The braking current of a relay is calculated based on the relay's pickup current, CT ratio, and plug setting multiplier (PSM), ensuring selective fault clearing and proper coordination with upstream and downstream relays.

Understanding Key Parameters

1. Pickup Current (I_pickup): The minimum current at which the relay starts to operate. It is usually set above the maximum load current to avoid nuisance tripping. Formula: I_pickup = PSM × Rated CT secondary current ( ).
2. Plug Setting Multiplier (PSM): Ratio of the desired pickup current to the rated CT current. It adjusts the relay sensitivity: PSM = I_pickup / I_rated ( ).
3. Time Multiplier Setting (TMS): Determines the operating time of the relay according to the selected inverse time characteristic (Standard, Very Inverse, Extremely Inverse, or Long Time Inverse) defined in IEC 60255 ( ).
4. Fault Current (I_fault): The prospective short-circuit current at the relay location, calculated using IEC 60909 or system analysis tools ( ).

Step-by-Step Calculation

1. Determine the CT ratio: Identify the primary and secondary ratings of the current transformer (CT). For example, a 200/5 A CT converts 200 A primary current to 5 A secondary current.
2. Set the Pickup Current: Decide the relay pickup current based on load current and safety margin. For instance, if the load current is 150 A and you choose 125% of load, the pickup current is 187.5 A. Round to the nearest standard plug setting, e.g., 200% → 250 A ( ).
3. Calculate PSM: PSM = I_pickup / I_CT_secondary. Using the example above, if CT secondary is 5 A, PSM = 250 / 5 = 50.
4. Select Relay Curve and TMS: Choose the appropriate inverse time characteristic and calculate the operating time using: Operating Time = TMS × Time from relay curve for given PSM ( ).
5. Determine Braking Current: The braking current is the minimum fault current at which the relay will operate within the desired time. It is calculated by comparing the fault current to the pickup current and applying the relay's time-current characteristic: I_braking ≥ I_pickup × Coordination Factor, where the coordination factor ensures downstream relays operate first ( ).

Practical Considerations

• Use time grading to ensure selectivity: the relay closest to the fault operates first, while upstream relays provide backup ( ).
• For inverse time relays, the operating time decreases as fault current increases, which helps in faster clearing of high-magnitude faults.
• Always verify calculations against CT saturation limits, relay thermal ratings, and system short-circuit withstand capacities ( ). By following these steps, you can calculate the braking current accurately, ensuring reliable and selective protection of the electrical network.