TESTING OF ADVANCED TRANSMISSION LINEPROTECTION RELAYS

A. Apostolov*, B. Vandiver t

*OMICRON electronics, USA, alex.apostolov@omicronusa~comt OMICRON electronics, USA, benton.vandiver@omicronusa~com

Keywords: distance protection, communication schemes,testing.

AbstractModem distance protection relays are complexmultifunctional devices based on classical or advancedoperating principles that impose special requirements for theirtesting. The paper analyzes the functionality, the fuinctionalhierarchy and the schemes typically available in distancerelays. It then discusses the methodology for testing of suchrelays. Testing of individual protection and non-protectionelements, superimposed components based functions,complex protection functions and communications basedtransmission line protection schemes are described in detail.

1 IntroductionThe testing of transmission protection relays with advanceddistance characteristics, fault and directional detectionfunctions and multiple communication-based schemes can notbe successfully performed using the methods applicable forelectromechanical distance relays. The order of testing ofbasic and complex protection functions needs to start with theindividual protection elements -distance, directional,overcurrent and then move into testing of more complexfunctional elements such as distance characteristics with loadencroachment or with directional supervision. These elementsneed to be tested not only under basic fault conditions, butalso under evolving faults and power swing conditions.Distance relays, with communication accelerated schemes,operate based on the state of multiple monitored signals suchas permissive signals, breaker status signals, andcommunication channel status signals. Time coordination ofthese signals and synchronization with the pre-fault and faultanalogue signals is required in order to perform adequatetesting of these types of schemes.The paper later discusses the challenges in the testing ofcommunication based schemes in transmission line protectionrelays. A detailed example is used to describe this approachfrom the perspective of testing requirements

-i.e., hardwarerequirements for simulation of breaker status, communicationand state of channel signals and how they should besynchronized with the analog signals simulating fault andpost-fault conditions.

Testing of distance relays used on double circuit transmissionlines expands the challenges for testing due to the impact ofmutual coupling on the behavior of the distance relay. Testingof the communications based schemes used on double circuitlines will require more complex simulation to cover the casesof current reversal during sequential tripping of breakers.Cross-country faults can have significant impact on theoperation of distance relays used on double circuit lines. Thisis especially important in the case of single pole trip andreclosing. The testing of the relays under such conditionsrequires proper simulation and monitoring of the response ofthe individual single phase tripping and closing contacts.The testing of distance relays under out-of-step conditions isalso challenging due to the requirements for realisticsimulation of such system conditions. The tests also need tocover the cases when there is a fault during the power swing.

2 Modern Transmission Line Protection DeviceFunctionsThe main purpose of any multifunctional distance protectionLED is to detect and clear as quickly as possible short circuitfaults that can damage substation equipment or createconditions that adversely affect system stability or sensitiveloads. This is achieved through the use of instantaneousdistance elements or communications based protectionschemes.The distance elements can be simple or complex, withdifferent operating characteristic, with or without directionalsupervision.The functions in the distance relay have a hierarchy that needsto be considered for the testing of the device (see Figure 1).First of all, the secondary currents and voltages that areapplied to the distance protection relay are filtered andprocessed in the analogue input module and provideinstantaneous sampled values to the internal digital data busof the IED. These sampled values can be logged when anabnormal system condition is detected or used to calculatevarious measurements (e.g. current and voltage phasors orsuperimposed components) used by the different protectionfunctions.The outputs of the measurement elements become inputs toprotection or other functional elements of the device. Each

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basic protection element operates based on a specificmeasured 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.

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Distance Protection ReaI I ,,

Fig. 1 Distance protection block diagram

When a protection element detects an abnormal condition, itmay operate and issue a trip coimmand to clear a fault. It mayalso interact with other protection elements in an advanceddistance protection scheme used for acceleration or adaptationof the relay to changing configuration or system conditions.

The multifunctional distance protection relays also performautomatic functions such as multi-shot reclosing and localbackup protection such as breaker failure protection.

Power swing detection functions can be used for separation ofdifferent parts of the system during wide area disturbances orto prevent that in parts of the system where it will furtherdeteriorate the system conditions.

The successful detection and clearing of any abnormal systemcondition is affected not only by the correct configuration andoperation of the protection elements, but it needs healthysecondary current and voltage circuits, as well as breaker tripor close circuits. This requires the relays to also performmonitoring functions, such as trip circuit supervision, currentand voltage circuit supervision or different breakermonitoring functions.

Last, but not least, the relays are also used as the first level inthe hierarchy of a substation or system analysis funiction.Based on the pre-fault and fault currents and voltages theycalculate the location of the fault, magnitude and angle of thecurrents and voltages before and after the fault, duration ofthe fault and other parameters.

In case of double circuit transmission lines the relays mayhave mutual current compensation that improves theperformance of the fault location functions.

