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The introduction of computer technology means that theProtection Engineer must now be familiar with a rangeof technical terms in this field, in addition to the termslong associated with Protection and Control. Below is a
list of terms and their meanings that are now commonlyencountered in the Protection and Control field.
ACAlternating Current
ACBAir Circuit Breaker
AccuracyThe accuracy of a transducer is defined by the limits of intrinsic error and by the limits of variations.
Accuracy c lass
A number used to indicate the accuracy range of ameasurement transducer, according to a definedstandard.
Act ive power (watt ) t ransducerA transducer used for the measurement of activeelectrical power
ADCAnalogue to Digital Converter
A/D Convers ionThe process of converting an analogue signal into an
equivalent digital one, involving the use of an analogueto digital converter
AdjustmentThe operation intended to bring a transducer into a stateof performance suitable for its use
AGCAutomatic Gain Control
AIAnalogue Input
AISAir Insulated Switchgear
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AlarmAn alarm is any event (see below) tagged as an alarmduring the configuration phase
Al l-or-nothing re layAn electrical relay which is intended to be energised bya quantity, whose value is either higher than that atwhich it picks up or lower than that at which it drops out
Ant i-pumping deviceA feature incorporated in a Circuit Breaker or reclosingscheme to prevent repeated operation where the closingimpulse lasts longer than the sum of the relay and CBoperating times
AOAnalogue Output
ARAuto Reclose: A function associated with CB,implemented to carry out reclosure automatically to try
to clear a transient faultARBITERProprietary protocol for time synchronisation fromARBITER Systems, Inc. Paso Robles, California USA
Arcing t imeThe time between instant of separation of the CBcontacts and the instant of arc extinction
Auto-t ransformerA power transformer that does not provide galvanicisolation between primary and secondary windings
AUXAuxiliary
Auxi l ia ry c i rcu i tA circuit which is usually energised by the auxiliarysupply but is sometimes energised by the measuredquantity
Auxi l ia ry re layAn all-or-nothing relay energised via another relay, forexample a measuring relay, for the purpose of providinghigher rated contacts, or introducing a time delay, or
providing multiple outputs from a single input.Auxi l ia ry supplyAn a.c. or d.c. electrical supply other than the measuredquantity which is necessary for the correct operation of the transducer
AVRAutomatic Voltage Regulator
Back-up protect ionA protection system intended to supplement the mainprotection in case the latter should be ineffective, or to
deal with faults in those parts of the power system thatare not readily included in the operating zones of themain protection
BaySet of LV, MV, or HV plant and devices, usually controlledby a bay computer
BCBay Computer. Computer dedicated to the control of oneor several bays within a substation
BCD
Binary Coded Decimal
BCPBay Control Point. A local keypad at bay level to controlthe elements of a single bay
Biased re layA relay in which the characteristics are modified by theintroduction of some quantity other than the actuatingquantity, and which is usually in opposition to theactuating quantity
Bias current
The current used as a bias quantity in a biased relay
BIOSBasic Input/Output System (of a computer ormicroprocessor)
BTBooster Transformer
Booster TransformerA current transformer whose primary winding is in serieswith the catenary and secondary winding in the returnconductor of a classically-fed a.c. overhead electrified
railway. Used at intervals to ensure that stray tractionreturn currents, with their potential to causeinterference in nearby communication circuits, areminimised
BurdenThe loading imposed by the circuits of the relay on theenergising power source or sources, expressed as theproduct of voltage and current (volt-amperes, or watts if d.c.) for a given condition, which may be either at‘setting’ or at rated current or voltage. The rated outputof measuring transformers, expressed in VA, is always atrated current or voltage and it is important, in assessingthe burden imposed by a relay, to ensure that the valueof burden at rated current is used
CCapacitance
CADComputer Aided Design
Cal ibrat ionThe set of operations which establish, under specifiedconditions, the relationship between values indicated bya transducer and the corresponding values of a quantityrealized by a reference standard. (This should not beconfused with ‘adjustment’, q.v.)
