This Article is About the Electrical Component

8/3/2019 This Article is About the Electrical Component

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This article is about the electrical component. For other uses, seeRelay(disambiguation).

Automotive-style miniature relay, dust cover removed

A relay is an electrically operated switch. Electric currentthrough the coil of the relay

creates a magnetic field which attracts a lever and changes the switch contacts. The

coil current can be on or off so relays have two switch positions and they are double-

throw (changeover) switches.

Contents

[hide]

1 Basic design and operation

2 Types of relay

o 2.1 Latching relay

o 2.2 Reed relay

o 2.3 Mercury-wetted relay

o 2.4 Polarized relay

o 2.5 Machine tool relay

o 2.6 Contactor relay

o 2.7 Solid-state relay

o 2.8 Solid state contactor relay

o 2.9 Buchholz relayo 2.10 Forced-guided contacts relay

o 2.11 Overload protection relay

3 Pole and throw

4 Applications

5 Relay application considerations

6 Protective relay

o 6.1 Overcurrent relay

o 6.2 Induction disc overcurrent relay

7 Distance relay

8 Motor protection relay

9 Railway signallingo 9.1 General
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o 9.2 Double switching

o 9.3 Proving

10 See also

11 References

12 External links

[edit] Basic design and operation

Simple electromechanical relay

Small relay as used in electronics

A simple electromagnetic relay, such as the one taken from a car in the first picture, is

an adaptation of an electromagnet. It consists of a coilof wire surrounding a soft iron

core, an iron yoke, which provides a lowreluctance path for magnetic flux, a movable

iron armature, and a set, or sets, of contacts; two in the relay pictured. The armature is

hinged to the yoke and mechanically linked to a moving contact or contacts. It is held

in place by a spring so that when the relay is de-energised there is an air gap in the

magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured

is closed, and the other set is open. Other relays may have more or fewer sets of

contacts depending on their function. The relay in the picture also has a wire

connecting the armature to the yoke. This ensures continuity of the circuit between the

moving contacts on the armature, and the circuit track on thePrinted Circuit Board

(PCB) via the yoke, which is soldered to the PCB.

When an electric currentis passed through the coil, the resulting magnetic field

attracts the armature, and the consequent movement of the movable contact or

contacts either makes or breaks a connection with a fixed contact. If the set of

contacts was closed when the relay was de-energised, then the movement opens the

contacts and breaks the connection, and vice versa if the contacts were open. When

the current to the coil is switched off, the armature is returned by a force,approximately half as strong as the magnetic force, to its relaxed position. Usually
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this force is provided by a spring, but gravity is also used commonly in industrial

motor starters. Most relays are manufactured to operate quickly. In a low voltage

application, this is to reduce noise. In a high voltage or high current application, this is

to reduce arcing.

If the coil is energized with DC, adiodeis frequently installed across the coil, todissipate the energy from the collapsing magnetic field at deactivation, which would

otherwise generate avoltage spikedangerous to circuit components. Some automotive

relays already include that diode inside the relay case. Alternatively a contact

protection network, consisting of a capacitor and resistor in series, may absorb the

surge. If the coil is designed to be energized with AC, a small copper ring can be

crimped to the end of the solenoid. This "shading ring" creates a small out-of-phase

current, which increases the minimum pull on the armature during the AC cycle.[1]

By analogy with the functions of the original electromagnetic device, a solid-state

relay is made with a thyristoror other solid-state switching device. To achieve

electrical isolation anoptocouplercan be used which is alight-emitting diode (LED)coupled with a photo transistor.

[edit] Types of relay

[edit] Latching relay

Latching relay, dust cover removed, showing pawl and ratchet mechanism. The

ratchet operates a cam, which raises and lowers the moving contact arm, seen edge-on

just below it. The moving and fixed contacts are visible at the left side of the image.

