Overhead high voltage power lines are not just a simple copper wire but it is actually a combination of aluminium and steel i.eACSR (aluminium conductor steel reinforced) conductors which has steel rods and so the weight of the conductor is already high so that would add up to the weight of the line resulting into intense pressure on towers.
You see a section of Path 15 which connects northern and southern California. Note that there are three towers, each carrying six conductors, so there are 18 conductors to carry three phases. The left two towers with 12 conductors are carrying 345 kV three-phase alternating current, and the rightmost tower with six conductors is carrying 500 kV direct current. But still — why six conductors per tower? This is a clue to the original question.
Path 15 here has a south-to-north capacity of 5400 MW, which is slightly more than 860 A per conductor. The current a conductor can carry is limited by two things:
If you add an insulator to a wire, the electrical insulator will also act as a thermal insulator, so that the conductor will be hotter than the surface. The larger diameter insulator will absorb more sunlight as well. However, the larger surface area will convect away more heat, and it turns out this effect almost perfectly cancels the thermal insulation effect up to a few inches of insulation.
12 kV requires only 6 mm of polyethylene insulation. Your 12 kV neighborhood distribution wires (the two or three bare wires at the top of the poles) can be replaced with off-the-shelf insulated wires, typically called “tree wire”. These are about 3 times as heavy, 2.6 times the diameter, and probably twice as expensive. These insulated lines are already used in congested areas like alleyways or near trees that cannot be trimmed as usual.
An insulated 400 kV line requires 27 to 30 mm of high-purity plastic insulation, which makes it enormous. I wasn’t able to find off-the-shelf insulated 400 kV overhead lines. If built, they might look a bit like the insulated 400 kV ground cables that are sold today (shown above), but with an added carbon fiber core to take the tension. These cables are utterly massive: the equivalent of the 1 inch diameter conductors on Path 15 is a cable 5 inches in diameter and 10 times as heavy. A better choice would be cables capable of twice the current, which are 6 inches in diameter and 7 times as heavy as the pair of conductors they replace.
Aesthetically they’d look quite different: the towers would be more like wind turbine towers, and the three phases of the conductors would not need to be spaced apart, and so would probably be bundled into a fat pipe around a foot in diameter. The fat wires would have significantly more drag in crosswinds, adding to the already larger loads on the towers. At this point I’d wonder if there’s any point in having it aerial at all… why not bury it?
Those insulated 400 kV ground cables top out at 1770 A rated current , limited by heat generation. That big cable has a conductor 63 mm in diameter, in an effort to reduce the heat buildup by reducing conductor losses. This would lead to a skin effect problem, but the conductors on these big cables are divided into five segments with a little insulation between them, so that the current can’t be forced out of the center of the conductor.
Bottom line: Why are overhead high voltage power lines not insulated? Because they’d be way too heavy if they were.
Why do we use Aluminum to substitute Copper in transmission lines?
Another reason is that the transmission line is carrying extreme high voltage and it would require a whole lot of thick insulation to absorb the heat produced by such high voltage.You see a section of Path 15 which connects northern and southern California. Note that there are three towers, each carrying six conductors, so there are 18 conductors to carry three phases. The left two towers with 12 conductors are carrying 345 kV three-phase alternating current, and the rightmost tower with six conductors is carrying 500 kV direct current. But still — why six conductors per tower? This is a clue to the original question.
Path 15 here has a south-to-north capacity of 5400 MW, which is slightly more than 860 A per conductor. The current a conductor can carry is limited by two things:
- The alternating current skin effect. Basically the oscillating current in the wires pushes the current out of the core of the wire and into the skin. Skin depth in aluminum at 60 Hz is 10.6 mm, which is the depth at which the current is 1/e=37% of the current at the surface. Most conductors are ACSR construction: Aluminum outer wires around a galvanized steel core, so the lack of current in the center isn’t immediately terrible. The skin effect limits AC conductors to approximately 30 mm in diameter. Southwire has a handy catalog here which lists the various bare overhead conductor cables they make, ranging from 499 to 1768 rated amps. Note that on the right hand side of the chart, they list the resistance of the wire to 60 Hz AC and DC currents. The resistance to AC is greater than to DC, due to the skin effect, and the ratio gets larger with increasing size, and the ratio of AC ampacity to weight gets worse with larger size. Since you pay for cable by weight, you can deliver more power per dollar by using multiple smaller cables instead of one large cable.
- Sag due to resistive heating. That same catalog has a little note at the bottom of the page which reads “ampacity based on 25 °C ambient temperature, 2 ft/sec perpendicular wind, in sun, emissivity of 0.5, solar absorption of 0.5, at sea level”, under which conditions the cables will reach 75 °C while transmitting their rated load. As the conductors heat up, they expand (in length as well as diameter), and therefore droop lower. The towers are designed to hold the cables sufficiently high off the ground in worst-case conditions.
If you add an insulator to a wire, the electrical insulator will also act as a thermal insulator, so that the conductor will be hotter than the surface. The larger diameter insulator will absorb more sunlight as well. However, the larger surface area will convect away more heat, and it turns out this effect almost perfectly cancels the thermal insulation effect up to a few inches of insulation.
12 kV requires only 6 mm of polyethylene insulation. Your 12 kV neighborhood distribution wires (the two or three bare wires at the top of the poles) can be replaced with off-the-shelf insulated wires, typically called “tree wire”. These are about 3 times as heavy, 2.6 times the diameter, and probably twice as expensive. These insulated lines are already used in congested areas like alleyways or near trees that cannot be trimmed as usual.
An insulated 400 kV line requires 27 to 30 mm of high-purity plastic insulation, which makes it enormous. I wasn’t able to find off-the-shelf insulated 400 kV overhead lines. If built, they might look a bit like the insulated 400 kV ground cables that are sold today (shown above), but with an added carbon fiber core to take the tension. These cables are utterly massive: the equivalent of the 1 inch diameter conductors on Path 15 is a cable 5 inches in diameter and 10 times as heavy. A better choice would be cables capable of twice the current, which are 6 inches in diameter and 7 times as heavy as the pair of conductors they replace.
Aesthetically they’d look quite different: the towers would be more like wind turbine towers, and the three phases of the conductors would not need to be spaced apart, and so would probably be bundled into a fat pipe around a foot in diameter. The fat wires would have significantly more drag in crosswinds, adding to the already larger loads on the towers. At this point I’d wonder if there’s any point in having it aerial at all… why not bury it?
Those insulated 400 kV ground cables top out at 1770 A rated current , limited by heat generation. That big cable has a conductor 63 mm in diameter, in an effort to reduce the heat buildup by reducing conductor losses. This would lead to a skin effect problem, but the conductors on these big cables are divided into five segments with a little insulation between them, so that the current can’t be forced out of the center of the conductor.
Bottom line: Why are overhead high voltage power lines not insulated? Because they’d be way too heavy if they were.
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