What is a secondary relay protection device

A secondary relay protection device is a backup system that activates when primary protection fails, ensuring safe isolation of faults in electrical power systems.

Overview

Secondary relay protection devices are part of a secondary protection system, which serves as a backup to the primary protection system in electrical networks. Their main purpose is to detect faults such as overcurrent, short circuits, or grounding issues and intervene if the primary protection fails, thereby preventing damage to equipment and maintaining system stability . These devices are critical for system reliability and safety, providing a safety net for unexpected faults.

Key Functions

• Fault Detection: Secondary relays continuously monitor current and voltage through sensors like current transformers (CTs) and voltage transformers (VTs). They detect abnormal conditions that the primary protection may have missed .
• Signal Transmission: Once a fault is detected, the relay sends a signal to circuit breakers or operators to isolate the affected section .
• Timing and Coordination: Secondary relays typically have a delayed operation to allow the primary protection to act first, ensuring proper coordination .
• System Monitoring: They provide continuous supervision of the power system, often integrating with digital substation systems for enhanced monitoring and control .

Types and Examples

Modern secondary protection relays are digital or numerical, offering multifunctional capabilities and programmable settings:

• ABB Relion Series: Includes REF601 for feeder protection, REJ603 for self-powered substation protection, and REX615 for comprehensive protection and control in primary and secondary distribution .
• Siemens SIPROTEC Series: Offers modular relays like 7SJ86 for overcurrent backup, 7SL87 for combined differential and distance protection, and 7UT82 for transformer protection .
• SEL Relays: Provide high-speed protection for feeders, lines, transformers, generators, and motors, with digital secondary systems reducing cabling and improving safety .

Integration and Configuration

Secondary relays are often configured using engineering tools such as ABB's PCM600 or Siemens DIGSI 5, which allow mapping of inputs/outputs, setting protection parameters, and ensuring IEC 61850 compliance for communication between devices . This integration enables bay-to-bay interlocking, remote monitoring, and automated fault response, enhancing overall system reliability.

Importance

Secondary relay protection devices are essential in medium- and high-voltage systems, industrial power networks, and substations. They ensure continuity of service, protection of critical equipment, and safety of personnel by acting as a reliable backup when primary protection fails . Their use is a standard practice in modern power system design to meet regulatory and operational requirements.