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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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