A latching relay has two relaxed states (bistable). These are also called "impulse",

"keep", or "stay" relays. When the current is switched off, the relay remains in its last

state. This is achieved with asolenoidoperating a ratchet and cam mechanism, or by

having two opposing coils with an over-center spring or permanent magnet to hold the

armature and contacts in position while the coil is relaxed, or with a remanent core. In

the ratchet and cam example, the first pulse to the coil turns the relay on and the

second pulse turns it off. In the two coil example, a pulse to one coil turns the relay on

and a pulse to the opposite coil turns the relay off. This type of relay has the

advantage that it consumes power only for an instant, while it is being switched, and it

retains its last setting across a power outage. A remanent core latching relay requires a

current pulse of opposite polarity to make it change state.
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[edit] Reed relay

A reed relayhas a set of contacts inside a vacuumorinert gasfilled glass tube, which

protects the contacts against atmosphericcorrosion. The contacts are closed by a

magnetic field generated when current passes through a coil around the glass tube.

Reed relays are capable of faster switching speeds than larger types of relays, buthave low switch current and voltage ratings. See also reed switch.

[edit] Mercury-wetted relay

A mercury-wetted reed relay is a form of reed relay in which the contacts are wetted

with mercury. Such relays are used to switch low-voltage signals (one volt or less)

because of their low contact resistance, or for high-speed counting and timing

applications where the mercury eliminates contact bounce. Mercury wetted relays are

position-sensitive and must be mounted vertically to work properly. Because of the

toxicity and expense of liquid mercury, these relays are rarely specified for new

equipment. See also mercury switch.

[edit] Polarized relay

A Polarized Relay placed the armature between the poles of a permanent magnet to

increase sensitivity. Polarized relays were used in middle 20th Centurytelephone

exchanges to detect faint pulses and correct telegraphic distortion. The poles were on

screws, so a technician could first adjust them for maximum sensitivity and then apply

a bias spring to set the critical current that would operate the relay.

[edit] Machine tool relay

A machine tool relay is a type standardized for industrial control of machine tools,

transfer machines, and other sequential control. They are characterized by a large

number of contacts (sometimes extendable in the field) which are easily converted

from normally-open to normally-closed status, easily replaceable coils, and aform

factorthat allows compactly installing many relays in a control panel. Although such

relays once were the backbone of automation in such industries as automobile

assembly, theprogrammable logic controller(PLC) mostly displaced the machine tool

relay from sequential control applications.

[edit] Contactor relay

A contactoris a very heavy-duty relay used for switchingelectric motorsand lighting

loads. High-current contacts are made with alloys containing silver. The unavoidable

arcing causes the contacts to oxidize and silver oxide is still a good conductor. Such

devices are often used for motor starters. A motor starter is a contactor with overload

protection devices attached. The overload sensing devices are a form of heat operated

relay where a coil heats a bi-metal strip, or where a solder pot melts, releasing a

spring to operate auxiliary contacts. These auxiliary contacts are in series with the

coil. If the overload senses excess current in the load, the coil is de-energized.

Contactor relays can be extremely loud to operate, making them unfit for use wherenoise is a chief concern.
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[edit] Solid-state relay

Solid state relay, which has no moving parts

25 A or 40 A solid state contactors

A solid state relay (SSR) is a solid stateelectronic component that provides a similar

function to an electromechanical relay but does not have any moving components,

increasing long-term reliability. With early SSR's, the tradeoff came from the fact that

every transistor has a small voltage drop across it. This voltage drop limited the

amount of current a given SSR could handle. As transistors improved, higher currentSSR's, able to handle 100 to 1,200 Amperes, have become commercially available.

Compared to electromagnetic relays, they may be falsely triggered by transients.

[edit] Solid state contactor relay

A solid state contactor is a very heavy-duty solid state relay, including the necessary

heat sink, used for switching electric heaters, small electric motors and lighting loads;

where frequent on/off cycles are required. There are no moving parts to wear out and

there is no contact bounce due to vibration. They are activated by AC control signals

or DC control signals from Programmable logic controller(PLCs), PCs,Transistor-

transistor logic (TTL) sources, or other microprocessor and microcontroller controls.

