Relay protection time is adjusted using the Time Setting Multiplier (TSM) and Plug Setting Multiplier (PSM), which together determine the relay's operating time for different fault currents.

Understanding Key Parameters

• Plug Setting Multiplier (PSM): Indicates how many times the actual current exceeds the relay's pickup current. A higher PSM results in faster relay operation because it represents a more severe fault condition .
• Time Setting Multiplier (TSM): Scales the base operating time derived from the relay's characteristic curve. Increasing TSM delays the relay operation, while decreasing it speeds up the trip .

Steps to Adjust Relay Time

1. Determine Relay Pickup Current: Identify the minimum current at which the relay should operate. This is often calculated using the CT ratio and system load .
2. Set Plug Setting Multiplier (PSM): Adjust the plug position to define the fault current level at which the relay will respond. For example, if the relay should trip at 2.5 times the rated CT current, set PSM = 2.5 .
3. Select Time Setting Multiplier (TSM): Use the relay's time-current characteristic curve to determine the base operating time for the chosen PSM. Multiply this base time by the TSM to get the actual operating time .
4. Check Coordination: Ensure upstream relays have longer operating times than downstream relays to maintain selectivity and prevent unnecessary outages .
5. Verify Settings: Test the relay under simulated fault conditions to confirm that it operates within the desired time range and does not violate equipment thermal limits .

Practical Considerations

• Inverse Time Relays: The operating time decreases as fault current increases, so both PSM and TSM must be coordinated carefully .
• System Changes: If CTs are replaced or system parameters change, recalculate PSM and TSM to maintain proper protection .
• Time Margin: Maintain a small time margin (typically 0.3–0.5 seconds) between relays to ensure proper grading . By carefully adjusting PSM and TSM, you can control the relay's operating time to ensure reliable protection, proper coordination, and minimal disruption to healthy parts of the electrical system.