What are the set values ​​for relay protection

Relay protection settings are defined by parameters such as pickup current, plug setting multiplier (PSM), time setting multiplier (TSM), overload (OL), earth leakage (EL), and multiplying factor (MF), which together determine when and how a relay trips during a fault.

Key Relay Settings

1. Pickup Current (Current Setting): This is the minimum current at which the relay will operate. It is usually expressed as a percentage of the rated secondary current of the current transformer (CT). For overcurrent relays, typical settings range from 50% to 200% in steps of 25%, while earth fault relays are often set between 10% and 70% in steps of 10% . The pickup current ensures the relay only responds to currents above normal operating levels. 2. Plug Setting Multiplier (PSM): PSM indicates how many times the actual fault current exceeds the relay's pickup current. It is calculated as: PSM = Fault Current / Pickup Current For example, if a CT has a ratio of 400/5 A, the relay current setting is 120%, and the secondary fault current is 10 A, the pickup current is 6 A, giving a PSM of 1.67 . A higher PSM results in a faster relay operation according to the inverse definite minimum time (IDMT) curve. 3. Time Setting Multiplier (TSM): TSM scales the operating time derived from the relay's characteristic curve. It allows coordination between relays so that the closest relay to the fault trips first, while backup relays operate with a delay. TSM is selected according to IEC 60255-3 standards and depends on the type of IDMT curve (standard inverse, very inverse, extremely inverse), . 4. Overload Setting (OL): This setting protects equipment from prolonged overcurrent conditions. It is typically based on the thermal rating of the equipment and ensures the relay trips before overheating occurs . 5. Earth Leakage / Earth Fault Setting (EL): EL defines the current threshold for detecting ground faults. It is often set as a percentage of the rated current or as an absolute value, depending on system requirements. For example, a typical earth fault pickup might be 125 A with a time delay of 0.8 seconds . 6. Multiplying Factor (MF): MF, also called the metering or scaling factor, adjusts the relay readings to account for CT ratios or transformer tap settings. It ensures that the relay interprets the measured current correctly in per-unit or actual values .

Practical Application

• Coordination: Pickup current and TSM are adjusted to ensure selective tripping, so only the relay closest to the fault operates immediately, while upstream relays act as backup.
• Example: For a transmission line, zone 1 reach may be set to 80% of line impedance, with overcurrent backup relay pickup at 120% of line current and a time delay of 0.8 seconds .
• Relay Curves: Choose the appropriate IDMT curve type (standard, very inverse, extremely inverse) based on system characteristics and fault clearing requirements . By carefully setting these parameters, relays provide fast, selective, and reliable protection, preventing equipment damage and maintaining system stability.