Calculation Principle of Relay Protection Setting

Relay protection settings are calculated to ensure selective, fast, and reliable fault isolation by determining pickup current, plug setting multiplier, and time multiplier according to system fault currents and relay characteristics.

Key Principles

1. Pickup Current (Ip or Current Setting): The pickup current is the threshold at which a relay begins to operate, overcoming the controlling force of the relay mechanism. It is typically set slightly above the maximum load current to avoid nuisance tripping while ensuring sensitivity to fault currents. The current setting can be adjusted by changing the relay coil turns or using a percentage of the CT's rated secondary current . 2. Plug Setting Multiplier (PSM): PSM is the ratio of the actual fault current to the relay's pickup current. It determines how fast the relay operates according to the inverse definite minimum time (IDMT) curve. A higher PSM results in faster tripping, which is critical for protecting equipment during high fault currents . 3. Time Setting Multiplier (TSM or TMS): TSM scales the base operating time derived from the relay's characteristic curve. It is used to achieve proper coordination between upstream and downstream relays, ensuring that the relay closest to the fault operates first. Lower TSM values result in faster tripping, while higher values provide backup protection . 4. Inverse Time Characteristics: Relays often use inverse time-current characteristics, where the operating time decreases as fault current increases. Common curves include Standard Inverse (SI), Very Inverse (VI), Extremely Inverse (EI), and Long Time Inverse (LTI), defined by IEC 60255 standards. These curves help match relay operation to the thermal withstand of protected equipment . 5. Coordination and Grading: Time grading ensures selectivity by setting incremental delays between relays along a feeder. The grading time is the difference in operating times between consecutive relays, allowing the relay nearest the fault to trip first while upstream relays act as backup. Inverse time relays generally require longer grading times to account for measurement inaccuracies and high fault currents . 6. Zone and Impedance Settings: For distance or differential protection, zone reach is set based on line impedance or transformer ratings. Zone 1 is typically set to 80–90% of the line impedance for instantaneous operation, while Zone 2 includes adjacent line impedance with a time delay to provide backup protection .

Calculation Workflow

1. Determine maximum load current and fault current at each relay location.
2. Set pickup current slightly above load current to avoid false trips.
3. Calculate PSM using the ratio of fault current to pickup current.
4. Select the appropriate relay curve type (SI, VI, EI, LTI) based on system requirements.
5. Determine base operating time from the curve and apply TSM for coordination.
6. Define grading times between relays to ensure selectivity.
7. Verify settings against equipment thermal limits and system stability criteria. By following these principles, relay settings ensure fast fault clearance, minimal supply interruption, and protection selectivity, which are essential for safe and reliable operation of electrical networks .