it followed to our indian standard

due to thhe form factor=1.11, so output voltage must be 
mulipy of formfactor,so we get 11,22,33

it is because the form factor=1.11 so everything should be 
multiple of 11.

form factor of alternating current is 1.1 not 1.11, which 
is the ratio of rms value to average value of alternating 
current.so generation of ac voltage is in multiple of 11, 
like 11kv,33kv,44kv,66kv etc

so every conductor has disc so the minimum range of disc is 
11kv and if the voltage is high so we use another 11kv disc 
so it increases number of discs

The answers mentioning the form factor 1.1 are correct.

Just one clarification...

1.1 is form factor of a Sine wave...hence ac voltage being 
a sine wave holds Form Factor of 1.1

FORM FACTOR of sine wave is 1.11
rms/avg=(im/root2)/(2*im/pi) = 1.11
FF of square wave is 1.00 n triangular is 1.15
so i think form factor is not the only logic behind 
11kv/33kv but while the generator o/p formula suggest there 
are certain terms multiplied with the form factor(1.11) n 
it is forcibly made to a standadadised value by changing 
certain parameters..

Pl. give me the right answar

Above answers are not appropiately correct.Because we can 
varry a voltage from 1kv to 132kv or 220kv.

since form factor for sinusoidal wave is 1.11 which is  a 
multiple of 11 so generation and transmission is done at 
11,33,66kv

As per our Indian standard it is derived from the form 
factor I quite agrree to this answer because one question 
arise why not 22,44,55,77..... & why 400 KV?