Answer / piyush rana

All the power generation units generates energy which is
the average value.
We required to use RMS value of the electrical quantities.
So all the average value is multiplies by the form factor
(1.11) to give RMS value, so the all the voltage levels are
multiple of 1.11.

Answer / lev stroud

Firstly, not all of them are.

400kV is the top voltage transmitted in GB, and it is not a
multiple of 11. Similarly 13kV is commonly used by the
distribution networks, which also is clearly not a multiple
of 11kV.
The rail network uses 25kV for most of its connections to
the grid, also not a multiple of 11.
Countries with longer connections are known to use other
even higher HV values, again most of which are not multiples
of 11.


The voltages in common use that are multiples of 11kV are
33, 66, 132 and 275.

These are a result of many generators- particularly in the
early days of the electrical industry- working at 11kV,
which when being stepped up to transmission voltages and
down again to distribution voltages meant that the networks
at the time would use simple transformer coil ratios- 3:1,
6:1 etc.

The reason for them working at 11kV came primarily due to
the limits of such factors as insulation and eddy currents
etc in the generator modules themselves. As technology has
progressed many have stepped up to higher voltages. Often
this is 22kV as, again, the transformer ratios are simple.
In fact not only are the transformer ratios simple, but
insulator stacks can simply be doubled up on.

To change the network voltages from those that have arisen
historically would be both a vast cost and incredibly
disruptive, which is why it has not been done. It would
also require the re-tooling of almost every factory that
makes HV equipment.

However, when the higher voltage 400kV (and higher)
transmission networks were added there was no requirement
for the transformer ratios to be kept so simplistic and the
voltage was chosen based on other engineering factors instead.

Contrary to the statements above, the form factor of 1.11
for sine wave power compared to transmission losses was not
an influence.
Losses are well below 10%. 1/10 losses in a 1000MVA line
would destroy most transmission systems by melting the
conductors with 100MW of heat being dissipated along it. In
fact the entire point of HV AC transmission is to keep the
losses to a minimum.

The form factor is simply the ratio of the RMS to the
average of the absolute values. This ratio gives the
ability to calculate the DC equivalent power in a system by
comparison to the AC voltage.
The figure of approximately 1.11 would still apply at the
users end, regardless of any losses, because it is a ratio.

Answer / khongserbu

since all the voltage generation are in average and we use
rms quantities for computation. so we need to convert the
average to rms simply by multiplying the form factor 1.11.
thats how the voltage levels are in the multiple of 11.

So stop fighting both of u

Dr. khongserbu
university of lungzor
poktor
Chazzam

Answer / sriman

Hi Johince can u please explain in detail i didnt
understood
how can u say that transmission line consumes 10% of power

Answer / soumya ranjan mania

Dear friends,for your kind information form factor of full
sine wave is infinite,but it is 1.11 for halfwave,how
halfwave formfactor is related to our topic,as we need
fullsine wave so form factor has no related to generation of
voltage multiple of 11,so only answer is only the historical
perspective ie In early days when technology has not
advanced this much,the transmission lines consumes around
10% of the totalpower transferred. Normally loads are
designed as multiples of 10 or 100 KW. So the generation
done for required load power + transmission loss. ie 11KW,
110KW etc. So generated voltage also was designed as
multiples of 11.

Answer / shreekar

Let us say transmission voltage is Vt and generated voltage is Vg.
According to some people Vt is 10% more than Vg [because 10% is transmission line drop]. Hence Vt = 1.1 * Vg ----eq1.
According to some people Vt is a multiple of form factor i.e Vt = 1.11 * a ----eq1.
where 'a' is some voltage.
When we see these two equations i.e eq1 and eq2 there is lot of resemblance.

Why form factor theory is correct?

Now in India transmission line voltages are 11kV, 33kV, 66kV, 220kV.
Here the transformer ratio is greater than or equal to 2, between two consecutive transmission line voltage, so electrical engineers use 3- PHASE TRANSFORMER. Hence line voltage is a multiple of form factor (i.e 1.11).

But this is not always true for higher transmission voltage like 400kV and 765kV (Note: 400 and 765 are not multiple form factor). The transformer ratio between 400kV and 765kV is less than 2, this is true as we move to even higher transmission voltage 800kV and so on. If transformer ratio is less than 2, then Electrical Engineers use AUTO-TRANSFORMER (this is because of economical reasons). That is why you have to take phase voltages after 220kV.
Hence, Phase voltage [The reasons to take phase voltages is because of AUTO-TRANSFORMER] of 400kV = 400/1.73 = 231.2kV
231.2kV is near to 220kV and 220 is a multiple of form factor. (this approximation because of corona discharge and it is related to Insulator string efficiency, Tower size and
dimensions of Conductors). [Note: Corona loss depends on transmission line voltage]
Similarly, Phase voltage of 765kV = 765/1.73 = 462kV which is near to 440kV and 440kV is a multiple of form factor.

In the contrary,
you cannot say transmission losses is always below 10%. Some time it may exceed 10%. In major cases it dose not exceed, but who knows what happens the whole power system is a dynamic system.

Conclusion:
Hence transmission line or phase voltage are multiples of form factor. We have to consider phase voltage only when we encounter AUTO-TRANSFORMERS and not the line voltages.

Answer / sachin

Because form factor is 1.11 so all transmission either LT
And HT is in multipules of 1.11

Answer / shariff

It is just the standard being maintained by all the transmission utilities, with form factor 1.11, voltage not even multiple of 11 is being under use, like 132kV,400kV etc, if this old standard is changed then it should be changed all over, from generating to distribution level, which is not possible in practical, hence the same std being maintained.

Answer / ranjith

because output equation of transformer depend upon
formfactor so it is in multiples of 11

Answer / johince

Multiples of 11 has no role decoded from form factor and
anything like that (I believe). the relation to form factor
is just a coincidence.
In early days when technology has not advanced this much,
the transmission lines consumes around 10% of the total
power transferred. Normally loads are designed as multiples
of 10 or 100 KW. So the generation done for required load
power + transmission loss. ie 11KW, 110KW etc. So generated
voltage also was designed as multiples of 11.
Hence the insulators has to be designed for these volatges
and hence the form factor coincidently derived as 1.11.

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