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Numeric Relay Overview: Working and Types

Author: Apogeeweb
Date: 15 Jan 2021
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.


3. What is meant by numerical relay?

In utility and industrial electric power transmission and distribution systems, a numerical relay is a computer-based system with software-based protection algorithms for the detection of electrical faults. Such relays are also termed microprocessor-type protective relays.


4. What is numerical overcurrent relay?

A 'Numerical over Current Relay' is a type of protective relay which operates when the load current exceeds a preset value. ... The overcurrent relay of IDMT is the relay that starts to operate after the intended time delay. The time delay is also known as operation time.


5. What are the advantages of numerical relay?

• Compact Size. 

• Flexibility. 

• Reliability. 

• Multi-Function Capability. 

• Different types of relay characteristics. 

• Digital communication capabilities. 

• Modular frame.

• Low burden.


6. Which transistor is used in the numerical relay?

The high-powered analog signals are stepped down with the current transformer and Potential transformer. It is fed to the numeric relay using a lowpass filter. The low pass filter is used to eliminate the noisy signal in the system due to the corona or induction effect from a nearby high voltage line.


7. What is the difference between numerical relay and static relay?

A big difference between conventional electromechanical and static relays is how the relays are wired. ... Electromechanical and static relays have fixed wiring and the setting is manual. Numeric relays, on the other hand, are programmable relays where the characteristics and behavior can be programmed.


8. How does a numerical relay work?

Numerical relays use a specialized digital signal processor (DSP) as the computational hardware, along with associated software tools. The relaying voltage and currents are passed through an isolation transformer.


9. What do you mean by a numerical protection scheme?

Numerical protection relays are digital systems in constant communication with substation automation systems through menu-driven interfaces. They have configurable binary inputs, outputs, and programmable logic. They monitor, measure, and record electrical values, faults and disturbances, and events.


10. What are the demerits of numerical relay?

1 relay can perform only 1 function. There are some disadvantages of the microprocessor are given below, The microprocessor has a limitation on the size of data. Wide Range of setting, more accurate, Low burden hence low VA of CT is required which minimizes the cost.


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