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CBCircuit Breaker
CBCCompact Bay Controller. Small capacity bay computerfor Medium Voltage applications
CBCTCore Balance Current Transformer
CCRCentral Control Room
CDMConceptual Data Modelling is an activity whose aims are:
• to define objects and links and naming conventions fortheir identifications
• to guarantee interoperability between subsystems
• to define standard exchange formats between systemconfigurator and subsystem configurators
Character i s t ic angleThe angle between the vectors representing two of theenergising quantities applied to a relay and used for thedeclaration of the performance of the relay
Character i s t ic curveThe curve showing the operating value of thecharacteristic quantity corresponding to various valuesor combinations of the energising quantities
Character i s t ic Impedance Rat io (C . I .R . )The maximum value of the System Impedance Ratio up
to which the relay performance remains within theprescribed limits of accuracy
Character ist ic quantityA quantity, the value of which characterises theoperation of the relay, for example, current for anovercurrent relay, voltage for a voltage relay, phase anglefor a directional relay, time for an independent timedelay relay, impedance for an impedance relay
Check protect ion systemAn auxiliary protection system intended to prevent
tripping due to inadvertent operation of the mainprotection system
CHPCombined Heat and Power
Circuit insulat ion voltageThe highest circuit voltage to earth on which a circuit of a transducer may be used and which determines itsvoltage test
Class indexThe number which designates the accuracy class
Clos ing Impulse t imeThe time during which a closing impulse is given to theCB
Clos ing T imeThe time for a CB to close, from the time of energisationof the closing circuit to making of the CB contacts
C o mp l ia nc e v o lt ag e ( a cc ur a cy l i m it in goutput vo l tage)For current output signals only, the output voltage up towhich the transducer meets its accuracy specification
Conjunct ive testA test of a protection system including all relevantcomponents and ancillary equipment appropriatelyinterconnected. The test may be parametric or specific
Convers ion coeff ic ientThe relationship of the value of the measurand to thecorresponding value of the output
Core Ba lance Current TransformerA ring-type Current Transformer in which all primaryconductors are passed through the aperture of the CBCT.
Hence the secondary current is proportional only to anyimbalance in current. Used for sensitive earth-faultprotection
Count ing RelayA relay that counts the number of times it is energisedand actuates an output after a desired count has beenreached.
CSV Character (or Comma) Separated Values format. Awidely used format for the exchange of data betweendifferent software, in which the individual data items aseparated by a known character – usually a comma
CTCurrent Transformer
Current t ransducerA transducer used for the measurement of a.c. current
CVTCapacitor Voltage Transformer. A voltage transformerthat uses capacitors to obtain a voltage divider effect.Used at EHV voltages instead of an electromagnetic VTfor size/cost reasons
DACDigital to Analogue Converter
DARDelayed auto-reclose
DATDigital Audio Tape
DBMSData Base Management system
DCF77
LF transmitter located at Mainflingen, Germany,broadcasting a time signal on a 77.5kHz frequency
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DCPDevice Control Point: local keypad on device level tocontrol the switchgear, often combined withlocal/remote switch
DCSDistributed Control System
Dead T ime (auto-rec lose)
The time between the fault arc being extinguished andthe CB contacts re-making
De-ionisat ion t ime (auto-reclose)The time required for dispersion of ionised air after afault is cleared so that the arc will not re-strike on re-energisation
Delayed Auto-Rec loseAn auto-reclosing scheme which has a time delay inexcess of the minimum required for successful operation
Dependent t ime measur ing re lay
A measuring relay for which times depend, in a specifiedmanner, on the value of the characteristic quantity
DFTDiscrete Fourier Transform
Digi ta l S ignal ProcessorA microprocessor optimised in both hardwarearchitecture and software instruction set for theprocessing of analogue signals digitally, through use of the DFT and similar techniques
Digi ta l S ignal Process ing
A technique for the processing of digital signals byvarious filter algorithms to obtain some desiredcharacteristics in the output. The input signal to theprocessing algorithm is usually the digital representationof an analogue signal, obtained by A/D conversion
Direct ional re layA protection relay in which the tripping decision isdependent in part upon the direction in which themeasured quantity is flowing
