07 relay technology

8/14/2019 07 Relay Technology

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Introduction 7.1

Electromechanical relays 7.2

Static relays 7.3

Digital relays 7.4

Numerical relays 7.5

Additional features of numerical relays 7.6

Numerical relay issues 7.7

References 7.8

7 R e l a y T e c h n o l o g y

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N e t w o r k P r o t e c t i o n & A u t o m a t i o n G u i d e 1 0 0

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application at this time, all other types having beensuperseded by more modern equivalents.

7.2.1 Attracted Armature Relays

These generally consist of an iron-cored electromagnetthat attracts a hinged armature when energised. A

restoring force is provided by means of a spring orgravity so that the armature will return to its originalposition when the electromagnet is de-energised.Typical forms of an attracted armature relay are shownin Figure 7.1. Movement of the armature causes contactclosure or opening, the armature either carrying amoving contact that engages with a fixed one, or causesa rod to move that brings two contacts together. It isvery easy to mount multiple contacts in rows or stacks,and hence cause a single input to actuate a number of outputs. The contacts can be made quite robust andhence able to make, carry and break relatively largecurrents under quite onerous conditions (highly inductivecircuits). This is still a significant advantage of this typeof relay that ensures its continued use.

The energising quantity can be either an a.c. or a d.c.current. If an a.c. current is used, means must beprovided to prevent the chatter that would occur fromthe flux passing through zero every half cycle. Acommon solution to the problem is to split the magneticpole and provide a copper loop round one half. The fluxchange is now phase-shifted in this pole, so that at notime is the total flux equal to zero. Conversely, for relaysenergised using a d.c. current, remanent flux mayprevent the relay from releasing when the actuatingcurrent is removed. This can be avoided by preventingthe armature from contacting the electromagnet by anon-magnetic stop, or constructing the electromagnetusing a material with very low remanent flux properties.

Operating speed, power consumption and the numberand type of contacts required are a function of thedesign. The typical attracted armature relay has anoperating speed of between 100ms and 400ms, but reedrelays (whose use spanned a relatively short period in thehistory of protection relays) with light current contactscan be designed to have an operating time of as little as1msec. Operating power is typically 0.05-0.2 watts, butcould be as large as 80 watts for a relay with severalheavy-duty contacts and a high degree of resistance tomechanical shock.

Some applications require the use of a polarised relay. This

S N

Permanentmagnet

Core

Armature

Coil

Figure 7.2: Typical polarised relay

Figure 7.3: Typical attracted armature relay mounted in case

(a) D.C. relay (c) Solenoid relay

(b) Shading loop modification to pole of relay (a) for a.c. operation

(d) Reed relay

Figure 7.1: Typical attracted armature relays

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can be simply achieved by adding a permanent magnet tothe basic electromagnet. Both self-reset and bi-stableforms can be achieved. Figure 7.2 shows the basicconstruction. One possible example of use is to providevery fast operating times for a single contact, speeds of lessthan 1ms being possible. Figure 7.3 illustrates a typicalexample of an attracted armature relay.

7 .3 STATIC RELAYS

The term static implies that the relay has no movingparts. This is not strictly the case for a static relay, as the

output contacts are still generally attracted armaturerelays. In a protection relay, the term static refers to theabsence of moving parts to create the relaycharacteristic.

Introduction of static relays began in the early 1960s.Their design is based on the use of analogue electronicdevices instead of coils and magnets to create the relaycharacteristic. Early versions used discrete devices suchas transistors and diodes in conjunction with resistors,capacitors, inductors, etc., but advances in electronicsenabled the use of linear and digital integrated circuitsin later versions for signal processing and

implementation of logic functions. While basic circuitsmay be common to a number of relays, the packagingwas still essentially restricted to a single protectionfunction per case, while complex functions requiredseveral cases of hardware suitably interconnected. Userprogramming was restricted to the basic functions of adjustment of relay characteristic curves. They thereforecan be viewed in simple terms as an analogue electronicreplacement for electromechanical relays, with someadditional flexibility in settings and some saving in spacerequirements. In some cases, relay burden is reduced,making for reduced CT/VT output requirements.

