Line transmission system depends on the receiving end. Suppose, receiving end requires 10kV and transmission station needs to meet receivers demand. In order to meet the demand, transmission station takes transmission losses (voltage drop across line) into consideration and transmit 10% extra voltage.
If receiving end requires 10kV, 20kV, 132kV
10kV x 110% = 11kV
30kV x 110% = 33kV
60kV x 110% = 66kV
120kV x 110% = 132 kV
This how transmission voltage is delivered.
Note:
- Transmission line drop is not exactly 10% since current is not constant and it changes from peak value to zero. They have considered 10% is most satisfying value.
- Even 55kV, 77kV can also be transmitted but there is no point of having all voltage values for transmission distance. And its is also not cost effective.
some says ;
The live voltage numbering happens because of the value of Form Factor is 1.11 for sinusoidal A.C.
10kV x 1.11=11.1kV (11kV)
30kV x 1.11=33.3kV (33kV)
60kV x 1.11 =66.6kV (66kV)
120kV x 1.11=133.2kV
From the above, we can conclude form factor concept is invalid, because we have 66kV not 66.6kV.
