Application of Relay Protection Setting Calculation

Relay protection settings are determined by calculating pickup currents, time multipliers, and coordination parameters to ensure selective and reliable fault isolation in electrical networks.Key Parameters in Relay Settings1. Plug Setting Multiplier (PSM): PSM represents how many times the actual current exceeds the relay's pickup current. It is crucial for IDMT (Inverse Definite Minimum Time) relays, as it determines the operating time based on the fault current magnitude. A higher PSM results in faster tripping, while a lower PSM delays operation to allow downstream relays to act first . 2. Time Setting Multiplier (TSM): 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. Lower TSM values produce faster trips, while higher TSM values provide backup protection . 3. Overload (OL) and Earth Leakage (EL) Settings:OL protects against thermal overloads by tripping when current exceeds a thermal limit.EL or Earth Fault settings detect ground faults and isolate the affected section . 4. Multiplying Factor (MF): MF is used to scale relay readings for metering or to convert secondary currents into per-unit values for accurate relay operation .Calculation Principles1. Pickup Current Calculation: The pickup current is typically set above the maximum load current to avoid nuisance tripping. For example, a relay may be set at 125% of full load current, then rounded to the nearest standard plug setting . 2. Time-Current Coordination: Using IEC 60255 standard curves (Standard Inverse, Very Inverse, Extremely Inverse), the relay operating time is calculated as a function of PSM and TSM. The goal is to ensure selectivity, where the relay closest to the fault trips first, and upstream relays act as backup . 3. Distance Protection Settings (for EHV lines):Mho characteristics are preferred for phase faults due to high-speed operation.Quadrilateral characteristics are used for phase-to-ground faults.Zone 1 reach is typically set to 80–90% of line impedance, including resistive components like tower footing and arc resistance .Zero-sequence compensation factors can be applied to adjust for line impedance variations. 4. TAP Scaling for Transformer Protection: For transformer differential or overcurrent relays, TAP scaling converts secondary currents into per-unit values, ensuring consistent relay operation across multiple windings. The ratio TAPmax/TAPmin ≤ 7.5 is recommended for proper scaling .Practical Steps for Relay SettingDetermine maximum load current and fault current at the relay location.Calculate pickup current using PSM and load margin.Select relay characteristic curve and compute operating time using TSM.Verify coordination with downstream and upstream relays to maintain selectivity.Adjust settings for line impedance, transformer ratings, and system topology.Document settings and perform commissioning tests to validate calculations .Tools and ReferencesInteractive calculators can compute PSM, TMS, operating time, and coordination intervals using fault current and CT ratios .Relay manuals and IEC 60255 standards provide guidance for IDMT and distance relay settings .Coordination studies ensure reliable operation across HV, MV, and LV networks . By carefully calculating and coordinating these settings, protective relays can isolate faults quickly and selectively, minimizing equipment damage and maintaining system stability.