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Few Words About Digital Protection Relay
Few Words About Digital Protection Relay (on photo: Siprotec microprocessor based protective relay)

History of Protective Relay

Around 1980s the digital relay entered the market. Compared to the Solid State Relay, the digital relay takes the advantages of the development of microprocessors and microcontrollers. Instead of using analog signals, the digital relay converts all measured analog quantities into digital signals.

Digital protection relays is a revolution step in changing Relay technology.

In Digital Relay Microprocessors and micro controllers are used in replacement of analogue circuits used in static relays to implement relay functions. Digital protection relays introduced in 1980.

However, such technology will be completely superseded within the next five years by numerical relays.

By the mid-1990s the solid state and electromechanical relay had been mostly replaced by digital relay in new construction. In distribution applications, the replacement by the digital relay proceeded a bit more slowly.

While the great majority of feeder relays in new applications today are digital, the solid state relay still sees some use where simplicity of the application allows for simpler relays, and which allows one to avoid the complexity of digital relays.

Measuring principles

Compared to static relays, digital relays introduce Analogue to Digital Convertor (A/D conversion) of all measured analogue quantities and use a microprocessor to implement the protection algorithm.

The microprocessor may use some kind of counting technique, or use the Discrete Fourier Transform (DFT) to implement the algorithm.

The Microprocessors used in Digital Relay have limited processing capacity and memory compared to that provided in numerical relays.

Function of Relay

The functionality tends therefore to be limited and restricted largely to the protection function itself. Additional functionality compared to that provided by an electromechanical or static relay is usually available, typically taking the form of a wider range of settings, and greater accuracy.

A communications link to a remote computer may also be provided.

SEPAM relays in medium voltage switchgear
SEPAM relays in medium voltage switchgear

The limited power of the microprocessors used in digital relays restricts the number of samples of the waveform that can be measured per cycle. This, in turn, limits the speed of operation of the relay in certain applications. Therefore, a digital relay for a particular protection function may have a longer operation time than the static relay equivalent.

However, the extra time is not significant in terms of overall tripping time and possible effects of power system stability.

Operation of Relay

Digital relay consists of:

  1. Analogue input subsystem,
  2. Digital input subsystem,
  3. Digital output subsystem,
  4. A processor along with RAM (data scratch pad),
  5. main memory (historical data file) and
  6. Power supply
Operation diagram of digital relay
Operation diagram of digital relay

Digital relaying involves digital processing of one or more analog signals in three steps:

  1. Conversion of analogue signal to digital form
  2. Processing of digital form
  3. Boolean decision to trip or not to trip.

 Advantages of Digital Relay

  • High level of functionality integration.
  • Additional monitoring functions.
  • Functional flexibility.
  • Capable of working under a wide range of temperatures.
  • They can implement more complex function and are generally more accurate
  • Self-checking and self-adaptability.
  • Able to communicate with other digital equipment (pear to pear).
  • Less sensitive to temperature, aging
  • Economical because can be produced in volumes
  • More Accurate.
  • plane for distance relaying is possible
  • Signal storage is possible

Limitations of Digital Relay

  • Short lifetime due to the continuous development of new technologies.
  • The devices become obsolete rapidly.
  • Susceptibility to power system transients.
  • As digital systems become increasingly more complex they require specially trained staff for Operation.
  • Proper maintenance of the settings and monitoring data.


  • Handbook of Switchgear –Bhel
  • Digital/Numerical Relays -T.S.M. Rao

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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. Muhammad Arok
    Dec 19, 2022

    Hi, I would like to know industrial average of digital relay operating life cycle currently? MTBF data? You also mentioned less sensitive of temperature however what is the best range of temperature to get optimum operating life cycle? Thanks

  2. Veeraraghavan VS
    Dec 24, 2012

    Digital relays are here to stay. Mostly because of their connectivity. In case a more reliable method of transferring CT and PT information through a different mode is designed, all metering will be at one place! Utility supplier need not send anybody to each house to take meter reading!

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