Project Description

SEL-787-3/-4 Transformer Protection Relay

SEL-787-3S Transformer Protection Relay

Protect and monitor most three- and four-terminal transformers with the versatile SEL-787 Transformer Protection Relay platform. Select SEL-787-3E/-3S models, and apply three-phase current and voltage inputs with options for either a single-phase restricted earth fault (REF) input or a single-phase voltage input for comprehensive transformer protection. Select the SEL-787-4X for current-based, four-winding differential protection. Benefits include advanced automation and flexibility, single or dual copper or fiber Ethernet communications ports, asset management data, and easy retrofit of most electromechanical relays.

Functional Overview

24 Volts/Hertz
25 Synchronism Check
27P Undervoltage (Phase)
27S Undervoltage (Synchronism or Battery Voltage)
32 Directional Power
49R RTD Thermal
50BF Overcurrent (Breaker Failure)
50N Neutral Overcurrent
50 (P,G,Q) Overcurrent (Phase, Ground, Neg. Seq.)
59 (P,G,Q) Overvoltage (Phase, Ground, Neg. Seq.)
59S Overvoltage (Synchronism or Battery Voltage)
81 (O,U) Over-/Underfrequency
87 Current Differential
87G Restricted Earth Fault (REF) Differential
85 SEL Mirrored Bits Communications
BF Breaker Failure
BW Breaker Wear Monitoring
DFR Event Reports
ENV SEL-2600 RTD Module Support
LDP Load Data Profiling
LGC SELogic Control Equations
LOP Loss-of-Potential
MET High-Accuracy Metering
RTD 10 Internal or 12 External (see ENV) RTD Inputs
RTU Remote Terminal Unit
SER Sequential Events Recorder
TFE Through Fault Event Monitor
PMU Synchronized Phasor Measurement


Front- and Rear-Panel Diagrams

SEL-787-3S Front Panel SEL-787-3S Rear Panel
Front Rear


Differential Protection

The SEL-787-3/-4 provides dual-slope differential protection with harmonic blocking and restraint for as many as four terminals, and as many as three independent REF elements for sensitive ground-fault detection in grounded-wye transformers. Standard overcurrent elements provide backup protection, including phase, negative-sequence, residual-ground, and neutral-ground elements. Breaker failure protection for as many as four three-pole breakers is available.

Transformer Monitoring

Measure and track accumulated through-fault current levels, and use optional 4 to 20 mA or resistance temperature detector (RTD) thermal inputs to monitor ambient, load tap changer (LTC) tank, or transformer oil temperatures.

Flexible Communications

Advanced protocols, such as IEC 61850, IEC 60870-5-103, Modbus® (RTU and TCP/IP), Simple Network Time Protocol (SNTP), DNP3 (serial and LAN/WAN), ASCII, Telnet, FTP, and DeviceNet™, support communications using legacy and modern supervisory and control systems.

Proven Hardware

The SEL-787-3/-4 Relays operate in extreme conditions, with an operating temperature range of –40° to +85°C (–40° to +185°F). They are designed to work in harsh substation environments and tested to verify that they exceed requirements for reliable operation in the presence of vibration, electromagnetic interference, and other adverse environmental conditions. Optional conformal coating provides extra protection in caustic environments.


487E 387E 387 387A 787 787-3E 787-3S 787-4X 587
Breaker Failure Protection f f f f
Transformer and Machine Current Differential
Low-Impedance Bus Differential
Underfrequency Load Shedding f +
Undervoltage Load Shedding f +
Three-Phase Current Inputs 5 3 4 2 2 3 3 4 2
Three-Phase Voltage Inputs 2 1 1+ 1 1
24 Overexcitation (Volts/Hertz) +
25 Synchronism Check
27/59 Under-/Overvoltage +
32 Directional Power +
46 Current Unbalance
49 Equipment Thermal Monitoring +
50FO Flashover Protection f f f f 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 (P,G,Q) Directional Overcurrent (Phase, Ground, Neg. Seq.)
81 Under-/Overfrequency +
81R Change-in-Rate-of-Frequency f
87 Current Differential
REF Restricted Earth Fault + +
SELogic Control Equations
Voltage Check on Closing f f f
Transformer Cooling Fan Control f f f f f
Nonvolatile Latch Control Switches
SELogic Remote Control Switches
SELogic Local Control Switches
Display Points
Multiple Settings Groups
Substation Battery Monitor
Breaker Wear Monitor
Event Report (Multicycle Data)
Sequential Events Recorder
Instantaneous Meter
Demand Meter
Load and Temperature Profile Report
RTD (Resistance Temperature Detector) Inputs + + + +
Built-In Web Server
IEEE C37.118 Synchrophasors
IEC 61850 + + + + + +
IEC 60870-5-103 + + +
Simple Network Time Protocol (SNTP)
Parallel Redundancy Protocol (PRP)
Through-Fault Monitor +
2600 RTD Module Communications
Thermal Model +
Connectorized (Quick Disconnect) Available + + + + + + +
Standard Feature
+ Model Option
f This function may be created using settings

Product Literature