We can not test 21st century IEDs with 20th century testing technology.pdf

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Development

(EN ISO 9001)

Customer n

Project n

* Optional

Project x

System test

Prototype series

Type test

Verification*

Customer x

Customer 1

Market approval 1 Delivery and putting online

Project 1

ConformanceTest

Routine

test ofproducts

FAT*

of systemequipment

Sitecommissioning

Site acceptancetest

Trial operation*

Warranty

Decommissioning

Maintenance

Custome

r'slifecycle

Productrealization

As we can see from the History articles in themagazine, different forms of protection have been usedin electric power systems for more than a century. With theincrease in the importance of protection, it has been necessaryto ensure that the relays are able to perform their functionsas expected, when necessary, and do not operate when notrequired. Throughout the years different testing tools havebeen used successfully with electromechanical, solid-state andmicroprocessor based relays. Protection engineers and testingspecialists from around the world have developed confidencein traditional methods such as constant current or constantvoltage that have served them well for over a century. Thesemethods were more or less the things you could do with thetest devices available through most of the twentieth century:

Fix the current at a specific level and then start reducingthe voltage until the relay operates.

Fix the voltage and increase the current.This works great with electromechanical relays, but

unfortunately does not help us if we need to test advancedprotection IEDs. This brings us to the issue of Questions.

As with any human activity, before we start, it is importantfirst to ask ourselves some questions that will help us do abetter job, i.e. not only to get the job done, but also to do it inthe most efficient way possible.

Why are we testing?

This is the first question that we should ask. The reasonis, because the requirements for testing can be very different,

for example if a manufacturer is performing type testingof a new relay or a user is considering the acceptance of amultifunctional protection device, a certain set of things will

need to be tested:Characteristics of every functionPerformance of different functions under various

operating conditionsPerformance of different functions under various

non-operating conditionsThese tests can be performed also with settings of the

tested functions within the whole available range. There isa significant difference if we are testing a multifunctionalprotection IED during commissioning. In this case it willbe sufficient to do testing only of the used functions withtheir application specific settings. Testing of the completecharacteristic is not necessary. Checking of a couple of pointsjust to ensure that the relay has the right settings and thewiring between the test switch and the relay is correct, issufficient.

Things get more interesting when we are testing substationautomation systems. There is a difference in the requirements,methods and tools if we are doing factory acceptance testingversus site acceptance testing.

What are we testing?

This second question has a lot to do with the complexityof the new world of protection, automation and control. Andthere are different answers to it. The requirements for testingtools and methods will depend on what we are testing:

Electromechanical relayMultifunctional IEDDistributed protection schemeSubstation Automation SystemCommunications based protection scheme in the labCommunications based protection scheme in the field

If we look at some of the examples above, testingof an electromechanical relay can be performed using a

We can not test21st century IEDs with20th century testingtechnology.

1 Quality assurance process

coverstory

by Alex Apostolov, USA

ProtectionTesting

PAC.WINTER.2009

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020-025_cover_story_winter09_E_OK.indd 20 2/17/09


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Distance Protection

DataBus

I

WaveformRecording

AnalogInputsModule

OptoInputsModule

ProtectionSchemeModule

DistanceProtectionModule

RelayOutputsModule

V, I,V0, I0,

V2, I2

by the user, engineering, integration, commissioning andmaintenance of a substation protection, automation andcontrol system (SPACS). Each party manufacturer, integrator

and customer has a different role in the process that has tobe clearly defined for every stage of a project.

As can be seen from Figure 1, the manufacturer isresponsible for the development of devices according to ISO9001, type testing and system testing of all IEDs that can beintegrated in a SPACS. After a product is made available on themarket, a vendor still needs to perform regularly, specific teststo ensure the quality of the products delivered to customers.

Once a product is available on the market, the user makesa decision if it can be applied in the system using acceptancetesting. In principle, acceptance testing is similar to typetesting it is used to prove that the IED performs as statedin its technical specification. What is included in the testingis based on the acceptance criteria of the user. It may coverevery function in the tested device, but at least all featuresto be used should be tested. In the case of microprocessorbased relays, this is the time when the user should test the fullcharacteristics distance, inverse-time overcurrent, etc. Thisis because digital algorithms do not deteriorate with time.Once we know that the characteristics are within the statedtolerances, we do not need to check the full characteristic, justa few points to make sure that the wiring and settings are OK.Once a device is accepted for use in an electric power system,we need to make sure that it can operate within a SPACS. Thisrequires interoperability and integration testing.

