Everything You Need to Know About Frequency Relays

Frequency relays are considered one of the essential tools in the field of electricity and electronics. These devices control and manage currents and voltages, and have established their place in various industries for decades using physical and electronic principles. The primary applications of this technology are in industrial power, automation, control of electrical devices, industrial automation systems, and protection against electrical hazards.

In this article, we introduce frequency relays, explore their developmental history, discuss their different types, advantages and disadvantages, leading brands, applications, operating mechanisms, and structures. Aimed at providing a comprehensive review of this technology, this article offers readers insights into one of the important tools in the field of electricity and electronics.

Frequency relays are electronic devices sensitive to various frequencies and are used to control and manage electrical and electronic equipment. These devices operate based on electronic and physical principles and are used to transmit electrical signals of different frequencies.

The performance of frequency relays is influenced by electrical inputs, typically including voltage and current. With inputs of varying frequencies, their performance and output change accordingly. These relays are made from components such as transistors, MOSFETs, and thyristors and are available in various types for diverse applications. They are used in controlling electrical devices across various industries, including the automotive industry, the power industry, and control and automation systems.

The operating method of frequency relays is explained through the following stages:

Frequency Measurement: Frequency relays measure the electrical system’s frequency using specialized sensors or electronic circuits. This measurement can be direct, meaning the frequency is directly measured by the relay, or indirect, meaning the frequency is calculated based on observing the voltage or current waveform.

Comparison with the Predetermined Range: The measured frequency is compared with a predetermined range. If the frequency is within the specified range, no action is taken, and the system continues to operate normally. However, if the frequency falls outside this range, the next stage is activated.

Activation of Protective Function: If the frequency deviates from the predetermined range, the protective component of the relay is activated. This component could be a circuit breaker, a switch, or any other device controlled by the relay. The purpose of this stage is to prevent damage to electrical equipment and maintain system stability.

Execution of Protective Operations: Once the protective component is activated, the necessary protective operations are carried out. This includes interrupting or temporarily cutting off the power, disconnecting equipment from the network, or performing any other operations required to prevent damage to the electrical equipment or system.

In general, frequency relays contribute to the stability and safety of the system by measuring and comparing the electrical system’s frequency and performing necessary protective operations. These devices operate automatically and reduce the risks and damages caused by frequency fluctuations without the need for human intervention.

History of Frequency Relays

The history of frequency relays dates back to the early 20th century when electrical engineers began developing these devices. Over the years, these developments transitioned from electromechanical to electronic and then to digital technologies.

Early Development: The concept of frequency protection originated in the early 20th century. The German-American electrical engineer Charles Proteus Steinmetz invented the frequency relay. This development was crucial for managing the operation of generators within the lower frequency range.

Selective Protection: The advent of selective protection is attributed to around 1899 when the Niagara power plants required selective line protection for their 11 kV network.

Directional Relays: Known as induction relays, directional relays were introduced around 1903. These devices were primarily used to shut down generators operating in parallel by reversing the direction of energy flow, marking a significant advancement in protective relay technology.

Solid-State Relays: In the 1950s, solid-state relays emerged, composed of electronic components like diodes, transistors, and operational amplifiers. These types of relays offered greater flexibility and eliminated many of the drawbacks of electromechanical relays.

Evolution of Digital Protection Relays

In the 1960s and 1970s, digital protection relays entered the market. The first fully digital protective relay was introduced commercially in 1984 by the Schweitzer Engineering Laboratories (SEL). These relays represent a significant technological leap, offering the capability to emulate the performance of many discrete electromechanical relays in a single device.

General Structure: A relay consists of several key components that work together to perform its protective function. These components include the coil, armature, contacts, spring, and yoke. The coil creates a magnetic field when an electric current passes through it, activating the armature. The armature is connected to the relay contacts and moves when the magnetic field is activated. The contacts create or break the circuit path. The spring holds the armature in place and regulates the switching characteristics of the relay. Finally, the yoke provides a low-reluctance path for the magnetic flux to activate the armature.

Frequency Relay Components

In addition to the primary components, frequency relays are comprised of additional or modified components integrated for monitoring the electrical system’s frequency and executing protective actions. These components include:

1. Frequency Measurement Component: This component is designed to measure the frequency of the electrical system and may be either a direct frequency measuring device or an indirect method that calculates frequency based on the voltage or current waveform.

2. Comparator or Processor: This component compares the measured frequency with a predefined range and activates protective actions if the frequency falls outside this range.

3. Protective Action Component: This component can be a circuit breaker, a switch, or another device controlled by the relay to isolate faults or disturbances from the rest of the system.

The operational principles of a frequency relay involve continuous monitoring of the electrical system’s frequency. When the frequency deviates from a predefined range, the comparator or processor of the relay activates the protective action component to isolate the fault or disturbance, thereby preventing damage to electrical equipment and maintaining system stability.

The primary application of frequency relays is in the power and energy industry. These devices are used as an essential part of protective systems. They monitor and protect power plants and their equipment. Additionally, in case of frequency loss or disturbance, they automatically execute protective actions to prevent equipment damage.

Protection of Power Generators: Frequency relays are utilized for monitoring and safeguarding power generators such as generators and turbines. They detect changes resulting from power load variations, network irregularities, or internal faults and take necessary actions to prevent equipment damage.

