We are Apogeeweb Semiconductor Electronic


Home arrow Relays arrow Numeric Relay Overview: Working and Types

arrow left

arrow right

Numeric Relay Overview: Working and Types

Author: Apogeeweb Date: 15 Jan 2021  87

Multiplexer and Analog to digital converter

Ⅰ Introduction

As the technology evolved, several improvements from a standard fuse to the circuit breaker have also been made to the safety devices. We have been using static relays and magnetic relays for years to secure an electrical network, and now the safety systems have also changed as the microprocessors have evolved.

We've heard about various kinds of relays before, and Numerical Relay was one of them, so we're going to concentrate more on this kind of relay today. The formed type of a static and electromagnetic relay is numeric relays. They are a system used in an electrical network to calculate electrical parameters and transform them into numerical data that is mathematically and logically interpreted to determine whether to activate an electrical network.


A numerical relay's primary function is to protect the electrical network from unpredictable currents of failure. Due to their flexible features, numerical relays are often favored. A single numerical relay can track various parameters, such as current, voltage, frequency, time of onset, time of offset, etc. And for the analysis and control of multiple faults such as over current, over flux, different current and more, the same relay can be used.


Ⅰ Introduction

Ⅱ Working and Hardware Architecture of Numerical Relay

Ⅲ Types of Numerical Relays

  3.1 Based on Logic

  3.2 Based on Characteristics

  3.3 Based on Actuating Parameters

  3.4 Based on Application

Ⅳ Conclusion


Ⅱ Working and Hardware Architecture of Numerical Relay

Since they both have identical hardware architecture with minor variations, the numeric relay can be considered a miniature device.

working of relay

Their architecture can seem overwhelming, but all of the architecture in these major categories can be simplified.

• Input Module


• Memory

• Multiplexer and Analog to digital converter

• Output module

• Digital input/Communication module


Input Module

The power system uses analog parameters to operate. With existing transformers and future transformers, the high-powered analog signals are stepped down. Using lowpass filters, it is fed to the numeric relay. Owing to the corona or induction effect from a nearby high voltage line, the low pass filter is used to remove the noisy signal in the device.



The central processing unit (CPU) is the system's brain, which processes and filters all data protection algorithms and digital inputs.




There are two memories, RAM and ROM, in the numerical relay. Random Access Memory (RAM) is responsible for the retention and processing of input data to the relay during compilation.

Read-Only Memory (ROM) is the relay's storage unit. It stores the required software and other data related to events and disturbances. The Storage Unit is a must because it allows during the occurrence of a fault to evaluate and troubleshoot any incident.


Multiplexer and Analog to digital converter

Only digital data can be processed by the CPU, but the feedback from the current transformer and future transformer is analog. The Analog to Digital converter is then used to translate the signal to digital data. A multiplexer is used to select the necessary analog input for conversion if multiple analog signals need to be converted.


Output Module

The digital contacts that are actuated when a trip command is provided by the CPU are the output module. Pulses that are produced as a response signal are these digital contacts. According to the application of the relay, the response time may be modified.


Digital input/Communication module

As with a computer, a relay also has serial and parallel ports to link the relay to the substation's control and communication systems. To extend the tripping command, the Auxiliary relays can be attached to the digital output contacts.

Ⅲ Types of Numerical Relays

For different types of safety, numerical relays are used and are graded based on characteristics, logic, parameters of action and application. Although they are categorized under different circumstances, their function remains the same, in the event of a fault in the electrical network, to enable the travel system.

3.1 Based on Logic

Such classifications are made based on the relay's logical operation.

• Over Current/ Earth Fault: It will cause the circuit breaker when excessive current flows through a device. Used for protection against transformers and feeders.

over current relay

• Directional overcurrent: When the fault forces the power to flow in a specific direction, it is controlled (Opposite to the specified direction). Used for the safety of transformers, generators, and bus bars.

• Differential:  When the phase difference of two or more equivalent electric quantities exceeds the stated value, the differential relay is set to trip. It can protect transformers from localized faults and generators.

• Under/ Over Voltage: Under such conditions, the voltage in an electric network may drop or rise below or above a fixed value, the circuit is tripped.

• Distance: The function of this type of relay is dependent on the distance between the fault impedance and the location of the relay. They are primarily used to safeguard transmission lines.

