Setting of relay protection parameters
Relay protection settings are critical for ensuring system reliability, selectivity, and fast fault isolation, with key parameters including PSM, TSM, impedance reach, and transformer differential thresholds.
Key Parameters
1. Plug Setting Multiplier (PSM) PSM indicates how many times the actual current exceeds the relay's pickup current. It is essential for IDMT (Inverse Definite Minimum Time) relays to determine operating time. The formula is: PSM = Fault Current / Pickup Current, where Pickup Current = Current Setting × CT Secondary Rating. A higher PSM results in faster relay operation, and settings must comply with IEC 60255-151 standards for overcurrent relays (e.g., CT: 400/5A, Plug Setting: 120%, Fault Current: 10A → PSM = 1.67) . 2. Time Setting Multiplier (TSM) TSM scales the base operating time derived from the relay's characteristic curve. It ensures proper coordination with downstream relays and is calculated according to IEC 60255-3, which defines standard inverse, very inverse, and extremely inverse time-current characteristics . 3. Impedance/Distance Settings For transmission line protection, relays use impedance-based characteristics (Mho or Quadrilateral). Zone 1 reach is typically set to 80–90% of line impedance to avoid overreaching. Resistive components like tower footing and arc resistance are included in calculations . 4. Transformer Differential Settings Differential relays (e.g., SEL-787) use TAP scaling to convert secondary currents into per-unit values. The sum of incoming currents equals 1.0 and outgoing currents –1.0 under full-load conditions. The ratio TAPmax / TAPmin ≤ 7.5 ensures stability and sensitivity .
Practical Considerations
- Coordination and Selectivity: Relay settings must ensure that only the faulty section is isolated while maintaining system stability .
- Fault Level Calculations: Determine maximum and minimum fault currents for proper relay sensitivity .
- Testing and Commissioning: Settings should be verified during commissioning using actual measured values to ensure reliability and correct operation .
- Relay Types: Settings vary for overcurrent, differential, distance, directional, and special protection relays, each requiring specific parameter calculations .
Summary
Relay protection parameter settings involve careful calculation of PSM, TSM, impedance reach, and differential thresholds, coordinated with system design and fault analysis. Properly configured relays ensure fast, selective, and reliable fault isolation, protecting transformers, lines, and other critical substation equipment while maintaining overall system stability .
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