Factory and site acceptance testing play a key role inensuring that the system will operate correctly under allpossible conditions. The system hierarchy and distributionof functions between multiple devices require the use ofnew methods and tools that become more complex whencommunications are used for exchange of signals between the

individual devices.It is very important to understand that the definition of

test procedures, methods and tools should be part of thedesign of the system. The description of the functionality ofa protection, automation and control scheme should comewith a specification of the tests to be performed to verifythat it performs as designed. Once the SPACS is in service,we need to make sure that all components of the systemare operating properly at any moment in time. The qualityof IED based systems can be ensured successfully using theadvanced monitoring tools that are built into most of thesedevices. Analysis of the monitoring functions can help theuser determine which components states are not know inorder to define the requirements for maintenance relatedtesting. Once we understand the quality assurance process,we can start looking into how it can be implemented. This iseasier to achieve by using a specific example multifunctionaltransmission line protection.

Transmission Line Protection FunctionsWe can start our analysis by looking into what we are

testing. The main purpose of any multifunctional distanceprotection IED is to detect and clear as quickly as possible

constant current or constant voltage, while the testingof an advanced multifunctional IED may require the use ofelectromagnetic transient simulation.

How are we testing?This third question is related to the selection of the testing

tools. It is clear that we can not answer it if we dont alreadyhave the answers to the first two questions. But this is notsufficient. We need to also know very well the capabilities ofthe test equipment available, as well as the functionality of theavailable testing tools.

For example, if we are testing a high-burdenelectromechanical ground overcurrent protection relay,we will need to use a test set that can deliver the requiredcurrent at the necessary compliance voltage. But if we aretesting a relay using the IEC 61850 sampled values andGOOSE messages a test device with communications onlycapabilities and support of the protocol may be used. Manualtesting may be OK if we are just verifying the connectionsof a relay during commissioning, while execution of large,object-oriented test plans is needed for acceptance testing.

We will try to explain in mode details the answers to theabove questions and discuss some modern methods and toolsthat can be used for twenty first century testing.

Quality assurance process

The requirements for testing that can be defined by aquality assurance process used by a manufacturer, utility oranyone else, can vary significantly from one entity to anotherand depend on philosophy, experience, available tools andother factors. One of the benefits from the introduction ofIEC 61850 is that it not only defines a new communicationsprotocol, but also describes in Part 4 of the standard a qualityassurance process that can be applied for any device or system,not only to the ones supporting the standard.

The quality assurance is a process that requires

involvement of all participants in the development andmanufacturing of individual devices and their acceptance

2 Transmission line protection block diagram

Alex Apostol

received MSEE

MSAM and Ph.

degrees from tTechnical Univ

in Sofia, Bulgar

He has more t

30 years exper

ence in protec

automation an

communicatio

He is presently

Principal Engin

OMICRON

electronics in L

Angeles, CA. H

is IEEE Fellow a

Member of the

Power System

Relaying Comm

and Substation

Subcommittee

serves on man

PES working gr

and is Chairma

of Working Gro

C9. He is Mem

of CIGRE and is

Convener of C

WG B5.27. He

representative

IEC TC 57 WG

17, 18. He hold

three patents

has authored apresented mor

than 280 tech

papers.

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Prefault

PowerSwingStart

Current

Fault

+

= imemirly

= Superimposedir

PowerSwingDetection

3Superimposed components calculation

Advanced

protection

functions

based on

superimposed

components

require the use

of different

testing

methods

compared

to electro-

mechanical

or solid state

relays.

The successful detection and clearing of any abnormalsystem condition is affected not only by the correctconfiguration and operation of the protection elements, butit also needs healthy secondary current and voltage circuits,as well as breaker trip or close circuits. This requires therelays to also perform monitoring functions, such as tripcircuit supervision, current and voltage circuit supervision ordifferent breaker monitoring functions.

