June 26, 2012

Why we are use 11KV / 22KV / 33KV / 66KV / 110KV / 230KV / 440KV this type of ratio. Why can’t we use other voltage ratio like 54KV / 99KV etc?


When an alternator generates voltage, we always use a multiple of 1.11 because for a pure sine wave the FORM FACTOR is the  ratio of rms value of voltage or current with the avg. value of voltage or current and for pure sine wave rms value of current is Imax/root '2' and avg. value is 2Imax/pie and which comes out to be 1.1;


We can't have a combination of other then a multiple of 1.11*.
So we can see all the voltages are made inevitably multiple of this value (1.1, which is the form factor of ac wave).

Also it provides us the best economic construction of step up and step down transformers.


* In the case of a Square Wave ie. a digital wave, the RMS and the average value are equal; therefore, the form factor is 1.

comments

22 Comment(s):

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  2. WHAT U HAD GIVEN ANSWER IS PARTIALLY CORRECT BUT I WAN A POSE A QUESTION.IN GENERATING STATIONS WE ILL USE 400KV RIGHT SO UR LOGIC WILL NOT BE APPLIED.400KV IS NOT A MULTIPLE OF 1.11

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  4. The multiples of 11 does not have any significance with form factor.

    Only thing is that most of the generators are at 11kV, the next stage transformers are of 11kV to 33kv, 66kv etc. You can even have 11kV to 40kV or any such multiplication, but the problem is that if your secondary happens to be 40kV, you would not be in a position to connect this 40kV to our present system, where there is no 40kV bus!!!!!!

    This means that all on a sudden you cannot change the voltage ratios as you desire, because, the whole existing power system has to be revamped which is not possible. However flexible ac transmission system (FACTS) can overcome this problem, which is being used now..

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  8. There is a misconception that the line voltage should be a multiple of 11 because the form factor is 1.11. The real reason is something historical. In historical days when electricity was becoming popular people had a misconception that in the transmission line there would be transmission loss of 10%. So to get 100 at the load point they started sending 110 from the supply side.This is the reason it has nothing to do with the form factor.
    Now a days that thought has changed and we are using 400v instead of 440v 230v instead of 220v
    Also alternators are available which can generate voltage levels in the range 10.5Kv to 15.5Kv so generation in multiples of 11 does not arise.
    Now a days we have voltage correction systems , power factor correction capacitors which can boost/correct voltages to a desired level. We are using exact voltages of 400Kv instead of 400Kv.


    ReplyDelete
  9. Yes u r right..... but a doubt arise that.,. How you confirm about its historical reasons?

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  10. Yes u r right..... but a doubt arise that.,. How you confirm about its historical reasons?

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  11. Hi Sirs,
    Have a question, if I have 6.6kV system, can I use 11 or 12 kV rated VCB instead for the system?

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  12. Hi Sirs,
    If I have 6.6kV system, can I use 11kV or 12 kV Vacuum circuit breaker instead of the nearest 7kV rated available in the market?

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  13. Hi Sirs,
    If I have 6.6kV system, can I use 11kV or 12 kV Vacuum circuit breaker instead of the nearest 7kV rated available in the market?

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  14. but y 400KV and 765KV

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