A number of design problems had to be solved with staticrelays. In particular, the relays generally require areliable source of d.c. power and measures to preventdamage to vulnerable electronic circuits had to bedevised. Substation environments are particularly hostileto electronic circuits due to electrical interference of various forms that are commonly found (e.g. switchingoperations and the effect of faults). While it is possibleto arrange for the d.c. supply to be generated from themeasured quantities of the relay, this has thedisadvantage of increasing the burden on the CTs or VTs,and there will be a minimum primary current or voltagebelow which the relay will not operate. This directlyaffects the possible sensitivity of the relay. So provisionof an independent, highly reliable and secure source of relay power supply was an important consideration. Toprevent maloperation or destruction of electronic devicesduring faults or switching operations, sensitive circuitryis housed in a shielded case to exclude common mode

and radiated interference. The devices may also besensitive to static charge, requiring special precautionsduring handling, as damage from this cause may not beimmediately apparent, but become apparent later in theform of premature failure of the relay. Therefore, radicallydifferent relay manufacturing facilities are requiredcompared to electromechanical relays. Calibration andrepair is no longer a task performed in the field withoutspecialised equipment. Figure 7.4 shows the circuit boardfor a simple static relay and Figure 7.5 shows examples of simple and complex static relays.

Figure 7.4: Circuit board of static relay

Figure 7.5: Selection of static relays

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7 .4 DIGITAL RELAYSDigital protection relays introduced a step change intechnology. Microprocessors and microcontrollersreplaced analogue circuits used in static relays toimplement relay functions. Early examples began to beintroduced into service around 1980, and, withimprovements in processing capacity, can still be regardedas current technology for many relay applications.However, such technology will be completely supersededwithin the next five years by numerical relays.

Compared to static relays, digital relays introduce A/Dconversion of all measured analogue quantities and usea microprocessor to implement the protection algorithm.The microprocessor may use some kind of countingtechnique, or use the Discrete Fourier Transform (DFT) toimplement the algorithm. However, the typicalmicroprocessors used have limited processing capacityand memory compared to that provided in numerical

relays. The functionality tends therefore to be limitedand restricted largely to the protection function itself.Additional functionality compared to that provided by anelectromechanical or static relay is usually available,typically taking the form of a wider range of settings,and greater accuracy. A communications link to aremote computer may also be provided.

The limited power of the microprocessors used in digitalrelays restricts the number of samples of the waveformthat can be measured per cycle. This, in turn, limits thespeed of operation of the relay in certain applications.Therefore, a digital relay for a particular protection

function may have a longer operation time than thestatic relay equivalent. However, the extra time is notsignificant in terms of overall tripping time and possibleeffects of power system stability. Examples of digitalrelays are shown in Figure 7.6.

7 .5 NUMERICAL RELAYS

The distinction between digital and numerical relay restson points of fine technical detail, and is rarely found inareas other than Protection. They can be viewed asnatural developments of digital relays as a result of advances in technology. Typically, they use a specialised

digital signal processor (DSP) as the computationalhardware, together with the associated software tools.The input analogue signals are converted into a digitalrepresentation and processed according to the appropriatemathematical algorithm. Processing is carried out using aspecialised microprocessor that is optimised for signalprocessing applications, known as a digital signalprocessor or DSP for short. Digital processing of signals inreal time requires a very high power microprocessor.

In addition, the continuing reduction in the cost of microprocessors and related digital devices (memory, I/O,etc.) naturally leads to an approach where a single item

of hardware is used to provide a range of functions(one-box solution approach). By using multiplemicroprocessors to provide the necessary computationalperformance, a large number of functions previouslyimplemented in separate items of hardware can now beincluded within a single item. Table 7.1 provides a list of typical functions available, while Table 7.2 summarisesthe advantages of a modern numerical relay over thestatic equivalent of only 10-15 years ago. Figure 7.7shows typical numerical relays, and a circuit board isshown in Figure 7.8. Figure 7.9 provides an illustration of the savings in space possible on a HV feeder showing the

space requirement for relays with electromechanical andnumerical relay technology to provide the samefunctionality.

