Numerical RelayHAMED NAJAFIPOUR

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SRTTU

December 2018

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Overview

A protection relay is a device thatsenses any change in the signal it isreceiving, usually from a currentand/or voltage source. If themagnitude of the incoming signal isoutside a pre-set value the relay willcarry out a specific operation,generally to close or open electricalcontacts to initiate some furtheroperation, for example the tripping ofa circuit breaker.

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3Two ball-shaped floats and

two glass-enclosed reed

switches are visible inside

this cutaway view of a

Buchholz relay

https://en.wikipedia.org/wiki/Buchholz_relay

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Centrifugal switch

Bimetallic

thermostat

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Attraction:

Induction:

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has no moving mechanical parts

The Solid Static relays use analogue

electronic devices instead of

magnetic coils and mechanical

components to create the relay

characteristics. The measurement is

carried out by static circuits

consisting of comparators, level

detectors, filter etc while in a

conventional electromagnetic relay it

is done by comparing operating

torque (or force) with restraining

torque (or force).

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RACID Microprocessor time

overcurrent relay (ABB)

year: 1981

Measuring principles:

A2D

Microprocessor : counting technique, use the Discrete Fourier Transform

(DFT),limited processing capacity and memory compared to that provided in

numerical relays.

Function:

wider range of settings, and greater accuracy. A communications link to a

remote computer may also be provided

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), main memory (historical data

file)

and Power supply

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.

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Limitations of Digital Relay:

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Typical numerical relay hardware architecture

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Commissioning Numerical Protection and Procedure For

Carrying Out Performance Tests

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Set-up system configuration of a numerical protection

system (Beckwith M-3425)

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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

Advantages of Numerical relays:

Compact Size

Flexibility

Multi Function Capability

Different types of relay characteristics

Digital communication capabilities

Modular frame

Sensitivity

Speed

Fast Resetting

Data History

(1)Nature of fault, (2) Magnitude of fault level, (3) Breaker problem, (4)

C.T. saturation , (5) Duration of fault

Auto Resetting & Self Diagnosis

Limitations of Numerical Relay:

potential risk of

hacking

shares common functions

Thanks!

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