[edit] Buchholz relay

A Buchholz relay is a safety device sensing the accumulation of gas in large oil-filled

transformers, which will alarm on slow accumulation of gas or shut down the

transformer if gas is produced rapidly in the transformer oil.

[edit] Forced-guided contacts relay

A forced-guided contacts relay has relay contacts that are mechanically linkedtogether, so that when the relay coil is energized or de-energized, all of the linked
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contacts move together. If one set of contacts in the relay becomes immobilized, no

other contact of the same relay will be able to move. The function of forced-guided

contacts is to enable the safety circuit to check the status of the relay. Forced-guided

contacts are also known as "positive-guided contacts", "captive contacts", "locked

contacts", or "safety relays".

[edit] Overload protection relay

One type ofelectric motoroverload protection relay is operated by a heating element

in series with the electric motor. The heat generated by the motor current operates a

bi-metal strip or melts solder, releasing a spring to operate contacts. Where the

overload relay is exposed to the same environment as the motor, a useful though

crude compensation for motor ambient temperature is provided.

[edit] Pole and throw

Circuit symbols of relays. "C" denotes the common terminal in SPDT and DPDT

types.

The diagram on the package of a DPDT AC coil relay

Since relays are switches, the terminology applied to switches is also applied to

relays. A relay will switch one or morepoles, each of whose contacts can be thrownby energizing the coil in one of three ways:

Normally-open (NO) contacts connect the circuit when the relay is activated;

the circuit is disconnected when the relay is inactive. It is also called a Form

A contact or "make" contact.
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Normally-closed (NC) contacts disconnect the circuit when the relay is

activated; the circuit is connected when the relay is inactive. It is also called a

Form B contact or "break" contact.

Change-over (CO), or double-throw (DT), contacts control two circuits: one

normally-open contact and one normally-closed contact with a common

terminal. It is also called a Form C contact or "transfer" contact ("breakbefore make"). If this type of contact utilizes a "make before break"

functionality, then it is called a Form D contact.

The following designations are commonly encountered:

SPST - Single Pole Single Throw. These have two terminals which can be

connected or disconnected. Including two for the coil, such a relay has four

terminals in total. It is ambiguous whether the pole is normally open or

normally closed. The terminology "SPNO" and "SPNC" is sometimes used to

resolve the ambiguity.

SPDT - Single Pole Double Throw. A common terminal connects to either oftwo others. Including two for the coil, such a relay has five terminals in total.

DPST - Double Pole Single Throw. These have two pairs of terminals.

Equivalent to two SPST switches or relays actuated by a single coil. Including

two for the coil, such a relay has six terminals in total. The poles may be Form

A or Form B (or one of each).

DPDT - Double Pole Double Throw. These have two rows of change-over

terminals. Equivalent to two SPDT switches or relays actuated by a single coil.

Such a relay has eight terminals, including the coil.

The "S" or "D" may be replaced with a number, indicating multiple switches

connected to a single actuator. For example 4PDT indicates a four pole double throw

relay (with 14 terminals).

[edit] Applications

Relays are used to and for:

Control a high-voltage circuit with a low-voltage signal, as in some types of

modems or audio amplifiers,

Control a high-current circuit with a low-current signal, as in the starter

solenoid of an automobile, Detect and isolate faults on transmission and distribution lines by opening and

closing circuit breakers (protection relays),
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A DPDT AC coil relay with "ice cube" packaging

Isolate the controlling circuit from the controlled circuit when the two are at

different potentials, for example when controlling a mains-powered device

from a low-voltage switch. The latter is often applied to control office lighting

as the low voltage wires are easily installed in partitions, which may be oftenmoved as needs change. They may also be controlled by room occupancy

detectors in an effort to conserve energy,

Logic functions. For example, the boolean AND function is realised by

connecting normally open relay contacts in series, the OR function by

connecting normally open contacts in parallel. The change-over or Form C

contacts perform the XOR (exclusive or) function. Similar functions for

NAND and NOR are accomplished using normally closed contacts. The

Ladder programming language is often used for designing relay logic

networks.

o Early computing. Before vacuum tubes and transistors, relays were

used as logical elements indigital computers. SeeARRA (computer),Harvard Mark II,Zuse Z2, and Zuse Z3.

o Safety-critical logic. Because relays are much more resistant than

semiconductors to nuclear radiation, they are widely used in safety-

critical logic, such as the control panels of radioactive waste-handling

machinery.

