Protective relays typically operate on secondary voltages of 110V AC or 120V DC for voltage relays, and 1A or 5A for current relays, depending on the system and CT/VT ratings.

Voltage Relay Considerations

Voltage relays are designed to detect abnormal voltage conditions such as overvoltage, undervoltage, and voltage imbalance. The voltage applied to the relay is usually derived from a voltage transformer (VT), which steps down the high system voltage to a safe, standardized secondary voltage. Common secondary voltages for relays are 110V AC or 120V DC, which are compatible with most protective relay designs and ensure safe operation while maintaining accuracy .

Current Relay Considerations

Current relays receive input from current transformers (CTs), which reduce the line current to a manageable secondary current, typically 5A or 1A. The CT ratio is selected slightly higher than the maximum expected line current to ensure proper relay operation without saturation . The secondary current level must match the relay's rated input to maintain sensitivity and selectivity.

Key Factors Affecting Voltage Selection

1. System Voltage Level: High-voltage systems require VTs to step down to standard relay voltages, while low-voltage systems may connect directly to relays or use smaller VTs .
2. Relay Type: Numerical relays are more tolerant of voltage variations, whereas electromechanical relays may require precise secondary voltages to avoid misoperation .
3. Accuracy and Burden: The relay's burden and the CT/VT class (e.g., C-class for low leakage, T-class for high leakage) determine the maximum voltage that can be applied without affecting performance .
4. Protection Coordination: Voltage levels must ensure that relays operate correctly under all fault conditions, including minimum fault voltage scenarios, to maintain selectivity and prevent nuisance tripping .

Practical Guidelines

• Use 110V AC or 120V DC for voltage relays in HV/MV substations.
• Use 5A secondary for current relays in most applications; 1A may be used for compact or low-burden systems.
• Ensure CT and VT ratings are compatible with relay input and system fault levels.
• Verify relay operation under minimum and maximum voltage conditions to ensure reliable protection . By following these guidelines, protective relays can reliably detect abnormal conditions and coordinate with other relays to isolate faults efficiently, minimizing system disruption.