How to configure relay protection time
Relay protection time is set by calculating the Plug Setting Multiplier (PSM), determining the Time Setting Multiplier (TSM), and using the relay's time/PSM curve to achieve proper coordination.
Key Parameters
- Plug Setting Multiplier (PSM): This is the ratio of the fault current to the relay's pickup current. It determines how much the actual current exceeds the relay's pickup and directly affects the operating time on IDMT (Inverse Definite Minimum Time) curves. Higher PSM values result in faster relay operation .
- Time Setting Multiplier (TSM): TSM scales the base operating time obtained from the relay's characteristic curve. It allows coordination between upstream and downstream relays. A lower TSM results in faster tripping, while a higher TSM delays operation to ensure selective fault clearance .
- Time/PSM Curve: This graph shows the relationship between relay operating time and PSM. By plotting the curve, you can determine the base operating time for a given fault current and then adjust it using TSM to achieve the desired trip time .
Step-by-Step Procedure
- Calculate Pickup Current: Determine the relay's pickup current based on the system's current transformer (CT) ratio and the maximum load current. This ensures the relay does not trip under normal operating conditions .
- Convert Fault Current to Relay Coil Current: Divide the expected fault current by the CT ratio to find the current that flows through the relay coil .
- Determine PSM: Divide the relay coil current by the pickup current to obtain the Plug Setting Multiplier .
- Read Base Operating Time: Using the relay's time/PSM curve, find the operating time corresponding to the calculated PSM at TSM = 1 .
- Apply TSM: Multiply the base operating time by the chosen Time Setting Multiplier to get the actual relay operating time. Adjust TSM to ensure proper coordination with upstream and downstream relays .
- Check Coordination: Ensure that the relay nearest to the fault trips first and that upstream relays operate with a sufficient time margin to provide backup protection. Plotting all relay curves on a common scale helps verify selective operation .
Practical Tips
- Use relays with the same operating characteristic in series whenever possible for easier coordination .
- Maintain a time margin between relays to prevent simultaneous tripping of upstream and downstream devices .
- For numerical relays, you can directly set the operating time using the relay's time dial or software interface, which simplifies the process compared to electromechanical relays . By following these steps and carefully adjusting PSM and TSM, you can set relay protection times that ensure selective, reliable, and safe operation of the power system.
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