Last, but not least, the relays are also used as the first levelin the hierarchy of a substation automation system, eventrecording and analysis functions. Based on the pre-fault andfault currents and voltages they calculate the location of thefault, magnitude and angle of the currents and voltages before

and after the fault, duration of the fault and other parameters.The interaction of different logical and functional elements

needs to be well understood, since there are differencesbetween the implementation of some protection functionsin electromechanical and microprocessor based relays. Forexample, a directional ground overcurrent protection in amicroprocessor-based relay is achieved as a combination ofovercurrent and directional elements.

short circuit faults thatcan damage substationequipment or create

conditions that adverselyaffect system stability orsensitive loads. This isachieved through theuse of instantaneousdistance elements orcommunications basedprotection schemes.

The distance elements can be simple or complex,with different operating characteristics, with or withoutdirectional supervision. The functions in the transmissionline protection relay have a hierarchy that needs to beconsidered for the testing of the device (Figure 2). The IEDactually works with an image of the electric power systemcurrents and voltages provided through several conversions the instrument transformers and secondary circuits inthe substation, as well as the analog inputs and internalprocessing in the device. The secondary currents and voltagesthat are applied to the distance protection relay are filteredand processed in the analog input module and provideinstantaneous sampled values to the internal digital databus of the IED. These sampled values are used to calculatevarious measurements (e.g. current and voltage phasors orsuperimposed components) used by the different protectionfunctions. The outputs of the measurement elements becomeinputs to protection or other functional elements of thedevice. Each basic protection element operates based on aspecific measured value phase or sequence current, voltage,frequency, etc. Measurements of active, reactive and apparentpower or power factor are often available from the relays ifrequired in the substation automation system.

When a protection element detects an abnormal condition,it may operate and issue a trip command to clear a fault. Itmay also interact with other protection elements in a distanceprotection scheme used for acceleration or adaptation of therelay to changing configuration or system conditions.

The multifunctional distance protection relays alsoperform automatic functions such as multi-shot reclosing andlocal backup protection such as breaker failure protection.

4Superimposed components elementoperation during power swing

The testing m

coverstory

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different functions based on these quantities:Fault detectionFaulted phase selectionDirectional detectionPower swing detection

Testing Requirements

Understanding the algorithms of the devices we aretesting answers the question what we are testing. This canbe used to determine how we are going to perform the tests.Conventional methods for testing of the main functions intransmission line protection relays the distance and thedirectional have been used for many years based on therequirements for testing of electromechanical or solid staterelays. They are also influenced by the technology availableat the time. As a result the Constant Current and Constant

Voltage methods are the ones usually applied. The maincharacteristic of these methods is that one of the parameters isfixed at a pre-selected value, and then the second parameter ischanged until an operation of the tested elements is detected.

From the description of the superimposed componentsbased functions in transmission line protection relays, it isclear that using these methods is not going to work for thetesting of such devices. This is due to the simple fact that therelays are designed to detect faults in real life conditions, i.e.when there is simultaneous change in the magnitude andangle of both the faulted phases currents and voltages.

The requirements for testing of such advanced functionsclearly point towards dynamic testing. We still need to becareful with regard to the understanding of this term. Insome cases a state change from pre-fault to fault condition

may be sufficient. However, if this is represented as a stepchange in the fault injection to the distance relay under test,it still may result in an operating time slower than expecteddue to the fact that the current waveform is not realistic. Thatis why electromagnetic transient simulation is the best wayto generate the signals used for the testing of the distanceelement.

Advanced functionsThe technology for protection

of transmission lines has changedsignificantly in the last two decades due to the advancementsof microprocessor based hardware and new algorithmsimplemented in the relay software. We can not cover a lotin this article due to the limited space, so we will just use asan example some protection or protection related functions

based on superimposed components - fault, directional andpower swing detection, as well as faulted phase selection.

When a fault, such as a short circuit, occurs in the electricpower system, it leads to a dynamic transition from thenormal system condition to a fault system condition. Thecurrents and voltages measured by the relay will changeas a function of the pre-fault system configuration andthe parameters of the fault - fault type, fault location, faultresistance, etc.