Discr iminat ion
The ability of a protection system to distinguish betweenpower system conditions for which it is intended tooperate and those for which it is not intended to operate
Distort ion factorThe ratio of the r.m.s. value of the harmonic content tothe r.m.s. value of the non-sinusoidal quantity
DNPDistributed Network Protocol. A proprietarycommunication protocol used on secondary networksbetween HMI, substation computers or Bay Computersand protective devices
DOLDirect-on-Line
Direct-on-LineA method of motor starting, in which full line voltage isapplied to a stationary motor
Drop-out (or drop-off )A relay drops out when it moves from the energisedposition to the un-energised position
Drop-out/pick-up rat io
The ratio of the limiting values of the characteristicquantity at which the relay resets and operates. Thisvalue is sometimes called the differential of the relay
DSPDigital Signal Processing
DTDefinite time
Earth fau l t protect ion systemA protection system which is designed to respond only tofaults to earth
Earthing transformerA three-phase transformer intended essentially toprovide a neutral point to a power system for thepurpose of earthing
Effect ive rangeThe range of values of the characteristic quantity orquantities, or of the energising quantities to which therelay will respond and satisfy the requirementsconcerning it, in particular those concerning precision
Effect ive set t ing
The ‘setting’ of a protection system including the effectsof current transformers. The effective setting can beexpressed in terms of primary current or secondarycurrent from the current transformers and is sodesignated as appropriate
Elect r i ca l re layA device designed to produce sudden predeterminedchanges in one or more electrical circuits after theappearance of certain conditions in the electrical circuitor circuits controlling it
NOTE: The term ‘relay’ includes all the ancillaryequipment calibrated with the device
Elect romechanica l re layAn electrical relay in which the designed response isdeveloped by the relative movement of mechanicalelements under the action of a current in the inputcircuit
EMCElectro-Magnetic Compatibility
Embedded generat ion
Generation that is connected to a distribution system(possibly at LV instead of HV) and hence poses particularproblems in respect of electrical protection
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e .m.f .Electro-motive Force ( or voltage)
Energis ing quantityThe electrical quantity, either current or voltage, whichalong or in combination with other energising quantities,must be applied to the relay to cause it to function
EPROM
Electrically Programmable Read Only Memory
Error (of a t ransducer)The actual value of the output minus the intended valueof the output, expressed algebraically
EventAn event is any information acquired or produced by thedigital control system
FATFactory Acceptance Test. Validation procedureswitnessed by the customer at the factory
Fault Passage IndicatorA sensor that detects the passage of current in excess of a set value (i.e. current due to a fault) at the location of the sensor. Hence, it indicates that the fault liesdownstream of the sensor
FBDFunctional Block Diagram: One of the IEC 61131-3programming languages
Fiduc ia l va lueA clearly specified value to which reference is made in
order to specify the accuracy of a transducer. (Fortransducers, the fiducial value is the span, except fortransducers having a reversible and symmetrical outputwhen the fiducial value may be either the span or half the span as specified by the manufacturer. It is stillcommon practice, however, for statements of accuracyfor frequency transducers to refer to ‘percent of centre-scale frequency’ and, for phase angle transducers, to anerror in electrical degrees.)
FPIFault Passage Indicator
Frequency t ransducerA transducer used for the measurement of the frequencyof an a.c. electrical quantity
Ful l duplex communicat ionsA communications system in which data can travelsimultaneously in both directions
GatewayThe Gateway is a computer which provides interfacesbetween the local computer system and one or severalSCADA (or RCC) systems
GISGas Insulated Switchgear (usually SF6)
Global Posit ioning SystemA system used for locating objects on Earth precisely,using a system of satellites in geostationary orbit inSpace. Used by some numerical relays to obtain accuratetime information
GMTGreenwich Mean Time
GPSGlobal Positioning System
GTOGate Turn-off Thyristor
Half- duplex communicat ionsA communications system in which data can travel inboth directions, but only in one direction at a time
High-speed rec los ingA reclosing scheme where re-closure is carried outwithout any time delay other than that required for de-
ionisation, etc.
