Project Description

SEL-421 Distance Relay

SEL-421 Distance Relay

Apply the SEL-421 Protection, Automation, and Control System 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 Capacitance Voltage Transformer (CVT) 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 ultra-high-speed (subcycle) operation for application on critical lines requiring high-speed fault clearing.

Functional Overview

SEL-421 Functional Overview
21 Phase and Ground Distance
25 Synchronism Check
27 Undervoltage
50 Overcurrent
50BF Dual Breaker Failure Overcurrent
51 Time-Overcurrent
59 Overvoltage
67 Directional Overcurrent
68 Out-of-Step Block/Trip
79 Single-/Three-Pole Reclosing
81 (O, U) Over-/Underfrequency
DFR Event Reports
ENV SEL-2600
LGC Expanded SELOGIC Control Equations
MET High-Accuracy Metering
PMU Synchrophasors
SER Sequential Events Recorder
BRM Breaker Wear Monitor
LDE Load Encroachment
LOC Fault Locator
SBM Station Battery Monitor


Front- and Rear-Panel Diagrams

SEL421- Front Panel SEL421- Rear Panel
Front Rear



Select an SEL-421 Protection, Automation, and Control System with operating times to match your system requirements. Use standard-speed or subcycle elements for critical applications. Use Mirrored Bits® communications and high-speed outputs with all SEL-421 Relay models to improve operating times.


Integrate a two-breaker, four-shot recloser and SELogic® control equations into a complete automation system. Initiate reclosing based on fault type, protection zone, or external conditions.


View data across the system with 10 μs time-stamping accuracy. Verify load angles and system impedances with measured values.


Apply Ethernet (FTP, Telnet, DNP3 LAN/WAN, SNTP, and IEC 61850) connectivity to simplify and speed information distribution. Integrate expanded SELogic control equations into your complete station automation system.


Improve reliability with continuous checks of station batteries and relay health. Reduce costs through tailored maintenance intervals based on accumulated breaker duty and operating times. View relay read-only data, including settings, metering, event reports, relay status, and configuration, with built-in web server over Ethernet.


411L 421 311C 311L 387L
Distance Protection
Line Current Differential
Breaker Failure Protection f f
Undervoltage Load Shedding f f f f
Series-Compensated Lines + +
21G Mho Ground Distance
21XG Quad Ground Distance
21P Mho Phase Distance
25 Synchronism Check
27/59 Under-/Overvoltage f
49 Thermal f f
50 (N,G) Overcurrent (Neutral, Ground)
50P Phase Overcurrent
50Q Negative-Sequence Overcurrent
51 (N,G) Time-Overcurrent (Neutral, Ground)
51P Phase Time-Overcurrent
51Q Negative-Sequence Time-Overcurrent
67 (N,G) Directional Overcurrent (Neutral, Ground)
67P Phase Directional Overcurrent
67Q Negative-Sequence Directional Overcurrent
81 Under-/Overfrequency
87L Line Current Differential
Programmable Analog Math
Out-of-Step Block and Trip
Load-Encroachment Supervision
Single-Pole Trip + +
Zone/Level Timers
Pilot Protection Logic
79 Automatic Reclosing
Number of Controlled Breakers 2 2 1 1 1
Fault Locating
Traveling Wave Fault Location +
Subcycle Distance Elements + + +
SELogic Control Equations
Nonvolatile Latch Control Switches
SELogic Remote Control Switches
SELogic Local Control Switches +
Display Points +
Mirrored Bits Communications
Substation Battery Monitor
Breaker Wear Monitor
Trip Coil Monitor f f f f
Event Report (Multicycle Data)
Sequential Events Recorder
Instantaneous Meter
DNP3 Level 2 Outstation + +
IEC 61850 Communications + + + +
Synchrophasors (SEL Format)
Synchrophasors (IEEE C37.118 Format)
Accepts Delta Voltage Transformers +
Connectorized (Quick Disconnect) Available + + +
Configurable Labels +
Standard Feature
+ Model Option
f This function may be created using settings

Product Literature