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Figure 7.6: Selection of digital relays Table 7.1: Numerical distance relay features

Distance Protection- several schemes including user definable)Overcurrent Protection (directional/non-directional)

Several Setting Groups for protection valuesSwitch-on-to-Fault Protection

Power Swing Blocking Voltage Transformer Supervision

Negative Sequence Current ProtectionUndervoltage ProtectionOvervoltage Protection

CB Fail ProtectionFault LocationCT Supervision VT Supervision

Check SynchronisationAutoreclose

CB Condition MonitoringCB State MonitoringUser-Definable Logic

Broken Conductor DetectionMeasurement of Power System Quantities (Current, Voltage, etc.)

Fault/Event/Disturbance recorder

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Because a numerical relay may implement thefunctionality that used to require several discrete relays,the relay functions (overcurrent, earth fault, etc.) arenow referred to as being relay elements, so that asingle relay (i.e. an item of hardware housed in a singlecase) may implement several functions using several

relay elements. Each relay element will typically be asoftware routine or routines.

The argument against putting many features into onepiece of hardware centres on the issues of reliability andavailability. A failure of a numerical relay may causemany more functions to be lost, compared to applicationswhere different functions are implemented by separatehardware items. Comparison of reliability and availabilitybetween the two methods is complex as inter-dependency of elements of an application provided byseparate relay elements needs to be taken into account.

With the experience gained with static and digital relays,most hardware failure mechanisms are now well

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Table 7.2: Advantages of numerical protection relays over static

Several setting groupsWider range of parameter adjustment

Remote communications built inInternal Fault diagnosis

Power system measurements availableDistance to fault locator

Disturbance recorder

Auxiliary protection functions ( broken conductor, negative sequence, etc.)CB monitoring (state, condition)

User-definable logicBackup protection functions in-built

Consistency of operation times - reduced grading margin

Figure 7.7: Typical numerical relays

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the design. It must work at high speed, use lowvoltage levels and yet be immune to conducted andradiated interference from the electrically noisysubstation environment. Excellent shielding of therelevant areas is therefore required. Digital inputs areoptically isolated to prevent transients beingtransmitted to the internal circuitry. Analogue inputsare isolated using precision transformers to maintainmeasurement accuracy while removing harmful

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understood and suitable precautions taken at the designstage. Software problems are minimised by rigorous useof software design techniques, extensive prototypetesting (see Chapter 21) and the ability to downloadamended software into memory (possibly using a remotetelephone link for download). Practical experienceindicates that numerical relays are at least as reliableand have at least as good a record of availability asrelays of earlier technologies.

As the technology of numerical relays has only becomeavailable in recent years, a presentation of the conceptsbehind a numerical relay is presented in the followingsections.

7.5.1 Hardware Architecture

The typical architecture of a numerical relay is shownin Figure 7.10. It consists of one or more DSPmicroprocessors, some memory, digital and analogueinput/output (I/O), and a power supply. Wheremultiple processors are provided, it is usual for one of them to be dedicated to executing the protection relayalgorithms, while the remainder implements anyassociated logic and handles the Human MachineInterface (HMI) interfaces. By organising the I/O on aset of plug-in printed circuit boards (PCBs), additionalI/O up to the limits of the hardware/software can beeasily added. The internal communications bus linksthe hardware and therefore is critical component in

Figure 7.8: Circuit board for numerical relay

Figure 7.9: Space requirements of different relay technologies for same functionality

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transients. Additionally, the input signals must beamplitude limited to avoid them exceeding the powersupply voltages, as otherwise the waveform will appeardistorted, as shown in Figure 7.11.

