why do we use multiples of11 as voltage in transmission lines?

Question

wat is form factor of ac

Answer 1

They don't any more. The modern voltages used today: 69, 138, 230, 345, 500, and 765 kV are not multiples of eleven.

The very first "transmission lines" were 11 kV. They then tripled the voltage to 33 kV. However utilities tended to run the equipment around 5% above the nominal rating. This became the standard voltage of 34.5 kV. Future transmission voltages tended to be a doubling of 34.5 kV.

This question appears every couple months... Almost as if a naive professor keeps asking every class the same question.

Below is a history of the evolution of distribution voltages in the US. This article appeared in the July/August 2007 issue of IEEE Power Enginering Society (PES) magazine.

Ever mindful of the economies in construction required to keep the costs to the customers as low as possible, a voltage of 2,400-V phase-phase delta, requiring the minimum of three conductors, was chosen early on by many systems. As load grew and the need for either larger conductors or a higher operating voltage became necessary, the conversion of 2,400-V delta systems to 2.4/4.16-kV grounded-wye systems occurred. This type of conversion allowed the amount of power transported over the existing conductors to be increased by a factor of three as well as the use of the existing step-down transformers by connecting them phase-ground instead of phase-phase.
Other companies also facing growth in a local area, but wanting to retain the perceived benefits of a delta system, accepted a slightly more costly upgrade by doubling the voltage to 4,800-V delta, or in those areas with rapid growth, tripling the lower voltage to 7,200-V delta. Converting to 7.2 kV resulted in a nine-fold increase in the capacity of the existing conductors but did require the replacement of all step-down transformers. As the loads and load densities increased further, the need for more capacity on the distribution lines saw the conversion from 7,200-V delta to 7.2/12.47-kV grounded-wye, another tripling of the line capacity. It should be noted that many of the 2.4/4.16-kV systems have also been converted to 7.2/12.47-kV as the loads on those systems increased. Two paths were taken to reach the same destination, each appropriate according to their economic factors driven by the load growth.
Other voltages derived from these systems were frequently used for distribution systems. For most of the regulated history of electric utilities, the voltages used in the distribution delivery systems were allowed to vary within defined limits, mostly plus or minus 5%. As systems operated constantly at the upper 5% limit, these limits became the new nominal operating level. Therefore, 7.2/12.47-kV systems became 7.6/13.2-kV systems, which then evolved to 7.9/13.8-kV systems. In addition, the 7.2/12.47-kV system was doubled to provide the 14.4/25-kV system often used by the aforementioned REA delivery systems serving remote, sparsely populated areas.
The desire for even higher distribution primary delivery voltages required an evolution of the transmission voltages used in the early years of electric system development. The 11-kV delta lines were upgraded, again by a factor of three, to 33 kV. As these were consistently operated at a 5% operating level, to reduce losses and offset voltage drop on the lines, a new nominal level of 34.5 kV was established. In 1957, Virginia Electric Power Company (VEPCo) added a multigrounded neutral to create the first 19.9/34.5-kV distribution line.

Answer 2

Are you talking of 110 kV, 220kV and 330 kv? Remember there is also 500 kV. This voltage are set as standard ratings by equipment manufacturers. Furthermore, these ratings are from ANSI Standards, the Europeans have their own standard.

Answer 3

because Alternating current wave form has a form factor of 1.1 , and we generate sine wave in our power generation