In the previous section we saw the basics about production of electricity. In this section, we will see it's transmission and distribution.
• We have seen that electricity is obtained from a generator.
• What is the energy conversion taking place in an AC generator?
Ans: We have seen that, mechanical energy is required to rotate the armature. (Details here).
• So we can say: Mechanical energy is converted into electrical energy.
But where does this mechanical energy come from?
There are several sources. Let us see some of them:
Method 1:
1. Water can be allowed to flow from a height.
2. This flowing water will rotate a turbine, which in turn will rotate the armature.
• When the armature rotates, we get electricity
3. The required water is stored in huge dams. These dams are constructed at elevated areas.
4. So the water stored will have potential energy.
5. This potential energy is converted into kinetic energy when the 'flow of water' takes place.
6. This kinetic energy causes the turbine to rotate.
7. A power station which uses this method is called hydroelectric power station.
8. So, in a hydroelectric power station, the energy change taking place is:
Potential energy ➡️ kinetic energy ➡️ Mechanical energy ➡️ Electrical energy
Method 2:
1. Water can be heated to make steam.
2. Steam under high pressure can be used to rotate the turbine.
3. A power station which uses this method is called thermal power station.
4. Coal can be used as a fuel to heat water.
5. But burning coal can cause environmental pollution.
6. In coal, energy is stored as chemical energy
7. So, in a thermal power station, the energy change taking place is:
Chemical energy ➡️ heat energy ➡️ Mechanical energy ➡️ Electrical energy
Method 3:
1. Instead of coal, nuclear energy can be used to heat water.
2. A power station which uses this method is called a nuclear power station.
7. So, in a nuclear power station, the energy change taking place is:
Nuclear energy ➡️ heat energy ➡️ Mechanical energy ➡️ Electrical energy
■ Electricity produced in a power station is transmitted to distant places. This process is called power transmission.
■ Special conducting wires are used for this purpose. Such wires are called transmission lines.
The various stages of power transmission and distribution are shown schematically in the fig.12.1 below:
1. We can see that the generator produces electricity of 11 kilo volts
2. This is sent to a power transformer
• So the input into this Power transformer is 11 kV.
3. The output from the power transformer is shown as 220 kV.
• That means, the 11 kV is increased 20 times to become 220 kV.
4. Why do we need such an increase?
We will write the answer in steps:
(i) When current flows through a conductor, heat is generated.
• To reach the consumer, from the power station, current has to flow through large distances
• So a large quantity of heat will be generated
■ We can say:
When current flows from the power station to the consumers, a portion of electrical energy is lost in the form of heat.
• We have to find ways to reduce this energy loss.
(ii) We have seen from Joules law that: Heat generated H = I2Rt (Details here)
• So, when resistance increases, heat also increases.
(iii) If we can reduce the 'resistance of the conducting wires', heat can be reduced
• To reduce the 'resistance of the conducting wires', we have to increase the 'area of cross section' of those wires. (We have seen the reason here)
(iv) But when the area of cross section is increased, two disadvantages occur:
(a) The conducting wires will become very expensive
(b) The conducting wires will become very heavy. Then, 'very strong and huge supports' will be required to keep them in position
• So 'increasing the area of cross section' is not an option here. We have to look for other methods
(v) Consider again the equation H = I2Rt
• We see that, the heat generated depends upon the square of the current (I)
• So, if we can reduce the current, heat generated will be low.
(vi) Consider the equation for power: P = VI (Details here)
• We do not want any 'energy loss'. That is., we do not want any 'power loss'
• So the left side 'P' must remain the same
• At the same time, the current 'I' should decrease
• There is only one way to satisfy both the conditions:
'Increase the voltage (V)'
An example:
• Let P1 = V1I1 and P2 = V2I2
• But we want: P1 = P2
• So we get: V1I1 = V2I2
• If the current is reduced to one tenth, we get: I2 = I1⁄10
• Substituting this, we get:
V1I1 = V2×I1⁄10 ⟹ V1 = V2×1⁄10 ⟹ V2 = 10V1
■ That is: The voltage should be increased 10 times
Thus we can write:
• If the voltage is increased 10 times, we can reduce the current to one tenth
• When the current is thus reduced, heat generated will be low
• That is the reason for stepping up the voltage from 11 kV to 220 kV in the 'stage 1' shown in fig.12.1 above
5. Now we can get back to our main discussion:
The current at 220 kV travels long distances through 'transmission towers' (images here).
• Then they reach a substation. The name of this substation is '220 kV substation'
• Here the current is split into different branches
• Each branch enters into it's own designated step down transformer
• The outputs from these different step down transformers are:
110 kV, 66 kV, 33 kV, 11 kV etc.,
• The voltages 66 kV, 33 kV, 11 kV etc., are given to large scale industries because, they require such high voltages
6. The 110 kV continues the journey and reach another substation. The name of this substation is '110 kV substation'
• Here, all the current enters into a step down transformer.
• The output from this transformer is 11 kV
• This 11 kV current is split into two parts.
♦ One 11 kV part is given to small scale industries
7. The other 11 kV part continues it's journey and reach a transformer.
• It is a step down transformer. It's name is 'distribution transformer'
•The output from this transformer is 230 V
• 230 V is just what is required for house hold appliances
• We have seen that electricity is obtained from a generator.
• What is the energy conversion taking place in an AC generator?
