Transmission month: how to make the existing grid work better

(If you’d rather listen to this post than read it, just click play above.)

Welcome back to Volts, where every week is Transmission Week!

In my three transmission posts so far, I have focused mostly on the challenges of building new long-distance energy transmission lines in the US — the poor planning, the inefficient financing, the permitting and siting hassles.

Today I’m going to turn to a different subject: the various ways that the performance of the existing transmission system could be upgraded and improved through so-called “grid-enhancing technologies” (GETs).

To be honest, I probably should have tackled this subject first. Though new lines are going to be needed regardless, it is faster and cheaper to upgrade the existing system, with fewer regulatory barriers. GETs can achieve short-term relief from grid congestion while new lines are being developed.

There are three techs that are typically classified as grid-enhancing technologies, and I will focus on them in this post. In my next post, I’ll cover a couple of extra options that I haven’t found any other way to fit in.

Let’s jump in.

(I should note here up top that I will be drawing heavily from a 2019 report on GETs from the Brattle Group and Grid Strategies.)

Closer monitoring to improve line performance

When electricity passes through transmission lines, they heat up. As they heat up, they sag. If too much electricity is run through a line, it can exceed its maximum operating temperature or sag to the point that it brushes up against trees or other structures, potentially sparking fires.

Grid operators want to avoid that, so they do not load lines to their full rated capacity. They set an operational limit well below theoretical capacity, to create a safety margin.

But how far below capacity should the limit be set? That is the question.

The heat and sag of a given line are changing in subtle ways all the time. They vary with the ambient temperature, humidity, barometric pressure, and wind speed. If it’s warmer, the line will heat up faster; if there’s a breeze, it will heat more slowly. Because the heat and sag are in constant flux, so too is the maximum safe capacity of the line.

“The number we love to quote is, an increase in wind blowing across a power line of three feet per second results in a 44 percent increase in the capacity of that power line,” says Jonathan Marmillo, co-founder of LineVision, a company that makes equipment for monitoring lines. “That's the equivalent of a light breeze.”

(Note: this means that the capacity of transmission lines increases as the production of wind energy increases. Handy!)

But transmission system operators do not generally have that kind of real-time information about the heat and sag of their lines. They are forced to estimate, to use an average. In some cases, they assign a line a single “static rating,” well below full capacity. In some cases, they assign the lines seasonal ratings, adjusting for seasonal conditions. These estimates are, necessarily, conservative.

As a result, “most transmission lines are loaded at 40 or even 30 percent of their rated capacity,” says Marmillo. That’s an enormous amount of usable capacity going unused, to hedge against the lack of information.

That has changed with the development of “dynamic line ratings” (DLRs), whereby lines are continuously monitored and their capacity continuously updated.

DLRs have been around for a couple of decades, but the first generations of devices were cumbersome. They were installed directly on the power lines (which involved taking the lines out of commission) and proved unreliable in operation.

Technology marches on, though, and the latest generation of DLRs is vastly improved. LineVision’s DLR devices, for instance, have “no-contact” installation, which means no messing with the lines; they attach to the transmission tower. They are topped with LIDAR — the same technology used by autonomous vehicles — which gathers fine-grained data that is then crunched to determine the “net effective perpendicular windspeed,” the most important variable for determining line temperature. “We essentially use the conductor as a giant hot wire anemometer,” says Marmillo.

Of course, if you abandon averages in favor of real-time measurement, sometimes capacity will be below what the static average would have indicated. But “we see capacity above static [ratings] about 97 percent of the time,” says Marmillo. It turns out those static ratings are extremely conservative.

Allowing more power to travel through lines relieves grid congestion, which is valuable to grid operators. Marmillo says a recent installation of LineVision’s device on a PJM line paid itself back in three months.

DLRs are particularly cheap if you compare them to more dramatic solutions to grid congestion. “The cost of deploying a DLR system on a transmission line,” says Marmillo, “is less than 5 percent the unit cost of reconstructing or rebuilding the line.”

(Note: there’s an open FERC proposal on the subject of line ratings, in which the commission plans to require seasonal line ratings within two years, and for RTOs and ISOs to put in place systems that are prepared for DLRs within a year.)

So that’s technology one: DLRs to better understand and exploit the real-time capacity of existing lines.

Controlling the flow of electricity to ease congestion

The Brattle report says: “Power flow through an AC line is proportional to the sine of the difference in the phase angle of the voltage between the transmitting end and the receiving end of the line,” and I’ll just go ahead and trust them on that.

Left uncontrolled, power will simply cascade through the system according to Kirchhoff's laws. But it is useful for grid operators to be able to route power away from congested areas and toward less congested areas. To do that, they need flow control devices.

The first kind are special transformers called “Phase Angle Regulators” (PARs) that directly manipulate the phase angle to control the flow of power. They are well-known and accepted in the industry, but they are expensive, to the tune of millions of dollars a year, which has limited their deployment to a select few high-traffic lines. Plus, the accelerating pace of change in the electricity system has made their size and inflexibility more problematic. “These are 40-plus-year fixed assets,” says Jenny Erwin, marketing director for Smart Wires Inc., a company that makes flow control devices, and these days, “it's just much harder to plan out what you need 40 years from now.”

The other family of flow control devices are Flexible Alternating Current Transmission Systems (FACTS), which are generally power-electronics devices that control the flow of power through a line or voltage on a system by, for example, increasing or decreasing reactance on a line.

Older versions of FACTS were also quite large and expensive, but due to advances in electronics and control software, they have been made muc