Setting values ​​in relay protection

Relay protection values are set by calculating the pickup current, plug setting multiplier (PSM), and time setting multiplier (TSM) to ensure proper fault detection and coordination.

Key Parameters

1. Pickup Current (I_pickup): This is the minimum current at which the relay begins to operate. It is determined based on the system load and the current transformer (CT) ratio. For example, if a CT has a ratio of 200/1 A and the relay is set at 150%, the secondary current for a 1000 A primary fault is 5 A, which becomes the reference for relay operation . 2. Plug Setting Multiplier (PSM): PSM is the ratio of the actual fault current in the relay to its pickup current. It determines how fast the relay operates according to the inverse definite minimum time (IDMT) curve. Higher PSM values result in faster tripping. PSM is calculated as: PSM = Fault Current / Pickup Current Adjusting the plug position changes the number of active turns in the relay coil, which modifies the pickup current . 3. Time Setting Multiplier (TSM): TSM scales the base operating time obtained from the relay's characteristic curve. It is used to coordinate upstream and downstream relays, ensuring selective tripping. Lower TSM values result in faster tripping, while higher values provide backup protection. The actual operating time is: Operating Time = Base Time from PSM Curve × TSM .

Steps to Set Relay Protection Values

1. Collect System Data:
   • Full-load current, short-circuit current, CT ratios, and cable/transformer ratings.
   • Determine the maximum and minimum fault currents expected.
2. Determine Pickup Current:
   • Set the relay pickup above the maximum load current to avoid nuisance tripping.
   • Use the formula: I_pickup = PSM × Rated CT secondary current.
3. Calculate PSM:
   • Based on expected fault currents, calculate the PSM to ensure the relay trips within the desired time.
   • Adjust the plug setting to achieve the required PSM.
4. Set TSM:
   • Use the relay's time-current characteristic curve to select TSM for proper coordination.
   • Ensure downstream relays have lower TSM for faster tripping, while upstream relays have higher TSM for backup.
5. Verify Coordination:
   • Plot time-current curves for all relays in the system.
   • Ensure no overlap that could cause miscoordination or delayed tripping.
6. Test and Commission:
   • Simulate fault conditions to verify relay operation.
   • Adjust PSM and TSM if necessary to meet protection and coordination requirements .

Additional Considerations

• Overload (OL) and Earth Leakage (EL) Settings: Set according to thermal limits and earth fault requirements.
• Multiplying Factor (MF): Used for scaling metering or relay readings.
• Standards Compliance: Follow IEC 60255 series for IDMT relays and IEEE/IEC guidelines for overcurrent protection . By carefully calculating and adjusting pickup current, PSM, and TSM, and verifying coordination with system curves, relay protection can be effectively set to isolate faults quickly while minimizing disruption to the rest of the network.