Secondary circuit of relay protection device

Secondary circuit protection relays monitor electrical parameters and command circuit breakers to isolate faults, ensuring safety and reliability in power distribution systems.

Overview

A secondary circuit protection relay is a device used to detect abnormal conditions such as overcurrent, short circuits, voltage imbalances, or other faults in the secondary distribution network. Unlike primary devices, these relays do not directly interrupt current; instead, they receive signals from current transformers (CTs) and voltage transformers (PTs), process the measurements, and send a trip signal to a circuit breaker to isolate the faulted section, minimizing damage and maintaining system stability .

Working Principle

Secondary protection relays operate by continuously monitoring electrical quantities such as current, voltage, frequency, and impedance. When a measured parameter exceeds a preset threshold (pickup value), the relay activates its logic and closes its contacts to complete the trip circuit of the breaker. The breaker then interrupts the fault current, isolating the affected section . Key elements in the relay operation include:

• Sensing circuits (CTs/PTs)
• Relay logic and settings
• Trip output to the breaker
• Breaker operation and isolation Proper coordination with upstream and downstream devices is essential to ensure selective and reliable protection .

Types of Secondary Protection Relays

Secondary relays can be classified based on their operating principle or function:

• Electromechanical Relays: Use moving parts and electromagnetic forces; traditional and robust.
• Static Relays: Use electronic components without moving parts; faster and more reliable.
• Numerical (Digital) Relays: Microprocessor-based, multifunctional, capable of advanced monitoring, communication, and configurable protection schemes . Functionally, relays may provide:
• Overcurrent protection
• Differential protection
• Distance or impedance protection
• Voltage and frequency monitoring
• Multi-function protection for feeders, transformers, and switchgear .

Examples of Secondary Protection Relays

Modern secondary relays are often used in medium-voltage distribution systems and can be integrated into switchgear or ring main units:

• ABB Relion REF601: Entry-level feeder protection relay with CT or Rogowski sensor interfaces, pre-configured for fast commissioning, and supports serial communication (Modbus RTU, IEC-103), .
• ABB Relion REJ603: Self-powered relay for remote locations, ideal for ring main units and secondary distribution switchgear .
• ABB Relion REX615: All-in-one protection and control relay for primary and secondary distribution, offering modular hardware and software for flexible applications .

Key Considerations

When selecting or implementing secondary protection relays:

• Ensure proper CT/PT ratios and accuracy.
• Verify relay settings, time delays, and coordination studies.
• Confirm trip circuit health and breaker compatibility.
• Consider communication capabilities for monitoring and integration with SCADA systems . Secondary protection relays are critical for reliable, safe, and efficient operation of electrical distribution systems, providing fast fault detection and isolation while minimizing downtime and equipment damage.