Superimposed components can be used for systemanalysis if the fault system condition is caused by a singleevent (the fault inception) and no other simultaneous eventhas occurred. In this case the faulted network state can beconsidered as the result of the superposition of the pre-faultand the fault generated quantities (Figure 3).

There are different approaches to the derivation of thesuperimposed components. The general aim is to estimatewhat the expected no-fault current or voltage sample shouldbe at this moment and then subtract that from the latestsample captured.

Once the superimposed components of the currents andvoltages have been calculated, the relay can run in parallel the

The requirements for testing ofprotection IEDs depend on the purpose of

the test.

Failures of

different types

of substation or

system equipment

result in 'natural'

testing of

protection devices.

and tools should eliminate the need for

'natural' testing.

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The test

system includes

different

simulation,

performance

evaluation and

documentation

tools.

WaveformRecord

NetworkSimulator

TestComputer

Comtrade file

Trip

TestDevice

MultifunctionalProtection IED

TestSequence

V, I, binarysignals

52aIV

Te st in g of Mu lt ifu n ct ion al d ist an ce

protection relays

Wh en we an al yz e th e co mp le xi t y of mo de rnmultifunctional distance protection devices, it is clear thattheir testing requires the use of advanced tools and softwarethat can simulate the different system conditions and statusof primary substation equipment and other multifunctionalIEDs. The test system should be able to replay COMTRADEfiles from disturbance recorders or produced fromelectromagnetic transient analysis programs. It should be ableto apply user defined current and voltage signals with settablephase angles, as well as execute a sequence of pre-definedpre-fault, fault and post-fault steps.

The testing of the different IED elements has to start fromthe bottom of the functional hierarchy and end with the mostcomplex logic schemes implemented in the device.

Protective relays with such schemes operate based on thestate of multiple monitored signals such as permissive orblocking signals, breaker status signals, and relay status signals.Time coordination of these signals and synchronization withthe pre-fault and fault analog signals is required in order toperform adequate testing of these types of schemes. Thetest device needs to be able to properly simulate the distanceprotection environment from Figure 2, as well as to monitorthe operation of the relay under the simulated conditions.

Testing of the analog signal processing andmeasurement functions

The analog signal processing is the first critical step in thetesting of a transmission line protection relay because if any

problems exist at this level, they will be reflected at any otherstep up the functional hierarchy. The only problem is that thedata bus of the IED is usually not directly accessible or visible

through the relay communications or user interface. That iswhy an indirect method is recommended. If we configurethe testing software to generate pure sinusoidal waveformsof balanced currents and voltages with their nominal valuesand no phase shift (zero degrees) between the currents andvoltages in the same phases, and record the applied waveformswith the tested relay, extracting and analyzing the records willallow us to evaluate if there is any problem with the analogsignal processing and recording functions.

The testing of the measurement functions of the relay isthe next step. It can use the same setup as described above, atleast as the initial measurements test condition. The measuredphase currents and voltages in this case need to be as close aspossible to the nominal balanced values applied to the relayby the test device (within the accuracy range specified by therelay manufacturer). The positive sequence measurementsshould be within tolerance of the phase values. Sincethe applied phase currents and voltages are balanced, themeasured negative and zero sequence values should be closeto zero (again within the expected tolerance range).

Testing of the main protection functions

When testing the individual protection elements in aconventional fashion, it is very important that they are theonly enabled protection function (if all protection elementsshare the same relay output). If the IED has multiple relayoutputs and different protection elements are mapped todifferent outputs, we need to make sure that the test devicemonitors the correct relay output during the test.

For a modern test system, such mappings shouldnt benecessary. A good fault model will correctly generate a systemcondition that the relay should distinguish, indicate, and

trip correctly for based on the enabled protection elementcharacteristic.

If we (based on the measurement functions tests) assumethat the relay measures accurately the applied current andvoltage signals, the testing of the distance elements shouldgive us an indication of what is the characteristic of the testedzone and expected relay operating time when the apparentimpedance seen by the distance element based on the appliedcurrents and voltages is within the operating characteristic.

