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Testing of Distance Protection Relays
Alexander Apostolov, Benton Vandiver
OMICRON electronics
I. INTRODUCTION
Distance protection relays have changed dramatically in the last two decades from simple single
function distance protection relays into multifunctional IEDs with primary transmission line
protection functions based on classical or advanced operating principles. Testing of such devices
requires the availability of a set of tools that will simplify the testing process, while at the same
time will ensure the required high quality of that testing process.
Testing of distance protection relays and IEDs is one of the key requirements to ensure their
correct operation under short circuit faults during abnormal system conditions. Since protection
technology is becoming more and more complex, with protective relays evolving to
multifunctional devices with integrated pre-programmed control logic and additional functions
like metering, fault and disturbance recording, programmable scheme logic, etc., ensuring that
they are properly configured and implemented requires adequate testing of their functionality. At
the same time the operation of the transmission systems close to their stability limit requires
significant reduction in the fault clearing times that can be only achieved by using the advanced
logic schemes available in modern transmission protection relays. In this case verification of the
relay operating times through testing is critical.
The relays are also used as a front end to the substation automation system and provide different
logging functions for analysis of their operation or different power system events. That is why
testing of such devices requires excellent understanding of the available protection and nonprotection functions, as well as the operational logic of the different schemes that require testing.
The paper first analyzes the functionality of modern state-of-the-art distance protection relays.
Several groups of functions are identified:
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Basic transmission line fault protection
•
Advanced protection schemes
•
Abnormal system conditions detection
•
Automation
•
Monitoring
•
Recording
•
Analysis
Considering the fact that there is a possibility for simultaneous occurrence of different types of
abnormal system conditions, it is clear that it is necessary to be able to test the distance
protection relay’s operation under such conditions. The requirements for the testing of all above
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listed functions are presented in the paper.
Different methods for testing are described, with special attention being paid to the testing tools
for advanced transmission line protection functions and schemes.
II. MODERN DISTANCE PROTECTION IED FUNCTIONS
The main purpose of any multifunctional distance protection IED is to detect and clear as
quickly as possible short circuit faults that can damage substation equipment or create conditions
that adversely affect system stability or sensitive loads. This is achieved through the use of
instantaneous distance elements or communications based protection schemes.
The distance elements can be simple or complex, with different operating characteristics, with or
without directional supervision.
Fig. 1 Distance protection block diagram
The functions in the distance relay have a hierarchy that needs to be considered for the testing of
the device (see Figure 1). First of all, the secondary currents and voltages that are applied to the
distance protection relay are filtered and processed in the analog input module and provide
instantaneous sampled values to the internal digital data bus of the IED. These sampled values
can be logged when an abnormal system condition is detected or used to calculate various
measurements (e.g. current and voltage phasors or superimposed components) used by the
different protection functions.
The outputs of the measurement elements become inputs to protection or other functional
elements of the device. Each basic protection element operates based on a specific measured
value – phase or sequence current, voltage, frequency, etc. Measurements of active, reactive and
apparent power or power factor are often available from the relays if required 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. It may also interact with other protection elements in a distance
protection scheme used for acceleration or adaptation of the relay to changing configuration or
system conditions. Some of the most common distance protection logic schemes are:
•
Permissive Underreaching
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• Permissive Overreaching
• Blocking Scheme
• Directional Comparison
• Weak Infeed
• Echo Logic
• Current Reversal
• Breaker Failure Protection
The multifunctional distance protection relays also perform automatic functions such as multishot reclosing and local backup protection such as breaker failure protection.
Several distance protection relay functions can be used for separation of different parts of the
system during wide area disturbances or to prevent that in parts of the system where it will
further deteriorate the system conditions.
The successful detection and clearing of any abnormal system condition is affected not only by
the correct configuration and operation of the protection elements, but it needs healthy secondary
current and voltage circuits, as well as breaker trip or close circuits. This requires the relays to
also perform monitoring functions, such as trip circuit supervision, current and voltage circuit
supervision or different breaker monitoring functions.
Fig. 2 Distance relay configuration
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Last, but not least, the relays are also used as the first level in the hierarchy of a substation or
system analysis function. Based on the pre-fault and fault currents and voltages they calculate
the location of the fault, magnitude and angle of the currents and voltages before and after the
fault, duration of the fault and other parameters.
In case of double circuit transmission lines the relays may have mutual current compensation
that improves the performance of the fault location functions.
The interaction of different logical and functional elements needs to be well understood, since
there are differences between the implementation of some protection functions in
electromechanical and microprocessor based distance protection relays. For example, a
directional ground overcurrent protection is a single electromechanical device, while in the
microprocessor-based relay it is achieved as combination of overcurrent and directional
elements.
Figure 2 shows the configuration of a distance protection relay with the different available
functions that can be enabled or disabled depending on the requirements of the specific
application. This is something that needs to be carefully considered before, during and after the
testing of a distance protection relay.
