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Home / Technical Articles / Flexibility and Reliability of Numerical Protection Relay

History of Protection Relays

The first protection devices based on microprocessors were employed in 1985. The widespread acceptance of numerical technology by the customer and the experiences of the user helped in developing the second generation numerical relays in 1990.

Flexibility and Reliability of Numerical Protection Relay
Flexibility and Reliability of Numerical Protection Relay (on photo: ABB’s numerical relay type SPAD 330 C designed to be used as a fast interwinding short-circuit and interturn fault protection for two-winding power transformers and power plant generator-transformer units.

Conventional electromechanical and static relays are hard wired relays. Their wiring is fixed, only their setting can be manually changed. Numeric relays are programmable relays. The characteristics and behaviour of the relay are can be programmed.

First generation numerical relays were mainly designed to meet the static relay protection characteristic, whereas modern numeric protection devices are capable of providing complete protection with added functions like control and monitoring.

Numerical protection devices offer several advantages in terms of protection, reliability, troubleshooting and fault information.

The distinction between digital relay and numerical relay rests on points of fine technical detail, and is rarely found in areas other than Protection. They can be viewed as natural developments of digital relays as a result of advances in technology.

Typically, they use a specialized digital signal processor (DSP) as the computational hardware, together with the associated software tools.

Measuring Principles

The input analogue signals are converted into a digital representation and processed according to the appropriate mathematical algorithm. Processing is carried out using a specialized microprocessor that is optimized for signal processing applications, known as a digital signal processor or DSP for short. Digital processing of signals in real time requires a very high power microprocessor.

The measuring principles and techniques of conventional relays (electromechanical and  static) are fewer than those of the numerical technique, which can differ in many aspects like the type of protection algorithm used, sampling, signal processing, hardware selection, software discipline, etc.

These are microprocessor-based relays in contrast to other relays that are electromechanically controlled.

Function of Relay

Numerical Directional Overcurrent Relay
Numerical Directional Overcurrent Relay (Relay operating time is determined by selecting definite time characteristics or one of the four inverse time characteristics i.e. 3s normal inverse, 1.3s normal inverse, very inverse and extremely inverse.)

Modern power system protection devices are built with integrated functions. Multifunction like protection, control, monitoring and measuring are available today in numeric power system protection devices.

Also, the communication capability of these devices facilitates remote control, monitoring and data transfer.

Traditionally, electromechanical and static protection relays offered single-function, single characteristics, whereas modern numeric protection offers multi-function and multiple characteristics. Numerical protection devices offer several advantages in terms of protection, reliability, and trouble shooting and fault information.

Numerical protection devices are available for generation, transmission and distribution systems.

Numerical relays are microprocessor based relays and having the features of recording of parameter used as disturbance recorder flexibility of setting & alarms & can be used one relay for all type of protections  of one equipment hence less area is required.

Wide Range of setting, more accurate, low burden hence low VA of CT is required which minimize the cost.

Numeric relays take the input analog quantities and convert them to numeric values.  All of the relaying functions are performed on these numeric values.

The following sections cover:

  1. Relay hardware,
  2. Relay software,
  3. Multiple protection characteristics,
  4. Adaptive protection characteristics,
  5. Data storage,
  6. Instrumentation feature,
  7. Self-check feature,
  8. Communication capability,
  9. Additional functions,
  10. Size and cost-effectiveness.

The disadvantages of a conventional electromechanical relay are overcome by using microcontroller for realizing the operation of the relays.

Microcontroller based relays perform very well and their cost is relatively low.

Operation of Relay

A current signal from CT is converted into proportional voltage signal using I to V converter.

The AC voltage proportional to load current is converted into DC using precision rectifier and is given to multiplexer (MUX) which accepts more than one input and gives one output.

Microprocessor sends command signal to the multiplexer to switch on desired channel to accept rectified voltage proportional to current in a desired circuit.

Microprocessor Based Numerical Relay

Microprocessor Relay - Operation diagram
Microprocessor Relay – Operation diagram

Output of Multiplexer is fed to analog to digital converter (ADC) to obtain signal in digital form. Microprocessor then sends a signal ADC for start of conversion (SOC), examines whether the conversion is completed and on receipt of end of conversion (EOC) from ADC, receives the data in digital form.

The microprocessor then compares the data with pick-up value.

If the input is greater than pick-up value the microprocessor send a trip signal to circuit breaker of the desired circuit.

In case of instantaneous overcurrent relay there is no intentional time delay and circuit breaker trips instantly. In case of normal inverse, very inverse, extremely inverse and long inverse overcurrent relay the inverse current-time characteristics are stored in the memory of microprocessor in tabular form called as look-up table.

 Advantages of Numerical relays

Compact Size

Electromechanical Relay makes use of mechanical comparison devices, which cause the main reason for the bulky size of relays. It uses a flag system for the indication purpose whether the relay has been activated or not.

While numerical relay is in compact size and use indication on LCD for relay activation.

Digital protection can be physically smaller, and almost always requires less panel wiring than equivalent functions implemented using analog technology.


A variety of protection functions can be accomplished with suitable modifications in the software only either with the same hardware or with slight modifications in the hardware.