Analogue signals are converted to digital form using anA/D converter. The cheapest method is to use a singleA/D converter, preceded by a multiplexer to connect

each of the input signals in turn to the converter. Thesignals may be initially input to a number of simultaneous sample-andhold circuits prior tomultiplexing, or the time relationship betweensuccessive samples must be known if the phaserelationship between signals is important. Thealternative is to provide each input with a dedicated A/Dconverter, and logic to ensure that all convertersperform the measurement simultaneously.

The frequency of sampling must be carefully considered,as the Nyquist criterion applies:

f s 2 x f h

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Figure 7.10: Relay modules and information flow

Batterybacked-up

SRAMSRAM

FlashEPROM

Front LCD panel RS232 Front comms port

Parallel test port

Main processor boardLEDs

CPU

Present valuesof all settings

CPU code& datasetting database

data

Alarm, event, fault &maintenance records

Default settings &parameters language text

software code

IRIG - B board(optional)

IRIG - B signal

Fibre opticrear commsport optional

Comms betwenmain & compressor

boards

CPU code& data

FPGA SRAM

CPU

Coprocessor board

S e r i a

l d a

t a b u s

( s a m p

l e d a

t a )

O p

t o - i s o

l a t e d

i n p u

t s

Input boardADC

D i g i t a

l i n p u

t s ( x 8 o r x

1 6 )

O u

t p u

t r e

l a y s

Relay board

O u

t p u

t r e

l a y c o n

t a c t s

( x 1 4 o r x

2 1 )

Timingdata

Parallel data busPower supply, rear comms,data, output relay status Digital input values

Power supplyboard

Transformerboard

Power supply (3 voltages),rear comms data Analogue input signals

Current & voltage inputs (6 to 8)Rear RS485communication port

Fieldvoltage

Watchdogcontacts

Powersupply

E2 PROM

Legend:

SRAM - Static Read Only MemoryCPU - Central Procesing UnitIRIG-B - Time Synchronisation SignalFPGA - Field Programmable Logic Array

ADC - Analog to Digital ConverterE 2 PROM - Electrically Erasable Programmable Read Only MemoryEPROM - Electrically Programmable Read Only MemoryLCD - Liquid Crystal Display

where:

f s = sampling frequency

f h = highest frequency of interest

If too low a sampling frequency is chosen, aliasing of theinput signal can occur (Figure 7.12), resulting in highfrequencies appearing as part of signal in the frequencyrange of interest. Incorrect results will then be obtained.The solution is to apply an anti-aliasing filter, coupledwith an appropriate choice of sampling frequency, to theanalogue signal, so those frequency components thatcould cause aliasing are filtered out. Digital sine andcosine filters are used (Figure 7.13), with a frequencyresponse shown in Figure 7.14, to extract the real andimaginary components of the signal. Frequency trackingof the input signals is applied to adjust the samplingfrequency so that the desired number of samples/cycle isalways obtained. A modern numerical relay may sampleeach analogue input quantity at between 16 and 24samples per cycle.

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All subsequent signal processing is carried out digitally insoftware, final digital outputs use relays to provideisolation or are sent via an external communications busto other devices.

7.5.2 Relay Software

The software provided is commonly organised into aseries of tasks, operating in real time. An essentialcomponent is the Real Time Operating System (RTOS),whose function is to ensure that the other tasks areexecuted as and when required, on a priority basis.

Other task software provided will naturally vary

according to the function of the specific relay, but can begeneralised as follows:

a. system services software this is akin to the BIOSof an ordinary PC, and controls the low-level I/Ofor the relay (i.e. drivers for the relay hardware,boot-up sequence, etc.)

b. HMI interface software the high level softwarefor communicating with a user, via the front panelcontrols or through a data link to anothercomputer running suitable software, storage of setting data, etc.

c. application software this is the software thatdefines the protection function of the relay

d. auxiliary functions software to implement otherfeatures offered in the relay often structured asa series of modules to reflect the options offered toa user by the manufacturer

