SEL-787-3/-4 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

SEL-787-3E SEL-787-3S SEL-787-4X

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

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.