HMIHuman Machine Interface. The means by which a humaninputs data to and receives data from a computer-basedsystem. Usually takes the form of a Personal Computer(PC) (desktop or portable) with keyboard, screen andpointing device
HRCHigh Rupturing Capacity (applicable to fuses)
HSR
High Speed Reclosing
HV High Voltage
HVDCHigh Voltage Direct Current
ICurrent
ICCPTerm used for IEC 60870-6-603 protocol
ICTInterposing Current Transformer (software implemented)
I .D.M.T.Inverse Definite Minimum Time
IGBTInsulated Gate Bipolar Transistor
I /OInput/Output
IED
Intelligent Electronic Device. Equipment containing amicroprocessor and software used to implement one ormore functions in relation to an item of electrical
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equipment (e.g. a bay controller, remote SCADAinterface/protocol converter). A microprocessor-basednumerical relay is also an IED. IED is a generic term usedto describe any microprocessor-based equipment, apartfrom a computer
IGBTInsulated Gate Bipolar Transistor
Independent t ime measur ing re layA measuring relay, the specified time for which can beconsidered as being independent, within specified limits,of the value of the characteristic quantity
Inf luence quantityA quantity which is not the subject of the measurementbut which influences the value of the output signal for aconstant value of the measurand
Input quantityThe quantity, or one of the quantities, which constitute
the signals received by the transducer from the measuredsystem
Instantaneous re layA relay that operates and resets with no intentional timedelay.
NOTE: All relays require some time to operate; it ispossible, within the above definition, to discuss theoperating time characteristics of an instantaneous relay
Insulated Gate Bipolar TransistorA special design of transistor that is suitable for handling
high voltages and currents (relative to an ordinarytransistor). Frequently used in static power controlequipment (inverters, controlled rectifiers, etc) due to theflexibility of control of the output
In t r ins ic er rorAn error determined when the transducer is underreference conditions
Inverse t ime delay re layA dependent time delay relay having an operating timewhich is an inverse function of the electricalcharacteristic quantity
Inverse t ime re lay with def in i te min imumtime ( I .D.M.T.)An inverse time relay having an operating time thattends towards a minimum value with increasing valuesof the electrical characteristic quantity
IR IG-BAn international standard for time synchronisation
ISOInternational Standards Organisation
K-bus (K-bus Cour ier )Term used for the Courier protocol on K-Bus interface forK-Relay range manufactured by AREVA
Knee-point e .m.f .That sinusoidal e.m.f. applied to the secondary terminalsof a current transformer, which, when increased by 10%,causes the exciting current to increase by 50%
LInductance
LAN
Local Area Network
LCDLiquid Crystal Display
LEDLight Emitting Diode
LDLadder Diagram. One of the IEC 61131-3 programminglanguages
LDC
Line drop compensatorLimit ing value of the output currentThe upper limit of output current which cannot, bydesign, be exceeded under any conditions
Loca l Contro l ModeWhen set for a given control point it means that thecommands can be issued from this point
Lock-out (auto-rec lose)Prevention of a CB reclosing after tripping
Long-term stabi l i ty
The stability over a period of one year
Low-speed auto-rec loseSee Delayed Auto-Reclose
LV Low Voltage
Main protect ionThe protection system which is normally expected tooperate in response to a fault in the protected zone
Maximum permissible values of the inputcurrent and vol tage
Values of current and voltage assigned by themanufacturer which the transducer will withstandindefinitely without damage
MCBMiniature Circuit Breaker
MCCBMoulded Case Circuit Breaker
Mean-sens ing t ransducerA transducer which actually measures the mean
(average) value of the input waveform but which isadjusted to give an output corresponding to the r.m.s.value of the input when that input is sinusoidal
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MeasurandA quantity subjected to measurement
Measur ing e lementA unit or module of a transducer which converts the
measurand, or part of the measurand, into a
corresponding signal
Measur ing range
That part of the span where the performance complieswith the accuracy requirements
Measur ing re layAn electrical relay intended to switch when its
characteristic quantity, under specified conditions and
with a specified accuracy attains its operating value
Metering (non-tariff) Values computed depending on the values of digital or
analogue inputs during variable periods
Meter ing ( tar i f f )Energy values computed from digital and/or analogue
inputs during variable periods and dedicated to energymeasurement for billing (tariff) purposes
Mid Point Sect ioning Substat ion
A substation located at the electrical interface of twosections of electrified railway. It contains provision for
the coupling of the sections electrically in the event of
loss of supply to one section
ModBusProprietary communication protocol used on secondary
networks between HMI, substation computers or BayComputers and protective devices
MPSSMid Point Sectioning Substation (electrified railways)
Mult i-element transducerA transducer having two or more measuring elements.