The interaction of different logical and functional elementsneeds to be well understood, since there are differencesbetween the implementation of some protection fuinctions inelectromechanical and microprocessor based distance

protection relays. For example, a directional groundovercurrent protection is a single electromechanical device,while in the microprocessor-based relay it is achieved as acombination of an overcurrent and directional element.

The configuration of a distance protection relay with thedifferent available fuinctions that can be enabled or disableddepending on the requirements of the specific application hasto be taken under consideration before, during and after thetesting of a distance protection relay.

3 Superimposed Components Based Functions

Since the dynamic stability is a fuinction of the loading of theline and the duration of the fault, the operating time of thedistance relay will affect the level of loading of the protectedline. Superimposed components based fimrctions 'are one ofthe main characteristics of modem transmission lineprotection relays that also have significant impact on therequirements for their testing.

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 change at thesame time as a fuinction of the pre-fault system configuration,as well as the parameters of the fault - fault type, faultlocation, fault resistance, etc. The changes in the currents andvoltages measured are directly related to the fault. In general,Ai#t) and Av#~) can be considered as containing twocomponents - a steady-state component and a transientcomponent. However, directly after the fault inception thetransient is the prevailing component.

Once the superimposed components of the currents andvoltages have been calculated, the relay can run in parallel thedifferent applications based on these quantities:

" Faulted phase selection

" Directional detection

" Power swing detection

3.1 Faulted phase selection

Faulted phase selection is an important function intransmission line protection relays. For example, it isnecessary to ensure that the right distance mho orquadrilateral elements are allowed to decide whether to tripaccording to the fault type.

Since the superimposed components are directly related to thechanges in system parameters caused by the fault, faultedphase selection can be based on superimposed quantities. Onemethod is to use the superimposed components of the threephase-to-phase currents.

Figure 2 shows the changes in two of the phase-to-phasecurrents (IBC and ICA) for a phase C-G fault. This faultproduces the same superimposed component in the BC andCA currents and zero in the AB current.

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'NoChan ge!

AB

BC

CA

I I I

1 CycleComparison I CycleComparison

Ground Fault,Phase C

Fig. 2 Superimposed components based faulted phaseselectionIn the case of a phase-to-phase or two-phase-to-ground faultone of the phase-to-phase superimposed currents will begreater than the other two, while for a three-phase fault allthree superimposed components will be similar.If the faulted phase selection is required to be maintained fortime longer than the superimposed components calculation,buffer time (one or two cycles), the pre-fault samples fromthe buffer can be "recycled" and reused.One of the big advantages of the superimposed componentsbased faulted phase selection method is that it does notrequire any settings and is not significantly affected by themagnitude of the pre-fault load current. It also works verywell under evolving fault conditions.

3.2 Directional detection

Conventional directional detection methods are based on thechanges in the phase relationship between a measured phasorand a reference (polarizing) phasor. This approach has beenapplied successfully for many years. However, because ofextensive filtering requirements in order to obtain correctmeasurement, it is not applicable when very high speed faultclearing is required, as is the case with the distributed busprotection application. Alternative solutions for ultra high-speed directional detection based on the transient response ofthe power system when a fault occurs have been consideredand successfully implemented for more than twenty years inseveral generations of protective relays. The forward directionfor the relay is considered to be from the bus into the line. Fora forward fault the Ai and Av have different polarity. This isdue to the fact that the transient current and voltage wavestravelling from the fault location towards the system causechanges with different polarity in the currents and voltages atthe relay location.For a reverse fault the changes of the currents and voltagesmeasured by the relay have the same polarity.

A further development of the method based on the sign of Aiand Av is to use the product of Ai and Av , i.e. the transientpower or even further integrate this transient power overcertain period of time directly after the fault inception toderive the transient energy.The direction of the transient energy is based on the threephase products of the superimposed phase currents andvoltages. The direction of the fault is determined by the signof the transient energy caused by the inception of the fault,given by

S =fU.1ldt (1)The three phase energy transition is given byS = fAMa. Ala -sAUb. Alb +AUc. Alc )dtwhich is calculated in the relay asS = (AUai. Alai +AIJbi. ANb +AUci. Alci)

(2)

Based on the earlier discussions on the effect of the fictitioussource at the fault location on the changes of the phasecurrents and voltages measured at the relay location, we canconclude that it is always negative for forward faults andpositive for reverse faults.This method allows accurate directional detection undervarying system conditions and is not affected by seriescompensated transmission lines or mutual coupling.Therefore, it reduces the probability of relay misoperation andprovides a very fast (between V4 to 'A of a cycle) and reliabledirectional decision that can be used by a distributed busprotection system.

