Electricity Transmission & Distribution – Complete Answer

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How this electricity is brought to our homes?

Electricity is produced using electricity generators. These generators convert mechanical power such as hydro power, steam power, etc. into electricity. These generators produce electricity at 11 kV level.

 

This 11kV level is stepped up to 100kV, 132kV, 220kV, 400kV, and 765kV using Power Transformers installed at Substations. Then this extra high voltage level electricity is transmitted over long distances using long conductors supported by huge towers called as power transmission system.

Now, at consumer regions, this extra high voltage 100kV, 132kV, 220kV, 400kV, and 765kV is stepped down to 11kV, 22kV & 33kV using power transformers at substations. This stepped-down power is brought near the consumer areas. At consumer ends this  11kV, 22kV, and 33kV level is again stepped down to 440 volts using distribution transformers.

 

And finally, this 440 volts three-phase power is distributed to homes in a single-phase format of which the voltage is 230 volts. This 230 volts of power is used in our homes.

In brief, electricity is generated at the 11kV level. Then it is stepped up to 100kV, 132kV, 220kV, 400kV, and 765kV levels, and then transmitted over long distances. At the consumer end electricity is again stepped down to 11kV, 22kV, and 33kV levels. Further, it is stepped down to a three-phase 440 volts, which is 230 volts single-phase. And electricity is used by consumers at these voltage levels.

Since we generate electricity at 11kV level. Why can’t we just transmit the electricity at this same 11kV voltage level over long distances? And then step it down to desired 440 volts level at the consumer end. It would be relatively easy. Right?

Why do we first Step-up & then Step-down Electricity Voltage levels?

This step-up and step-down of electricity is done to save electric power loss. Let’s understand this with an example. 
Let’s consider that electricity generated at the generation plant is to be transmitted & distributed to the consumers.
The length of this transmission line to be 1 km,
& its total resistance R = 1 ohm
Let’s assume the power factor pf = 1
Now, Case – I
Consider that 1 MW of electricity is generated at this generation plant at the 11kV voltage level in 1 second. And the same power is supplied to the consumers at the same 11kV voltage level without stepping up the voltage. 
We know the formula for electric power in Watts 
                          P = √3 x VL x IL x pf
In this case,
Power P = 1 MW, & Voltage V = 11kV,
hence current I (flowing through transmission line) = 52.49 Amperes.
We know Joule’s equation of electrical heating due to resistance in a conductor 
                           Power Loss = I² x R x t
In this case resistance R = 1 ohm.
And time t = 1 second.
         Hence loss due to heating = 52.49² x 1 ohm x 1 seconds = 2755  Joules.
 
Now, Case II
Consider the same scenario with the same parameters as in the previous case. The only difference is that the voltage is stepped up to 220kV for electricity transmission.
Hence, in this case,
the current I (flowing through transmission line to transmit 1 MW power) = (1 MW) / (√3 x 220 kV) = 2.624 Amperes. 
resistance R = 1 ohm.
And time t = 1 seconds.
      Hence loss due to heating = 2.624² x 1 ohm x 1 seconds = 6.889 Jules.
After discussing both cases, we can see that the loss is reduced by 400 times, just by increasing the voltage level by 20 times. This way loss can be further reduced by approximately 1400 times by stepping up the 11kV voltage level to 400kV level.

Simply put, as per the Jules equation of electrical heating, the loss is proportional to the square of the current. That means, the loss is inversely proportional to the square of the voltage. Hence loss can be reduced proportionally to the square of the voltage step-up ratio.
In conclusion, we can say, by simply stepping up the voltage level for electricity transmission over long distances, huge electricity loss is reduced. This is the key reason to transmit electric power in AC and not in DC because DC power voltage can not be stepped up or stepped down by power transformers. Because power transformer works on only AC power. Hence losses in DC transmission can not be reduced like AC. Nowadays HVDC technology is used in which electricity voltage is stepped up by the power transformer in AC form and then AC is converted into DC. This DC power is transmitted over long distances, and at the consumer end again it is converted back to AC form, and then its voltage is stepped down again. But this technology requires very high capital cost and maintenance cost.

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