Time delay functions. Relays can be modified to delay opening or delay

closing a set of contacts. A very short (a fraction of a second) delay would use

a copper disk between the armature and moving blade assembly. Current

flowing in the disk maintains magnetic field for a short time, lengthening

release time. For a slightly longer (up to a minute) delay, adashpot is used. Adashpot is a piston filled with fluid that is allowed to escape slowly. The time

period can be varied by increasing or decreasing the flow rate. For longer time

periods, a mechanical clockwork timer is installed.

[edit] Relay application considerations
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A large relay with two coils and many sets of contacts, used in an old telephone

switching system.

Several 30-contact relays in "Connector" circuits in mid 20th century 1XB switch and

5XB switch telephone exchanges; cover removed on one

Selection of an appropriate relay for a particular application requires evaluation of

many different factors:

Number and type of contacts - normally open, normally closed, (double-throw)

Contact sequence - "Make before Break" or "Break before Make". For

example, the old style telephone exchanges required Make-before-break so

that the connection didn't get dropped while dialing the number.

Rating of contacts - small relays switch a few amperes, large contactors are

rated for up to 3000 amperes, alternating or direct current

Voltage rating of contacts - typical control relays rated 300 VAC or 600 VAC,

automotive types to 50 VDC, special high-voltage relays to about 15 000 V

Coil voltage - machine-tool relays usually 24 VAC, 120 or 250 VAC, relays

for switchgear may have 125 V or 250 VDC coils, "sensitive" relays operate

on a few milliamperes Coil current - Usually in the range of 40 - 200 mA for 0 - 24 VDC coils.[2]

Package/enclosure - open, touch-safe, double-voltage for isolation between

circuits, explosion proof, outdoor, oil and splash resistant, washable for

printed circuit board assembly

Assembly - Some relays feature a sticker that keeps the enclosure sealed to

allow PCB post soldering cleaning agents. Which is removed once assembly is

complete.

Mounting - sockets, plug board, rail mount, panel mount, through-panel

mount, enclosure for mounting on walls or equipment

Switching time - where high speed is required

"Dry" contacts - when switching very low level signals, special contactmaterials may be needed such as gold-plated contacts
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Contact protection - suppress arcing in very inductive circuits

Coil protection - suppress the surge voltage produced when switching the coil

current

Isolation between coil circuit and contacts

Aerospace or radiation-resistant testing, special quality assurance

Expected mechanical loads due to acceleration- some relays used inaerospaceapplications are designed to function in shockloads of 50gor more

Accessories such as timers, auxiliary contacts, pilot lamps, test buttons

Regulatory approvals

Stray magnetic linkage between coils of adjacent relays on a printed circuit

board.

[edit] Protective relay

A protective relay is a complex electromechanical apparatus, often with more than

one coil, designed to calculate operating conditions on an electrical circuit and tripcircuit breakers when a fault was found. Unlike switching type relays with fixed and

usually ill-defined operating voltage thresholds and operating times, protective relays

had well-established, selectable, time/current (or other operating parameter) curves.

Such relays were very elaborate, using arrays of induction disks, shaded-pole

magnets, operating and restraint coils, solenoid-type operators, telephone-relay style

contacts, and phase-shifting networks to allow the relay to respond to such conditions

as over-current, over-voltage, reversepowerflow, over- and under- frequency, and

even distance relays that would trip for faults up to a certain distance away from a

substation but not beyond that point. An important transmission line or generator unit

would have had cubicles dedicated to protection, with a score of individual

electromechanical devices. The various protective functions available on a given relayare denoted by standard ANSI Device Numbers. For example, a relay including

function 51 would be a timed overcurrent protective relay.

