Disclaimer: This post contains information written by AI.
There are three towers on Path 15, the connecting route between northern and southern California. Each tower carries six conductors, totaling 18 conductors for the three phases. The left two towers accommodate 345 kV three-phase alternating current, while the rightmost tower carries 500 kV three-phase alternating current. The use of six conductors per tower raises the question of why this configuration is chosen. Let's delve into the explanation.
Path 15 has a south-to-north capacity of 5400 MW, which translates to slightly over 860 A per conductor. Two factors calculate the current-carrying capacity of a conductor:
- Alternating current skin effect: When the current oscillates within the wires, it concentrates on the outer layers, known as the skin, rather than flowing uniformly throughout the core. In the case of aluminum conductors at a frequency of 60 Hz, the skin depth is approximately 10.6 mm. This means the current is strongest at the surface and gradually diminishes towards the center. Since most conductors follow the ACSR construction (Aluminum Conductor Steel Reinforced), with aluminum outer wires surrounding a galvanized steel core, the reduced current at the center is not a significant issue. The skin effect restricts the diameter of AC conductors to approximately 30 mm. Southwire offers a useful catalog detailing various bare overhead conductor cables they produce, ranging from 499 to 1768 rated amps. Notably, the resistance of the wire to 60 Hz AC current is higher than DC current due to the skin effect. Furthermore, this ratio increases with larger conductors, and the AC ampacity-to-weight ratio worsens as the conductor size increases. Since cable cost is typically determined by weight, utilizing multiple smaller cables delivers higher power per dollar more economically than employing a single large cable.
- Sag caused by resistive heating: The same catalog mentioned earlier provides additional information about ampacity, specifying the conditions under which the cables can reach a temperature of 75 °C while transmitting their rated load. As the conductors heat up, they expand both in length and diameter, resulting in lower sag. The towers are designed to maintain a sufficient height above the ground, even under worst-case conditions.
By adding an insulator to a wire, not only does it serve as an electrical insulator, but it also acts as a thermal insulator. Consequently, the conductor tends to be hotter than the surface. While larger-diameter insulators absorb more sunlight, their larger surface area facilitates enhanced heat dissipation. This counteracts the thermal insulation effect up to a few inches of insulation.
For 12 kV lines, only 6 mm of polyethylene insulation is necessary. The conventional bare wires found in neighborhood distribution systems can be replaced with readily available insulated wires, commonly called "tree wires." These insulated wires are approximately three times heavier, 2.6 times larger in diameter, and potentially twice as expensive. Insulated lines are already utilized in congested areas or near trees that cannot be regularly trimmed.
In contrast, insulated 400 kV lines require 27 to 30 mm of high-purity plastic insulation, making them significantly larger. While I could not find off-the-shelf insulated 400 kV overhead lines, if they were to be constructed, they might resemble the insulated 400 kV ground cables currently available, possibly incorporating a carbon fiber core to handle tension. These cables are extremely massive; for instance, the equivalent of the 1-inch diameter conductors on Path 15 would be a cable 5 inches in diameter and ten times heavier. Alternatively, cables capable of carrying twice