Ans: We have seen that, mechanical energy is required to rotate the armature. (Details here).
• So we can say: Mechanical energy is converted into electrical energy.
But where does this mechanical energy come from?
There are several sources. Let us see some of them:
Method 1:
1. Water can be allowed to flow from a height.
2. This flowing water will rotate a turbine, which in turn will rotate the armature.
• When the armature rotates, we get electricity
3. The required water is stored in huge dams. These dams are constructed at elevated areas.
4. So the water stored will have potential energy.
5. This potential energy is converted into kinetic energy when the 'flow of water' takes place.
6. This kinetic energy causes the turbine to rotate.
7. A power station which uses this method is called hydroelectric power station.
8. So, in a hydroelectric power station, the energy change taking place is:
Potential energy ➡️ kinetic energy ➡️ Mechanical energy ➡️ Electrical energy
Method 2:
1. Water can be heated to make steam.
2. Steam under high pressure can be used to rotate the turbine.
3. A power station which uses this method is called thermal power station.
4. Coal can be used as a fuel to heat water.
5. But burning coal can cause environmental pollution.
6. In coal, energy is stored as chemical energy
7. So, in a thermal power station, the energy change taking place is:
Chemical energy ➡️ heat energy ➡️ Mechanical energy ➡️ Electrical energy
Method 3:
1. Instead of coal, nuclear energy can be used to heat water.
2. A power station which uses this method is called a nuclear power station.
7. So, in a nuclear power station, the energy change taking place is:
Nuclear energy ➡️ heat energy ➡️ Mechanical energy ➡️ Electrical energy
■ Electricity produced in a power station is transmitted to distant places. This process is called power transmission.
■ Special conducting wires are used for this purpose. Such wires are called transmission lines.
The various stages of power transmission and distribution are shown schematically in the fig.12.1 below:
 |
| Fig.12.1 |
2. This is sent to a power transformer
• So the input into this Power transformer is 11 kV.
3. The output from the power transformer is shown as 220 kV.
• That means, the 11 kV is increased 20 times to become 220 kV.
4. Why do we need such an increase?
We will write the answer in steps:
(i) When current flows through a conductor, heat is generated.
• To reach the consumer, from the power station, current has to flow through large distances
• So a large quantity of heat will be generated
■ We can say:
When current flows from the power station to the consumers, a portion of electrical energy is lost in the form of heat.
• We have to find ways to reduce this energy loss.
(ii) We have seen from Joules law that: Heat generated H = I2Rt (Details here)
• So, when resistance increases, heat also increases.
(iii) If we can reduce the 'resistance of the conducting wires', heat can be reduced
• To reduce the 'resistance of the conducting wires', we have to increase the 'area of cross section' of those wires. (We have seen the reason here)
(iv) But when the area of cross section is increased, two disadvantages occur:
(a) The conducting wires will become very expensive
(b) The conducting wires will become very heavy. Then, 'very strong and huge supports' will be required to keep them in position
• So 'increasing the area of cross section' is not an option here. We have to look for other methods
(v) Consider again the equation H = I2Rt
• We see that, the heat generated depends upon the square of the current (I)
• So, if we can reduce the current, heat generated will be low.
(vi) Consider the equation for power: P = VI (Details here)
• We do not want any 'energy loss'. That is., we do not want any 'power loss'
• So the left side 'P' must remain the same
• At the same time, the current 'I' should decrease
• There is only one way to satisfy both the conditions:
'Increase the voltage (V)'
An example:
• Let P1 = V1I1 and P2 = V2I2
• But we want: P1 = P2
• So we get: V1I1 = V2I2
• If the current is reduced to one tenth, we get: I2 = I1⁄10
• Substituting this, we get:
V1I1 = V2×I1⁄10 ⟹ V1 = V2×1⁄10 ⟹ V2 = 10V1
■ That is: The voltage should be increased 10 times
Thus we can write:
• If the voltage is increased 10 times, we can reduce the current to one tenth
• When the current is thus reduced, heat generated will be low
• That is the reason for stepping up the voltage from 11 kV to 220 kV in the 'stage 1' shown in fig.12.1 above
5. Now we can get back to our main discussion:
The current at 220 kV travels long distances through 'transmission towers' (images here).
• Then they reach a substation. The name of this substation is '220 kV substation'
• Here the current is split into different branches
• Each branch enters into it's own designated step down transformer
• The outputs from these different step down transformers are:
110 kV, 66 kV, 33 kV, 11 kV etc.,
• The voltages 66 kV, 33 kV, 11 kV etc., are given to large scale industries because, they require such high voltages
6. The 110 kV continues the journey and reach another substation. The name of this substation is '110 kV substation'
• Here, all the current enters into a step down transformer.
• The output from this transformer is 11 kV
• This 11 kV current is split into two parts.
♦ One 11 kV part is given to small scale industries
7. The other 11 kV part continues it's journey and reach a transformer.
• It is a step down transformer. It's name is 'distribution transformer'
•The output from this transformer is 230 V
• 230 V is just what is required for house hold appliances
• This completes the description about the schematic diagram shown in figure 12.1.
• So now we know the basics about power transmission.
■ Consider the last transformer in figure 12.1.
• We see that 3 lines go into that transformer.
• But there are 4 lines coming out. why is that so?
We will see the reason in the next section
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