Reconductoring: Building Tomorrow’s Grid Today

What happens when you build the largest machine in the world, but it’s still not big enough? That’s the situation the North American transmission system, the grid that connects power plants to substations and the distribution system, and which by some measures is the largest machine ever constructed, finds itself in right now. After more than a century of build-out, the towers and wires that stitch together a continent-sized grid aren’t up to the task they were designed for, and that’s a huge problem for a society with a seemingly insatiable need for more electricity.

There are plenty of reasons for this burgeoning demand, including the rapid growth of data centers to support AI and other cloud services and the move to wind and solar energy as the push to decarbonize the grid proceeds. The former introduces massive new loads to the grid with millions of hungry little GPUs, while the latter increases the supply side, as wind and solar plants are often located out of reach of existing transmission lines. Add in the anticipated expansion of the manufacturing base as industry seeks to re-home factories, and the scale of the potential problem only grows.

The bottom line to all this is that the grid needs to grow to support all this growth, and while there is often no other solution than building new transmission lines, that’s not always feasible. Even when it is, the process can take decades. What’s needed is a quick win, a way to increase the capacity of the existing infrastructure without having to build new lines from the ground up. That’s exactly what reconductoring promises, and the way it gets there presents some interesting engineering challenges and opportunities.

Bare Metal

Copper is probably the first material that comes to mind when thinking about electrical conductors. Copper is the best conductor of electricity after silver, it’s commonly available and relatively easy to extract, and it has all the physical characteristics, such as ductility and tensile strength, that make it easy to form into wire. Copper has become the go-to material for wiring residential and commercial structures, and even in industrial installations, copper wiring is a mainstay.

However, despite its advantages behind the meter, copper is rarely, if ever, used for overhead wiring in transmission and distribution systems. Instead, aluminum is favored for these systems, mainly due to its lower cost compared to the equivalent copper conductor. There’s also the factor of weight; copper is much denser than aluminum, so a transmission system built on copper wires would have to use much sturdier towers and poles to loft the wires. Copper is also much more subject to corrosion than aluminum, an important consideration for wires that will be exposed to the elements for decades.

Aluminum has its downsides, of course. Pure aluminum is only about 61% as conductive as copper, meaning that conductors need to have a larger circular area to carry the same amount of current as a copper cable. Aluminum also has only about half the tensile strength of copper, which would seem to be a problem for wires strung between poles or towers under a lot of tension. However, the greater diameter of aluminum conductors tends to make up for that lack of strength, as does the fact that most aluminum conductors in the transmission system are of composite construction.

The vast majority of the wires in the North American transmission system are composites of aluminum and steel known as ACSR, or aluminum conductor steel-reinforced. ACSR is made by wrapping high-purity aluminum wires around a core of galvanized steel wires. The core can be a single steel wire, but more commonly it’s made from seven strands, six wrapped around a single central wire; especially large ACSR might have a 19-wire core. The core wires are classified by their tensile strength and the thickness of their zinc coating, which determines how corrosion-resistant the core will be.

In standard ACSR, both the steel core and the aluminum outer strands are round in cross-section. Each layer of the cable is twisted in the opposite direction from the previous layer. Alternating the twist of each layer ensures that the finished cable doesn’t have a tendency to coil and kink during installation. In North America, all ACSR is constructed so that the outside layer has a right-hand lay.

ACSR is manufactured by machines called spinning or stranding machines, which have large cylindrical bodies that can carry up to 36 spools of aluminum wire. The wires are fed from the spools into circular spinning plates that collate the wires and spin them around the steel core fed through the center of the machine. The output of one spinning frame can be spooled up as finished ACSR or, if more layers are needed, can pass directly into another spinning frame for another layer of aluminum, in the opposite direction, of course.

Fiber to the Core

While ACSR is the backbone of the grid, it’s not the only show in town. There’s an entire beastiary of initialisms based on the materials and methods used to build composite cables. ACSS, or aluminum conductor steel-supported, is similar to ACSR but uses more steel in the core and is completely supported by the steel, as opposed to ACSR where the load is split between the steel and the aluminum. AAAC, or all-aluminum alloy conductor, has no steel in it at all, instead relying on high-strength aluminum alloys for the necessary tensile strength. AAAC has the advantage of being very lightweight as well as being much more resistant to core corrosion than ACSR.

Another approach to reducing core corrosion for aluminum-clad conductors is to switch to composite cores. These are known by various trade names, such as ACCC (aluminum conductor composite core) or ACCR (aluminum conductor composite reinforced). In general, these cables are known as HTLS, which stands for high-temperature, low-sag. They deliver on these twin promises by replacing the traditional steel core with a composite material such as carbon fiber, or in the case of ACCR, a fiber-reinforced metal matrix.