TESTING OF MULTIFUNCTIONAL DISTANCE PROTECTION DEVICES
When we analyze the complexity of modern multifunctional distance protection devices it is
clear that their testing requires the use of advanced tools and software that can simulate the
different system conditions and status of primary substation equipment and other multifunctional
IEDs. The test system should be able to replay COMTRADE files from disturbance recorders or
produced from electromagnetic transient analysis programs. It should be able to apply user
defined current and voltage signals with settable phase angles, as well as execute a sequence of
pre-defined pre-fault, fault and post-fault steps.
Fig. 3 Test system block diagram
The testing of the different IED elements has to start from the bottom of the functional hierarchy
and end with the most complex logic schemes implemented in the device.
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Protective relays with such schemes operate based on the state of multiple monitored signals
such as permissive or blocking signals, breaker status signals, and relay status signals. Time
coordination of these signals and synchronization with the pre-fault and fault analog signals is
required in order to perform adequate testing of these types of schemes.
Fig. 4 Testing of distance protection IED
Figure 4 shows in a simplified way the need for the test device to be able to properly simulate
the distance protection environment from Figure 1, as well as to monitor the operation of the
relay under the simulated conditions.
As can be seen from the figure, the testing should include any visible behavior of the tested
distance relay. This requirement is taken into consideration in the following sections of the
paper.
1) Testing of the analog signal processing
The analog signal processing is the first critical step in the testing of a distance protection relay
because if any problems exist at this level, they will be reflected at any other step up the
functional hierarchy.
The only problem is that the data bus of the IED is usually not directly accessible or visible
through the relay communications or user interface. That is why an indirect method is
recommended.
If we configure the testing software to generate pure sinusoidal waveforms of balanced currents
and voltages with their nominal values and no phase shift (zero degrees) between the currents
and voltages in the same phases (as shown in Figure 5) and record the applied waveforms with
the tested relay, extracting and analyzing the records will allow us to evaluate if there is any
problem with the analog signal processing.
Using any COMTRADE viewer to analyze the waveform record extracted from the relay will
immediately show us if there are any deviations from the expected sine waveform, if there is any
phase shift or if the amplitude is different from the expected value.
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Fig. 5 Test configuration for analog signal processing and measurement functions tests
COMTRADE viewers are usually readily available as part of the relay software or the testing
software itself. They also typically calculate and display the magnitude and phase angles of the
currents and voltages, so just by looking at these values and comparing them with the expected
nominal values and balanced phase angles it is quite easy to determine if there are problems with
the analog signal processing of the tested device.
This step does not have to be used every time because it takes some time and it also requires the
availability of COMTRADE viewer and communications with the relay in order to extract the
recorded waveforms. A much easier way of detecting potential problems in the analog signals
processing is the testing of the measurements as described in the next section.
2) Testing of the measurement functions
The testing of the measurement functions of the relay is the next step. It can use the same set up
as described in the previous section, at least as the initial measurements test condition.
The nice thing about this test is that it does not require the use of relay communications, since
the relay measurements are normally available through the front panel user interface.
The measured phase currents and voltages in this case need to be as close as possible to the
nominal balanced values applied to the relay by the test device (within the accuracy range
specified by the relay manufacturer).
The positive sequence measurements should be within tolerance of the phase values. Since the
applied phase currents and voltages are balanced, the measured negative and zero sequence
values should be close to zero (again within the expected tolerance range).
At the same time the power factor should be close to 1 and the frequency close to the nominal
frequency of the applied signals to the relay.
Depending on the measurements available in the tested relay, it is quite simple to calculate the
nominal balanced values and to compare and see if the measured values are within the expected
range and tolerance.
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If we are interested to check the accuracy of the relay measurements at sub-nominal levels, we
can configure the test software to apply 10% or 1% of the nominal values and follow a similar
procedure to the one described above.
3) Testing of the main protection functions
As discussed earlier, the main protection functions of a distance protection relay are the phase
and ground distance elements.
The testing of the instantaneous and time delayed elements is different and also should follow a
specific order.
When testing in a conventional fashion the individual protection elements, it is very important
that they are the only enabled protection function (if all protection elements share the same relay
output). If the IED has multiple relay outputs and different protection elements are mapped to
different outputs, we need to make sure that the test device monitors the correct relay output
during the test.
For a modern test system, such mappings shouldn’t be necessary. A good fault model will
correctly generate a system condition that the relay should distinguish, indicate, and trip
correctly for based on the enabled protection element characteristic.
If we (based on the measurement functions tests) assume that the relay measures accurately the
applied current and voltage signals, the testing of the distance elements should not provide any
surprises from the accuracy point of view, but will rather give us an indication of what is the
characteristic of the tested zone and expected relay operating time when the apparent impedance
seen by the distance element based on the applied currents and voltages is within the operating
characteristic.
