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iguparmar 

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 digita

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

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However, such technology will be completely superseded within the next five years by numerical relays.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

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 provide

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 th

form of a wider range of settings, and greater accuracy.

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

The limited power of the microprocessors used in digital relays restricts the number of samples of the waveform tha

can be measured per cycle. This, in turn, limits the speed of operation of the relay in certain applications. Therefore

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

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SEPAM relays in medium voltage switchgear 

Operation diagram of digital relay

6. Power supply

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 form3. 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.

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References

Handbook of Switchgear –Bhel

Digital/Numerical Relays -T.S.M. Rao

About Author //

Jignesh Parmar 

iguparmar  - Jignesh Parmar has completed his B.E(Electrical) from Gujarat University. He

is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more

than 12 years experience in Transmission -Distribution-Electrical Energy theft detection-

Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-

coordination -Execution). He is Presently associate with one of the leading business group

as a Assistant Manager at Ahmedabad,India. He has published numbers of Technical

 Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"

(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance

Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger 

and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and

help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

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2 Comments

1.

Flexibility and Reliability of Numerical Protection Relay | EEP 

Dec 27, 2012

[...] 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 [...]

(reply)

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 oneplace! Utility supplier need not send anybody to each house to take meter reading!

(reply)

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