How should each level of relay protection coordinate

Each level of relay protection should be coordinated so that the device closest to the fault operates first, while upstream relays act as backups with appropriate time or current delays to ensure selectivity.

Principles of Relay Coordination

Relay coordination ensures selective tripping, meaning only the faulted section of the network is disconnected while the rest remains operational . The key principles include:

• Downstream-first operation: The protective device nearest to the fault must operate before any upstream device.
• Backup protection: Upstream relays provide backup in case the downstream device fails.
• Time and current grading: Relays are set with time delays (time grading) or current thresholds (current grading) to maintain proper sequence .
• Grading margin: A safety margin, typically 0.2–0.4 seconds, is added to account for breaker opening time, relay overshoot, and measurement inaccuracies .

Coordination Methods

1. Time-Graded Protection:
   • Uses overcurrent relays with definite time or inverse time characteristics.
   • Definite time relays operate after a fixed delay, while inverse time relays operate faster for higher fault currents.
   • Suitable for radial networks where fault current variations are significant .
2. Time- and Current-Graded Protection:
   • Combines time delays with current thresholds to improve selectivity.
   • Ensures that relays do not operate for faults outside their zone, even if the fault current is high .
3. Coordination Across Multiple Levels:
   • For a three- or four-level system (e.g., subcircuit fuse → MCCB → feeder relay → main relay), each device's time-current curve must lie below and to the left of the upstream device's curve across all fault currents .
   • Instantaneous elements of upstream relays are set above the maximum fault current seen by downstream devices to prevent premature tripping .

Practical Steps

1. Perform a Protection Coordination Study:
   • Analyze the network to determine fault currents at each point.
   • Plot time-current curves for all devices and adjust settings to maintain selectivity .
2. Set Grading Times and Margins:
   • Determine the Coordination Time Interval (CTI) between devices.
   • Adjust relay time multipliers (TMS) to achieve the required operating sequence .
3. Verify Selectivity:
   • Ensure that for any fault, only the nearest device operates.
   • Check for curve overlaps and adjust relay characteristics or CTI as needed .
4. Consider Device Types:
   • Digital relays allow tighter CTI and curve adjustments.
   • Mixing electromechanical and digital relays requires accounting for mechanical “coast” times .

Standards and Guidelines

• IEC 60255 and IEC 60947: Define relay performance, time-current characteristics, and coordination principles .
• IEEE Std 242 and C37 series: Provide recommended practices for industrial and utility systems .
• OSHA and NFPA 70E: Require coordination studies for personnel safety and arc flash mitigation .

Summary

Effective relay coordination involves systematic analysis, proper grading of time and current, and adherence to standards. Each level of protection must operate in a sequence that isolates only the faulted section, with upstream devices providing reliable backup. This ensures network reliability, safety, and minimal service interruption.