A significant improvement in the relay reliability is obtained because the use of fewer components results in less interconnections and reduced component failures.

Multi Function Capability

Traditional electromechanical and static protection relays offers single-function and single characteristics. Range of operation of electromechanical relays is narrow as compared to numerical relay.

Different types of relay characteristics

It is possible to provide better matching of protection characteristics since these characteristics are stored in the memory of the microprocessor.

Digital communication capabilities

The microprocessor based relay furnishes easy interface with digital communication equipment. Fibre optical communication with substation LAN.

Modular frame

The relay hardware consists of standard modules resulting in ease of service.

Low burden

The microprocessor based relays have minimum burden on the instrument transformers.


Greater sensitivity and high pickup ratio.


With static relays, tripping time of ½ cycle or even less can be obtained.

Fast Resetting

Resetting is less.

Data History

Availability of fault data and disturbance record. Helps analysis of faults by recording details of:

  1. Nature of fault,
  2. Magnitude of fault level,
  3. Breaker problem,
  4. C.T. saturation,
  5. Duration of fault.

Auto Resetting and Self Diagnosis

Electromechanical relay do not have the ability to detect whether the normal condition has been attained once it is activated thus auto resetting is not possible and it has to be done by the operating personnel, while in numerical relay auto resetting is possible.

Other Advantages

  • By combining several functions in one case, numerical relays also save capital cost and maintenance cost over electromechanical relays
  • Separate connection is not required, zero sequence voltages and currents can be derived inside the processor
  • Basic hardware is shared between multiple functions, the cost of individual protection functions can be reduced significantly.
  • Loss of voltage feature helps block the relay in case of momentary/permanent loss of voltage.

Limitations of Numerical Relay

Protection quality

Numerical relay offers more functionality, and greater precision. Unfortunately, that does not necessarily translate into better protection.

Faster Decisions

Numerical Relay can make faster decisions. However, in the real world, faster protection itself is of no value because circuit breakers are still required to interrupt at the direction of the protective equipment, and the ability to make circuit breakers interrupt faster is very limited.

Risk Of Hacking

Numerical Relay protection often relies on non-proprietary software, exposing the system to potential risk of hacking.


Numerical Relay protection sometimes has exposure to externally-sourced transient interference that would not affect conventional technology.

Failure Impact

Numerical Relay protection shares common functions. This means that there are common failure modes that can affect multiple elements of protection.

For example, failure of a power supply or an input signal processor may disable an entire protective device that provides many different protection functions.

This problem has receive a lot of design attention, and experience generally has supported the notion that the equipment has a very high reliability once it is past the infant mortality stage.

But it remains something to be aware of.


A multifunction numeric relay can provide three phase, ground, and negative sequence directional or non-directional overcurrent protection with four shot recloser, forward or reverse power protection, breaker failure, over/under frequency, and over/under voltage protection, sync check, breaker monitoring and control.

It would take 10 – 11 single function Solid state or electromechanical relays at least 5 to 6 times the cost.

Additionally Numeric relays have communications capabilities, sequence-of-events recording, fault reporting, rate-of-change frequency, and metering functions, all in an integrated system.

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More Information

Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 20 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.


  1. Prateek Bakshi
    Jul 19, 2019

    Thank you sir. It is a very precise and illustrative information.

  2. Hareesh
    May 04, 2017

    Thank you Jignesh sir The way you have explained is good. fresh graduates will easily understand the concept .

  3. LarsenToubro
    Oct 23, 2015

    Thanks for sharing this information with us.This post has shared good concept of Relay.We can totally understand working of Relay,functionality of Relay ,importance of Relay,types of Relay.Relays are very important in sensing any kind of fault in power system & it is very reliable.Larsen & Toubro has some registered product.You may visit at Larsen & Toubro

  4. Pushpinder kansal
    Jun 02, 2015

    sir , i want to study configure of numeric relays i-e different types of settings in different types of numeric relays(DPR, Differential, ref etc)

  5. dhiru
    Jan 05, 2013

    i’m really grateful to u sir for this kind information.i m a 8th semester student but b4 this i hv no idea ab8 working of numerical relays. thanx a lot sir……

  6. Thomas J. Zazulak
    Dec 28, 2012

    Reliability of numerical relays is still questionable. There have been far too many failures to say they are reliable. The manufacturers need better burn in and quality check of the parts they are using to assemble the printed circuit boards.
    However, I do agree that as an industry that uses a great many relays we still have to keep going forward and use these numerical relays because they save us a lot of money. Second the life of the internal parts is still in question. Maybe going to IEC 61850 design that less of these questional internal parts will be used.

    • Edvard
      Jan 05, 2013

      Well. at the time they were actual, numerical relays were very reliable. This serie of technical articles about protection relays from mr. Jignesh is actually the overview of all types of relays we had in last 50 years.

  7. senthemekmr
    Dec 28, 2012

    wow edward sir nice.really i got more information this more about relay information….

    • Edvard
      Dec 28, 2012

      Thanks, but this technical article is written by mr. Jignesh Parmar ;) All thanks goes to him!

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