html{display:none} Time-Domain Line Protection | Schweitzer Engineering Laboratories

Time-Domain Line Protection

Time-domain line protection sets a new standard for transmission line protection.

Time Domain hero

Using a combination of traveling-wave and incremental-quantity protection technologies, SEL time-domain protection relays trip securely at speeds as fast as 1 ms, record events at 1 MHz, and locate faults to within a single tower span.

Faster fault clearing and more accurate fault locating improve the safety of utility personnel and the public, widen transient stability margins, limit equipment wear, improve power quality, and mitigate potential property damage.

Fast, Secure Line Protection

Deployed together with the SEL-411L Advanced Line Differential Protection, Automation, and Control System, the SEL-T401L Ultra-High-Speed Line Relay provides a complete dual transmission line protection solution.

The underreaching distance (TD21) element in SEL time-domain protective relays trips using incremental voltages and currents at the relay location. Not dependent on a protection channel, the TD21 element operates as fast as 2 ms for close-in, high-current faults.

Dependably protect the entire line by using a permissive or blocking pilot protection scheme over a standard digital or analog protection channel with traveling-wave (TW32) and incremental-quantity (TD32) directional elements, which operate as fast as 0.1 ms and 1.5 ms, respectively.

The traveling-wave differential (TW87) protection scheme uses current traveling waves to detect in-zone faults with operating times in the range of 1 to 5 ms, depending on the line length.

Dependable Protection in Challenging Applications

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.

The SEL-T401L trips 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 re-ignition and other high-frequency transients with 1 MHz resolution.

Accurate Fault Locating

SEL time-domain protection relays include an integrated traveling-wave fault locator, which is accurate to within a single tower span.

The robust SEL traveling-wave technology is backed up by impedance-based fault locating to provide the most accurate and dependable fault location possible under any combination of operating conditions and fault types.

Using the precise fault location results, you can dispatch crews directly to the tower nearest to the fault, reducing maintenance expenses, outage time, and associated penalty costs.

Improve the Resiliency of Hybrid Lines

Accurate, real-time fault location data allows selective reclosing, which improves resiliency in temporary fault conditions on overhead lines and helps avoid cable damage by inhibiting reclosing for faults on cable sections.

Use the double-ended traveling-wave fault location method with adaptive autoreclose cancel (ARC) logic on hybrid lines to:

  • Adaptively control autoreclosing and single-pole tripping by asserting a control bit based on the fault location obtained at the line terminals.
  • Allow single-pole tripping and reclosing for faults on overhead line sections.
  • Force three-pole tripping and inhibit autoreclosing for faults on cable sections.

Time-Domain Line Protection Products


SEL-T401L Ultra-High-Speed Line Relay

The SEL-T401L delivers subcycle operating times and includes a complete suite of primary and backup line protection functions, plus line monitoring, high-resolution event reporting, and accurate fault location.

  • Ultra-high-speed line protection with traveling-wave and incremental-quantity-based elements and schemes.
  • High-performance distance protection with five zones of phase and ground elements selectable as either mho or quadrilateral.
  • Traveling-wave fault locator that is accurate to a single tower span and allows the use of selective reclosing.
  • Line monitoring function that enables condition-based maintenance.

SEL-T400L Time-Domain Line Protection

Improve the reliability of your system with ultra-high-speed protection, accurate fault locating, and ultra-high-resolution transient recording.

  • Ultra-high-speed line protection with traveling-wave and incremental-quantity-based elements and schemes.
  • Traveling-wave fault locator that is accurate to a single tower span and allows the use of selective reclosing.
  • Simplified settings and ease of use in a 3U package.

SEL-2730M Managed 24-Port Ethernet Switch

Ethernet connectivity and secure network management for SCADA, engineering access, and control. Deploy the SEL-2730M with the SEL-T400L or SEL-T401L to integrate with your substation Ethernet network.


SEL ICON Integrated Communications Optical Network

A wide-area networking multiplexer optimized for utility and industrial applications. The ICON is configurable for SONET or Ethernet transport. It is also ideal for migrating legacy network technologies to a converged network solution. Deploy the ICON with the SEL-T400L or SEL-T401L to implement double-ended traveling-wave fault locating over a multiplexed network.


SEL-2488 Satellite-Synchronized Network Clock

Receive Global Navigation Satellite System (GNSS) time signals, and distribute precise time via multiple output protocols, including IRIG-B and the Network Time Protocol (NTP).

Products and Service You Can Rely On

SEL products are designed and manufactured for the world’s most challenging environments, exceeding all industry standards for temperature, shock, and electric stress. An optional conformal coating for circuit boards adds an extra level of protection against contaminants in extreme environments.

Every SEL product comes with a ten-year warranty—the best in the industry. If any product fails under warranty, we’ll repair or replace it for free.

SEL Solutions in Action

TOUA featured

Lighting up the Desert Nation

In the Sonoran Desert, intense summer heat and sudden microburst storms caused repeated power outages for TOUA, the Tohono O’odham Utility Authority—until they modernized their system with the SEL-T400L. See how SEL time-domain and traveling-wave technology answered the challenge of electric power transmission in this unforgiving environment.


The Modernization Story of PNM

In May of 2018, the Public Service Company of New Mexico (PNM) became the first transmission utility in the world to use SEL-T400L relays with time-domain technology for tripping line breakers—a significant step in the next stage of electric power protection.

Questions? Contact Us!

We’re happy to answer all your questions and help develop the best solution for your needs.

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Unbeatable Support

SEL support teams are stationed in regional offices around the world and staffed with application engineers who are experts in our products and in power system applications.

Technical support for SEL-manufactured devices is always free. No matter how often you need to call or how long your SEL products have been in service, you’ll reach an SEL expert who can provide the service and support you need.

Our cybersecurity team is always ready with the information and resources needed to keep your OT networks and critical systems secure and working effectively. Cyber services support contracts can include incident response, audits, system hardening, and more, depending on your anticipated needs.