html{display:none} SEL-421 Protection, Automation, and Control System | Schweitzer Engineering Laboratories


Protection, Automation, and Control System

Apply the SEL-421 for high-speed distance and directional protection and complete control of a two-breaker bay.

Protect any transmission line using a combination of five zones of phase- and ground-distance and directional overcurrent elements. A graphical user interface system provides logic and application templates for typical line protection schemes. Patented coupling capacitor voltage transformer (CCVT) transient overreach logic enhances the security of Zone 1 distance elements. Best Choice Ground Directional Element logic optimizes directional element performance and eliminates many directional settings. Optional additional logic prevents Zone 1 overreach on series-compensated lines. Optional incremental components provide subcycle operation for application on critical lines requiring high-speed fault clearing.

Transform the way you modernize your substation with optional Time-Domain Link (TiDL) technology or Sampled Values (SV)-based technology using IEC 61850-9-2. 

SEL-400 Series Relay Mimic Diagrams

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Subcycle Distance Protection—Protect critical transmission lines with high-speed quadrilateral, mho distance, and directional elements. Optional subcycle distance elements and series compensation logic reduce operating times to improve system stability. You can invert individual or grouped CT or PT polarities to account for field wiring or protection zone changes.

Single- or Three-Pole Operation—Incorporate programmable single- and three-pole tripping and reclosing of one or two circuit breakers into an integrated substation control system. Provide complete bay control with synchronism and voltage checks from multiple sources.

Series-Compensated Transmission Lines—Detect faults beyond a series capacitor and prevent Zone 1 overreach on series-compensated lines using the optional logic in the SEL-421-5 Protection, Automation, and Control System.

Communications-Assisted Tripping Schemes—Configure protection for transmission lines without any need for external coordination devices. The SEL-421 includes settings for permissive overreaching transfer trip (POTT), two-channel permissive overreaching transfer trip (POTT2), phase-segregated permissive overreaching transfer trip (POTT3), directional comparison unblocking (DCUB), and directional comparison blocking (DCB) schemes.

Out-of-Step Blocking and Tripping—Select from either out-of-step blocking of distance elements or out-of-step tripping during power swings. The relay includes multizone elements and logic for detection of an out-of-step condition.

Dual CT Applications—Apply the SEL-421 with ring-bus, breaker-and-a-half, or other two-breaker schemes. Combine currents within the relay from two sets of CTs for protection functions, but keep them separately available for monitoring and station integration applications.

Thermal Overload Protection—Use the three independent IEC 60255-149 thermal elements to activate a control action, issue an alarm, or trip when equipment overheats as a result of adverse operating conditions. The SEL-2600 RTD Module can provide ambient temperature measurements.

Underfrequency Load Shedding—Operate six levels of frequency elements as either an underfrequency or an overfrequency element. The frequency elements are suited for applications such as underfrequency load shedding and restoration control systems.

Bay Control, Reclosing, and Breaker Failure Protection—Apply complete bay control, reclosing, and breaker failure protection for two breakers. Monitor circuit breaker performance, including average and last tripping time, compressor run time, and contact-interrupting duty.

Digital Secondary System Technologies—SEL TiDL and SV-based solutions replace copper with fiber to increase safety, improve reliability, and limit the impact of an electromagnetic pulse. Apply the SEL-421-7 TiDL relay with the SEL-TMU TiDL Merging Unit in the field to provide remote I/O and digitize analog signals. This simple point-to-point solution with a strong cybersecurity posture is easy to implement. Apply the IEC 61850-compliant SEL-421-7 SV relay to receive SV data from a remote merging unit via a network, or apply the SEL-421-7 SV merging unit to publish SV data to SV relays in the control house. Improve system availability in SV applications with the Parallel Redundancy Protocol (PRP) or seamless failover using a five-port Ethernet card.

Flexible Communications Options—Use small form-factor pluggable (SFP) transceivers on designated process bus, station bus, and engineering access ports in the five-port Ethernet card. Choose between single- and multimode communications options for increased flexibility. All ports support 100 Mbps SFP transceivers, and process bus ports also support 1 Gbps SFP transceivers.


    1. 1

      EIA-232 serial port is quick and convenient for system setup and local access.

    2. 2

      The display allows operators to control and view the status of disconnects and breakers.

    3. 3

      The user-selectable mimic screens show the system configuration in one-line diagram format.

    4. 4

      Easy-to-use keypad aids simple navigation.

    5. 5

      LEDs indicate custom alarms and provide fast and simple information to assist dispatchers and line crews with rapid power restoration.

    6. 6

      Programmable operator pushbuttons with user-configurable labels allow customization.

    1. 1

      Choose from a vertical or horizontal, panel-mount or rack-mount chassis and different size options.

    2. 2

      High-current interrupting output contacts increase contact robustness and reliability.

    3. 3

      Use the three EIA-232 ports for Mirrored Bits communications, DNP3, SCADA, and engineering access.

    4. 4

      Communications protocols include FTP, Telnet, synchrophasors, DNP3 LAN/WAN, PRP, IEC 61850 Edition 2 (optional feature), and the IEEE 1588 Precision Time Protocol Version 2 (PTPv2).

