What is the Voltage Relay and How does It Work?

A voltage relay, also known as a voltage protection relay, is a type of protective relay that monitors the voltage level in an electrical system and initiates appropriate actions when certain voltage conditions are met. It is commonly used in substations and power systems to protect equipment and ensure system stability. 

The Voltage Relay and Its Working Principle

The voltage relay operates based on predetermined voltage thresholds and specific settings programmed by the operator or engineer.

Here's a general overview of how a voltage relay works:

Voltage Sensing: The voltage relay continuously monitors the voltage level of the electrical system it is connected to. It typically has voltage sensing inputs that are connected to potential transformers (PTs) or voltage transformers (VTs) to measure the system voltage. These transformers step down the voltage to a level suitable for the relay's input.

Voltage Thresholds: The voltage relay is configured with predetermined voltage thresholds. These thresholds define the acceptable voltage range for normal operation and trigger points for abnormal conditions. Common voltage thresholds include overvoltage (high voltage) and Undervoltage (low voltage) settings.

Protective Actions: When the monitored voltage level exceeds or falls below the programmed thresholds, the voltage relay initiates protective actions. These actions can include activating alarms, tripping circuit breakers to isolate the affected section, or sending signals to other protective relays for coordinated protection.

Time Delays: Voltage relays often incorporate time delay settings to prevent false trips during transient voltage fluctuations. Time delays allow the relay to observe the voltage condition for a specific duration before initiating any protective action. This helps differentiate between temporary disturbances and sustained abnormal voltage conditions.

Resetting and Reclosing: Once the voltage returns to within the acceptable range, the voltage relay resets and prepares for normal operation. In some cases, if the voltage condition stabilizes within certain criteria, the relay may initiate automatic reclosing of the circuit breakers to restore power after a temporary fault.

It's worth noting that voltage relays can have additional features and functionalities based on the specific application and system requirements. Some relays may incorporate communication capabilities to exchange information with other relays or a supervisory control and data acquisition (SCADA) system, enabling remote monitoring and control.

Overall, voltage relays play a crucial role in protecting electrical systems from voltage-related abnormalities and ensuring the reliability and stability of the power infrastructure.

Types of Voltage Relay Used in Electrical Substation

In substations, various types of voltage relays are used to monitor and protect the electrical system. Here are some commonly used types of voltage relays in substations:

Overvoltage Relays: These relays are designed to detect and protect against excessive voltage levels in the system. They are used to prevent equipment damage caused by voltage spikes or transient overvoltages.

Undervoltage Relays: Undervoltage relays monitor the system for voltage drops below a certain threshold. They are used to detect power supply failure, phase imbalances, or abnormal voltage conditions that can lead to equipment malfunction or tripping of protective devices.

Overfrequency Relays: These relays monitor the frequency of the electrical system and protect against overfrequency conditions. They are typically used in generators and grid-connected systems to prevent excessive rotational speeds or damage to connected equipment.

Underfrequency Relays: Underfrequency relays are used to monitor the frequency of the electrical system and protect against underfrequency conditions. They detect a decrease in frequency below a specified threshold, which can indicate a power supply shortage or an abnormal operating condition.

Voltage Balance Relays: These relays are used to monitor the balance between the phases of a three-phase system. They detect voltage imbalances between phases and can trip protective devices to prevent equipment damage or phase-to-phase faults.

Synchronizing Relays: Synchronizing relays are used in substation switchgear to ensure the synchronization of two or more power sources before connecting them. They monitor the voltage, frequency, and phase angle of the sources and provide indications to operators for safe synchronization.

Voltage Differential Relays: These relays detect voltage differences between two points in an electrical system. They are commonly used to protect transformers and can quickly detect faults such as internal winding faults or phase-to-phase short circuits.

It's worth noting that the specific types of voltage relays used in a substation depend on the design, configuration, and requirements of the electrical system. Different substations may employ different combinations of relays to achieve the desired protection and control functions. Different Types of Relays and Their working Procedure are discussed in detail in another episode.