Relay Coordination Study
Time-current coordination, selectivity verification, TCC development, and relay settings optimization for reliable power system protection.
What is Relay Coordination?
Relay coordination ensures protective devices operate in the correct sequence to isolate faults while maintaining service to unaffected areas. The study develops relay settings, verifies selectivity through time-current curve (TCC) analysis, and documents coordination margins between upstream and downstream devices.
Proper coordination prevents cascading outages, reduces equipment damage, and maintains system reliability. Time-current curves graphically demonstrate device operating characteristics and verify adequate separation between protection zones under various fault conditions.
This study is essential for new installations, equipment changes, generator interconnection, and NERC compliance. It supports PRC-005 maintenance programs and ensures protection systems perform as designed.
When This Study Is Required
Generator/BESS Interconnection
Protection scheme design and utility coordination requirements
New Equipment or Modifications
Transformers, feeders, breakers, or relay replacements
Protection System Upgrades
Relay modernization or settings optimization
NERC Compliance Support
PRC-005, PRC-023, PRC-025, PRC-027 documentation
What eGridSync Delivers
Protection zone identification and one-line review
Short-circuit analysis (if not already available)
Device characteristic curves and CT/PT ratio verification
Time-current curve (TCC) plots showing coordination
Relay settings calculations and recommendations
Selectivity verification and margin documentation
Coordination constraints identification and mitigation
Settings tables and programming files (where applicable)
Final coordination report with TCCs and recommendations
Inputs Required (Data Request Checklist)
| Item | Examples | Why Required |
|---|---|---|
| One-line diagram | Protection zones, relay locations, device types | Define coordination scope |
| Short-circuit currents | Max/min fault duties at key buses | Required for TCC development |
| Relay types & models | Manufacturer, model, firmware | Device capabilities and curves |
| CT/PT ratios | Current/voltage transformer ratios | Primary to secondary conversion |
| Existing settings | Current relay settings files | Baseline and validation |
| Coordination criteria | Time intervals, philosophy | Design requirements |
Relay Coordination Principles
Selectivity
Downstream devices clear faults before upstream devices operate
Time Coordination
Adequate time margin between device operations (0.2-0.4s typical)
TCC Analysis
Graphical verification of coordination under all fault conditions
Speed vs Selectivity
Balance fast fault clearing with proper coordination
Common Coordination Challenges
Insufficient Time Margin
Curves too close causing potential miscoordination and unnecessary outages
Curve Crossover
Protection curves intersecting at certain fault levels losing selectivity
Outdated Settings
System changes invalidating existing coordination requiring updates
Missing Fault Current Data
Inadequate short-circuit data preventing proper coordination analysis
Frequently Asked Questions
What is relay coordination?
Ensuring relays operate in proper sequence to isolate faults with minimal system impact.
What is a TCC?
Time-Current Curve showing when protective devices operate at different fault levels.
How much margin is needed?
Typically 0.2-0.4 seconds between upstream and downstream device operations.
Can you coordinate with utility?
Yes, if utility provides relay data; otherwise we coordinate to the POI.
Do you provide settings files?
Yes—relay programming files where applicable.
What if curves cross?
We recommend adjustments or document the constraint and mitigation options.
How does this relate to PRC-005?
Coordination supports protection system performance and maintenance documentation.
Can you verify existing settings?
Yes—we review current settings against system conditions.
What software do you use?
Industry-standard tools for coordination and protection analysis.
How long does this study take?
Typically 2-4 weeks depending on system complexity.
Ready for Relay Coordination?
Contact eGridSync for TCC development and relay settings optimization.