How to calculate the current of a relay protection device

The current in relay protection is calculated by determining the secondary current seen by the relay using CT ratios, relay pickup settings, and fault current levels.

Step 1: Determine the Relay Pickup Current

The pickup current is the minimum current at which the relay begins to operate. It is set as a percentage of the CT's rated secondary current. For example, if a relay is connected to a CT with a 1 A secondary rating and the desired pickup is 150%, the relay pickup current is: Pickup Current = CT Secondary Rating × Current Setting Pickup Current = 1 A × 150% = 1.5 A

Step 2: Convert Primary Fault Current to Relay Secondary Current

The relay sees a reduced current based on the CT ratio. If the primary fault current is known, the secondary current is calculated as: Relay Secondary Current = Primary Fault Current ÷ CT Ratio For instance, if the primary fault current is 1500 A and the CT ratio is 100:1: Relay Secondary Current = 1500 ÷ 100 = 15 A

Step 3: Calculate the Plug Setting Multiplier (PSM)

The PSM is the ratio of the relay secondary current to the relay pickup current: PSM = Relay Secondary Current ÷ Relay Pickup Current Using the previous example: PSM = 15 ÷ 1.5 = 10

Step 4: Determine Relay Operating Time

The relay operating time is obtained from the Time vs. PSM curve (IEC 60255 standard curves: Standard Inverse, Very Inverse, Extremely Inverse, Long Time Inverse). The operating time can be adjusted using the Time Multiplier Setting (TMS): Actual Operating Time = Time from Curve × TMS If the curve indicates 3 seconds for PSM = 10 and TMS = 0.1: Actual Operating Time = 3 × 0.1 = 0.3 seconds

Step 5: Consider Coordination and System Settings

For proper protection, ensure relay settings are coordinated with downstream relays to isolate only the faulted section. This involves calculating maximum load current, minimum fault current, and adjusting pickup and time settings to maintain selectivity and reliability .

Summary

To calculate the current in relay protection:

1. Set the relay pickup current based on CT secondary rating and desired percentage.
2. Convert the primary fault current to the relay secondary current using the CT ratio.
3. Compute the PSM to determine how many times the fault exceeds the pickup.
4. Use the relay's time-current curve and TMS to find the operating time.
5. Verify coordination with other relays to ensure selective fault clearing. This method applies to both overcurrent and distance relays, with distance relays additionally considering voltage inputs and impedance calculations for line protection .