Network Protection: Frequency relays are employed to protect power grids, so that in the event of incidents like frequency reduction, voltage increase, or load imbalance, they undertake protective measures to prevent hazards and damages caused by these events.

Frequency Control: Frequency relays serve as tools for controlling and managing the frequency of electrical systems. They impose changes on the system’s frequency to keep it within the desired range and prevent unintended fluctuations.

Types of Frequency Relays:

Frequency relays are divided into two main types:

1. Under-Frequency Relay:

   – Designed to monitor the frequency of the electrical system.

   – Activated when the frequency decreases below a predetermined threshold.

   – Initiates protective actions such as load shedding to prevent system collapse.

   – Crucial for maintaining network stability and ensuring the continuous and reliable operation of power generation and distribution systems by protecting generators against frequency conditions.

2. Over-Frequency Relay:

   – Used for controlling the system frequency and activated when the frequency exceeds a predetermined upper limit.

   – Initiates actions such as reducing generator output or shedding load to prevent overloading and maintain system stability.

   – Essential for managing changes in power sources, especially in conditions of excessive generation or sudden load reduction.

Protection of Power Generators: Frequency relays provide effective protection for power generators such as generators and turbines by ensuring frequency within a safe and executable range. These devices detect disturbances caused by power load variations, network fluctuations, or internal faults and prevent equipment damage.

Network Protection: Frequency relays safeguard power grids as a reliable response to frequency deviations. In cases of voltage increases, frequency reductions, or load imbalances, these relays automatically activate protective measures to prevent hazards and damages resulting from these events.

Frequency Control: In addition to protection, frequency relays assist in controlling and managing the frequency of electrical systems. By ensuring frequency within a safe range, these relays prevent unintended fluctuations and enhance network stability.

Advantages and Disadvantages of Frequency Relays:


1. System Protection: Frequency relays offer the capability to protect the system against frequency deviations, preventing system instability and equipment damage.

2. Stability Enhancement: By controlling and monitoring the network frequency, frequency relays contribute to improving the stability of power systems, particularly in transmission and distribution networks.

3. Equipment Protection: These relays safeguard electrical equipment against damage resulting from undesirable frequency performance, including generators and transformers.

4. Increased Reliability: By ensuring frequency within a safe range, frequency relays help enhance the overall reliability of the power grid.


1. Complexity and Cost: Designing and implementing frequency relays can be complex and costly, especially in systems with high-frequency fluctuations or where precise frequency control is required.

2. False Trip Risk: The risk of false trips can lead to unnecessary shutdowns of equipment, as relays may mistakenly identify normal frequency conditions as deviations.

3. Limited Response Time: The response time of frequency relays may be limited in systems requiring rapid responses to frequency changes.

4. Maintenance Requirements: Frequency relays require regular maintenance and repairs for accuracy and reliability, which can be time-consuming and costly.

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1. ABB:

ABB is a global leader in the electrical technology industry, offering high-quality and reliable protection relays for medium voltage applications. ABB products are renowned and trusted in the electrical industry due to their durability, performance, and compliance with international standards. This brand stands out for its commitment to customer satisfaction through innovative technology advancements and advanced innovations.

2. Siemens:

Siemens, a reputable company in the power industry, provides advanced and highly accurate protection relays for medium voltage power systems. Siemens products are well-known for their fast response time, durability, and ease of installation, making them valuable for protecting vital equipment and ensuring a stable and reliable power system.

3. Schneider Electric:

Schneider Electric offers advanced protection relays with high precision for medium voltage power systems. Their relays are known for their reliability, advanced features, and ease of installation, providing effective protection for critical equipment and ensuring a stable power system.

4. Emerson:

Emerson is recognized for providing innovative solutions in the power industry, including high-quality protection relays for medium voltage applications. Emerson relays are valued for their reliability, advanced technology, and performance, contributing to the safety, reliability, and efficiency of power systems.

5. General Electric (GE):

General Electric (GE) is a leading manufacturer of protection relays with advanced features and high reliability for medium voltage power systems. GE relays are known for their fast response time, durability, and ease of use, making them a trusted choice for ensuring the safety and reliability of power systems.

When selecting protection relays, it is essential to consider the performance characteristics of each brand, compliance with international standards, and the reputation of the brand for quality and reliability. These brands play a significant role in improving the safety, reliability, and performance of power systems through active participation in the electrical industry and the development of protection relays, making them a secure and effective choice for the electrical industry.

Frequency relays play a vital and fundamental role in the electronics industry, contributing significantly to the control, management, and protection of electrical and electronic systems. With the long-term development of this technology and the establishment of principles of physics and electronics, frequency relays have been employed in various industries and have had considerable impact.

A precise familiarity with the types of frequency relays, their advantages and disadvantages, their functionality, and applications is highly important as it enables users to make better decisions in selecting, using, and operating these vital tools for their systems.

Moreover, recognizing reputable brands and utilizing modern and quality technologies in the manufacture of frequency relays is of special importance as these factors contribute to improving the efficiency and reliability of electrical and electronic systems.

Ultimately, this article, by providing comprehensive information about frequency relays, allows readers to make better decisions and achieve better performance for their systems in the field of electricity and electronics.

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