The signals from the individual elements are combined toproduce an output signal corresponding to the
measurand
Mult i-sect ion t ransducerA transducer having two or more independent measuring
circuits for one or more functions
Mult i-shot rec los ingA reclosing scheme that permits more than one reclosing
operation of a CB after a fault occurs before lock-out
occurs
MV Medium Voltage
N/C
Normally Closed
N/ONormally Open
Nominal range of useA specified range of values which it is intended that aninfluence quantity can assume without the output signalof the transducer changing by amounts in excess of those specified
Notching relayA relay which switches in response to a specific number
of applied impulsesNPSNegative Phase Sequence
NSNeutral Section (electrified railways)
Numer ica l re layA protection relay which utilises a Digital SignalProcessor to execute the protection algorithms insoftware
OCB
Oil Circuit Breaker
Off-Load Tap ChangerA tap changer that is not designed for operation whilethe transformer is supplying load
OHLOverhead line
OLTCOn Load Tap Changer.
On Load Tap Changer
A tap changer that can be operated while thetransformer is supplying load.
Opening t imeThe time between energisation of a CB trip coil and theinstant of contact parting
Operat ing current (of a re lay)The current at which a relay will pick up
Operat ing t ime (CB)The time between energisation of a CB trip coil and arcextinction
Operat ing t ime ( re lay)With a relay de-energised and in its initial condition, thetime which elapses between the application of acharacteristic quantity and the instant when the relayoperates
Operat ing t ime character i s ti cThe curve depicting the relationship between differentvalues of the characteristic quantity applied to a relayand the corresponding values of operating time
Operat ing va lue
The limiting value of the characteristic quantity at whichthe relay actually operates
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OPGWOptical Ground Wire – a ground wire that includesoptical fibres to provide a communications link
OSI 7- layer modelThe Open Systems Interconnection 7-layer model is amodel developed by ISO for modelling of acommunications network.
Output common mode interference vol tageAn unwanted alternating voltage which exists betweeneach of the output terminals and a reference point
Output current (of a transducer)The current produced by the transducer which is ananalogue function of the measurand
Output loadThe total effective resistance of the circuits andapparatus connected externally across the outputterminals
Output power (of a transducer)The power available at the transducer output terminals
Output ser ies mode interference vol tageAn unwanted alternating voltage appearing in seriesbetween the output terminals and the load
Output s ignalAn analogue or digital representation of the measurand
Output span (span)The algebraic difference between the lower and uppernominal values of the output signal
Overcurrent re layA protection relay whose tripping decision is related tothe degree by which the measured current exceeds a setvalue.
Overshoot t imeThe overshoot time is the difference between theoperating time of the relay at a specified value of theinput energising quantity and the maximum duration of the value of input energising quantity which, whensuddenly reduced to a specific value below the operating
level, is insufficient to cause operationParametric conjunct ive testA conjunctive test that ascertains the range of values of each parameter for which the test meets specificperformance requirements
PCBPrinted Circuit Board
PCCPoint of Common Coupling
PED
Power Electronic Device
Phase angle t ransducerA transducer used for the measurement of the phaseangle between two a.c. electrical quantities having thesame frequency
Pick-upA relay is said to ‘pick-up’ when it changes from the de-energised position to the energised position
Pi lot channelA means of interconnection between relaying points forthe purpose of protection
PLCProgrammable Logic Controller. A specialised computerfor implementing control sequences using software
PLCCPower Line Carrier Communication
Point of Common Coupl ingThe interface between an in-plant network containing
embedded generation and the utility distributionnetwork to which the in-plant network is connected
POWPoint-on- Wave. Point-on-wave switching is the processto control moment of switching to minimise the effects(inrush currents, overvoltages)
Power Electronic DeviceAn electronic device (e.g. thyristor or IGBT) or assemblyof such devices (e.g. inverter). Typically used in a powertransmission system to provide smooth control of output
of an item of plantPower factorThe factor by which it is necessary to multiply theproduct of the voltage and current to obtain the activepower
Power Line Carr ier Communicat ionA mean of transmitting information over a powertransmission line by using a cariier fraquencysuperimpozed on the normal power frequency.