7.5.3 Application SoftwareThe relevant software algorithm is then applied. Firstly,the values of the quantities of interest have to bedetermined from the available information contained inthe data samples. This is conveniently done by theapplication of the Discrete Fourier Transform (DFT), and

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+V ref

V ref

V ref

V out

-V ref

V in

Figure 7.11: Signal distortiondue to excessive amplitude

Apparent signal

Actual signal

Sample points

Figure 7.12: Signal aliasing problem

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b. timer expired action alarm/trip

c. value returned below setting reset timers, etc.

d. value below setting do nothing

e. value still above setting increment timer, etc.

Since the overall cycle time for the software is known,

timers are generally implemented as counters.

7 .6 ADDITIONAL FEATURESOF NUMERICAL RELAYS

The DSP chip in a numerical relay is normally of sufficient processing capacity that calculation of therelay protection function only occupies part of theprocessing capacity. The excess capacity is thereforeavailable to perform other functions. Of course, caremust be taken never to load the processor beyondcapacity, for if this happens, the protection algorithm

will not complete its calculation in the required time andthe protection function will be compromised.

Typical functions that may be found in a numerical relaybesides protection functions are described in this section.Note that not all functions may be found in a particularrelay. In common with earlier generations of relays,manufacturers, in accordance with their perceivedmarket segmentation, will offer different versionsoffering a different set of functions. Functionparameters will generally be available for display on thefront panel of the relay and also via an external

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(a) Sine filter

(b) Cosine filter

+ +++0 082 X s = 2

X 12

X 3 -2

X 52

X 7 X 2 - - X 6

- -++ X 482 X c = 2

X 1 X 0 2 X 3 -

2 X 5

2 X 70 + +0

Figure 7.13: Digital filters

Figure 7.14: Filter frequency response

Alias of fundamental

f 0 2f 0 3f 0 4f 0 5f 0 6f 0 7f 0 8f 0 9f 0

0

1

Gain

Frequency

the result is magnitude and phase information for theselected quantity. This calculation is repeated for all of the quantities of interest. The quantities can then becompared with the relay characteristic, and a decisionmade in terms of the following:

a. value above setting start timers, etc.

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communications port, but some by their nature may onlybe available at one output interface.

7.6.1 Measured Values Display

This is perhaps the most obvious and simple function toimplement, as it involves the least additional processortime. The values that the relay must measure to performits protection function have already been acquired andprocessed. It is therefore a simple task to display themon the front panel, and/or transmit as required to aremote computer/HMI station. Less obvious is that anumber of extra quantities may be able to be derivedfrom the measured quantities, depending on the inputsignals available. These might include:

a. sequence quantities (positive, negative, zero)

b. power, reactive power and power factor

c. energy (kWh, kvarh)

d. max. demand in a period (kW, kvar; average andpeak values)

e. harmonic quantities

f. frequency

g. temperatures/RTD status

h. motor start information (start time, total no. of starts/reaccelerations, total running time

i. distance to fault

The accuracy of the measured values can only be as goodas the accuracy of the transducers used (VTs CTs, A/D

converter, etc.). As CTs and VTs for protection functionsmay have a different accuracy specification to those formetering functions, such data may not be sufficientlyaccurate for tariff purposes. However, it will besufficiently accurate for an operator to assess systemconditions and make appropriate decisions.

7.6.2 VT/CT Supervision

If suitable VTs are used, supervision of the VT/CT suppliescan be made available. VT supervision is made morecomplicated by the different conditions under whichthere may be no VT signal some of which indicate VTfailure and some occur because of a power system faulthaving occurred.

CT supervision is carried out more easily, the generalprinciple being the calculation of a level of negativesequence current that is inconsistent with the calculatedvalue of negative sequence voltage.

7.6.3 CB Control/State Indication /Condition Monitoring

System operators will normally require knowledge of thestate of all circuit breakers under their control. The CB

position-switch outputs can be connected to the relaydigital inputs and hence provide the indication of statevia the communications bus to a remote control centre.

