Possible myth, current/voltage in relation to wire size

[Brian Dugas]

I've always heard it said that the super high voltage transmission lines
don't need real thick wire because the voltage is so high, there's not a lot
of current. Yet, the formula E=IxR tells us that as voltage goes up, so
does current.
Now I know transmission wire is thick, but it doesn't seem to be
proportionate to what it should be for so many volts. Any good reasons for
this?
Thanks.
Brian

 

[Charles Perry]

I've always heard it said that the super high voltage transmission lines
don't need real thick wire because the voltage is so high, there's not a
lot of current. Yet, the formula E=IxR tells us that as voltage goes up,
so does current.
Now I know transmission wire is thick, but it doesn't seem to be
proportionate to what it should be for so many volts. Any good reasons
for this?
Thanks.
Brian

You are using the wrong formula. Try P=ExI (power equals voltage times
current). For a given amount of power that you want to transmit, the higher
the voltage, the lower the current. The calculation of how much power a
transmission line can transmit is a little more complicated, but that
formula holds true enough to answer your question.

Charles Perry P.E.

 

[Brian Dugas]

Thanks Charles for clarifying my formula usage.
I can accept that formula, but my brain wants to know why. I guess what I'm
looking for is an analogy or something to help me visualize. A lot of my
learning recently has been through analogies, because I can't see EM fields
yet and I tend to be a visual learner. I also don't like the old fashioned
response of "because I said so." Just because a formula says it, I still
want to know why.
Thanks.

 

[operator jay]

I've always heard it said that the super high voltage transmission lines
don't need real thick wire because the voltage is so high, there's not a lot
of current. Yet, the formula E=IxR tells us that as voltage goes up, so
does current.
Now I know transmission wire is thick, but it doesn't seem to be
proportionate to what it should be for so many volts. Any good reasons for
this?
Thanks.
Brian

A better relation to consider is

P = I * V

so, for a given amount of power that is required to be sent down a
transmission line, higher voltage allows a lower current. Some transmission
lines are thick, some are thin. Skin effect is taken into account in large
transmission lines. Often lines are ACSR (Aluminum Conductor Steel
Reinforced) and the center is steel, which is relatively high resistance
compared to aluminum, but not much current wants to travel in the center
anyways, due to skin effect.

j

 

[Petri Vatilo]

When was studying we were given analogy of water pipes. Voltage is same as
the height of a water reservoir would be. Current obvously is the amount of
water going through the pipes. Hence if you raise the level of water
reservoir the energy of the flowing water to i.e. turbine will be increased.

I hope this analogy is what you sere looking for...

Reagrds,
Petri Vatilo

 

[Brian Dugas]

When was studying we were given analogy of water pipes. Voltage is same
as
the height of a water reservoir would be. Current obvously is the amount
of
water going through the pipes. Hence if you raise the level of water
reservoir the energy of the flowing water to i.e. turbine will be
increased.

I hope this analogy is what you sere looking for...

Reagrds,
Petri Vatilo

I like where you're going with this analogy. However, I'm stumped on
something.
Let's say we have two water towers like this:
http://www.converttapes.com/other/example.bmp
Now, the height of the tower would represent voltage (potential). That
makes sense. The size of the pipe going into the ground would be the
resistance, I drew them equal diameter so we could have a constant. Now,
for example, let's say we put a paddle wheel at the bottom of these two
towers. The wheel on the left would have more work done on it because the
pressure would be higher. However, that tank would drain it's water faster
(highter current) because of the pressure being higher. So in this case,
higher pressure leads to higher flow. So this analogy doesn't quite hold up
with electric current flow in relation to potential. That is, unless I have
something messed up in my head about the two water towers.

 

[Brian Dugas]

I think I answered my own problem in my head just now.
Given the two water towers, with paddle wheels at the bottom, we should keep
in mind the formula P=IxE which means we keep the paddle wheels spinning at
the same rate. To do this, we need to slow the flow of water down the
higher tower, because it is hitting the wheel with more
force/pressure/potential (whatever you wish to call it). So as the pressure
goes up, the current needs to slow down to keep the power constant. That
sound about rigth?

