Adjusting relay protection values ​​in power systems

Relay protection values are adjusted by calculating fault currents, selecting appropriate pickup and time settings, and coordinating relays to ensure selective and reliable operation. Key Parameters for Relay Adjustment1. Plug Setting Multiplier (PSM): PSM defines how many times the actual current exceeds the relay's pickup current.

It is crucial for inverse definite minimum time (IDMT) relays, determining the basic operating time. A higher PSM results in faster tripping, while a lower PSM slows the relay response.

PS

M settings must comply with IEC 60255-151 standards to ensure proper relay performance under fault conditions. 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, ensuring backup protection.

Lower TS

M values result in faster tripping, while higher values delay operation to allow downstream relays to clear faults first. 3. Overload and Earth Fault Settings: Overload (OL) and Earth Leakage (EL) settings define the relay's sensitivity to thermal overloads and ground faults.

These values are selected based on maximum load currents, minimum fault currents, and system voltage levels to maintain both sensitivity and selectivity.

Steps for Adjusting Relay Values

Fault Current Calculations: Determine maximum and minimum fault currents for all types of faults (single line-to-ground, line-to-line, three-phase). This ensures the relay can detect faults without tripping under normal load conditions. Select Pickup Currents: Set the relay pickup current slightly above the maximum load current to avoid nuisance tripping.

For overcurrent relays, typical settings are 120–125% of line current. Time Coordination: Use TSM to create a time grading scheme. Downstream relays should trip faster than upstream relays to maintain selectivity.

For distance relays, zone reach and impedance settings are calculated to cover the intended line segment without overreaching. Transformer and Bus Protection: For transformer differential relays, use per-unit scaling (TAP settings) to normalize currents from different windings. Ensure TAPmax/TAPmin ratios comply with manufacturer recommendations to maintain sensitivity and stability.

Validation and Testing: After setting values, perform relay coordination studies and simulation tests to verify that relays operate correctly under all fault scenarios. Adjust settings iteratively if necessary.

Best Practices

Always follow IEC standards for relay characteristics and coordination. Consider system changes such as new lines or transformers, which may require recalculating settings. Document all settings and maintain a relay setting record for future reference and maintenance.

Use digital or numerical relays where possible, as they allow easier adjustment, monitoring, and coordination analysis. By carefully calculating fault currents, selecting appropriate PSM/TSM values, and coordinating relays, power system engineers can ensure reliable, selective, and safe operation of the electrical network.