Circuit breakers also require periodic maintenance of their operating mechanisms and contacts to ensure theywill operate when required and that the fault capacity is

not affected adversely. The requirement for maintenanceis a function of the number of trip operations, thecumulative current broken and the type of breaker. Anumerical relay can record all of these parameters andhence be configured to send an alarm when maintenanceis due. If maintenance is not carried out within definedcriteria (such as a pre-defined time or number of trips)after maintenance is required, the CB can be arranged totrip and lockout, or inhibit certain functions such asauto-reclose.

Finally, as well as tripping the CB as required under faultconditions, it can also be arranged for a digital output to

be used for CB closure, so that separate CB close controlcircuits can be eliminated.

7.6.4 Disturbance Recorder

The relay memory requires a certain minimum number of cycles of measured data to be stored for correct signalprocessing and detection of events. The memory caneasily be expanded to allow storage of a greater timeperiod of input data, both analogue and digital, plus thestate of the relay outputs. It then has the capability to actas a disturbance recorder for the circuit being monitored,

so that by freezing the memory at the instant of faultdetection or trip, a record of the disturbance is availablefor later download and analysis. It may be inconvenient todownload the record immediately, so facilities may beprovided to capture and store a number of disturbances.In industrial and small distribution networks, this may beall that is required. In transmission networks, it may benecessary to provide a single recorder to monitor a numberof circuits simultaneously, and in this case, a separatedisturbance recorder will still be required.

7.6.5 Time Synchronisation

Disturbance records and data relating to energyconsumption requires time tagging to serve any usefulpurpose. Although an internal clock will normally bepresent, this is of limited accuracy and use of this clockto provide time information may cause problems if thedisturbance record has to be correlated with similarrecords from other sources to obtain a complete pictureof an event. Many numerical relays have the facility fortime synchronisation from an external clock. Thestandard normally used is an IRIG-B signal, which may bederived from a number of sources, the latest being froma GPS satellite system.

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7.6.6 Programmable Logic

Logic functions are well suited to implementation usingmicroprocessors. The implementation of logic in a relayis not new, as functions such as intertripping and auto-reclose require a certain amount of logic. However, byproviding a substantial number of digital I/O and making

the logic capable of being programmed using suitableoff-line software, the functionality of such schemes canbe enhanced and/or additional features provided. Forinstance, an overcurrent relay at the receiving end of atransformer feeder could use the temperature inputsprovided to monitor transformer winding temperatureand provide alarm/trip facilities to theoperator/upstream relay, eliminating the need for aseparate winding temperature relay. This is anelementary example, but other advantages are evident tothe relay manufacturer different logic schemesrequired by different Utilities, etc., no longer needseparate relay versions or some hard-wired logic toimplement, reducing the cost of manufacture. It is alsoeasier to customise a relay for a specific application, andeliminate other devices that would otherwise berequired.

7.6.7 Provision of Setting Groups

Historically, electromechanical and static relays havebeen provided with only one group of settings to beapplied to the relay. Unfortunately, power systemschange their topology due to operational reasons on aregular basis. (e.g. supply from normal/emergency

generation). The different configurations may requiredifferent relay settings to maintain the desired level of network protection (since, for the above example, thefault levels will be significantly different on parts of thenetwork that remain energised under both conditions).

This problem can be overcome by the provision within therelay of a number of setting groups, only one of which isin use at any one time. Changeover between groups canbe achieved from a remote command from the operator,or possibly through the programmable logic system. Thismay obviate the need for duplicate relays to be fittedwith some form of switching arrangement of the inputs

and outputs depending on network configuration. Theoperator will also have the ability to remotely programthe relay with a group of settings if required.

