Why Electric Power Transmission is Multiple of 11 i.e 11kV, 22kV, 66kV etc?

 

Why are all the Transmission Line Voltages Multiple of 11kv? Not 10kV?

First of all, It is not true that all the transmissions and distribution voltages are multiple of 11. In most case, they are multiple of 11 such as 11kV, 22,kV, 33kV, 66kV & 132kV. But 400kV, 765kV and 800kV etc are not multiple of 11 in case of electric power system (generation, transmission and distribution) in power lines from the generation station to the receiving point.

It is not due to Form Factor (1.11)

Most of you may opt for form factor (1.11) which is nothing to do related to this question. How? Lets see below:

What is Form Factor?

Form Factor is the ratio between the R.M.S (Root Mean Square) value and average value of electric quantity (Current or Voltage). Mathematically, It can be expressed asFor a sinewave, the form factor is given below:

This way, the value of form factor is 1.11.

Now, we know that 400kV and 800kV are not the multiple of 11. Also,

• 10kV x 1.11 = 11.1kV
• 20kV x 1.11 = 22.2kV
• 30kV x 1.11 = 33.3kV
• 60kV x 1.11 = 66.6kV
• 120kV x 1.11 = 133.2kV (But the transmitted voltage are 132kV)

The above calculation clearly shows that the results are different then the general values i.e. in case of 132kV, with the from factor multiplication, it is 133.2kV.

In addition, We use 230V instead of 220V (Single Phase) and 400V instead of 440V (Three Phase).

Also, alternator and generator with terminal voltages are available i range of 10kv – 15kV which is not to do with multiple of 11.

If it is not related to the form Factor, What is it then?

Now back to the point, the voltage at the receiving end are generally:

• 10kV
• 20kV
• 30kV
• 60kV
• 120kV and so on…

But we already know that there are some voltage drop due to the resistance of the transmission lines. For this reason, they transmit 10% extra voltage. For example:

Sending Voltage = 10kV x 10% = 11kV. While the receiving end voltage are 10kV due to voltage drop. More examples are given in below table.

Sending Voltage	Receiving Voltage
10kV x 10% = 11kV	10kV
20kV x 10% = 22kV	20kV
30kV x 10% = 33kV	30kV
60kV x 10% = 66kV	60kV
120kV x 10% = 132kV	120kV
200kV x 10% = 120kV	200kV