    5. 5

      Six current and six voltage analog inputs support complete bay control and protection as well as two-breaker bay applications.

    6. 6

      Power supply options include 24–48 Vdc; 48–125 Vdc or 110–120 Vac; or 125–250 Vdc or 110–240 Vac.

    1. 1

      4U chassis with mounting options (vertical or horizontal; panel or rack) accommodates your application needs.

    2. 2

      LEDs indicate the connection status to an SEL-TMU on a per-port basis.

    3. 3

      Eight SFP ports allow the TiDL relay to connect with eight remote SEL-TMU devices and to receive remote analog and digital data.

    4. 4

      Select a five-port Ethernet communications card for dedicated station bus and engineering access ports.

    1. 1

      The 4U chassis has various mounting options to accommodate users’ hardware needs.

    2. 2

      Select a five-port Ethernet communications card for dedicated process bus, station bus, and engineering access ports.

    1. 1

      Chassis options (for up to three I/O boards) and mounting options accommodate hardware needs.

    2. 2

      Select a five-port Ethernet card for dedicated process bus, station bus, and engineering access ports.

    3. 3

      Six current and six voltage analog inputs support signal digitization and local protection schemes.



Complete Subcycle Line Protection
Apply the SEL-421 for single- and three-pole tripping schemes, compensated lines, or uncompensated lines. Protect transmission lines using five zones of phase and ground-distance elements in communications-assisted schemes, with directional overcurrent backup protection. Use optional high-speed elements to achieve subcycle operating time for most fault types and source impedance ratios.

Bay Control for Two Breakers
Fully integrate complete breaker protection and control for two breakers. Use the reclosing, breaker failure protection, and complete monitoring to integrate protection and bay control for one or two breakers. Use configurable labels to customize relay controls to match your application.

Synchrophasor Measurements
Turn state estimation into state measurement with wide-area measurement of voltage and current phase angles and magnitudes. Improve system stability with high-speed, load-angle measurements. Increase stable loading with analysis of voltage and angular stability.

Digital Fault Recording (DFR)
Combine relaying reliability with true DFR functionality. The SEL-421 high-speed sampling (8 kHz) and COMTRADE output provide advanced oscillography. Use the acSELerator QuickSet SEL-5030 Software graphical user interface for harmonic analysis and easy report generation.

High-Accuracy Time Stamping
Time-tag binary COMTRADE event reports with real-time accuracy of better than 10 µs. View system state information at time of faults or with timed triggers, across the entire system. Use system state information to validate system models, improve transfer limits, and improve system stability. Simple Network Time Protocol (SNTP) is capable of 5 ms accuracy over Ethernet and makes a good backup to more accurate IRIG-B time synchronization.

Expanded SELogic Control Equations
Use math and logic combinations of analog and digital values for customized applications. Adapt system control based on prefault conditions. Scale analog values and latch momentary inputs for SCADA retrieval.

Built-In Web Server
Access basic relay information on a standard Ethernet network with the built-in web server. View relay status, Sequential Events Recorder (SER), metering information, and settings for easy access within a local network. Web server access requires relay password and is limited to read-only viewing of information.

SEL TiDL-Based Solutions
Apply a TiDL-based process bus solution, where SEL-TMU devices are placed in the yard close to the primary equipment to digitize discrete I/O signals and analog data. SEL-TMU units exchange these data with an SEL-421-7 TiDL relay in the control house. Each SEL-TMU can share data with as many as four TiDL relays, providing protection design flexibility.

TiDL combines the proven protection of the SEL-400 series relays with the purpose-built SEL-TMU, simplifying deployment, reducing training requirements, and increasing availability. TiDL technology uses point-to-point connections and a non-Ethernet protocol, providing a simple and secure solution. TiDL also provides built-in data synchronization and synchronous sampling, ensuring protection is available in the relay regardless of whether or not an external time signal is available.

SEL SV-Based Solutions
Based on the IEC 61850 standard, SV-based solutions maximize flexibility and interoperability. The SEL-421-7 can be ordered as a merging unit to digitize analog signals from primary equipment and send them using the IEC 61850-9-2 SV protocol to IEC 61850-compliant relays in the control house. The SEL-421-7 can also be ordered as a relay to perform protective functions and other control actions based on the SV data streams it subscribes to.

  • SV Merging Unit—The SEL-421-7 merging unit comes with complete line protection built in. This allows you to have the protection right next to the primary equipment for increased speed and reliability as well as to easily transmit the SV data streams to subscriber relays in the control house for redundant protection.
  • SV Relay—The SEL-421-7 relay offers complete line protection and can also receive SV data from devices such as the SEL-421-7 merging unit or the SEL-401 Protection, Automation, and Control Merging Unit.

Because all SEL SV devices are compliant with IEC 61850-9-2 and the UCA 61850-9-2LE guideline, the SEL-421-7 can be used with primary equipment that generates similar SV streams or with other manufacturers’ IEC 61850-compliant units.

Flexible Ethernet Communications
The two-, four-, and five-port Ethernet communications card options enable you to communicate using a variety of protocols, including FTP, HTTP, DNP3, the Precision Time Protocol Version 2 (PTPv2), PRP, Telnet, synchrophasors, and IEC 61850 Edition 2.