Constant voltage and constant current methods may beused for the distance characteristics testing. This is acceptablefor some microprocessor based relays that use distanceelements based on the relationship of current and voltagephasors. While such tests are related to checking the distancecharacteristic of the relay, they may not be suitable for thetesting of the relay tripping time. This is especially importantfor Zone 1. If the relay uses superimposed components forthe fault detection, faulted phase selection or directionaldetection, the ramping of the current or voltage in someof the conventional test methods is not going to be seenas a fault condition and the relay under test is not going tooperate. Electromagnetic transient simulation is the best

The testing processshould follow the functionalhierarchy of the testeddevice.

5 Test system block diagram

coverstory

ProtectionTesting

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way to generate the signals used for the performance testingof the distance element. Evaluation of the distance elementoperation for multiple points on the selected characteristic is

typically required. Figure 7 shows the configuration for thetesting of a distance relay with a complex characteristic.

If the results from the testing of the distance characteristicsand the operating time are within the expected range, thenext step is the testing of the different communications basedschemes.

Testing Of Distance Protection Schemes

The testing of distance protection schemes is the finalstep in the testing of a distance relay and is based on theassumption that all individual protection elements distance,overcurrent, directional, faulted phase selection, etc. havealready been tested and proven to be operating correctly. Animportant consideration is the purpose of the test. If the testof a distance scheme is performed as part of a relay acceptancetest, the complete test can be performed by the simulation of

the analog and binary signals that the relay is going to measureor monitor under the specific test case conditions. However, ifthe test is part of the commissioning of the protection systemof a transmission line before it is put in service, it may benecessary to test the complete protection system, includingthe communications channel. End-to-end testing using GPSsynchronization is the preferred method in this case.

When communication aided schemes are used in complexsystem configurations, including double circuit transmissionline or transmission line loops with or without mutualcoupling, sequential tripping of faults on adjacent lines mayresult in incorrect operation of the accelerated schemes. It isrequired to develop test sequences simulating such conditionsto verify that the protective relay is going to operate correctly.

The next step in the Distance Protection Scheme testing isthe extension of the same test cases into the full operational

protection system test. This is commonly referred to asEnd-to-End Testing or System Testing.

IEC 61850 can also be an integrated function intransmission line protection relays. It impacts the testingprocess by requiring the test system to be configurable usingthe Substation Configuration Language, as well as to be able tosimulate GOOSE and sampled values, as well as to subscribeto and process GOOSE messages from the tested relay.

Transient simulation based testing

Wh en we an al yz e th e co mp le xi t y of mo de rnmultifunctional distance protection devices, it is clear that

their testing requires the use of advanced tools that cansimulate the different system conditions and status of primarysubstation equipment and other multifunctional devices.The test system should be able to replay COMTRADEfiles from disturbance recorders or produced fromelectromagnetic transient analysis programs. The focus atthis time is determining the performance of the device underrealistic system conditions as required by the application. Ifnecessary, the test cases should also include synchronous orasynchronous out of step conditions simulation to test thepower swing blocking or tripping functions. Performance ofthe tested relay when a fault occurs during a power swingshould also be included in the test plan.

The testing tools should allow easy configuration andexecution of such transient simulations as part of the testing

process, as well as proper evaluation and reporting of theoperation of the tested device (Figure 6).The protection of double circuit or other parallel line

configurations need to be able to operate under differentsystem conditions, evolving and cross-country faults,sequential tripping conditions and under the influence ofmutual coupling. All of the above needs to be consideredin the testing of such protection relays or communicationsbased schemes.

If we are testing a relay used on a double circuit line, itis important to properly model not only the impedancesof the two circuits, but also the mutual coupling betweenthem. Generic programs such as EMTP or ATP can beused to produce such files. They require good knowledgeof the software and proper configuration of the model andsimulation. Specialized testing tools make this easier by

providing a template for the transient simulation of differentfault conditions on mutually coupled double circuit line(Figure 6).

This offers significant advantages over the testing basedon a sequence of steps programmed in the software bymanually entering voltage and current phasors calculated bya steady state fault analysis software which do not properlysimulate the dynamic transition from one state to another.

Transient

simulation

should be

used for the

testing of t

performanc

of advanced

protection

functions

that operat

based on

simultaneo

changes of

currents an

voltages.

6 Double circuit line transient simulation 7 Distance characteristic test configuration

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We can not test 21st century IEDs with 20th century testing technology.pdf