Transmission voltage is a multiple of 11 because of the following reasons¹:
1. All the power generation units generate energy which is the average value. We required to use RMS value of the electrical quantities.
So all the average value is multiplied by the form factor (1.11) to give RMS value, so all the voltage levels are multiple of 1.11.2. Transformer Induced voltage equation contains 4.44 factor. From the equation we see that E is proportional to 4.4 and it is in turn multiple of 11. So always transmission voltage is multiple of 11.
3. For a pure sine wave, the form factor is the ratio of RMS value of voltage or current with the average value of voltage or current. For pure sine wave RMS value of current is Imax/root ‘2’ and average value is 2Imax/pie and which comes out to be 1.1.
We can’t have a combination of other than a multiple of 1.11.4. The voltages in common use that are multiples of 11kV are 33, 66, 132 and 275.
These are a result of many generators- particularly in the early days of the electrical industry- working at 11kV, which when being stepped up to transmission voltages and down again to distribution voltages meant that the networks.