Low voltage in relay protection exists due to a drop in system voltage below preset thresholds, which can result from faults, overloads, or transformer issues.

Causes of Low Voltage

Low voltage, or under-voltage, occurs when the electrical potential in a system falls below the normal operating range. Common causes include:

• Electrical faults such as short circuits or line-to-ground faults that reduce voltage levels.
• Increased load demand, which can cause voltage drops if the system cannot supply sufficient power.
• Loss of incoming transformers or failure in transformer earthing, leading to insufficient voltage supply.
• Overloading of transformers, which reduces output voltage under heavy demand conditions .

Detection by Under-Voltage Relays

Under-voltage relays, also known as low voltage relays, are designed to monitor system voltage continuously. They operate using potential transformers (PTs) to sense phase-to-phase or phase-to-neutral voltages. When the voltage drops below a preset threshold (commonly 85–90% of the rated voltage), the relay triggers a trip signal to disconnect the affected circuit . This prevents equipment damage and maintains system stability.

Operational Principle

Voltage protection relays function by comparing the measured voltage against preset minimum and maximum limits. If the voltage falls below the minimum:

• The relay activates and sends a trip command to the circuit breaker.
• The system disconnects the affected section to prevent damage to generators, motors, or transformers.
• Time delays are often incorporated to avoid tripping due to transient dips . Modern relays may include microprocessors, logic units, and communication interfaces for precise monitoring, real-time analysis, and integration with SCADA systems, enhancing reliability and response speed .

Summary

Low voltage in relay protection arises from voltage differences caused by faults, overloads, or transformer failures. Protective relays detect these conditions using potential transformers and predefined voltage thresholds, ensuring that circuits are disconnected promptly to safeguard equipment and maintain system stability .