html{display:none} SEL-321 Phase and Ground Distance Relay | Schweitzer Engineering Laboratories


Phase and Ground Distance Relay

SEL recommends selecting the SEL-311C Transmission Protection System in place of the SEL-321.

Replacement product: SEL-311C

No Longer in Production

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Apply the SEL-321 Relay for primary and/or backup single-or three-pole trip transmission line protection, including series-compensated lines.

Use the advanced single-pole tripping logic with phase selection, open-pole interval protection, and phase discordance logic with three-pole trip to protect against excessive single-pole-open events.

Choose from among the many communications-assisted tripping schemes available (POTT, DCUB, DCB, PUTT, DUTT, and DTT), including unique Mirrored Bits communications.

Select the load-encroachment feature to prevent operation of three-phase elements under high load conditions.


Use the time-overcurrent elements to provide current-dependent, time-delayed fault detection along the protected line and backup protection for remote terminals.

Use weak-infeed logic to rapidly clear both line terminals for internal faults near the weak terminal.

Select the SEL-321-5 Relay for enhanced performance on 50 Hz systems, including single-pole tripping.

Use acSELerator Team SEL-5045 Software to automatically gather relay event reports on your PC and manage them with a database.

Use acSELerator QuickSet SEL-5030 Software to manage your relay settings.

Use synchroWAVe Event Software to display oscillograms and vector diagrams to analyze event reports.



Comprehensive and Secure Distance Protection
Four zones of phase and ground mho elements plus four zones ground distance quadrilateral elements, each reversible, with independent phase and ground timers for complete distance protection. Positive-sequence memory voltage polarization with long memory time constant for three-phase fault directional security. Negative-sequence impedance directional element polarization for sensitive and secure operation during unbalanced faults.

Extensive Supplemental Protection Elements and Logic
Four residual and negative-sequence overcurrent elements with negative-sequence directional control. Phase, sequence, and ground time-overcurrent elements for backup protection. Out-of-step elements for power swing blocking and tripping. Phase and sequence under-and over-voltage elements. Load encroachment logic to protect against false operations during heavy load conditions. Single-pole tripping logic. Built-in communications-assisted tripping scheme logic. SELogic control equations for custom scheme logic.

Multiple Settings Groups
Six settings groups, for maximum protection scheme flexibility, allow application with bus-tie breakers or for remote system reconfiguration.

Fault Locating
Use the built-in fault locator to efficiently dispatch line inspection and repair personnel.

Patented Mirrored Bits communications and communications-assisted tripping schemes.

  • Chassis available for one additional interface I/O board.
  • I/O boards with contact sensing inputs and standard control outputs, high-current interrupting outputs, or high-speed, high-current interrupting outputs. Connectorized (plug-in connectors). Vertical or horizontal, panel-mount or rack-mount hardware packages (panel-mount available in Connectorized version only).
  • Extra long time event reports, including oscillography, available in the SEL-321-2 Relay.

Minimum Software Version

Latest Firmware Versions

There are no firmware versions available for this product.

Product Revision Firmware ID Date Available Serial Number
SEL-321-0 Last update 4/2/99, see instruction manual for details. 4/2/99
SEL-321-1 Last update 9/9/08, see instruction manual for details. 9/9/08
SEL-321-2 Last update 7/21/00, see instruction manual for details. 7/21/00
SEL-321-3 Last update 9/7/05, see instruction manual for details. 9/7/05
SEL-321-4 Last update 9/7/05, see instruction manual for details. 9/7/05
SEL-321-5 Last update 4/15/99, see instruction manual for details. 4/15/99

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

Minimum Software Version

3xx software tab - no acSELerator Architect

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.

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.

Product Certifications