3.3 Power swing detection

Power swing detection is another important transmission lineprotection function. It can be used successfully either to blockdistance protection elements in order to prevent them fromtripping the protected line during power swings, or to issue atrip command to separate two systems and limit the spread ofa wide area disturbance.Power swing detection in conventional relays is based on thedetection of the impedance crossing of a band surrounding thedistance trip characteristic. It is clear that the time when therelay will detect the power swing will depend on the size andshape of the distance trip characteristic, as well as on thespecifics of the system disturbance.The superimposed components offer an advanced alternativeto the distance methods for power swing detection. Thisapproach is based on the fact that a power swing will result incontinuous change of current that will be seen as continuousoutput from the relay superimposed current elements PHI.This method offers some significant advantages, such as:

a Will detect all power swings whether fast or slow,and ensure correct blocking of zones.

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* Detects, and remains stable for 3 and 2 phase swings- the latter is especially important for the resulting 2phase swing during single pole autoreclose.

Using this method the relay is able to operate for faultsoccurring during a power swing. It is important to mentionthat depending on the location of the protective relay in thepower system the detection of a power swing may also beused to issue a trip signal to separate two parts of the system.

FaultPowerswing

:2.5 cYV'Ies

PHI-I !\IPSB active PSB removed

PH2--L

Fig. 3 Superimposed components elements operation duringpower swing

The change of the threshold and the following reset of thisphase selector element allows it to be used to detect a faultthat occurs during the power swing.

4 Testing Multifunctional Distance ProtectionDevicesWhen we analyze the complexity of modem multifunctionaldistance protection devices, it is clear that their testingrequires the use of advanced tools and software that cansimulate the different system conditions and status of primarysubstation equipment and other multifunctional IEDs.

Fig. 4 Test system block diagram

The test system should be able to replay COMITRADE filesfrom disturbance recorders or produced from electromagnetictransient analysis programs. It should be able to apply userdefined current and voltage signals with settable phase angles,as well as execute a sequence of pre-defined pre-fault, faultand post-fault steps. The testing of the different LED elementshas to start from the bottom of the functional hierarchy andend with the most complex logic schemes implemented in thedevice. Protective relays with such schemes operate based onthe state of multiple monitored signals such as permissive orblocking signals, breaker status signals, and relay statussignals. Time coordination of these signals andsynchronization with the pre-fault and fault analogue signalsis required in order to perform adequate testing of these typesof schemes.

The testing should include any visible behaviour of the testeddistance relay. This requirement is taken into consideration inthe following sections of the paper.

4.1 Testing of analogue signal processing

The analogue signal processing is the first critical step in thetesting of a distance protection relay because if any problemsexist at this level, they will be reflected at any other step upthe functional hierarchy. The only problem is that the data busof the IED is usually not directly accessible or visible throughthe relay communications or user interface. That is why anindirect method is recommended.

If we configure the testing software to generate puresinusoidal waveforms of balanced currents and voltages withtheir nominal values and no phase shift (zero degrees)between the currents and voltages in the same phases andrecord the applied waveforms with the tested relay, extractingand analyzing the records will allow us to detect problemswith the analogue signal processing.

Using any COMTRADE viewer to analyze the waveformrecord extracted from the relay will immediately show us ifthere are any deviations from the expected sine waveform, ifthere is any phase shift or if the amplitude is different fromthe expected value.

This step does not have to be used every time because it takessome time and it also requires the availability ofCOMTRADE viewer and communications with the relay inorder to extract the recorded waveforms. A much easier wayof detecting potential problems in the analogue signalsprocessing is the testing of the measurements as described inthe next section.

4.2 Testing of measurement functions

The testing of the measurement functions of the relay is thenext step. It can use the same set up as described in theprevious section, at least as the initial measurements testcondition.

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The nice thing about this test is that it does not require the useof relay communications, since the relay measurements arenormally available through the front panel user's interface.The measured phase currents and voltages in this case need tobe as close as possible to the nominal balanced values appliedto the relay by the test device (within the accuracy rangespecified by the relay manufacturer).The positive sequence measurements should be withintolerance of the phase values. Since the applied phase currentsand voltages are balanced, the measured negative and zerosequence values should be close to zero (again within theexpected tolerance range). At the same time the power factorshould be close to 1 and the frequency close to the nominalfrequency of the applied signals to the relay.

4.3 Testing of the distance protection characteristicAs discussed earlier, the main protection functions of adistance protection relay are the phase and ground distanceelements.The testing of the instantaneous and time delayed elements isdifferent and should follow a specific order.During the conventional testing of individual protectionelements it is very important that they are the only enabledprotection function (if all protection elements share the samerelay output). If the IED has multiple relay outputs anddifferent protection elements are mapped to different outputs,we need to make sure that the test device monitors the correctrelay output during the test.For a modem test system, such mappings should not benecessary. A good fault model will correctly generate asystem condition that the relay should distinguish, indicate,and trip correctly for based on the enabled protection elementcharacteristic.