PPS
Positive Phase SequenceProtected zoneThe portion of a power system protected by a givenprotection system or a part of that protection system
Protect ion equ ipmentThe apparatus, including protection relays, transformersand ancillary equipment, for use in a protection system
Protect ion re layA relay designed to initiate disconnection of a part of anelectrical installation or to operate a warning signal, in
the case of a fault or other abnormal condition in theinstallation. A protection relay may include more thanone electrical element and accessories
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Protect ion scheme
The co-ordinated arrangements for the protection of oneor more elements of a power system. A protectionscheme may comprise several protection systems
Protect ion systemA combination of protection equipment designed tosecure, under predetermined conditions, usually
abnormal, the disconnection of an element of a powersystem, or to give an alarm signal, or both
ProtocolA set of rules that define the method in which a functionis carried out – commonly used in respect of communications links, where it defines the hardware andsoftware features necessary for successfulcommunication between devices.
PSMPlug Setting Multiple – a term used in conjunction with
electromechanical relays, denoting the ratio of the faultcurrent to the current setting of the relay
PSTNPublic Switched Telephone Network
PT100Platinum resistance temperature probe
RResistance
R.M.S .-sens ing t ransducerA transducer specifically designed to respond to the true
r.m.s. value of the input and which is characterised bythe manufacturer for use on a specified range of waveforms
Rat io correct ionA feature of digital/numerical relays that enablescompensation to be carried out for a CT or VT ratio thatis not ideal
RatingThe nominal value of an energising quantity that appearsin the designation of a relay. The nominal value usually
corresponds to the CT and VT secondary ratingsRCDResidual Current Device. A protection device which isactuated by the residual current
RCPRemote Control Point. The Remote Control Point is aSCADA interface. Several RCP’s may be managed withdifferent communication protocols. Physicalconnections are done at a Gateway or at substationcomputers or at a substation HMI
React ive power (var ) t ransducerA transducer used for the measurement of reactiveelectrical power
Rec la im t ime (auto-rec lose)The time between a successful closing operation,measured from the time the auto-reclose relay closingcontact makes until a further reclosing sequence ispermitted in the event of a further fault occurring
REFRestricted Earth Fault
Reference condi t ionsConditions of use for a transducer prescribed forperformance testing, or to ensure valid comparison of results of measurement
Reference rangeA specified range of values of an influence quantitywithin which the transducer complies with therequirements concerning intrinsic errors
Reference va lueA specified single value of an influence quantity at which
the transducer complies with the requirementsconcerning intrinsic errors
RelaySee Protection relay
Reset t ing va lueThe limiting value of the characteristic quantity at whichthe relay returns to its initial position
Res idual currentThe algebraic sum, in a multi-phase system, of all theline currents
Res idual vo l tageThe algebraic sum, in a multi-phase system, of all theline-to-earth voltages
Response t imeThe time from the instant of application of a specifiedchange of the measurand until the output signal reachesand remains at its final steady value or within a specifiedband centred on this value
Revers ib le output currentAn output current which reverses polarity in response to
a change of sign or direction of the measurandRipple content of the outputWith steady-state input conditions, the peak-to-peakvalue of the fluctuating component of the output
r .m.s .Root Mean Square
RMURing Main Unit
ROCOFRate Of Change Of Frequency (protection relay)
RSVCRelocatable Static Var Compensator
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RTDResistance Temperature Detector
RTOSReal Time Operating System
RTURemote Terminal Unit. An IED used specifically forinterfacing between a computer and other devices.
