5 ms·

> Thicker wires will heat up less for a given amount of current, increasing the carrying capacity of the circuit. One solution to increasing the effective thick

by femto 7y ago

> Thicker wires will heat up less for a given amount of current, increasing the carrying capacity of the circuit. One solution to increasing the effective thickness of a conductor is to “bundle” several conductors a few inches apart from one another, allowing for a larger increase in current for less cost than a conductor that is simply double the size. In power engineering at university we were taught that the separation of the wires within a conductor, to increase the effective diameter, is primarily about reducing the self-inductance rather than dissipating heat. From memory, it's along the lines that the separation effectively increases the diameter of the conductor, which in turn reduces the intensity of the magnetic field, reduces the self-inductance and allows more power transfer for a given voltage and current. The article got it right in saying spacing the wires gives an increase in current for a given cross-section of conductor, but the main reason is reduced inductance rather than the reduction in resistance due to it running cooler. There will be some reduction in resistive losses due to cooler conductors, but the main gain is the reduction in reactive power, which otherwise causes current flow with no power transfer.

brandmeyer 7y ago
This is technically true, but a smallish effect at the scales its employed. The biggest reason for stranding transmission conductors is that its easier to bend them. Long spools of wire are much easier to transport and assemble than bars.
new4thaccount 7y ago
I was always taught that one of the main reason to use multiple conductors was due to reduce the losses caused by the corona effect of ionizing the air. As another commenter says, bending might also be a factor.
brandmeyer 7y ago
(I'd edit my post, but its long since too late for that... mea culpa) There's a couple of things being mixed up here. Stranded wire versus bar is a mechanical issue. But what you were talking about is the separators you sometimes see to run several separate cables alongside each other in the same phase. In this case, there are a couple of things being overcome. For starters, the skin depth of Al and Cu is only about 10mm at 50-60 Hz. However, the form of the limit is not the reactive power loss, its the resistive power loss of the conductor. Eventually, making it thicker just stops decreasing its resistance at that frequency. The article hinted at this in the discussion on cable construction, when the author was describing wraps of aluminum conductor around a steel core. Now, imagine an A/B comparison, where the A side is a very large diameter cylinder of Al strands around a large steel core. The B side is three smaller such cylinders separated by air, with the same total circumference of Al. The B side is much lighter overall for the same current-carrying capacity. You might like to hollow out the A side, except that you need to endure the compressive stress of the outer sheath of Al conductors bearing down on the structural core somehow. Enter the periodically spaced triangular separators that actually are present on the B side.

by femto 7y ago

brandmeyer 7y ago

This is technically true, but a smallish effect at the scales its employed. The biggest reason for stranding transmission conductors is that its easier to bend them. Long spools of wire are much easier to transport and assemble than bars.

new4thaccount 7y ago

I was always taught that one of the main reason to use multiple conductors was due to reduce the losses caused by the corona effect of ionizing the air. As another commenter says, bending might also be a factor.

brandmeyer 7y ago

(I'd edit my post, but its long since too late for that... mea culpa) There's a couple of things being mixed up here. Stranded wire versus bar is a mechanical issue. But what you were talking about is the separators you sometimes see to run several separate cables alongside each other in the same phase. In this case, there are a couple of things being overcome. For starters, the skin depth of Al and Cu is only about 10mm at 50-60 Hz. However, the form of the limit is not the reactive power loss, its the resistive power loss of the conductor. Eventually, making it thicker just stops decreasing its resistance at that frequency. The article hinted at this in the discussion on cable construction, when the author was describing wraps of aluminum conductor around a steel core. Now, imagine an A/B comparison, where the A side is a very large diameter cylinder of Al strands around a large steel core. The B side is three smaller such cylinders separated by air, with the same total circumference of Al. The B side is much lighter overall for the same current-carrying capacity. You might like to hollow out the A side, except that you need to endure the compressive stress of the outer sheath of Al conductors bearing down on the structural core somehow. Enter the periodically spaced triangular separators that actually are present on the B side.