3.2 Based On Characteristics

These classifications are based on their tripping property

• Instantaneous relay: If the trigger is triggered directly after a fault occurs, no time delay will occur.

• Definite Time Relay: Only activated if the fault stays in place after a certain time.

• Definite Minimum Time (IDMT) Inverse Time Relays: These relays are often used on transmission lines. When the line current is higher than the safe value, the circuit breaker is triggered.

• Voltage restraint over current relay: The relay is only triggered if the conditions of both under-voltage and over-current arise at the same time.

3.3 Based on Actuating Parameters

• Current relays

• Voltage relays

• Frequency relays

• Power relays Etc.

3.4 Based on Application

• Primary relay

• Backup relay

The entire network could crash if the security system fails, so they use the backup relay. And if the primary relay goes wrong, doing this would help us secure the machine.

Ⅳ Conclusion

Numeric relays are often used for automatic safety in the generating stations and substations. Different components such as feeder, engine, generator, transmission line, transformers and bus bars can be secured by such relays.


Relays are available from different firms, such as Siemens, ABB, Schnieder Electric, Alstom, Texas, etc. Each business has its own software that can help us communicate with their relays and program the security algorithm.


You can construct your own algorithm for security and feed it to the relay once you know about the parameter and the various types of faults that could occur in a power system. It doesn't take years of training and practice to become an expert in the defense of the power system to become one overnight. To become an expert, keep learning and keep on investigating.


1. What is numerical protection relay?

Numerical relay is the relay in which the measured AC quantities are sequentially sampled and converted into numerical data that is mathematically and/or logically processed to make trip decisions.


Numerical relay is actually the digital relay as a unit for which manufacturers has developed standardized hardware, which can be used in conjunction with suitably developed software to meet variety of production requirements and applications.


2. What is the difference between a relay and a fuse and a circuit breaker?

A relay is a control component used for signalling or switching according to control voltage applied to it’s terminals.


A fuse is a protective device to limit the let through energy based on the current limit being exceeded. These are used once & then disposed of (not re-usable.) The fuses can be selected according to application & rated current (IE a motor, transformer or capacitor protection device)


A circuit breaker (CB) is also a protection device used to limit let through energy on a fault, also with different thermal characteristics according to application & some LV units with a variable current threshold & tripping curve. A CB has limits - IE on LV systems, some are rated say 35kA, other larger units 60 or 80kA according to the system & calculated worst case fault current.

Related Articles

The Types of Common Relay and How to Choose Relay?

Apogeeweb 12 Mar 2019  2397

Warm hints: This article contains about 3000 words and reading time is about 15 mins. IntroductionThis article mainly introduces the commonly used relays. Firstly, it introduces the role of relays. S...

Continue reading »

Time Delay Relay Basics: Relay Circuit and Applications

Apogeeweb 20 Oct 2020  1563

IntroductionTime relay refers to a kind of relay whose output circuit needs to make an obvious change (or contact action) after adding (or removing) the input action signal in a sp...

Continue reading »

Basic Knowledge of Relay Electronics Tutorial with Video

Apogeeweb 31 Jan 2018  2627

Warm hints: The word in this article is about 3200 and reading time is about 20 minutes. IntroductionAs an electric control device, the electrical relay is a kind of electrical apparatus that causes ...

Continue reading »

The Role of the Relay and Its Working Principle

Apogeeweb 17 Jan 2019  5843

Warm hints: This article contains about 4000 words and the reading time is about 18 mins. IntroductionA relay is an electronic control device that has a control system (also called an input loop) and...

Continue reading »

How Relays Work? Relay Functions and Applications

Apogeeweb 5 Aug 2020  2659

IntroductionA relay is an electromagnetic switch operated by a relatively small electric current that can turn on or off a much larger electric current. It consists of a set of input terminals for a s...

Continue reading »

Solid State Relays: A Basic Overview

Apogeeweb 11 Jun 2020  1801

I IntroductionSolid-state relays(SSRs) have unparalleled advantages over other relays, because it can make and break the circuit without contact or spark. In addition, with the progress of techno...

Continue reading »

pinglun 0 comment

Leave a Reply

Your email address will not be published.

code image
Rating: poor fair good very good excellent

# 0 1 2 3 4 5 6 7 8 9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z