How to use a relay protection schedule

A relay protection schedule is used to coordinate protective relays by setting current and time parameters to ensure selective fault isolation and system stability.

Understanding the Relay Protection Schedule

A relay protection schedule lists all protective relays in a system along with their settings, including current pickup, time delays, and coordination hierarchy. Its purpose is to ensure that only the relay closest to a fault operates first, preventing unnecessary outages and protecting equipment from damage . Key elements in a relay protection schedule include:

• Relay type: Overcurrent, differential, distance, or directional relays.
• Plug Setting Multiplier (PSM): Indicates how many times the actual current exceeds the relay's pickup current. It determines the speed of operation on IDMT (inverse definite minimum time) curves .
• Time Setting Multiplier (TSM): Scales the base operating time from the relay's characteristic curve to achieve proper coordination with upstream or downstream relays .
• Earth Leakage (EL) and Overload (OL) settings: Define thresholds for earth faults and thermal overload protection .
• Multiplying Factor (MF): Adjusts metering or scaling for accurate relay operation .

Steps to Use a Relay Protection Schedule

1. Identify the system layout: Determine feeders, transformers, generators, and other equipment requiring protection.
2. Determine protection objectives: Decide which equipment or zones must be isolated first in case of a fault, considering continuity, personnel safety, and system stability .
3. Select relay types and characteristics: Choose relays suitable for the network topology (radial, looped, or meshed) and fault current levels .
4. Set PSM and TSM values: Calculate PSM using the formula PSM = Fault Current / Pickup Current and adjust TSM to achieve selective coordination with upstream relays .
5. Apply time grading: Ensure relays closer to the fault operate faster than upstream relays. Use definite time or inverse time characteristics depending on network configuration .
6. Verify coordination: Check that relay operating times do not overlap undesirably, ensuring selectivity and minimizing fault impact.
7. Test and document: Perform secondary injection tests or simulations to confirm relay operation matches the schedule. Update the schedule with any changes in system configuration .

Practical Tips

• Use inverse time relays for radial networks with varying fault currents to speed up operation at high fault levels .
• Ensure reliability and sensitivity: Relays must operate correctly under actual fault conditions without false trips .
• Maintain a clear hierarchy: Primary relays act first, and backup relays operate with a time delay to cover failures .
• Keep the schedule updated with system modifications to maintain protection integrity. By following the relay protection schedule, engineers can achieve selective, fast, and reliable fault isolation, protecting both equipment and personnel while maintaining system stability.