High voltage relay protection settings include specific current, time, and impedance thresholds tailored to system requirements, such as PSM, TSM, zone reach, and differential relay tap values.

Overcurrent Relay Settings

• Plug Setting Multiplier (PSM): Indicates how many times the relay secondary current exceeds the pickup current. Higher PSM results in faster tripping. Typical PSM values are selected according to IEC 60255-151 standards and depend on fault current levels and coordination requirements .
• Time Setting Multiplier (TSM): Scales the base operating time from the relay's characteristic curve. Typical TSM ranges from 0.1 to 1.0, with lower values giving faster tripping and higher values providing backup coordination .
• Overload (OL) Settings: Thermal overload protection is based on I²t characteristics to prevent sustained overcurrent damage. Values are set according to equipment ratings and IEC 60255-8 standards .

Distance Relay Settings

• Zone 1 Reach: Usually 80–90% of the line impedance for phase faults, providing instantaneous protection without intentional delay .
• Zone 2 Reach: Covers 100% of the protected line plus 50% of the adjacent line impedance, with a time delay to act as backup protection .
• Impedance Characteristics: Mho characteristics are preferred for EHV systems due to high speed and stability, while quadrilateral characteristics are often used for phase-to-ground faults .
• Zero Sequence Compensation: Applied to all zones if line impedance varies or hybrid circuits are used .

Transformer Differential Protection

• TAP Settings: Transformer differential relays (e.g., SEL-787) use per-unit scaling of secondary currents. The ratio TAPmax / TAPmin ≤ 7.5 ensures stability under through-fault conditions .
• Differential Current Thresholds: Set to detect internal faults while avoiding false trips from inrush currents or harmonics .

General Principles

• Selectivity: Only the faulty section is disconnected.
• Sensitivity: Relays detect minor abnormal conditions.
• Speed: Rapid operation minimizes fault damage.
• Reliability: Relays operate correctly without unnecessary tripping . These settings are typically validated during commissioning and may be adjusted based on measured system parameters to ensure proper coordination and protection of HV equipment .