html{display:none} Technology propels utility into new era of electric power protection | Schweitzer Engineering Laboratories
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Technology propels utility into new era of electric power protection


The Modernization Story of PNM

On the second day of testing, he knew. Henry Moradi had seen not only a solution for his power system but also a glimpse into the future of electric power protection. It’s called the SEL-T400L Time-Domain Line Protection. On May 7, 2018, PNM became the first utility in the world to use SEL-T400L relays with time-domain technology for circuit breaker tripping—a significant step in the next stage of electric power protection.

Moradi works for the Public Service Company of New Mexico (PNM) as the manager of system engineering. He and his management had initiated a major protection system upgrade in their power grid, and one of the first transmission lines in the project was already presenting a challenge.

It was a series-compensated 345 kV line that connected the northern part of New Mexico to the Albuquerque metro area. PNM was building an interconnection on this line for another transmission customer. However, the way they had to split the line resulted in overcompensation on the first portion of the line by about 150 percent. This portion of the line looked much shorter, electrically, which led to improper coordination and inherent gaps in the relay protection.

Line-current differential schemes with traditional protection relays could have worked, but if the relays lost communications, the phasor-based distance impedance schemes would pose a challenge. These schemes would be unable to ensure proper coordination and fast protection.

However, PNM had learned about a new, nontraditional protective relay developed by Schweitzer Engineering Laboratories (SEL).

On March 12, 2018, Moradi and his team arrived at the SEL headquarters in Pullman, Washington, for a week-long factory acceptance test using the SEL-T400L. These tests used a digital simulation to model replicas of the utility’s power system, allowing the engineers to test any real-life operating scenario with any number of variables before commissioning.

Instead of operating on phasor measurements, the new relay operates in the time domain, using incremental quantities and traveling-wave principles. It’s the first and only relay of its kind in the world. While there are over 70 of these relays installed worldwide, no utility had used one for live circuit breaker tripping.

The Public Service Company of New Mexico (PNM) is the state’s largest electricity provider, serving more than 500,000 New Mexico residents and business customers. They are committed to preserving the environment as they establish system-wide technology upgrades that will make a difference for generations to come.

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“Originally, we talked about using the relay, but only in monitoring mode,” said Moradi. “Then we did the simulation testing.”

Moradi called his boss, Alwyn Vanderwalt, director of transmission and distribution engineering at PNM, and described the results from the testing. The SEL-T400L was so fast it didn’t even see the overcompensation. It was a nonissue.  

“When we did the simulation testing, we realized that, if anything, the relay was designed especially for security,” Moradi said. “We saw that it will cover the gaps in traditional protection, and it will not overtrip. For us, it was a logical choice to deploy this leading-edge technology to address our system needs.”

Vanderwalt added, “Approving the use of the relay in tripping mode was an easy decision.” 

“I’ve been around a long time, and this is the first time since the digital relay that I’ve seen a major step change in relay functionality.”
—Todd Fridley
Vice President of New Mexico Operations, PNM

No Two Schemes Were the Same

PNM’s decision to use the SEL-T400L tackles a deeper, industry-wide issue: the need for grid modernization.

“Most of the challenges for the industry today are related to exactly what this project is about,” said Todd Fridley, vice president of PNM operations in New Mexico, “and that is trying to find a way to bring smarter, more effective technology to enhance our infrastructure to maintain extremely high degrees of reliability for the electric system and service to our customers.”

At its core, every utility is driven by the obligation to serve. People want to be connected, they want electricity without having to consciously think about it, they want low utility bills, and they want the lights to stay on.

The duty to provide this type of constant, 24/7 service falls to utilities. They must maintain, protect, and operate their power systems every day and night and often in the face of outdated equipment, numerous protection scheme designs, and continually changing industry standards.

“We have to be focused and strategic about how we reinvest in our grid modernization, and we have to think about how these kinds of new technologies integrate into our short-term and long-term goals for maintaining high levels of customer service,” Fridley explained. “It’s something our utility industry must consider as we pursue our mission to provide for the future energy needs of our customers.”

For Moradi, Vanderwalt, and Fridley, this situation is more energizing than a struggle. One thing they have in common is a penchant for modern technology and decisive action. When they looked at the PNM power system from the long-term perspective, they recognized the need for modernization.

“Over the decades there was very little effort to provide standardized protection design on PNM’s transmission system,” said Vanderwalt. “No two protection schemes were the same, and that resulted in complex schema in terms of maintenance and operation.”

Cabezon substation, the site where the first SEL-T400L relays were installed.

It’s an easy trap to fall into. Over time, engineers working in different periods of infrastructure build out and develop new protection schemes and put new relays in service. As personnel changes, each scheme gets modified just a little, a tweak added here or there.

The burden falls to the relay technicians who maintain these protection schemes and relays. Without standardization, they must understand and refamiliarize themselves with each scheme, configuration, tripping logic, and settings configuration. On top of that, many utilities, including PNM, still have a large percentage of electromechanical relays in operation.

“To me, electromechanical relays, with their relative age and limited post-fault information you receive from them, provide a very strong case to quickly transition to more accurate and modern system protection elements,” said Vanderwalt. “If there’s an outage linked to the electromechanical relays, it’s much more difficult to perform proper fault analysis and troubleshooting.”

He explained that some of the transmission outages were worse than necessary because of relay application errors and lack of proper functionality.

“Just one of these transmission outages can affect tens of thousands of customers,” Vanderwalt said. “That made it clear to me that we could not proceed on this track. We decided we needed a broader, more holistic approach to upgrade our protection systems.”

