First let us define form factor before we go into the
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.
The reason is some thing historical. In olden days when the
electricity becomes popular, the people had a misconception
that in the transmission line there would be a voltage loss
of around 10%. So in order to get 100 at the load point
they started sending 110 from supply side. 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.
The reason for these "odd" voltage figures dates back
years.The original idea was to make up for the voltage drop
in long transmission line conductors , hence 11KV was
designed to deliver 10 KV at the end of the line , 132 KV
was designed to deliver 130KV at the end of the line and so
on. It has nothing to do with form factor . Also this has
long been abandoned and nowadays the voltage supplied at
the end of the line should be the voltage stated plus or
minus a very small percentage. 400 KV voltage appeared
after this idea was abandoned , hence it is a " round "
figure . Otherwise it might have been 405 or 410 KV !
i think it is not related with form factor as many replied
because if so then why 400 kv or 765 kv is followed as
these r not multiple of 11.i think becuse of constrainst in
gen voltage that is between 10.5 to 15 kv we[earlier]to
choose 11 kv and 33 66 132 kv r multiple of 11 to suite
our transformation ratio.if once follewed it continuied
otherwise i dont find any reason that with 12 kv gen why v
cant have 44 144 kv transmission.