When the first handheld digital scientific calculator became available, it was one of those a-ha moments. Hard-core engineers and scientists had to have one. Interestingly, they always checked the calculator’s answers with their slide rules. For many, it was their first introduction to digital technology replacing analog. A larger sampling of us ran into this digital vs. analog tug-of-war when cell phones challenged the landline. Like the digital scientific calculator, people had to have one, but they are still backing up their smartphones with their landlines. Surveys show that about 44% of people in the United States still have both technologies.
This technical dichotomy has seen several similar cases in the power delivery system too. Smart grid technology has been digitizing the grid for several decades, but let’s look at one aspect getting attention lately: transmission line rating methodology. For about eight decades, the industry has been using a rating system based on conservative assumptions to determine how much power a transmission line or distribution circuit could safely carry. These are known as static line rating (SLR) systems. SLR systems rely on printed data for the conditions used to rate the lines.
Digital vs. Static
The smart grid brought sensors, sophisticated communications systems and data-gathering digital systems throughout the grid including the transmission and distribution lines. The line component called the digitalization a dynamic line rating (DLR) system. These schemes wired the lines with digital devices delivering continuous line information and constant environmental data needed for real time line loading calculations. These DLR systems consisted of sensors mounted on the conductors, the poles or a combination of both. Initially these systems were expensive and complicated, and many users utilized the old SLR methods rather than try to figure out the digital technology.
Sam Jack, director of sales and marketing for EDM International, Inc., said, “Those early DLR systems required an expert within the utility who had a thorough knowledge and understanding of the system and line loadings to interpret the data. Today’s DLR systems have evolved to the extent that the utility’s engineers can let the DLR system do the heavy lifting. These systems use algorithms and analytics to turn the big- data into actionable information and the engineers can decide how to implement that information.”
Modern DLR systems are not only getting the attention of transmission line and distribution circuit owners, but also the regulators. As a result, regulators have been conducting meetings and conferences to explore how to improve the grid’s power capacity by taking advantage of DLR technology.
Capacity vs. Tradition
Keep in mind, all of this technological tug-of-war is happening at a time when utilities and transmission owners are being challenged. They need to provide more capacity on the grid for renewable generation from both sides of the meter. With recent events like voluntary power shutdowns, customers are digging deeper into distributed generation and storage. Stakeholders are also challenged by budget constraints and increased maintenance with limited crews to do the work.
Before going on, some explanation of line ratings is needed for those not familiar with load limits. Basically, it’s the relationship between the current, the type of conductor, the voltage level, the structure, and the ambient environmental conditions. They all interact in one way or another when setting the line’s rating.
Heat is produced when electric current passes through the conductor due to the conductor’s resistance. If the heat gets too high, it can weaken the conductor, which leads to loss of life. It can also cause the conductor to sag and reduce the line’s clearance to ground. These conditions are serious concerns for the transmission line designers and those charged with maintaining these lines, so they place limits on the amount of power passing on the transmission and distribution lines.
A group of transmission line experts got together in the early 1930s to address safe operational limits of a transmission line. Their work resulted in the SLR system still in use today. It’s a very conservative system based on long-established assumptions concerning the current, conductors and most impactfully, cautious weather conditions.
Stakeholders recognized this conservative approach left a lot of transmission line capacity untapped, but available. In response, two methods have been developed to provide higher load ratings. They are the seasonally adjusted rating (SAR) system and the ambient adjusted rating (AAR) system, but they still rely on weather related assumptions rather than real-time data.
DOE and FERC acknowledge this has opened the door for owners to interpret the weather assumptions governing these static rating systems, which has led to some interesting business case developments. Consider this hypothetical scenario: two transmission lines that appear to be physically identical, but owned by different parties. Owner #1 rates their line 10% to 20% higher than owner #2.
An independent power producer approaches both parties with power projects. Owner #2 says there is no capacity on their line and owner #1 says welcome. This theoretical situation is getting a great deal of regulatory attention lately. The bottom line, however, is the amount of line capacity depends on who is doing the load limiting and which one of the numerous rating systems are being used.
The FERC (Federal Energy Regulatory Commission) published a staff paper titled, Managing Transmission Line Ratings in 2019 observing what is taking place on the grid technologically. The staff paper points out the industry hasn’t ignored the issue of various SLR based systems vs. DLR’s real-time capacity, but the stakeholders haven’t exactly taken full advantage of the DLR technology either. The paper did report that many regional transmission organizations (RTO) and independent system operators (ISO) are using SLRs, SARs, or AARs provided by transmission line owners, but there is still capacity available.
Jack McCall, executive vice president for Lindsey Manufacturing said, “Dynamic Line Rating (DLR) is the natural and logical extension of the seasonal and ambient adjusted ratings trend. Why assume a line has only four ratings a year based on seasons when real-time data and line behavior modeling can provide reliable daily or even hourly ratings?”
The driving force behind all of this discussion is cost. Take, for example, the current congested condition of the grid. In the example above, due to perceived congestion, the system operator may run more expensive generation over cheaper intermittent (i.e., renewables) for system reliability. There isn’t capacity enough for both resources.
According to the Department of Energy’s (DOE) Dynamic Line Rating Report to Congress published in 2019, “Congestion cost can be quite substantial; the sum of real-time congestion cost among major U.S. system operators in 2016 was US$4.8 billion.” As the DOE report mentions, congestion can often be easily addressed through the application of dynamic line ratings.
While DLR technology is relatively inexpensive (about 1% the cost of a reconductor), DOE’s report points out there are several barriers to widespread adoption of DLR technology. The DOE said, “In general, hesitation exists in the largely risk-averse utility industry related to the use of unfamiliar technologies.”
The DOE went on to say, “The U.S. lags behind other countries in the deployment of some advanced transmission technologies such as DLR. One of the variables is the difference in regulatory environments; the U.S. provides transmission owners little incentive to deliver more power over existing lines or to reduce transmission congestion,” but if DOE’s report and FERC’s staff paper are any indication, regulatory support is developing, and the industry is interested.
The problem is common to all new technologies; it is extremely hard to get them into the rate base. It wasn’t that long ago that energy storage was one of these technologies. Energy storage had been a bit player until FERC stepped up and set into motion several orders that encouraged storage deployment. It seems strange today that storage wasn’t a key technology. DLR needs that same consideration. How can regulators not encourage a technology that can on the average add 10%, 25%, or more of additional capacity to an existing transmission line or distribution circuits?
Also, some think DLR outputs are erratic in real-time and can’t be safely used. That was true in the early days, but modern DLR includes forecasting. Manufacturers like Ampacimon, EDM International, Lindsey Manufacturing, LineVision, and others use advanced statistical processes to develop transmission capacity forecasting (TCF). TCF uses algorithms, machine learning and artificial intelligence to make real-time streaming data useful.
TCFs can range from short durations such as an hour to longer time frames of a week or more ahead. DRL Rev-2019 technology provides forecasting abilities similar to that used with generation and load balancing, which have changed the way ISOs and RTOs see the grid. DOE’s report suggests that TCF-enabled DLR systems can relieve congestion, increase resilience, improve reliability, and enhance market operation. The capacity is out there, but is the industry going to break tradition and use it?