Siemens Siprotec 7SJ61 Overcurrent Protection Relay

The Siemens Siprotec 7SJ61 is a numerical overcurrent protection relay designed for feeder protection in medium voltage and low voltage power systems. It combines overcurrent protection, control, and monitoring functions in a compact unit, suitable for radial networks, ring mains, and industrial power distribution. The 7SJ61 is part of the Siprotec 4 family and supports flexible communication protocols including PROFIBUS, IEC 60870-5-103, and IEC 61850.

Technical Specifications

The relay features three-phase current inputs (1 A or 5 A nominal), optional voltage inputs, and a sensitive ground fault input. Protection functions include: directional or non-directional overcurrent (definite time, inverse time IEC/ANSI curves), ground fault protection, thermal overload protection, and under/overvoltage. Trip and close logic are fully programmable. The device has 8 binary inputs (24/48/60/110/220 V DC) and 8 binary outputs (relay type). A built‑in control panel with graphical LCD (240x160 pixels) provides local operation. Power supply range: 24‑250 V DC / 60‑230 V AC. Dimensions: 185 mm (H) x 102 mm (W) x 188 mm (D). The relay is rated IP40 for front panel, IP20 for rear. Operating temperature -20°C to +55°C.

Key Features

• Multi‑function overcurrent and ground fault protection

• Directional and non‑directional modes

• Programmable logic (CFC) for custom switching sequences

• Integrated control for up to 10 circuit breakers

• Digital fault recording (8 seconds at 1.6 kHz sampling)

• Communication interfaces: PROFIBUS DP, IEC 61850, Modbus

• Optional arc flash detection (via sensor)

• Self‑monitoring and event logging

• Graphical display with user‑friendly navigation

Protection Functions

The 7SJ61 provides three‑stage overcurrent protection (phase and ground) with independent time delays. Curves include IEC standard inverse, very inverse, extremely inverse, and ANSI long-time inverse. A fourth instantaneous element can be used for high‑set tripping. Sensitive ground fault (IEC 50N/51N) detects earth faults with as low as 1 mA setting. Thermal overload model includes ambient temperature compensation. Voltage protection (optional) includes under/overvoltage and residual voltage. The relay also supports cold load pickup and inrush detection (second harmonic blocking) for transformer feeders.

Control and Monitoring

The 7SJ61 includes integrated circuit breaker control with position monitoring, trip circuit supervision, and switching duty monitoring. Up to 10 switches can be controlled from the same relay. The programmable logic (Continuous Function Chart) allows users to create interlocking, automatic transfer schemes, or load shedding without external PLC. The relay stores up to 100 events and 8 fault records (with oscillography). Analog values (currents, voltages, frequency, power) are displayed in real‑time.

Communication and Integration

The 7SJ61 is equipped with an optional PROFIBUS DP interface (standard) or IEC 61850 (model dependent). All measured values, statuses, and control commands are available to a SCADA system. The relay supports time synchronization via IRIG‑B or SNTP. Using the Siprotec DIGSI software (version 4 or 5), engineers can configure protection parameters, logic, and communication settings. DIGSI also provides offline simulation and fault analysis tools.

Applications

Typical installations include:

• Feeder protection in industrial substations (steel, cement, chemical plants)

• Ring main units (RMU) in medium voltage distribution networks

• Main incomer and coupling bay protection

• Transformer secondary protection (with voltage option)

• Backup protection for larger HV systems

Installation and Commissioning

The 7SJ61 is designed for panel mounting (flush) or 19‑inch rack installation. Use a cut‑out of 165 mm x 97 mm. Connect current transformers (CTs) to terminals B1‑B6; voltage transformers (VTs) to B7‑B12. Binary inputs/outputs are screw terminals on the rear. The power supply should be stable and fused externally (max 4 A). Commissioning is performed via the front keypad or DIGSI. The relay includes a built‑in test mode that injects simulated values to verify protection logic without high voltage.

Standards and Certifications

The Siprotec 7SJ61 complies with IEC 60255‑1 (protection relays), IEC 61000 (EMC), and UL 508 (industrial control equipment). It meets CE and RoHS requirements. The relay is certified for use in substations according to IEC 61850‑3 and IEEE 1613 (environmental and EMI).

Conclusion

The Siemens Siprotec 7SJ61 offers reliable overcurrent protection combined with control and monitoring in a single device. Its flexible communication, programmable logic, and robust design make it a versatile choice for modern power distribution systems.