What is the Frequency Relay and How Does It Work?

A frequency relay is a protective device used in electrical systems to monitor the frequency of the power supply and initiate appropriate actions or alarms when the frequency deviates from the normal operating range. Its primary function is to detect abnormal frequency conditions and protect the power system from potential damage or instability.

Fig-Frequency Measurement Idea, credit Wikimedia

Frequency Relay and Its Working Principle

The frequency relay typically operates based on a set trip point or threshold. When the frequency exceeds or falls below the pre-determined threshold, the relay initiates a trip signal to activate circuit breakers or other protective devices. The trip threshold is usually set based on the normal operating frequency range of the system, which is typically around 50 or 60 Hertz, depending on the region.

The operation of a frequency relay involves the following basic steps:

Frequency Measurement: The relay continuously measures the frequency of the power supply using a built-in frequency measurement circuit. This circuit typically includes a frequency sensor or input module that detects the frequency of the electrical signal.

Threshold Comparison: The measured frequency is compared to the pre-set trip threshold. If the measured frequency exceeds or falls below the threshold, the relay recognizes it as an abnormal condition.

Relay Operation: When the measured frequency crosses the trip threshold, the frequency relay activates its output contacts. These contacts can be used to trigger alarms, send control signals to other devices, or directly trip circuit breakers to isolate the affected part of the power system.

Time Delay: In some cases, the frequency relay may include a time delay feature to avoid unnecessary trips caused by temporary or minor frequency variations. The time delay allows the relay to confirm that the frequency deviation is sustained and not a transient event before initiating the trip signal.

Frequency relays are essential for maintaining the stability and integrity of the power system. They play a crucial role in protecting generators, motors, and other sensitive equipment from damage due to frequency deviations. Additionally, frequency relays are used in load-shedding schemes to shed non-critical loads during low-frequency conditions, helping to restore system stability and prevent a complete blackout.

Types of Frequency Relay Used in Electrical Substations

There are several types of frequency relays used in electrical substations to monitor and protect the power system. The selection of a specific type of frequency relay depends on the requirements and characteristics of the substation. Here are some commonly used frequency relays:

Over/Under Frequency Relay: This type of relay is used to monitor the frequency of the power system and protect against abnormal frequency conditions. It operates when the frequency exceeds or falls below a pre-set threshold, indicating a potential issue with the system's stability.

Rate of Change of Frequency (ROCOF) Relay: ROCOF relays monitor the rate at which the frequency changes over time. They are designed to detect rapid changes in frequency, which can be an indication of a disturbance or fault in the power system. ROCOF relays are commonly used for detecting and responding to system events such as loss of generation or loss of load.

Under-Frequency Load Shedding Relay: This relay is specifically designed to shed or disconnect non-critical loads when the frequency of the power system falls below a predetermined set point. It helps maintain system stability by shedding loads and preventing further frequency decline.

Frequency Rate of Change Relay: This type of relay operates based on the rate of change of frequency (ROCOF) and is used to detect sudden frequency deviations. It is particularly useful in detecting events such as generator trips or load rejections, which can cause rapid frequency changes.

Vector Surge Relay: Vector surge relays are used to protect synchronous generators against loss of synchronism and excessive frequency deviations. They monitor the frequency and phase relationship between the generator and the power system and operate if a loss of synchronism condition is detected.

Automatic Generation Control (AGC) Relay: AGC relays are used to automatically adjust the power output of generators to maintain the desired system frequency. They receive frequency signals from the power system and provide control signals to the generators to regulate their output based on the system frequency.

These are just a few examples of frequency relays commonly used in electrical substations. The specific relay types and configurations may vary depending on the design and requirements of the substation and the power system it serves. Different Types of Relays and Their working Procedure are discussed in detail in another episode.