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Transformer Protection Relay

Protect and monitor most industrial transformer applications with the versatile SEL-787 Transformer Protection Relay. Apply 2 three-phase winding inputs, an optional single-phase restricted earth fault (REF) input, and three-phase voltage inputs for comprehensive transformer 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.

Starting At

2,950 USD

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Two-Winding Complete Protection—Apply two-winding, dual-slope differential protection with phase, negative-sequence, residual, and neutral-overcurrent elements for backup protection. Use breaker failure protection for two 3-pole breakers. Also, protect three-phase autotransformers with the SEL-787.

Cost-Saving Transformer Monitoring—Extend the life of your transformer by monitoring the ambient, load tap changer (LTC) tank, and transformer oil temperatures with optional 4–20 mA analog inputs or resistance temperature detector (RTD) thermal inputs. Monitor the number of through faults, accumulated I2t, and fault duration times to determine the frequency and impact of external faults on the transformer to prevent catastrophic failures.

Fan Bank Control—Provide fan bank control and protection with the optional single-phase CT channel in addition to the RTD thermal elements.

Beyond Basic Transformer Protection—Achieve comprehensive transformer protection with the optional single-phase sensitive REF input and the three-phase voltage inputs. Eliminate separately mounted metering devices with built-in metering functions, including harmonics, power, phase voltages and currents, neutral currents, and sequence voltages and currents.



Two-Winding Current Differential, Overcurrent, and Breaker Failure Protection
Apply two-winding dual-slope differential protection with phase, negative-sequence, residual, and neutral overcurrent elements for backup protection. Use breaker failure protection for two 3-pole breakers.

Secure and Dependable
Provide maximum security during external faults and transformer magnetizing inrush conditions. Detect internal faults quickly, during energization or normal operating conditions, using combined harmonic-blocking and -restraint differential elements.

Transformer Monitoring
Record and accumulate through-fault duty for use in SELogic control equations or manual monitoring. Monitor ambient, load tap-changer (LTC) tank and transformer oil temperature with optional 4–20 mA analog inputs or RTD thermal inputs to extend the life of your transformers.

Operator Control
Use four programmable front-panel pushbuttons, each with two programmable LEDs, for a wide variety of applications, including easy trip-and-close control and status indications for a breaker. Implement local and remote operator control schemes with 32 local and 32 remote control bits.

Metering and Reporting
Eliminate separately mounted metering devices with built-in metering functions. Analyze Sequential Events Recorder (SER) reports and oscillographic event reports for rapid commissioning, testing, and post-fault diagnostics. The unsolicited SER protocol allows station-wide collection of binary SER messages. SEL-787 metered quantities include harmonics, power, frequency, and energy; phase voltages and currents; neutral currents; sequence voltages and currents; and maximum/minimum logging of selected quantities.

Rugged Design
Trust the industry’s widest ambient operating temperature range, –40° to +85°C (–40° to +185°F). The SEL-787 withstands vibration, electrical surges, fast transients, and extreme temperatures, meeting stringent industry standards. The printed circuit boards are conformal-coated to provide an additional barrier to airborne contaminants, such as hydrogen sulfide, chlorine, salt, and moisture.

  • Advanced Protection
    Protect grounded-wye transformers with sensitive REF elements. Provide volts/hertz protection with frequency tracking from 20 to 70 Hz for generator step-up and other variable frequency applications. Implement load shedding and other control schemes with over- and underfrequency and over- and undervoltage elements.
  • Flexible and Easy Communications
    Apply copper or fiber, single or dual fiber Ethernet (DNP3, IEC 61850, Modbus), and up to 4 serial ports.
  • Flexible I/O for Local Control and System Applications
    The base unit includes three digital outputs and two digital inputs. There are three card slots for optional SELect I/O cards including the following:
    • 3 digital inputs (DI)/4 digital outputs(DO)/1 analog input (AI)
      4 DO/4 DI
      4 AI/ 4 analog outputs (AO)
      8 DI
      Serial communication (EIA-232/EIA-485)
      10 internal RTD inputs (or add 12 external RTD inputs with an SEL-2600 RTD Module)
      Neutral ac current input with three-phase ac voltage inputs (1 ACI/3 AVI)
      Neutral ac current input (1 ACI)
  • Conformal Coating
    Protect equipment from harsh environments and airborne contaminants, such as hydrogen sulfide, chlorine, salt, and moisture. Learn more about conformal coating.
  • Projection Mounting
    Projection, flush, and easily extractable mounting solutions are available.



Instruction Sheets
SELect Input Output Card Installation Instructions


Product Certifications