Building new transmission takes years, and the interconnection queue for solar keeps growing, so the pressing question for most utilities is, how much more the lines they already own can carry, safely, in actual weather, in real time.
Tiffany Menhorn, Prisma’s VP of Utilities for North America, recently joined the Solar Sean podcast to argue that nobody is really measuring the grid accurately enough, and Prisma Photonics wants to change it with dynamic line rating.
What FERC Order 881 and Ambient Adjusted Ratings Miss
FERC Order 881 moved utilities off static line ratings and onto ambient adjusted ratings, which now take the changing air temperatures into consideration. While this IS progress, an ambient adjusted rating still carries a static wind assumption across the seasons, and weather patterns keep shifting. A fixed seasonal wind figure cannot describe the wind that is actually impacting the line today.
As Tiffany describes it, an ambient-adjusted rating is a stepping stone, but to get the most accurate rating, you need to know how much the wind is cooling the line right now. The received rating matters most as a forecast, where operators can plan hours ahead. A rating they can forecast lets them plan on real capacity instead of a conservative fixed value, which is a cautious guess.
Effective Wind: The Biggest Variable in a Dynamic Line Rating
The IEEE 738 equation takes in several factors that decide how much power a line can carry:
- Distance to ground
- Solar radiance
- Ambient temperature
- Effective wind
Of these, the effective wind is the one that impacts the rating most, and the physics works in the operator’s favor. A conductor sheds heat as air moves across it, so a windy line can carry more current without passing its thermal limit. The wind does not even need to be strong, around 3 feet per second already gives the line a big boost of cooling.
Solar radiance and ambient temperature still matter, and operators gather those from the field or third parties. But effective wind is the variable worth measuring most carefully, because it is where the extra capacity hides.
Distributed Acoustic Sensing on Dark Fiber, Span by Span
Prisma Photonics does not add sensors to the line; it uses acoustic fiber that the utility already owns. A fiber bundle usually holds around a hundred strands, and most utilities keep some as dark fiber; these are unused strands not rented out or assigned to relays. Prisma takes one dark strand and sends light down it, a method called distributed acoustic sensing and distributed temperature sensing. That signal tells Prisma the wind and temperature at every point along the line.
The fiber never touches the electricity. It works as a monitoring asset, reading the tiny signals along its length and translating them, through deep-learning models, into effective wind and temperature at every span. Tiffany compares it to the nervous system in a human body – because the fiber runs the whole length of the line, the coverage is continuous, with every span monitored rather than a few sampled points.
The only hardware required is a server-sized box that Prisma places inside the substation and plugs into the dark fiber. Today, we’re already monitoring thousands of miles of line this way.
The Critical Span and a De-Risked Dynamic Line Rating
A transmission line is only as strong as its hottest, least-cooled span. Where the wind cools least is the weakest point and it sets the limit for the whole line. This critical span moves as the wind shifts through the day, shifting the weak point from one place to another. The problem is, a sensor fixed to one tower reads its own location and misses the span that has become the constraint somewhere else, while a software-only model can only estimate the gaps. Continuously reading every stretch allows you to always find the real critical span. And because we always rate the line by its worst stretch, operators can trust the numbers. Tiffany calls it a safer rating: you are never caught out by a hot spot you could not see.
Beyond Dynamic Line Rating: Wildfire, Lightning, and Real-Time Grid Alerts
A straightforward principle: one measurement asset, many purposes. The same fiber that reads cooling wind for capacity also reads acute wind, the fast gusts that move a fire toward a line. It can tell the difference between a lightning strike and a wildfire, and it flags these events in near real time, faster than a crew in the field would spot them.
That speed helps utilities act, for example when:
- A utility has to decide whether to shut off power for safety, knowing the wind and heat along the line is exactly what it needs; or
- During a sudden cold or hot snap, a fast alert gives operators time to react before people lose power.
Over time, the fiber builds a record of everything that has happened on the line, stretch by stretch, something most utilities never had. The usual protection systems still do their job. Prisma works alongside them, catching everything else almost as it happens.
How Much Transmission Capacity Can Dynamic Line Rating Add?
The honest answer depends on the wind and the conductor, but overall we’re seeing up to 40 percent more ampacity when wind is cooling the line. All of it comes from wires already in the ground, with better information going to the control room rather than new lines in the field.
While some might be promising even up to 60 percent capacity, conductor engineering sets limits and utilities will not load a line that hard even when the weather would allow it.
Substation Security: On-Prem Data, No Field Sensors
Grid data invites the obvious concern about hacking when you add data to the existing grid, but Prisma’s setup actually lowers that risk rather than adding to it. The box sits inside the substation, which is already a locked, secure building. Most of the work happens right there on site, with only weather data coming from the cloud. And there are no small sensors spread out in the field for anyone to reach. One box in a secure room is far easier to protect than a chain of devices along the line.
See What Dynamic Line Rating Can Do on Your Lines
Prisma already monitors lines for a set of public utility customers, including Pacific Gas and Electric, Great River Energy, The New York Power Authority and Southern Company.
The work runs in parallel with the solar, wind, and storage coming onto the grid: the generators feed the highways, and Prisma helps the utilities that own those highways carry more down them. If your team is weighing how to meet demand without a decade of new construction, a dynamic line rating on the dark fiber you already own is worth a serious look. Contact Prisma Photonics directly, to talk through your most constrained lines.
