During a recent job interview of mine, one of the panel asked me the following question Q) Why is the transmission voltage of overhead lines in our country in the multiples of 11? Some background information, I am from India where the transmission voltages are 11,33,66kV etc. I have found 2 answers on google one of them stating that it's due to form factor and other to account for some kind of losses, I don't feel convinced by both these answers. What is the actual technical reason?
The voltages are multiples of 11kV because that's the way Great Britain built their transmission and distribution network. Countries that followed English electrical standards (India, Australia, and South Africa, for example) inherited the same family of voltages.
There's no inherent reason why multiples of 11kV are better than any other choice of voltage. For example, the USA uses 15kV where we would use 11kV. This doesn't fundamentally change the way the equipment is designed or operated.
As to why all the voltages are neat multiples of 11kV - i.e. 3.3kV, 11kV, 22kV, 33kV, 66kV, 132kV - it makes for neat integer transformer ratios and makes the maths easy.