These protective relays provide various types of electrical protection by detecting

abnormal conditions and isolating them from the rest of the electrical system by

circuit breaker operation. Such relays may be located at the service entrance or at

major load centers.

Design and theory of these protective devices is an important part of the education of

anelectrical engineerwho specializes in power systems. Today these devices are

nearly entirely replaced (in new designs) with microprocessor-based instruments(numerical relays) that emulate their electromechanical ancestors with great precision

and convenience in application. By combining several functions in one case,

numerical relays also save capital cost and maintenance cost over electromechanical

relays. However, due to their very long life span, tens of thousands of these "silent

sentinels" are still protecting transmission lines and electrical apparatus all over the

world.

See also Protective Device Coordination.
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Top, middle: reed switches, bottom: reed relay

[edit] Overcurrent relay

An "Overcurrent Relay" is a type of protective relay which operates when the load

current exceeds a preset value. The ANSI Device Designation Number is 50 for an

Instantaneous OverCurrent (IOC), 51 for a Time OverCurrent (TOC). In a typical

application the overcurrent relay is used for overcurrent protection, connected to a

current transformer and calibrated to operate at or above a specific current level.

When the relay operates, one or more contacts will operate and energize a trip coil in

a Circuit Breaker and trip (open) the Circuit Breaker.

[edit] Induction disc overcurrent relay

These robust and reliable electromagnetic relays use the induction principle

discovered by Ferraris in the late 19th century. The magnetic system in induction disc

overcurrent relays is designed to detect overcurrents in a power system and operate

with a pre determined time delay when certain overcurrent limits have been reached.

In order to operate, the magnetic system in the relays produces rotational torque that

acts on a metal disc to make contact, according to the following basic current/torqueequation:

T = K x 1 x 2 Sin

Where

K is a constant

1 and 2 are the two fluxes

is the phase angle between the fluxes

The relay's primary winding is supplied from the power systems current transformer

via a plug bridge, which is also commonly known as the plug setting multiplier (psm).

The variations in the current setting are usually seven equally spaced tappings or

operating bands that determine the relays sensitivity. The primary winding is located

on the upper electromagnet. The secondary winding has connections on the upper

electromagnet that are energised from the primary winding and connected to the lower

electromagnet. Once the upper and lower electromagnets are energised they produce

eddy currents that are induced onto the metal disc and flow through the flux paths.

This relationship of eddy currents and fluxes creates rotational torque proportional to

the input current of the primary winding, due to the two flux paths been out of phase

by 90.
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Therefore in an overcurrent condition a value of current will be reached that

overcomes the control spring pressure on the spindle and the breaking magnet causing

the metal disc to rotate moving towards the fixed contact. This initial movement of

the disc is also held off to a critical positive value of current by small slots that are

often cut into the side of the disc. The time taken for rotation to make the contacts is

not only dependent on current but also the spindle backstop position, known as thetime multiplier (tm). The time multiplier is divided into 10 linear divisions of the full

rotation time.

Providing the relay is free from dirt, the metal disc and the spindle with its contact

will reach the fixed contact, thus sending a signal to trip and isolate the circuit, within

its designed time and current specifications. Drop off current of the relay is much

lower than its operating value, and once reached the relay will be reset in a reverse

motion by the pressure of the control spring governed by the braking magnet.

[edit] Distance relayThe most common form of protection on high voltage transmission systems is

distance relay protection. Power lines have set impedance per kilometre and using this

value and comparing voltage and current the distance to a fault can be determined.

The ANSI standard device number for a distance relay is 21. The main types of

distance relay protection schemes are:-

Three step distance protection

Switched distance protection

Accelerated or permissive intertrip protection

Blocked distance protection

In three step distance protection, the relays are separated into three separate zones of

impedance measurement to accommodate for over reach and under reach conditions.