We Don’t Want to Do “Just Enough”

“This transition will ensure we have consistent design and improved schema for our engineers and relay technicians so they can better manage our grid protection systems,” said Moradi.

This project included not only the protection relays but also protection scheme designs, templates, the documentation process, settings calculations, and hardware.

“We will reduce misoperations and outages, but when we do have an outage, our crews will have improved logic to better identify and locate the cause of the outage,” said Moradi. “They already know what the design looks like, they know the logic, they can perform better fault analysis, and we will reduce the number of maintenance hours and save costs for our customers.”

Take a look inside the Cabezon substation, where PNM’s first SEL-T400L relays were installed.

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Infrastructure upgrades are a huge undertaking, not to mention a significant investment. To some, they might even seem mundane. But for PNM, these upgrades and projects have injected new life and energy into the company, especially the transmission and distribution department. It’s exciting and it’s infectious.

“Our industry can have a tendency at times to just hang on and hope nothing terrible happens,” said Fridley. “What we should be doing is looking forward to the needs of our future and ensuring that our investments are strategic and targeted to provide the kind of service our customers want and deserve. We’re changing how the industry is positioned, and we shouldn’t be discouraged that it’s tough or that it’s going to take some focus and a bit of investment.”

Even though it’s not one relay that makes a power system, there’s no better representation for the progressive mindset of PNM than the SEL-T400L.

“We didn’t want to standardize on something that’s ‘just enough,’” said Moradi. “We decided that if we were going to put in the time and effort to standardize, we want to deploy the latest and most robust protection schema and standardize that into our ongoing design.”

There’s a Genius in the Simplicity

Originally, PNM wanted to take the same approach with the SEL-T400L as the other utilities that had purchased it. They wanted to use it—it sounded great—but only in monitoring mode, which means it wouldn’t be used for fault clearing.

It’s a safe route. The concern many utilities have is how fast the relay operates—they're worried it won’t operate securely, meaning that it would trip for an out-of-zone fault, which isn’t necessary. Along with that concern is a general nonchalance about tripping speed. Many don’t think there’s a benefit in a relay tripping in a few milliseconds.

If the power system still has to wait on the breaker to open anyway, does speed actually matter? For PNM, the short answer is “yes.”

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“The technology applied here with these new relaying capabilities is simply astounding in terms of fault-clearing time,” said Fridley. “And it’s not just a little speed gain; it’s multiples of speed gain. If we really bear down on what causes equipment failures and degradation over time, it’s the length of time that these extremely high levels of destructive fault currents remain on the system. Shorter clearing times and less fault current translate directly to less equipment degradation over time, extended lifecycles and, ultimately, lower maintenance and investment costs.”

New technologies can have a downside though. It’s like an exhausting game of catch-up with something new being released every day or week that will make your life easier or do something better.

But every once in a while, something comes out that captivates the industry and causes a shift or adaption in behavior. Much of that stop-and-stare quality is a result of design. That’s how the SEL-T400L is positioned in the electric power industry. 

“When I look at the SEL-T400L, that’s what I see—there’s a genius in the simplicity,” said Fridley. “It’s hard to relate it to folks who don’t understand relay protection, but to truly come up with something that’s that beautiful in design, functionality, and simplicity is what really sets it apart from our traditional system protection devices.”

Part of the design includes traveling-wave fault locating, a technology that can save utilities hours when searching for faults. The traveling wave technology in the SEL-T400L pinpoints a fault’s location to within one tower span.

“It’s incredibly accurate, and here in the west, that’s more important than ever,” Fridley explained. “We have large transmission lines that traverse extended distances over vast, unpopulated areas. Patrolling them takes extensive time and effort, so for us the fault location accuracy is monumental.”

After seeing the SEL-T400L’s fault-locating performance in the simulation lab, PNM decided to make that relay, combined with the SEL-411L Advanced Line Differential Protection, Automation, and Control System, the standard for all their 345 kV transmission lines. They are also in the process of evaluating this duo as the standard for other lines in the system.

With breakthrough time-domain technologies, the SEL-T400L trips securely in as fast as 1 ms, records events with a 1 MHz sampling rate, and locates faults to the nearest tower.

We Owe It to Our Future

Regardless of industry, there’s a good chance you’ve heard someone say, “well, this is how we’ve always done it,” in response to a proposed or imminent change.

People get comfortable. Maybe they are worried about not being able to keep up. Change means work. And there’s an element of the unknown.


“I always go back to earlier days when utilities were considering larger 345 kV and 500 kV transmission lines for the first time,” said Fridley. “I can guarantee the naysayers around the table were saying, ‘why would we need these great big high voltages, those are crazy and the breakers cost a lot and the transformers are expensive.’ I can hear it now, how they talk.

“And yet, that literally is the backbone of the system we enjoy today. If the progressives wouldn't have had the foresight to build, invest, and take on that challenge with higher voltages, larger structures, and higher-cost equipment, we would not be enjoying the kind of system we enjoy today.

“It’s that same mindset that we have to remind ourselves of today. We’re making these investments for the purpose of the future—it’s a good thing, it’s a modernization effort, and that’s our task. We owe it to the future to place those investments in now so that we can have a system that supports us.”


Led by this attitude, PNM is changing the idea of what it means to be a utility. These pioneers are bringing state-of-the-art equipment into their power systems, spearheading the adoption of new technologies, and embracing fresh ideas.

As they continue grid modernization and system protection enhancements, their end goal remains the same, and that is to be a source of unending reliance and service to their customers in New Mexico.

—The End—

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PNM Incorporates Time-Domain Protection in Modernization Project