The point of composite cores is to provide the conductor with the necessary tensile strength and lower thermal expansion coefficient, so that heating due to loading and environmental conditions causes the cable to sag less. Controlling sag is critical to cable capacity; the less likely a cable is to sag when heated, the more load it can carry. Additionally, composite cores can have a smaller cross-sectional area than a steel core with the same tensile strength, leaving room for more aluminum in the outer layers while maintaining the same overall conductor diameter. And of course, more aluminum means these advanced conductors can carry more current.

Another way to increase the capacity in advanced conductors is by switching to trapezoidal wires. Traditional ACSR with round wires in the core and conductor layers has a significant amount of dielectric space trapped within the conductor, which contributes nothing to the cable’s current-carrying capacity. Filling those internal voids with aluminum is accomplished by wrapping round composite cores with aluminum wires that have a trapezoidal cross-section to pack tightly against each other. This greatly reduces the dielectric space trapped within a conductor, increasing its ampacity within the same overall diameter.

Unfortunately, trapezoidal aluminum conductors are much harder to manufacture than traditional round wires. While creating the trapezoids isn’t that much harder than drawing round aluminum wire — it really just requires switching to a different die — dealing with non-round wire is more of a challenge. Care must be taken not to twist the wire while it’s being rolled onto its spools, as well as when wrapping the wire onto the core. Also, the different layers of aluminum in the cable require different trapezoidal shapes, lest dielectric voids be introduced. The twist of the different layers of aluminum has to be controlled, too, just as with round wires. Trapezoidal wires can also complicate things for linemen in the field in terms of splicing and terminating cables, although most utilities and cable construction companies have invested in specialized tooling for advanced conductors.

Same Towers, Better Wires

The grid is what it is today in large part because of decisions made a hundred or more years ago, many of which had little to do with engineering. Power plants were located where it made sense to build them relative to the cities and towns they would serve and the availability of the fuel that would power them, while the transmission lines that move bulk power were built where it was possible to obtain rights-of-way. These decisions shaped the physical footprint of the grid, and except in cases where enough forethought was employed to secure rights-of-way generous enough to allow for expansion of the physical plant, that footprint is pretty much what engineers have to work with today.

Increasing the amount of power that can be moved within that limited footprint is what reconductoring is all about. Generally, reconductoring is pretty much what it sounds like: replacing the conductors on existing support structures with advanced conductors. There are certainly cases where reconductoring alone won’t do, such as when new solar or wind plants are built without existing transmission lines to connect them to the system. In those cases, little can be done except to build a new transmission line. And even where reconductoring can be done, it’s not cheap; it can cost 20% more per mile than building new towers on new rights-of-way. But reconductoring is much, much faster than building new lines. A typical reconductoring project can be completed in 18 to 36 months, as compared to the 5 to 15 years needed to build a new line, thanks to all the regulatory and legal challenges involved in obtaining the property to build the structures on. Reconductoring usually faces fewer of these challenges, since rights-of-way on existing lines were established long ago.

The exact methods of reconductoring depend on the specifics of the transmission line, but in general, reconductoring starts with a thorough engineering evaluation of the support structures. Since most advanced conductors are the same weight per unit length as the ACSR they’ll be replacing, loads on the towers should be about the same. But it’s prudent to make sure, and a field inspection of the towers on the line is needed to make sure they’re up to snuff. A careful analysis of the design capacity of the new line is also performed before the project goes through the permitting process. Reconductoring is generally performed on de-energized lines, which means loads have to be temporarily shifted to other lines, requiring careful coordination between utilities and transmission operators.

Once the preliminaries are in place, work begins. Despite how it may appear, most transmission lines are not one long cable per phase that spans dozens of towers across the countryside. Rather, most lines span just a few towers before dead-ending into insulators that use jumpers to carry current across to the next span of cable. This makes reconductoring largely a tower-by-tower affair, which somewhat simplifies the process, especially in terms of maintaining the tension on the towers while the conductors are swapped. Portable tensioning machines are used for that job, as well as for setting the proper tension in the new cable, which determines the sag for that span.

The tooling and methods used to connect advanced conductors to fixtures like midline splices or dead-end adapters are similar to those used for traditional ACSR construction, with allowances made for the switch to composite cores from steel. Hydraulic crimping tools do most of the work of forming a solid mechanical connection between the fixture and the core, and then to the outer aluminum conductors. A collet is also inserted over the core before it’s crimped, to provide additional mechanical strength against pullout.

Is all this extra work to manufacture and deploy advanced conductors worth it? In most cases, the answer is a resounding “Yes.” Advanced conductors can often carry twice the current as traditional ACSR or ACCC conductors of the same diameter. To take things even further, advanced AECC, or aluminum-encapsulated carbon core conductors, which use pretensioned carbon fiber cores covered by trapezoidal annealed aluminum conductors, can often triple the ampacity of equivalent-diameter ACSR.

Doubling or trebling the capacity of a line without the need to obtain new rights-of-way or build new structures is a huge win, even when the additional expense is factored in. And given that an estimated 98% of the existing transmission lines in North America are candidates for reconductoring, you can expect to see a lot of activity under your local power lines in the years to come.