html{display:none} A New Standard for PNM | Schweitzer Engineering Laboratories

A New Standard for PNM

Continued Collaboration

Schweitzer Engineering Laboratories (SEL) and New Mexico’s PNM have a long history of collaboration. In 2018, PNM installed the first SEL-T400L Time-Domain Line Protection relay to be used for tripping line breakers at PNM’s Cabezon substation. This made the event of putting the SEL-T401L ultra-high-speed line relay into service on PNM’s 345 kV Taiban Mesa to Guadalupe (TG) line all the more exciting: the commissioning was about to open a world of possibility for the future of New Mexico’s power grid. 

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The Public Service Company of New Mexico (PNM) provides service for customers in all corners of the state. In need of a protection upgrade for its 345 kV transmission lines, the company partnered with SEL to install the first-ever SEL-T401L Ultra-High-Speed Line Relay to go into service. Paired with two SEL-411L Advanced Line Differential Protection, Automation, and Control System relays, the scheme would become the new standard on all of PNM’s 345 kV lines.

A New Standard

“Historically, we haven’t had a lot of standardization among our transmission line protection schemes,” said Alwyn VanderWalt, the division head of innovation and modernization for PNM. “Each new scheme was different, so our engineers would come up with a tweak to it every time an upgrade needed to be made.”

This lack of standardization created challenges in both maintenance and operation since even minor modifications required hours of relearning complex schemas.


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PNM needed a consistent design that could be applied across all its extra-high-voltage lines and that offered complete transmission protection systems on schemes across the board. By standardizing its protection schemes, PNM would increase reliability and decrease the potential for human error. And those bigger efforts to modernize PNM’s protection infrastructure were underway, including the successful commissioning of the ultra-high-speed line relays.

The TG line was a modernization priority for PNM—the 70.5-mile (113.5 km), 345 kV line runs as a part of New Mexico’s nearly 400-mile-long (644 km) electric power corridor that eventually connects the Western Transmission Grid to the Texas Interconnection power grid via an ac-dc-ac convertor station. Continual added capacity to the corridor, from interconnections via wind and solar generation, and the drive toward standardization, made the upgrade an urgent need.

The existing protection on the line did not always result in a successful reclose operation, which resulted in a significant disruption of service to the various wind generators connected to the line every time the line tripped. This new protection system, with ultra-high-speed fault clearing and single-pole trip and reclose capability, would greatly improve the ability of these wind generators to ride through a momentary fault.

So, with a planned outage on the line coming up, PNM looked to its ongoing partnership with SEL, who in turn devised a complete system upgrade with design drawings, studies, protection settings, equipment purchase, panel fabrication, installation, testing, and onsite commissioning.

“Having a close relationship with PNM makes us very alert to the needs and challenges faced and what the company was looking for in a system,” said Greg Smelich, the SEL lead product engineer who worked on the project.

The proposal? To pair the new SEL-T401L with two SEL-411L relays into one comprehensive line protection scheme.

“For us,” said VanderWalt, “the pairing of one SEL-T401L and two SEL-411L relays for our extra-high-voltage line protection schemes offers a winning combination that provides us the best of reliability, speed, and diversity due to utilizing the alternate technologies involved.”

The technology behind the SEL-T401L isn’t necessarily new, but the way it’s executed is what makes all the difference. As the first relay with phasor-based line protection elements and ultra-high-speed incremental-quantity and traveling-wave protection elements, the SEL-T401L offers the potential for faster single-pole tripping—meaning less damage to the system. Partnered with the integrated autoreclosing logic of the SEL-411L, this combination could improve power system performance and reduce equipment wear.

SEL was not only installing new ultra-high-speed line relays, but also devising a complete system upgrade and standardization—the results of which would affect hundreds of thousands of people in New Mexico.

To the Test

Onsite at SEL headquarters in Pullman, Washington, rigorous Real-Time Digital Simulator (RTDS) testing commenced, in which dozens of test scenarios that replicated faults in the PNM system were applied to the relays in a laboratory environment. Those signals get applied to the relays protecting both ends of the line, testing the entire scheme and ensuring the protection system—destined to protect PNM’s transmission lines—would work in any condition.

“We were able to literally model the whole system and test our single-pole trip and reclose schemes in a hardware-in-the-loop environment,” said Milind Malichkar, SEL project engineer III and tech lead on the project.

Back on the TG line, it was time for a month of onsite testing, installation, and commissioning of the new protection scheme. SEL and PNM crews headed to substations on either end of the line equipped with test sets time-synchronized to an absolute time reference using high-accuracy GPS clocks. It was the first time the SEL-T401L would undergo such end-to-end testing.

“The first time we ran an end-to-end test onsite and saw that the relays exchanged information over the communication channel, operated the tripping scheme, and provided the fault location was a really great moment,” said Smelich.

Results were analyzed, line current differential and distance elements verified, and the necessary adjustments made—the system was ready to go live.

“That was the moment when everything comes together,” said Malichkar, “when they close the breaker on the line and the system stays healthy and live—that’s when you know everything you’ve done for nearly the past year has paid off. It’s a great feeling.”

Pushing Forward

Today, the SEL-T401L provides enhanced protection and reliability to six different PNM 345 kV transmission lines and, going forward, the scheme will soon be standard on all of PNM’s 230 kV and 345 kV lines.

“This was the first ultra-high-speed, single-pole trip and reclose application for the T401L and 411L as a complete protection system,” said Malichkar, “and the logic SEL developed set the standard for PNM’s 345 kV system.”

A standard which PNM calls a big step forward in the push to modernize its complete power protection system.

“We’re really pleased with what we’ve accomplished to date,” said VanderWalt. “We’re on the right track to continue to build and get better every year, while modernizing our systems at the same time.”

More on the SEL-T401L Ultra-High-Speed Line Relay

The SEL-T401L trips in as fast as a quarter cycle, locates faults to within a single tower span, monitors for incipient line faults, locates fault precursors, allows selective reclosing based on real-time fault-locating logic to inhibit faults on cable sections of hybrid lines, and has an exceptional fault recorder that records breaker reignition and other high-frequency transients with 1 MHz resolution. Equipped with a full set of line protection functions, the SEL-T401L is well suited for today’s power system transmission lines with series compensation, wind turbine generators, inverter-based sources, low-inertia systems, and HVdc links.

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