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. Optional Time-Domain Link (TiDL) technology and SEL Sampled Values (SV) technology using IEC 61850-9-2 transform the way you modernize your substation.

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—Apply the TiDL-enabled SEL-421 with the SEL-2240 Axion TiDL nodes in the field to provide remote I/O and digitize analog signals over fiber optics. This simple and secure solution is easy to implement, with no external time source or network engineering required. Apply the SEL-421-7 with SV technology and built-in protection to publish or receive IEC 61850-9-2 SV data. TiDL and SEL SV technologies for digital secondary systems replace copper with fiber to increase safety, reduce costs associated with using copper, improve reliability, and limit the impact of an electromagnetic pulse.

SEL-421-4/-5 Protection, Automation, and Control System

421 Front for Carousel

From the front panel, control and view the status of disconnects and breakers, program operator pushbuttons, and configure labels. LEDs with custom alarms help dispatchers and line crews rapidly restore power.

SEL-421-4/-5 Without TiDL Technology

421 Back for Carousel

Six current and six voltage analog inputs support complete bay control and protection for dual breaker applications. Use the front and rear EIA-232 serial ports and the rear Ethernet port for communications.

SEL-421-4/-5 With TiDL Technology

421 TiDL Back for Carousel

Fiber-optic ports on the 4U chassis allow the relay to receive analog and digital data from remote TiDL nodes. Commissioning is easy with a press of a button. LEDs indicate a valid configuration as well as the connection status to a remote TiDL node on a per-port basis.

SEL-421-7 Merging Unit With IEC 61850-9-2 Sampled Values

421 Merging Unit Back for Carousel

Six current and six voltage analog inputs provide signal digitization and integrated protection schemes. Select fiber optics, copper, or mixed Ethernet with separate ports for SV data publishing and engineering access. Chassis options allow up to three I/O boards.

SEL-421-7 Relay With IEC 61850-9-2 Sampled Values

421 Distance Relay back for Carousel

Receive digitized current and voltage information from remote merging units. On this 4U chassis, select fiber optics, copper, or mixed Ethernet with separate ports for SV data and engineering access. The Ethernet links are self-monitoring.


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Complete Subcycle Line Protection
Apply the SEL-421 Relay 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.

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.

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.

An Ethernet Card Option Provides Two Copper or Fiber Ports for Failover Redundancy
Available Ethernet communications protocols include FTP, Telnet, DNP3 LAN/WAN, SNTP, the IEEE 1588 Precision Time Protocol Version 2 (PTPv2), IEC 61850 Edition 2, IEEE C37.118 synchrophasors, and the Parallel Redundancy Protocol (PRP). EIA-232 serial ports support SEL Fast Message, SEL ASCII, Compressed ASCII, SEL Fast Operate, SEL Fast Meter, SEL Fast SER, enhanced SEL Mirrored Bits communications, and DNP3 Level 2.

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.

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.

Digital Fault Recording
Combine relaying reliability with true DFR functionality. The SEL-421 Relay's 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.

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.

acSELerator QuickSet SEL-5030 Software
acSELerator QuickSet helps speed up relay applications by allowing you to develop relay settings offline, program SELogic control equations, and analyze post-fault event reports.

NOTE: SEL-421 Z010 Customers Using acSELerator QuickSet v4.1.30.0 or v4.1.34.0—Please verify your SITM setting BEFORE and AFTER upgrading to acSELerator QuickSet v4.2.3.0.

Time-Domain Link Technology
Apply a TiDL solution, where Axion TiDL nodes are placed in the yard close to the primary equipment to digitize discrete I/O signals and analog data and then transport them over a fiber-optic cable to the SEL-421 in the control house. This innovative technology uses point-to-point connections and a nonroutable protocol, providing a simple and secure solution. Because it does not need an external time source or Ethernet switches, it is easy to implement with no network engineering required. TiDL combines the proven protection of the SEL-400 series relays with the modularity of the Axion, reducing training requirements and providing a scalable and flexible solution. It also provides built-in time synchronization and synchronous sampling, ensuring protection is available in the relay regardless of whether or not an external time signal is available.

SEL Sampled Values Technology
Implement an SEL SV solution, where the SEL-421-7 Merging Unit (publisher) digitizes analog signals from primary equipment and then transmits them to an SEL-421-7 Relay (subscriber) in the control house via a fiber-based Ethernet network. The system uses precise time synchronization via IRIG-B or PTP.

  • SV Merging UnitThe SEL-421-7 Merging Unit is the only standalone merging unit in the world 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 duplicate the SV data streams for redundant protection.
  • SV RelayThe SEL-421-7 Relay offers the traditional protection available in the SEL-421-5 and can also receive SV data from devices such as the SEL-421-7 Merging Unit or the SEL-401 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’ SV-compliant units.

  • Digital secondary system solutions.
    • SEL-421-4/-5 Relay with TiDL technology.
    • SEL-421-7 Merging Unit (publisher) with IEC 61850-9-2 Sampled Values.
    • SEL-421-7 Relay (subscriber) with IEC 61850-9-2 Sampled Values.
  • Rack- or panel-mount, horizontal and vertical hardware packages.
  • Ethernet connection options for primary and standby connections.
    • 10/100BASE-T
    • 100BASE-FX
  • 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.


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