Zone 1 is instantaneous in operation and has a purposely set under reach of 80% of

the total line length to avoid operation for the next line. This is due to measurements

of impedance of lines not being entirely accurate, errors in voltage and current

transformers and relay tolerances. These errors can be up to 20% of the line

impedance, hence the zones 80% reach. Zone 2 covers the last 20% of the feeder line

length and provides backup to the next line by having a slight over reach. To prevent

mal-operation the zone has a 0.5 second time delay. Zone 3 provides backup for the

next line and has a time delay of 1 second to grade with zone 2 protection of the nextline.

[edit] Motor protection relay

AC motors need overcurrent protection against short circuits from external faults in

connecting cables and from internal faults in motor windings. In addition,

transformers are thermally rated and limited, and protective relays must be applied to

prevent overheating during operating conditions where no fault is present. [3]

[edit] Railway signalling
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[edit] General

Railway signalling relays are very big and cumbersome compared to the mostly small

voltages and currents that they switch. Contacts are widely spaced to prevent

dangerous flashovers and short circuits over a lifetime that may exceed fifty years.

The Q-series of plug-in relays are widely used on railways following English practice.

These are 120mm high, 180mm deep and 56mm wide and weigh about 1400 g.

[edit] Double switching

In railway signalling, relays energise to give a green light, so that if the power fails or

a wire breaks, the signal goes to red. This is called "fail-safe". To protect against false

feeds relay circuits are often cut on both the positive and negative side, so thattwo

false feeds are needed to cause a false green.

[edit] Proving

In railway signalling, many relays are proved down before a signal is allowed to show

green. For example, before a points normal detector is allowed to pick up (and allow

that signal to show green) the corresponding points reverse detector must be proved in

the de-energized position. Not all relays can be proved, for example repeat relays in

distant locations, so there needs to be is reliance on other features such as carbon to

silver contacts to resist lightning induced contact welding and AC immunity.

[edit] See also

Digital protective relay

Contactor

Dry contact

Wire spring relay

Race condition

Digital protective relay

From Wikipedia, the free encyclopedia

Jump to: navigation,search

A digital protective relay uses amicrocontrollerwith software-based protection

algorithms for the detection of electrical faults.

Contents

[hide]

1 Description and definition
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2 Basic principles

3 Protective element types

4 See also

5 External links / manufacturers

[edit] Description and definition

The digital protective relay, also called a numeric relay by some manufacturers and

resources, refers to a protective relay that uses an advanced microprocessor to analyze

power system voltages and currents for the purpose of detection of faults in an electric

power system. There are gray areas on what constitutes a digital/numeric relay, but

most engineers will recognize the design as having the majority of these attributes:

The relay applies A/D (analog/digital) conversion processes to the incoming

voltages and currents. The relay analyzes the A/D converter output to extract, as a minimum,

magnitude of the incoming quantity, most commonly usingFourier transform

concepts (RMS and some form of averaging are used in basic products).

Further, the Fourier transform is commonly used to extract the signal's phase

angle relative to some reference, except in the most basic applications.

The relay is capable of applying advanced logic. It is capable of analyzing

whether the relay should trip or restrain from tripping based on current and/or

voltage magnitude (and angle in some applications), complex parameters set

by the user, relay contact inputs, and in some applications, the timing and

order of event sequences.

The logic is user-configurable at a level well beyond simply changing frontpanel switches or moving of jumpers on a circuit board.

The relay has some form of advanced event recording. The event recording

would include some means for the user to see the timing of key logic

decisions, relay I/O (input/output) changes, and see in an oscillographic

fashion at least the fundamental frequency component of the incoming AC

waveform.

The relay has an extensive collection of settings, beyond what can be entered

via front panel knobs and dials, and these settings are transferred to the relay

via an interface with a PC (personal computer), and this same PC interface is

used to collect event reports from the relay.

The more modern versions of the digital relay will contain advanced metering

and communication protocol ports, allowing the relay to become a focal point

in a SCADA system.

