An undervoltage relay, also known as a voltage relay or under-voltage protection relay, is a protective device used in electrical systems to monitor and protect equipment from excessively low voltage conditions. It operates by detecting and responding to voltage levels below predetermined thresholds.
Here’s how an undervoltage relay typically operates:
- Voltage Monitoring: The undervoltage relay continuously monitors the voltage levels in the electrical system. It typically uses voltage transformers (VTs) or potential transformers (PTs) to obtain accurate voltage measurements.
- Voltage Thresholds: The relay is configured with pre-set voltage thresholds or settings. These thresholds define the minimum acceptable voltage levels for the protected equipment. The settings can be defined as absolute voltage values or as a percentage of the nominal voltage.
- Undervoltage Detection: If the measured voltage falls below the pre-set voltage thresholds, indicating an undervoltage condition, the undervoltage relay detects the anomaly. It compares the measured voltage with the pre-set thresholds to determine if the voltage level is below acceptable limits.
- Protective Actions: When an undervoltage condition is detected, the undervoltage relay initiates protective actions to mitigate the potential risks. These actions may include tripping the associated circuit breaker or contactor to isolate the affected equipment or activate other control mechanisms to restore or stabilize the voltage levels.
- Alarm and Indication: In addition to triggering protective actions, the undervoltage relay may also activate alarms or indicators to alert operators or maintenance personnel about the undervoltage condition. This helps ensure timely attention and appropriate corrective measures.
Undervoltage relays offer several benefits in power system protection:
- Equipment Protection: The primary purpose of an undervoltage relay is to protect sensitive electrical equipment from damage caused by excessively low voltage levels. It helps prevent equipment malfunction, reduced performance, and other adverse effects that can result from low voltages.
- System Stability: Undervoltage conditions can lead to system instability, motor stalling, and other operational issues. By promptly detecting and responding to undervoltage events, undervoltage relays help maintain the stability and reliability of the electrical system.
- Selectivity: Undervoltage relays provide selective protection by monitoring specific equipment or circuits. This allows them to target the protection where it is needed most, minimizing unnecessary tripping or intervention in unaffected parts of the system.
- Adjustable Settings: Undervoltage relays often have adjustable voltage settings, allowing for flexibility in adapting to different system requirements and voltage levels. This enables customization of protection schemes to suit specific equipment and operating conditions.
Undervoltage relays are commonly used in power distribution systems, industrial plants, and commercial buildings. They help protect equipment such as motors, generators, transformers, and other sensitive devices from voltage drops or abnormal voltage conditions. It is important to properly set and coordinate undervoltage relays with other protective devices in the system to ensure effective protection and reliable operation.
Here are some key points about undervoltage relays:
- Protect electrical equipment and systems from damage due to low voltage conditions.
- Monitor the voltage and trip when it drops below the preset pickup threshold.
- Can be electromechanical or digital (numeric) relays. Electromechanical types use coils and contacts.
- Operate on either a definite time or inverse definite minimum time basis. Definite time has a fixed delay, inverse time has reducing delay for lower voltages.
- Main types are single phase and triple pole relays for three phase monitoring. Can monitor phase-to-phase or phase-to-neutral voltages.
- Tripping can trigger an alarm, disconnect non-critical loads, or trip breakers to disconnect equipment or feeders.
- Provide protection against risks like motor stalling, erroneous tripping of other relays, equipment damage during undervoltage.
- Used in various applications like generators, transformers, motors, transmission lines, industrial plants.
- Setting the pickup voltage threshold and time delay is crucial to avoid nuisance tripping.
- Enhance reliability by isolating or shutting down equipment before severe damage due to low voltage.
- Coordinate with other relays like overload relays to distinguish between electrical faults and normal low voltage operation.