The TiDL and SV relays can be configured with a five-port Ethernet card supporting PRP or failover functionality to increase system availability, as well as the Precision Time Protocol Version 2 (PTPv2) on process bus or station bus ports for time synchronization. Use network monitoring features in the relay to further improve system availability by actively monitoring each redundant path to detect communications network failures.

The five-port card also features dedicated ports and IP addresses for station bus and engineering access networks to provide selective access control. All ports support SFP transceivers. Single- and multimode communications options provide increased flexibility. All ports support 100 Mbps SFP transceivers, and process bus ports also support 1 Gbps SFP transceivers.

Serial Communications
All four independent EIA-232 serial ports support ASCII, Compressed ASCII, Extended Fast Meter, Fast Operate, Fast SER, enhanced SEL Mirrored Bits communications, Ymodem, and DNP3 Level 2 Outstation as well as dial-out, Virtual Terminal, and communications with SEL-2600 series RTD modules.

Simplified Settings Management
Use acSELerator QuickSet SEL-5030 Software to develop relay settings offline and program SELogic control equations. Configure the TiDL and SV relays using SEL Grid Configurator, a software tool that increases efficiency and improves settings insights. It features a spreadsheet-style editor, protection visualization, comprehensive reporting, custom filters, and multiple-device settings management.

  • Digital secondary system solutions.
    • SEL-421-7 relay with TiDL technology.
    • SEL-421-7 merging unit (publisher) with IEC 61850-9-2 SV.
    • SEL-421-7 relay (subscriber) with IEC 61850-9-2 SV.
  • Rack- or panel-mount, horizontal and vertical hardware packages.
  • Ethernet connection options for primary and standby connections.
    • 10/100BASE-T
    • 100BASE-FX
  • Five-port Ethernet card supporting fiber-optic SFP transceivers and 100 Mbps or 1 Gbps data transfer rates.
  • IEC 61850 Edition 2 communications and IEEE 1588 Precision Time Protocol Version 2 (PTPv2).
  • DNP3 Level 2 outstation communications protocol.
  • One or two additional I/O boards with standard, high-interrupting or high-speed/ high-interrupting outputs.
  • User-configurable labels for operator controls and target LEDs.
  • Conformal coating protects equipment from harsh environments and airborne contaminants, such as hydrogen sulfide, chlorine, salt, and moisture.

Minimum Software Version


Protection and Monitoring In Spain

Latest Firmware Versions

There are no firmware versions available for this product.

Product Revision Firmware ID Date Available Serial Number
SEL-421-0 R204-V0 SEL-421-R204-V0-Z007007-D20090105 1/5/09 ~1090050001
SEL-421-1 R204-V0 SEL-421-1-R204-V0-Z007007-D20090105 1/5/09 ~1090050001
SEL-421-2 R131 SEL-421-2-R131-V0-Z100011-D20130627 7/10/13 ~1131910001
SEL-421-3 R131 SEL-421-3-R131-V0-Z100011-D20130627 7/10/13 ~1131910001
SEL-421-4 R332-V0 SEL-421-4-R332-V0-Z033015-D20240509 5/22/24 ~1241430001
SEL-421-5 R332-V0 SEL-421-5-R332-V0-Z033015-D20240509 5/22/24 ~1241430001
SEL-421-7 R411-V0 SEL-421-7-R411-V0-Z109003-D20240509 5/22/24 ~1241430001

The Firmware IDs for older versions of the firmware can typically be found in Appendix A of the instruction manual.

Minimum Software Version

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411L 421 451 487B 487E - Software Tab

Configuration Software

acSELerator QuickSet SEL-5030 Software

QuickSet is a tool for engineers and technicians to quickly and easily configure, commission, and manage power system devices. QuickSet includes a Graphical Logic Editor and a device template tool to reduce configuration time and streamline deployment.

Learn more and download on the QuickSet software page.

SEL-5037 Grid Configurator Software

SEL Grid Configurator is a no-charge software tool for engineers and technicians to create, manage, and deploy settings for SEL power system devices.

Learn more and download on the Grid Configurator software page.

acSELerator Architect SEL-5032 Software

Architect allows users to configure and document IEC 61850 systems that include GOOSE, Sampled Values (SV), or Manufacturing Message Specification (MMS) communications. Architect automatically compares SCL files with IEC 61850 requirements to ensure standard compliance.

Learn more and download on the Architect software page.

Automated Data Collection

SEL Data Management and Automation (DMA) Application Suite

Available on the Blueframe platform, DMA applications automatically collect, store, and manage device-specific information like oscillography, Sequence of Events (SOE) data, device settings, and property information. They also provide automated password rotation and streamlined device audits.

Learn more on the DMA software page.

Visualization and Analysis

SEL-5601-2 synchroWAVe Event Software

synchroWAVe Event helps engineers diagnose a protective relay’s behavior during a power system fault. It is a powerful and easy-to-use solution for displaying and analyzing SEL relay event reports and COMTRADE files.

Learn more and download on the synchroWAVe Event software page.

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