Fig. 5 Distance characteristic test configurationThe test system should be configured to apply currents andvoltages with magnitudes and phase angles calculated basedon the apparent impedance, type of fault and testing methodselected. It should measure the time between the start of thetest and the sensing of the operation of the relay output when

connected to a binary input of the test system. This timeshould be less than the maximum operating time in thetechnical specification of the tested relay. The testing ofdistance elements with complex characteristics also requiresaccurate modelling of the distance characteristic as part of thetest configuration process. Evaluation of the distance elementoperation for multiple points on the selected characteristic istypically required. Figure 5 shows the configuration for thetesting of a distance relay with a complex characteristic.Depending on the tested element the user should be able toconfigure the type of fault as single-phase-to-ground, phase-to-phase or three-phase and also select the testing method.If the results from the testing of the distance characteristicsare within the expected range, the next step is the testing ofthe performance of the transmission line protection for systemconditions related to the application of the tested relay. In thiscase it is also very important to understand that the methodsfor testing of modem distance relays are different then theconventional constant current or constant voltage methods.From the description of the superimposed components basedfunctions it is clear that both the current and voltages changesimultaneously when a fault occurs, and this is what isexpected by the relay to detect a fault. If the current orvoltage is constant during the test - as in the conventionalmethods - this is just not going to be seen as a fault and therelay will fail the test. That is why transient simulation is thebest method for testing of relays using such algorithms.

71 7 "7ArI4X

AM ItB 1 01 U

Fig. 6 Transient simulation based protection testingIn case of application of a relay for the protection of a doublecircuit line, the mutual coupling needs to be included in themodel.Testing of the power swing detection function in relays in

many cases is performed assuming that the relay uses the timeit takes for the apparent impedance seen by the relay to crossthe distance characteristics. The test can be based on a fewsteady state conditions - one outside the characteristics, onebetween the two characteristics and one inside.H-owever, this method is simply not going to work for thesuperimposed components based power swing detection,because it simulates a non-realistic condition that the relay isnot going to recognize as a power swing (which it is not. Thats why it is also appropriate to use transient simulation for the

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testing of the power swing detection function. Figure 7 showsan example of an asynchronous power swing transientsimulation used for testing.

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 testing of communication-aided schemes should beperformed also in a way that as closely as possible matchesreal life power system conditions (Figure 8). The test deviceis used to simulate both the analogue and the digital signalsreceived by the relay in the field. At the same time, its inputsare used to monitor the operation of different relay elementsas required by the scheme under test.

Fig. 7 Asynchronous power swing simulation for testing

4.4 Testing of communications based schemes

The testing of distance protection schemes [I] is the final stepin the testing of a distance relay and it 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.

The conventional test process requires the programming ofthe test system to perform pre-fault, fault and post-fault stepssimulating the changing power system conditions to evaluatethe performance of the selected transmission line protectionscheme logic. Different control signals are required by thedistance protection logic schemes and must be considered inthe test definition in order to verify the functionality and thecorrect settings of such schemes. The simulation of the relayenvironment is also affected by the location of the fault.

Different tests are designed to monitor the relay operation forZone 1 fault, Zone 2 fault on the protected line. Zone 2 faultoutside of the protected line, reverse faults, faults on a parallelcircuit of a double-circuit line, cross-country faults, etc.depending on the distance relay application. Because we aretesting communication based schemes, the relay reaction tothe receiving of correct and noise control signals under theabove listed fault conditions is tested as well.

Some more advanced communication aided schemes monitornot only the receiving of a control signal, but also theavailability of the carrier signal, which may be lost if the faultis on the phase used by the communication channel. Thecombined effect of carrier signal and control signal receivedhas to be also tested.

Fig. 8 Single-phase fault with current reversal simulation

The test system is used to simulate both the analogue and thedigital control signals received by the relay in the field. At thesame time, its inputs are used to monitor the operation ofdifferent relay elements as required by the scheme under test.

5 ConclusionsTesting of advanced transmission line protection relaysrequires good understanding of their functionality, operatingprinciples and different distance protection schemes.

The testing should follow the functional hierarchy of thedistance protection relay: start with testing of the analoguesignal processing and measurements, followed by individualprotection elements and finish with protection logic schemes.

Software and hardware tools to simulate appropriately the testconditions in a realistic way are required, especially for thetesting of functions based on superimposed components -fault detection, faulted phase selection, directional detectionand power swing detection.

References[1I] Apostolov, B. Vandiver, "Automated Testing of

Communications Based Schemes in Transmission LineProtection Relays", Power Industry ComputerApplications PICA 2001, Sydney, Australia, May 2001

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