As a point of comparison, an electromechanical relay converts the voltages and

currents to magnetic and electric forces and torques that press against spring tensions

in the relay. The tension of the spring and taps on the electromagnetic coils in the

relay are the main processes by which a user sets such a relay. In asolid state relay,

the incoming voltage and current waveforms are monitored by analog circuits, not

recorded or digitized. The analog values are compared to settings made by the user

via potentiometers in the relay, and in some case, taps on transformers.
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In some solid state relays, a simple microprocessor does some of the relay logic, but

the logic is fixed and simple. For instance, in some time overcurrent solid state relays,

the incoming AC current is first converted into a small signal AC value, then the AC

is fed into a rectifierand filter that converts the AC to a DC value proportionate to the

AC waveform. An op-amp and comparator is used to create a DC that rises when a

trip point is reached. Then a relatively simple microprocessor does a slow speed A/Dconversion of the DC signal, integrates the results to create the time-overcurrent curve

response, and trips when the integration rises above a setpoint. Though this relay has a

microprocessor, it lacks the attributes of a digital/numeric relay, and hence the term

"microprocessor relay" is not a clear term.

The digital/numeric relay was introduced in the early 1980s, with AREVA and ABB's

forerunners and SEL making some of the early market advances in the arena, but the

arena has become crowded today with many manufacturers. In transmission line and

generator protection, by the mid 1990's the digital relay had nearly replaced the solid

state and electromechanical relay in new construction. In distribution applications, the

replacement by the digital relay proceeded a bit more slowly. While the great majorityof 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.

[edit] Basic principles

Low voltage and low current signals (i.e., at the secondary of a VT and CT) are

brought into a low pass filter that removes frequency content above about 1/3 of the

sampling frequency (a relay A/D converter needs to sample faster than 2x per cycle of

the highest frequency that it is to monitor). The AC signal is then sampled by therelay's analog to digital converter at anywhere from about 4 to 64 (varies by relay)

samples per power system cycle. In some relays, the entire sampled data is kept for

oscillographic records, but in the relay, only the fundamental component is needed for

most protection algorithms, unless a high speed algorithm is used that uses subcycle

data to monitor for fast changing issues. The sampled data is then passed through a

low pass filter that numerically removes the frequency content that is above the

fundamental frequency of interest (i.e., nominal system frequency), and uses Fourier

transform algorithms to extract the fundamental frequency magnitude and angle. Next

the microprocessor passes the data into a set of protection algorithms, which are a set

of logic equations in part designed by the protection engineer, and in part designed by

the relay manufacturer, that monitor for abnormal conditions that indicate a fault. If afault condition is detected, output contacts operate to trip the associated circuit

breaker(s).

[edit] Protective element types

Protective Elements refer to the overall logic surrounding the electrical condition that

is being monitored. For instance, a differential element refers to the logic required to

monitor two (or more) currents, find their difference, and trip if the difference is

beyond certain parameters. The term element and function are quite interchangeable

in many instances.
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For simplicity on one-lines, the element/function is usually identified by what is

referred to as an ANSI device number, and hence there are three terms (element,

function, device number) in use for approximately the same concept. In the era of

electromechanical and solid state relays, any one relay could implement only one or

two protective elements/functions, so a complete protection system may have many

relays on its panel. In a digital/numeric relay, many functions/elements areimplemented by the microprocessor programming. Any one digital/numeric relay may

implement one or all of these device numbers/functions/elements.

A listing of device numbers is found at the site ANSI Device Numbers. A summary of

some common device numbers seen in digital relays is:

21 - Impedance (21G implies ground impedance)

27 - Under Voltage (27LL = line to line, 27LN = line to neutral/ground)

32 - Directional Power Element

46 - Negative sequence current

47 - Negative sequence voltage 50 - Instantaneous OverCurrent (subscript N or G implies Ground)

51 - Inverse Time Overcurrent (subscript N or G implies Ground)

59 - Over Voltage (59LL = line to line, 59LN = line to neutral/ground)

67 - Directional Over Current (typically controls a 50/51 element)

79 - Auto-reclosure

81 - Under/Over Frequency

87 - Current Differential (87L=transmission line diff; 87T=transformer diff;

87G=generator diff)
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