Relay protection time levels are set to ensure selective tripping, where relays closest to a fault operate first, using time-graded or time-current-graded coordination.

Time-Graded Protection

Time-graded protection is a method to achieve selective short-circuit protection by assigning different operating times to relays along a network. The relay closest to the fault operates first, while upstream relays operate with a delay to avoid unnecessary tripping of the entire system . This is commonly implemented using:

• Definite-time relays: Operate after a fixed time regardless of fault current magnitude.
• Inverse-time relays: Operate faster for higher fault currents, making them suitable for radial networks where fault current varies along the feeder .

Time Coordination and Levels

Relay time levels are coordinated by setting time delays between relays in series. The time interval between each relay's setting must be sufficient to allow the downstream relay to clear the fault before upstream relays operate . This ensures proper discrimination and prevents simultaneous tripping of multiple relays.

• Relay at the fault location: Shortest operating time.
• Upstream relays: Incrementally longer operating times (t1, t2, etc.) to allow downstream clearance.
• Time grading: Can be achieved using time setting multipliers (TSM) and plug setting multipliers (PSM), which adjust the relay's operating time based on fault current levels .

Practical Considerations

• Inverse-time relays are preferred when fault currents vary significantly along the feeder, as they reduce operating time for high fault currents while maintaining selectivity for lower currents .
• Definite-time relays are simpler but may result in longer fault clearance times for high-current faults.
• Coordination studies: Require knowledge of maximum and minimum fault currents, transformer inrush characteristics, motor starting currents, and CT performance to determine optimal relay settings .
• Graphical analysis: Plotting relay curves on a common scale helps visualize coordination and ensure proper time grading .

Summary

Relay protection time levels are essential for selective and reliable protection. By using time-graded or time-current-graded relays, engineers can ensure that the relay nearest the fault operates first, while upstream relays act as backups. Proper use of TSM, PSM, and inverse-time characteristics allows for faster fault clearance at high currents and maintains system stability and equipment safety .