How to adjust relay protection parameters

Adjusting relay protection parameters involves setting current, time, and thermal thresholds based on system characteristics, motor data, and coordination requirements to ensure safe and selective operation.

Key Parameters and Their Adjustment

1. Plug Setting Multiplier (PSM) PSM determines how many times the actual current exceeds the relay's pickup current. It is crucial for IDMT (inverse definite minimum time) relays to calculate operating time.

• Formula: PSM = Fault Current / Pickup Current
• Example: For a CT 400/5 A, relay current setting 120%, and fault current 10 A (secondary), Pickup = 1.2 × 5 = 6 A, PSM = 10 / 6 = 1.67
• Adjustment: Set PSM according to expected fault currents and IEC 60255-151 standards to ensure proper trip speed . 2. Time Setting Multiplier (TSM) TSM scales the base operating time from the relay's characteristic curve.
• Adjustment: Multiply the base time by TSM to coordinate with upstream and downstream protection. Follow IEC 60255-3 for inverse, very inverse, or extremely inverse curves . 3. Overload (OL) / Thermal Protection Thermal overload settings protect motors and transformers from long-duration overcurrent.
• Typical Settings: Motor protection: 90–110% of FLA; Transformer: 110–130% of rated current
• Adjustment: Use the motor's service factor and full-load amps (FLA) to calculate pickup. Modern relays use I²t algorithms to estimate winding temperature . 4. Earth Leakage (EL) / Earth Fault Pickup EL sets the threshold current for detecting ground faults.
• Adjustment: Set as a percentage of CT primary or secondary current, depending on residual or zero-sequence detection. Follow IEC 61008, IEC 61009, and IEC 60255-151 for sensitivity and trip limits .

Practical Steps for Motor Protection Relays

1. Collect Accurate Data: Motor nameplate (FLA, voltage, service factor, locked rotor current/time, NEMA design, insulation class), CT ratios, system grounding, and fault currents .
2. Set Thermal Overload Pickup: Typically 115–125% of FLA depending on service factor. Ensure the relay trips before thermal damage occurs .
3. Configure Start and Stall Protection: Set start current threshold slightly above running current (~1.5 × FLC) and maximum start time based on measured start time plus margin. Stall current typically 3–6 × FLC with short trip time to prevent rotor overheating .
4. Verify CT Connections: Confirm correct primary/secondary ratios, phase mapping, and polarity to avoid misoperation .
5. Coordinate with System Protection: Adjust TSM and zone settings to ensure selectivity with upstream and downstream relays, considering line impedance and breaker operating times .

Tips for Accurate Adjustment

• Measure actual motor start times and currents rather than assuming typical values to prevent nuisance trips .
• Enable thermal memory in relays to protect motors after short outages or repeated starts .
• Re-evaluate settings during commissioning based on measured system conditions .
• Document all settings and calculations for future reference and maintenance . By carefully following these steps and standards, relay protection parameters can be adjusted to provide reliable, selective, and safe operation of electrical systems.