Why all voltage levels are the multiples of 11KV?
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Answer Posted / 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.
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