 

[Bill Shymanski]

I was told by a lineman why high power lines were so thick, and he
said:

Electrical properties: minimize resistance( so mimimize voltage drop
and minimize heat build up)

Plus

Physical Properties: Strength ( so to span further distances, so to
have a longer working life, so to stand up to high winds and rough
weather).

Don't forget that at extra high voltages an important factor is
reduction of corona discharge. Since the conductor size required for
the current is usually too small to reduce the voltage gradient around
the conductor to a value low enough to provide acceptable corona loss,
EHV (365 kV and up) lines use bundled conductors to provide a larger
effective conductor diameter.

According to one reference I have, the annual average corona loss on a
500 kV AC transmission line will be 5.6 kW per km of line, though this
will vary substantially depending on weather conditions.

Bill

 

[Bill Shymanski]

A previous poster got this mixed up with "skin effect", which only
occurs at much higher frequencies. Many HV lines are made hollow to
increase the diameter without increasing the weight.

Skin effect occurs at all frequencies - but at 60 Hz the skin depth
(where the current density drops to 1/e of the value at the surface) is
about 8 mm, so only large conductors are much affected. Bus tubes for
arc furnaces, which have to carry 50,000 amperes, are hollow because of
the skin effect - a solid 8 inch (200 mm) conductor wouldn't effectively
carry much more 60 Hz current than a hollow conductor with a wall
thickness of 16 mm.

The skin effect is also important even in distribution-size conductors
for accurate calculation of the impedance of a circuit - such as for
estimating fault levels, etc.

I wonder when the economics will mandate DC lines. Maybe even buried
super-conductive! It's only a matter of time.

DC wins when the cost of the terminal equipment is offset by the lower
cost of the conductors and transmission right-of-way - this can make a
very short DC link economically preferable in situations such as urban
construction, or for buried cables or underwater cables. Drawbacks to
DC are the relative difficulty and expense of controlling multi-terminal
networks and lack of a good HVDC circuit breaker.

Superconducting transmission cables are still laboratory experiments,
from what I've read - maybe someday, but I don't think superconducting
cables will be a big factor in energy transmission for many years.

Bill

 

[[email protected]]

I think I answered my own problem in my head just now.
Given the two water towers, with paddle wheels at the bottom, we should keep
in mind the formula P=IxE which means we keep the paddle wheels spinning at
the same rate. To do this, we need to slow the flow of water down the
higher tower, because it is hitting the wheel with more
force/pressure/potential (whatever you wish to call it). So as the pressure
goes up, the current needs to slow down to keep the power constant. That
sound about rigth?

Nope. Your analogy is missing a component.
When pressure goes up, current ALWAYS goes up,
ALL OTHER CONDITIONS BEING EQUAL.

With power distribution at high voltage, we
CHANGE THE CONDITIONS with a transformer.

Your water example does not have a device analogous
to the transformer. If you add some gears to your fast
moving paddle wheel, then you'll have a good analogy.
The gears will transform the high speed, low torque
rotation of the paddle wheel to a low speed high torque
rotation on the output shaft driven by the gears, just
as the electrical transformer transforms the high voltage
low current to low voltage high current.

Ed

 

[Brian Dugas]

Nope. Your analogy is missing a component.

When pressure goes up, current ALWAYS goes up,
ALL OTHER CONDITIONS BEING EQUAL.

With power distribution at high voltage, we
CHANGE THE CONDITIONS with a transformer.

Your water example does not have a device analogous
to the transformer. If you add some gears to your fast
moving paddle wheel, then you'll have a good analogy.
The gears will transform the high speed, low torque
rotation of the paddle wheel to a low speed high torque
rotation on the output shaft driven by the gears, just
as the electrical transformer transforms the high voltage
low current to low voltage high current.

Ed

And now you have my gears turning.
I'll see if I can find some websites with visuals of this. If you know of
any, please post. Thanks.