Wire (read only)

Overhead lines are held high above the ground by metal towers

called pylons. If you look at a pylon carefully, you will see that theoverhead lines are held by a stack of discs hanging from the pylon.

This stack of discs is a series of suspended insulators which

prevents the line from being electrically connected to the

pylon. This prevents the electrical leakage from transmission wire

to the ground.


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Insulating disc prevents the line from being electrically

connected to the pylon.


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PROBLEMS INVOLVING ELECTRICITY

TRANSMISSION.

Power Loss During Transmission.

The high voltage transmission cable is very dangerous.

The cost of the cables are high.

Charge leakage may happen between cables and earth. Pylons may be struck by lightning.

Pylons and cables may be struck by light aircraft.


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NATIONALGRID NETWORK

What is a national Grid Network?

A national Grid Network is a network of cable that connects all thepower stations in a country to transmit electricity to the consumers

throughout the nation.

The network receives power from power stations and delivers it to

factories and homes. The output voltage from a generator which is

usually about 25kV is stepped up by a transformer as high as 132 kV

before being supplied to the National Grid Network.


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THEADVANTAGESOFTHE NATIONAL GRID

NETWORK

Reduces power lost during transmission. The potentialdifference is increased before transmission. This can reducethe current and hence reduces the energy lost duringtransmission.

Electricity supply is more stable and reliable. This ensures acontinuous supply of electrical energy to the whole country.

Electric current can be distributed to different users accordingto the voltage requirement. Transformer is used to step downthe voltage to certain level according to the needs of theconsumers.

Maintenance and repair work can be done at anytime. This isbecause any power stations can be shut down withoutaffecting users in other areas.


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EXAMPLE:

A power station generates 10 MW of electric power at

a voltage of 25 kV. This voltage is stepped up to

400 kV before being supplied to the National Grid

Network.

(a) What is the ratio Np : Ns of the transformer used?

(b) If the resistance of the cable is 0.5 Ohm per km,

calculate the power loss due to the 1000 km

length of the transmission cable.

ANSWER


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ANSWER:

(a) Vp= 25 kV, Vs= 400 kV

Np:Ns = Vp: Vs

= 25 000 : 400 000

= 1 : 16

(b) P = 10 MW

Ip= P/ Vp = 10x106 W 25x103 V = 400 A

By considering Vp Ip= Vs Is

Is = Vp Ip Vs = (25000x400) 400000 = 25 A

Total resistance of the cable,R = 1000 x 0.5 = 500 Ohm

Therefore, power loss due to the 100 km lenght of transmissioncable, P:

P = I2 R = 252 x 500 = 312 500 W


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EFFECTSONTHEENVIRONMENTCAUSEDBYTHEUSEOFVARIOUSSOURCESTOGENERATEELECTRICITY


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End

3.5generation & transmission of electricity

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