Changing relay protection settings requires careful calculation, coordination, and validation to ensure system safety and reliability.

Key Steps for Adjusting Relay Settings

1. Understand Relay Parameters

• PSM (Plug Setting Multiplier): Determines how many times the actual current exceeds the relay pickup current. Higher PSM results in faster tripping, and it must comply with IEC 60255-151 standards .
• TSM (Time Setting Multiplier): Scales the base operating time from the relay's characteristic curve. Proper TSM ensures coordination between upstream and downstream relays, with lower TSM leading to faster trips .
• EL (Earth Leakage) and OL (Overload): Define the relay's sensitivity to earth faults and thermal overloads, respectively. Adjustments must reflect system protection requirements .
• MF (Multiplying Factor): Scales internal measurements to match primary system values, compensating for CT/PT ratios and ensuring accurate relay operation . 2. Perform Calculations
• Fault Level Analysis: Determine maximum and minimum fault currents for single line-to-ground, line-to-line, and three-phase faults .
• Current and Voltage Sensing: Calculate expected normal and fault currents to set relay sensitivity .
• Time-Dial/Grading Calculations: Establish grading times between relays to maintain selectivity and avoid unnecessary delays, especially for inverse-time relays . 3. Coordination and Selectivity
• Ensure downstream relays trip before upstream relays to isolate faults locally.
• Adjust TSM and PSM to maintain proper time grading, considering inrush currents and thermal withstand limits .
• For transmission lines, consider mutual coupling, automatic reclosing, and communication-assisted schemes for coordination . 4. Testing and Validation
• Simulate fault conditions to verify relay response.
• Confirm that scaled measurements using MF match actual system values.
• Validate settings against system protection philosophy and IEEE/IEC standards . 5. Documentation and Compliance
• Record all new settings, calculations, and test results.
• Ensure compliance with relevant standards such as IEC 60255 series and IEEE relay guides . 6. Special Considerations for Modern Relays
• High-speed relays using incremental quantities or traveling-wave principles require setting propagation times and incremental thresholds .
• For series-compensated lines or lines with tapped loads, additional adjustments may be necessary to account for system dynamics . By following these measures—understanding parameters, performing calculations, coordinating relays, testing, and documenting changes—engineers can safely and effectively modify relay protection settings while maintaining system reliability and selectivity.