How to set the switching time for relay protection

The switching time of a protection relay is set by adjusting the pickup current, plug setting multiplier (PSM), and time setting multiplier (TSM) to achieve selective and coordinated tripping.

Key Concepts

Pickup Current (Iₚ): This is the minimum current at which the relay begins to operate. It is usually set slightly above the maximum load current to avoid nuisance tripping while ensuring the relay responds to faults . Plug Setting Multiplier (PSM): PSM is the ratio of the actual fault current to the relay's pickup current. It determines how quickly the relay will operate according to its inverse time characteristic. Higher PSM values result in faster tripping . Time Setting Multiplier (TSM): TSM scales the relay's base operating time derived from its characteristic curve. Adjusting TSM allows coordination between upstream and downstream relays, ensuring the relay closest to the fault trips first. Lower TSM values produce faster tripping, while higher values delay operation for backup coordination .

Steps to Set Switching Time

1. Determine Load and Fault Currents: Identify the maximum load current and the expected fault currents at each relay location .
2. Set Pickup Current: Choose a pickup current slightly above the maximum load current, typically 125–150% of full load, to prevent false trips .
3. Calculate PSM: Divide the expected fault current by the pickup current. This value will be used to reference the relay's time-current characteristic curve .
4. Select Relay Curve Type: Choose an appropriate inverse time characteristic (Standard, Very Inverse, Extremely Inverse, or Long-Time Inverse) based on network configuration and fault current variations .
5. Adjust TSM: Multiply the base operating time from the relay curve by the TSM to achieve proper coordination. Downstream relays should have smaller TSMs for faster tripping, while upstream relays have larger TSMs to act as backup .
6. Verify Coordination: Use selectivity diagrams or time-current curves to ensure that each relay trips in the correct sequence, isolating only the faulted section .
7. Test and Fine-Tune: Simulate fault conditions or use relay testing equipment to confirm that the relay operates at the desired time and current settings .

Practical Tips

• For radial networks, time-graded protection with inverse time relays is effective. Definite time relays can be used for instantaneous or high-set stages .
• Ensure the relay starts at 1.1–1.3 times the set current depending on the inverse characteristic used .
• Always consider the thermal and mechanical limits of network components when setting relay times to prevent equipment damage . By carefully setting pickup current, PSM, and TSM, and verifying coordination with selectivity diagrams, you can achieve reliable and selective relay protection that minimizes network disruption while protecting equipment.