All the power generation units generates energy which is the average value.
We required to use RMS value of the electrical quantities.So all the average value is multiplies by the form factor
(1.11) to give RMS value, so the all the voltage levels are multiple of 1.11.
Firstly, not all of them are.
400kV is the top voltage transmitted in GB, and it is not a multiple of 11. Similarly 13kV is commonly used by the
distribution networks, which also is clearly not a multiple of 11kV.
The rail network uses 25kV for most of its connections to the grid, also not a multiple of 11. Countries with longer connections are known to use other even higher HV values, again most of which are not multiples of 11.
The voltages in common use that are multiples of 11kV are 33, 66, 132 and 275.
– The form factor of an alternating current waveform (signal) is the ratio of the RMS (Root Mean Square) value to the average value (mathematical mean of absolute values of all points on the waveform). In case of a sinusoidal wave, the form factor is approximately 1.11.
This is the reason. It has nothing to do with form factor (1.11).
Nowadays that thought has changed and we are using 400 V instead of 440 V, or 230 V instead of 220 V.
Also alternators are now available with terminal voltages from 10.5 kV to 15.5 kV so generation in multiples of 11 does not arise.
– Form Factor = RMS voltage/Average Voltage For AC sine wave Form Factor is 1.11.