7.6.8 Conclusions

The provision of extra facilities in numerical relays mayavoid the need for other measurement/control devices tobe fitted in a substation. A trend can therefore bediscerned in which protection relays are provided withfunctionality that in the past has been provided usingseparate equipment. The protection relay no longer

performs a basic protection function; but is becoming anintegral and major part of a substation automationscheme. The choice of a protection relay rather thansome other device is logical, as the protection relay isprobably the only device that is virtually mandatory oncircuits of any significant rating. Thus, the functionspreviously carried out by separate devices such as baycontrollers, discrete metering transducers and similardevices are now found in a protection relay. It is nowpossible to implement a substation automation schemeusing numerical relays as the principal or indeed onlyhardware provided at bay level. As the power of microprocessors continues to grow and pressure onoperators to reduce costs continues, this trend willprobably continue, one obvious development being theprovision of RTU facilities in designated relays that act aslocal concentrators of information within the overallnetwork automation scheme.

7 . 7 N U M E R I C A L R E L AY I S S U E S

The introduction of numerical relays replaces some of theissues of previous generations of relays with new ones.Some of the new issues that must be addressed are asfollows:

a. software version controlb. relay data managementc. testing and commissioning

7.7.1 Software Version Control

Numerical relays perform their functions by means of software. The process used for software generation is nodifferent in principle to that for any other device usingreal-time software, and includes the difficulties of developing code that is error-free. Manufacturers musttherefore pay particular attention to the methodologyused for software generation and testing to ensure thatas far as possible, the code contains no errors. However,it is virtually impossible to perform internal tests thatcover all possible combinations of external effects, etc.,and therefore it must be accepted that errors may exist.In this respect, software used in relays is no different toany other software, where users accept that field usemay uncover errors that may require changes to thesoftware. Obviously, type testing can be expected toprove that the protection functions implemented by therelay are carried out properly, but it has been known forfailures of rarely used auxiliary functions to occur undersome conditions.

Where problems are discovered in software subsequentto the release of a numerical relay for sale, a new versionof the software may be considered necessary. Thisprocess then requires some form of software versioncontrol to be implemented to keep track of:

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a. the different software versions in existenceb. the differences between each versionc. the reasons for the changed. relays fitted with each of the versions

With an effective version control system, manufacturersare able to advise users in the event of reported problems

if the problem is a known software related problem andwhat remedial action is required. With the aid of suitable software held by a user, it may be possible todownload the new software version instead of requiringa visit from a service engineer.

7.7.2 Relay Data Management

A numerical relay usually provides many more featuresthan a relay using static or electromechanicaltechnology. To use these features, the appropriate datamust be entered into the memory of the relay. Users

must also keep a record of all of the data, in case of dataloss within the relay, or for use in system studies, etc.The amount of data per numerical relay may be 10-50times that of an equivalent electromechanical relay, towhich must be added the possibility of user-defined logicfunctions. The task of entering the data correctly into anumerical relay becomes a much more complex task thanpreviously, which adds to the possibility of a mistakebeing made. Similarly, the amount of data that must berecorded is much larger, giving rise potentially toproblems of storage.

The problems have been addressed by the provision of

software to automate the preparation and download of relay setting data from a portable computer connectedto a communications port of the relay. As part of theprocess, the setting data can be read back from the relayand compared with the desired settings to ensure thatthe download has been error-free. A copy of the settingdata (including user defined logic schemes where used)can also be stored on the computer, for later printoutand/or upload to the users database facilities.

More advanced software is available to perform theabove functions from an Engineering Computer in asubstation automation scheme see Chapter 24 for

details of such schemes).

7.7.3 Relay Testing and Commissioning

The testing of relays based on software is of necessityradically different from earlier generations of relays. Thetopic is dealt with in detail in Chapter 21, but it can bementioned here that site commissioning is usuallyrestricted to the in-built software self-check andverification that currents and voltages measured by therelay are correct. Problems revealed by such tests requirespecialist equipment to resolve, and hence field policy is

usually on a repair-by-replacement basis.

7 . 8 R E F E R E N C E S

7.1 Protective Relays Application Guide , 3rd edition.ALSTOM T& D Protection and Control, 1987.

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07 relay technology

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