Sometimes may include control/monitoring/storagefunctions
SATSite Acceptance Test. Validation procedures forequipment executed with the customer on site
SCADASupervisory Control and Data Acquisition
SCLSubstation Configuration Language. Normalisedconfiguration language for substation modelling (as
expected by IEC 61850-6)
SCPSubstation Control Point. HMI computers at substationlevel allowing the operators to control the substation
SCSSubstation Control System
Sett ingThe limiting value of a ‘characteristic’ or ‘energising’quantity at which the relay is designed to operate underspecified conditions. Such values are usually marked on
the relay and may be expressed as direct values,percentages of rated values, or multiples
SFCSequential Function Chart: One of the IEC 61131-3programming languages
Short-term stabi l i tyThe stability over a period of 24 hours
Simplex communicat ions systemA communications system in which data can only travelin one direction
Single-shot rec los ingAn auto-reclose sequence that provides only onereclosing operation, lock-out of the CB occurring if itsubsequently trips
S. I .R .System Impedance Ratio
Single e lement t ransducerA transducer having one measuring element
SOE
Sequence Of EventsSOTFSwitch on to Fault (protection)
Spec i f i c con junct ive testA conjunctive test using specific values of each of theparameters
Spr ing winding t imeFor spring-closed CB’s, the time for the spring to be fullycharged after a closing operation
ST
Structured Text: One of the IEC 61131-3 programminglanguages
Stabi l i ty (of a t ransducer)The ability of a transducer to keep its performancecharacteristics unchanged during a specified time, allconditions remaining constant
Stabi l i ty (of a protect ion system)The quantity whereby a protection system remainsinoperative under all conditions other than those forwhich it is specifically designed to operate
Stabi l i ty l imit s (of a protect ion system)The r.m.s. value of the symmetrical component of thethrough fault current up to which the protection systemremains stable
Start ing re layA unit relay which responds to abnormal conditions andinitiates the operation of other elements of theprotection system
STATCOMA particular type of Static Var Compensator, in which
Power Electronic Devices such as GTO’s are used togenerate the reactive power required, rather thancapacitors and inductors
Stat ic re layAn electrical relay in which the designed response isdeveloped by electronic, magnetic, optical or othercomponents without mechanical motion. Excludesrelays using digital/numeric technology
Stat ic Var CompensatorA device that supplies or consumes reactive power,comprised solely of static equipment. It is shunt-connected on transmission lines to provide reactivepower compensation
STCShort Time Current (rating of a CT)
Storage condi t ionsThe conditions, defined by means of ranges of theinfluence quantities, such as temperature, or any specialconditions, within which the transducer may be stored(non-operating) without damage
SVCStatic Var Compensator
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System di sturbance t ime (auto-rec lose)The time between fault inception and CB contactsmaking on successful re-closure
System impedance rat ioThe ratio of the power system source impedance to theimpedance of the protected zone
T101
Term used for IEC 60870-5-101 protocol
Tap changerA mechanism, usually fitted to the primary winding of atransformer, to alter the turns ratio of the transformer bysmall discrete amounts over a defined range
TCP/ IPTransmission Control Protocol/Internet Protocol. Acommon protocol for the transmission of messages overthe Internet
TCS
Trip Circuit Supervision
TC57Technical Committee 57 working for the IEC andresponsible for producing standards in the field of Protection (e.g. IEC 61850)
TFa) Transfer Function of a device (usually an element of acontrol system
b) Transient Factor (of a CT)
Through fau l t currentThe current flowing through a protected zone to a faultbeyond that zone
Time delayA delay intentionally introduced into the operation of arelay system
Time delay re layA relay having an intentional delaying device
TPITap Position Indicator (for transformers)
Transducer ( e l ec t r i ca l measu r ingt ransducer)A device that provides a d.c. output quantity having adefinite relationship to the a.c. measurand
Transducer wi th of f set zero ( l i ve zero)A transducer which gives a predetermined output otherthan zero when the measurand is zero
Transducer wi th suppressed zeroA transducer whose output is zero when the measurandis less than a certain value
Unit e lect r i ca l re layA single relay that can be used alone or in combinationswith others
Unit protect ionA protection system that is designed to operate only forabnormal conditions within a clearly defined zone of thepower system
Unrest r i c ted protect ionA protection system which has no clearly defined zone of operation and which achieves selective operation only by
time gradingUCAUtility Communications Architecture
UPSUninterruptible Power Supply
UTCUniversal Time Coordinates
V Voltage
VCB Vacuum Circuit Breaker
VDEWTerm used for IEC 60870-5-103 protocol. The VDEWprotocol is a subset of the IEC 60870-5-103 protocol
Vector group compensat ionA feature of digital and numerical relays thatcompensates for the phase angle shift that occurs intransformers (including VT’s) due to use of dissimilarwinding connections – e.g. transformers connecteddelta/star
Voltage trans ducerA transducer used for the measurement of a.c. voltage
VT Voltage Transformer
XReactance
ZImpedance • A1 •
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