Siemens SIPROTEC 5 7SJ81 Feeder & Overcurrent Protection Relay

The Siemens SIPROTEC 5 7SJ81 is a compact and cost-effective numerical relay designed for feeder and overcurrent protection in medium-voltage power systems. Part of the SIPROTEC Compact series, it is optimized for applications requiring reliable protection with minimal space and cost.

The relay supports low-power instrument transformers (LPIT / LPCT / LPVT), making it highly suitable for modern digital substations and retrofit projects. It ensures reliable detection and isolation of short circuits and ground faults across various network configurations, including radial, ring, and meshed systems.

With its robust protection algorithms, the 7SJ81 provides both directional and non-directional overcurrent protection along with advanced ground fault detection methods. Its compact design, conformal coating, and strong cybersecurity features ensure high availability even in harsh environments.

The relay is engineered using DIGSI 5, allowing flexible configuration, automation, and seamless integration into IEC 61850-based digital substations.

Main Function

• Feeder and overcurrent protection for MV/HV systems
• Suitable for lines, feeders, and backup protection applications
• Designed for LPIT (low-power CT/VT) applications
• Supports radial, ring, and meshed network configurations

Tripping

• 3-pole tripping for fast fault isolation
• Selective tripping of short circuits and ground faults
• High-speed operation for improved system stability

Inputs and Outputs

• 4 low-power current transformer (LPCT) inputs
• Optional 3 low-power voltage transformer (LPVT) inputs
• Binary inputs: approx. 11–23
• Binary outputs: approx. 9–16

Hardware Flexibility

• Compact SIPROTEC 5 design with fixed I/O configuration
• Multiple predefined I/O variants available
• Exchangeable communication interfaces
• Configurable via the DIGSI 5 engineering tool

Housing

• Compact 1/3 × 19-inch housing
• Available with a small or large display
• Conformal coating for operation in harsh environments

Function (Key Features)

• Directional and non-directional overcurrent protection
• Advanced ground fault detection:
  • Transient, harmonic, admittance-based methods
• Thermal overload and unbalanced load protection
• Circuit-breaker failure protection
• Automatic reclosing (optional)
• Voltage and frequency protection functions
• Fault recording (analog and binary signals)
• Basic power quality monitoring
• IEC 61850 communication with GOOSE support
• Cybersecurity features (RBAC, secure communication)