SEL-787 Transformer Protection Relay

Overview

The SEL-787 Transformer Protection Relay is a compact two-winding differential relay from Schweitzer Engineering Laboratories for power and distribution transformers, generator step-up units and large motor or feeder zones that need current differential protection. It pairs a dual-slope percentage differential characteristic with harmonic blocking and restraint, backup overcurrent for each winding, breaker failure protection for two breakers and optional voltage-based functions, all in one compact SEL-700 series relay.

Seven Star LLC supplies the SEL-787 to transmission and distribution companies, independent power producers, oil and gas facilities and industrial plants in Oman and the GCC, with engineering support in Muscat. It is a common choice for 11/0.433 kV, 33/11 kV and similar substation transformers where a single relay should provide main and backup protection with full event recording.

Differential Protection Principle

Transformer differential protection compares the currents entering and leaving the protected zone. The SEL-787 calculates three restrained differential elements (87R) from the Winding 1 and Winding 2 current inputs after compensating for CT ratio and transformer phase shift. Each element uses an operate quantity, which is the vector difference of the winding currents, and a restraint quantity derived from the through current. A single- or dual-slope percentage characteristic keeps the relay stable against CT mismatch, tap-changer movement and CT error during heavy external faults, while still tripping quickly for internal faults.

Magnetizing inrush and overexcitation produce differential current without a fault, so the relay offers second- and fourth-harmonic blocking and restraint for energization security and fifth-harmonic blocking for overexcitation. A separate fifth-harmonic alarm with its own threshold and timer warns of overexcitation near generating stations. Three unrestrained differential elements (87U), typically set at 8 to 10 per unit of operate current, trip without harmonic supervision for severe internal faults, and their published pickup time is about one cycle.

Engineering Design of the SEL-787

The base SEL-787 provides two three-phase current inputs, one per winding, with 1 A or 5 A nominal ratings, plus two contact inputs and three outputs. An optional current channel adds a neutral input for restricted earth fault (REF) and neutral overcurrent protection on grounded-wye windings. Optional three-phase voltage inputs add volts-per-hertz protection with frequency tracking from 20 to 70 Hz, over- and undervoltage, over- and underfrequency and directional power elements, which are useful on generator step-up transformers and for load shedding schemes.

Expansion slots accept additional digital and analog I/O, a 10-input internal RTD card, and serial or Ethernet communications cards. As many as 12 RTD-driven thermal elements, from the internal card or an external SEL-2600 RTD module, monitor ambient, tank, oil and load tap-changer temperatures with separate alarm and trip levels. The front panel carries a two-line LCD, navigation keys and four programmable pushbuttons with LEDs for breaker control, and settings are prepared in ACSELERATOR QuickSet SEL-5030 software, which also displays phasors to confirm CT polarity during commissioning.

Key Features

Engineers specify the SEL-787 for the following transformer protection functions:

• Two-winding dual-slope percentage differential protection with independent second-, fourth- and fifth-harmonic thresholds.
• Unrestrained differential elements for fast clearing of high-current internal faults regardless of harmonic content.
• Phase, negative-sequence, residual-ground and neutral overcurrent elements for each winding as backup protection, with IEC and US curves.
• Sensitive restricted earth fault protection for faults close to the neutral of grounded-wye windings.
• Breaker failure protection for two three-pole breakers and 32 local plus 32 remote control bits for operator schemes.
• Through-fault monitoring that accumulates fault current passing through the transformer to support condition-based maintenance.
• Thermal monitoring with as many as 12 RTD inputs or 4–20 mA analog inputs for oil, winding and ambient temperature.
• Sequential Events Recorder, oscillographic event reports and built-in metering that remove the need for separate panel meters.

Communications

Standard features include Modbus RTU, MIRRORED BITS communications, IRIG-B time input, IEEE C37.118 synchrophasors and an ST-connector fiber-optic serial port. Options include IEC 61850, Modbus TCP/IP, DNP3 serial and LAN/WAN, SNTP, DeviceNet, additional EIA-232 or EIA-485 ports and single or dual copper or fiber Ethernet ports. These allow the SEL-787 to report trips, alarms, metering and temperatures to a substation gateway, a SCADA RTU or a plant DCS.

Industrial Applications

Across Oman, the SEL-787 is suited to primary and grid substation transformers, step-up transformers at gas-fired and solar power plants, and unit transformers in oil field, refinery and petrochemical networks around Muscat, Sohar, Duqm and Salalah. The –40 to +85 °C IEC performance rating and IP65 front panel when enclosed in a panel support installation in hot switchrooms and outdoor kiosks, and UL Class I, Division 2 certification helps where the relay panel sits near classified areas.

Through-fault and RTD monitoring give asset managers data on cumulative fault stress and oil temperature, which helps schedule maintenance on aging transformers. Seven Star LLC can match the SEL-787 option string to winding CT ratings, voltage inputs, RTD count, auxiliary supply and communications protocol, and supply companion SEL relays for the outgoing feeders on the same board.

For feeder protection on the same switchboard, see the SEL-751 feeder protection relay.

Technical Specifications

The following SEL-787 data come from the SEL-787 data sheet (2025 revision). Confirm final values against the ordered model.

How to Order

For an SEL-787 offer, send Seven Star LLC the transformer rating and vector group, CT ratios and secondaries, the need for voltage, neutral or RTD inputs, the auxiliary supply and the required protocol. Our Muscat engineers will confirm the model, advise on delivery to sites in Oman and the GCC and share the SEL-787 data sheet for approval. Comparable differential and protection relays on our site include the ABB SPAD34 stabilized differential protection relay, the Schneider Electric PowerLogic P5 protection relay and the Schneider Electric Vamp 257 protection relay.