Why is electric power transmission lines 11KV OR 33KV, 66KV not in 10KV, 20KV, or 30KV?
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, 22kV, 33kV, 66kV & 132kV. But 400kV, 765kV and 800kV etc are not multiple of 11 in case of electric power system in power lines from the generation station to the receiving point.
Many people cite form factor a reason for so. Form Factor is defined as “RMS Value to Average value of a given AC voltage” and it is different for different waveforms.
For a sinewave, the form factor is given below:
The commonly used AC waveform we know is Sine Wave. A sine wave AC waveform has form factor of 1.11 (Approx.). So, the reason given is that the transmitted voltage of 10kV, 20kV, 60kV etc. is multiplied to this form factor to obtain such results which is described in the question like..
- 10kV → 10 x 1.11 = 11.1 kV
- 20kV → 20 x 1.11 = 22.2 kV
- 60kV → 60 x 1.11 = 66.6 kV
- 120kV →1.11 = 133.2kV
If we considered the value of first three cases approximately correct but there is a big difference in forth case of about 1.2kV.
So, at each subsequent step an amount of deviation has been seen which differs the actual and calculated result. Moreover, the deviation is not constant and increasing. Such results are intolerable and brings to the conclusion that form factor has nothing to do with voltage levels.
The electrical power generation companies tends to generate round figure voltages like 10kV, 20kV, 60kV, 120kV but this huge voltage needs to be transmitted over huge distance. The overhead line through which the power will be transmitted has its own impedance which will cause a considerable amount of voltage drop. This drop as being calculated is near about 10% based on all Physical factors. That’s why generation companies add 10% more in their actual target which neutralizes the line losses and the receiving end gets the targeted result.
|SENDING END VOLTAGES||RECIEVING END VOLTAGES|
|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|
What about 800kV, 765kV, 400kV Voltage Levels (Which is not multiple of 11)?
Modern extra high voltages (EHV) and ultra high voltages (UHV) lines operate at much higher voltage like 400 – 800kV. Higher voltage at same kVA reduces flow of current and thus reduces I²R loss. With decreased value of I²R losses, we don’t need to care much about line drop as that will automatically decrease. Increase in current causes increase in current density defined as (current/unit cross section area of the cable) and thus causes increase in electric field intensity in the surrounding (J=σE where J is the current density and E is electric field intensity). This E causes corona discharge and thus corona loss.
These aren’t the problems when it comes to extra high voltage transmission and thus voltages like 765kV or similar mid-range voltages are possible to transmit. The only main concern with EHV transmission is distance, insulation of the line and height of tower. Voltage drop becomes insignificant. Lower value of current reduces corona as well.