Introducing SEL-T401L Ultra-High-Speed Line Relay
The SEL-T401L is a new transmission line relay that combines time-domain technology and phasor-based protection in a single device.
Lighting up the Desert Nation
For the Tohono O'odham Utility Authority, that usually means outages for their customers, long nights, and overtime for the 16 technicians that cover over 4,400 square miles of territory.
But all of that is about to change.
This year marks a historic feat for both their region and the electric power industry worldwide. As one of the first utilities to install the SEL-T400L—a relay operating with time-domain technology—they were the first to experience the relay’s unprecedented tripping speed during a live event. The faster a relay trips, the less time dangerous fault currents remain on the system to cause damage. This speed, combined with advanced technologies like traveling-wave fault locating, make the SEL-T400L a relay that saves a utility precious time and money. The live event marked the entrance into a new era of electric power system protection for TOUA, where they will secure the reliability of their service for years to come.
In the remote Native American region of Arizona is the second-oldest tribally owned and operated electric utility in the United States: the Tohono O’odham Utility Authority (TOUA).
The Tohono O’odham, which means “Desert People” in their language, created TOUA in the late 1960s to boost their sovereignty and economic future. After purchasing an on-reservation distribution system from an outside electric cooperative, the utility’s first big undertaking was to connect more homes to the electric grid. In a hot, desolate area of dirt roads, no signage, and far-flung homes, it was a big accomplishment for a tiny utility. A half-century later, the utility has evolved to provide telephone, water, cellular, and internet services.
Ancient World, High-Tech
The Tohono O’odham Nation spans 4,460 square miles west of Tucson and south across the border into northern Mexico.
In the heart of their ancient homeland and off a winding dirt road is a TOUA metering station protected by a chainlink fence. Inside is a metal cabinet that houses the SEL-T400L. The site represents a cultural juxtaposition: hidden amid an environment steeped in history is a leading-edge technology
Monsoons peak in summer, when dramatic thunderstorms called microbursts erupt over the desert landscape—most often at night—bringing rain to the parched landscape but also strong winds.
“We get some bad storms here, including a lot of lightning, which can make it challenging to provide reliable electricity to our customers,” said Darrold Hobbs, operations manager of TOUA’s electric department. “To prevent outages and blackouts, we knew we had to modernize aging equipment. I heard about Schweitzer Engineering’s new relay—that it was both fast and simple—and we decided to upgrade with it.”
That decision made TOUA the second utility in the world to use SEL’s time-domain technology for tripping line breakers. The other utility is Public Service Company of New Mexico, which serves more than 500,000 customers, generates its own power, and has over 1,500 employees. By contrast, TOUA serves 3,000 customers, buys power from several outside companies, and has just 16 employees on its electric crew.
Installation of the SEL-T400L allows TOUA’s employees to spend their time and effort where it’s needed most.
“They work hard and they’re skilled, but we’re a small utility with limited resources trying to cover a lot of territory,” said Hobbs.
A disturbance on a line can mean crew members have to drive an hour and a half to isolate a fault, sometimes on washed-out roads during a monsoon storm, he added.
Part of the SEL-T400L design includes traveling-wave fault locating, a technology that can save utilities hours when searching for faults. The technology pinpoints a fault’s location to within one tower span.
The First Real-World Test
Not long after the SEL-T400L went online, an intense windstorm rolled across Tohono O’odham territory, giving the relay its first real-world test. In just over half an hour, there were seven faults, and each time the SEL-T400L tripped in under 2 ms. These events made TOUA the first utility ever to experience this relay’s unprecedented tripping speed.
“If not for this new technology, the power would have gone out and our crews would have been driving around and trying to restore it in bad weather,” said Hobbs. “Instead, it did the job so well that we didn’t even know about the disturbance until later.”
With this first-ever event, TOUA made electric transmission protection history.