Fig. 6 Phase comparator based distance characteristic
The test system should be configured to apply currents and voltages with magnitudes and phase
angles calculated based on the apparent impedance, type of fault and testing method selected. It
should measure the time between the start of the test and the sensing of the operation of the relay
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output when connected to a binary input of the test system. This time should be less than the
maximum operating time in the technical specification of the tested relay. It also depends on the
time delay setting of the distance zone.
Constant voltage and constant current methods may be used for the distance characteristics
testing. This is acceptable for electromechanical and solid state relays, as well as for some
microprocessor based relays that use distance elements based on the same principles, i.e. the
relationship of current and voltage phasors as shown in Figure 6.
The operating principles used in the distance relay also need to be taken into consideration in the
testing process. While the above described tests are related to checking the distance
characteristic of the relay, they may not be suitable for the testing of the relay tripping time. This
is especially important for Zone 1. If the relay uses superimposed components for the fault
detection, faulted phase selection (Figure 7) or directional detection, the ramping of the current
or voltage in some of the conventional test methods is not going to be seen as a fault condition
and the relay under test is not going to operate as expected.
Fig. 7 Superimposed components based faulted phase selection
In such cases dynamic testing will be required. We still need to be careful with regard to the
understanding of this term. In some cases a state change from pre-fault to fault condition may be
sufficient. However, if this is represented as a step change in the fault injection to the distance
relay under test, it still may result in an operating time slower than expected due to the fact that
the current waveform is not realistic. That is why electromagnetic transient simulation is the best
way to generate the signals used for the testing of the distance element.
The testing of distance elements with complex characteristics also requires accurate modeling of
the distance characteristic as part of the test configuration process. Evaluation of the distance
element operation for multiple points on the selected characteristic is typically required. Figure 8
shows the configuration for the testing of a distance relay with a complex characteristic.
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Depending on the tested element the user should be able to configure the type of fault as singlephase-to-ground, phase-to-phase or three-phase and also select the testing method.
Fig. 8 Distance characteristic test configuration
If the results from the testing of the distance characteristics and the operating time are within the
expected range, the next step is the testing of the different communications based schemes.
III. TESTING OF DISTANCE PROTECTION SCHEMES
The testing of distance protection schemes [1] is the final step in the testing of a distance relay
and it is based on the assumption that all individual protection elements – distance, overcurrent,
directional, faulted phase selection, etc. have already been tested and proven to be operating
correctly. An important consideration is the purpose of the test. If the test of a distance scheme is
performed as part of a relay acceptance test, the complete test can be performed by the
simulation of the analog and binary signals that the relay is going to measure or monitor under
the specific test case conditions. However, if the test is part of the commissioning of the
protection system of a transmission line before it is put in service, it may be necessary to test the
complete protection system, including the communications channel. End-to-end testing using
GPS synchronization is the preferred method in this case.
The conventional test process requires the programming of the test system to perform pre-fault,
fault and post-fault steps simulating the changing power system conditions to evaluate the
performance of the selected transmission line protection scheme logic.
There is a need for different options for testing of distance protection logic schemes based on the
purpose of the test. Three typical cases are:
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• Complete evaluation: all logic schemes are selected in a “point-and-click” manner and
the test software automatically executes a series of predefined tests, measures the
relay’s response, analyses the results and prepares the test report.
• Testing of a specific logic scheme: automatically executes all tests required for the
selected logic scheme, measures the relay’s response, analyses the results and prepares
the test report.
• Testing of a specific logic scheme for a specific condition: automatically executes a
single test required for the selected logic scheme, measures the relay’s response,
analyses the results and prepares the test report.
Different control signals are required by the distance protection logic schemes and must be
considered in the test definition in order to verify the functionality and the correct settings of
such schemes. The simulation of the relay environment is also affected by the location of the
fault.
A. What Should be Tested?
The testing of communication aided distance protection schemes is intended to evaluate the
performance of the relay under different fault, system and communication channel conditions.
Different tests are designed to monitor the relay operation for the following fault conditions:
• Zone 1 fault
• Zone 2 fault on the protected line
• Zone 2 fault outside of the protected line
• Reverse faults
• Faults on a parallel circuit of a double-circuit line
• No fault
Because we are testing communication based schemes, the relay reaction to the receiving of
correct and noise control signals under the above listed fault conditions is tested as well.
Some more advanced communication aided schemes monitor not only the receiving of a control
signal, but also the availability of the carrier signal, which may be lost if the fault is on the phase
used by the communication channel. The combined effect of carrier signal and control signal
received also has to be tested.
When communication aided schemes are used in complex system configurations, including
double circuit transmission line or transmission line loops with or without mutual coupling,
sequential tripping of faults on adjacent lines may result in incorrect operation of the accelerated
schemes. It is required to develop test sequences simulating such conditions to verify that the
protective relay is going to operate correctly.
B. How are the Tests Performed?
The fundamental requirement for advanced testing solutions in today’s utility environment is a
combination of efficiency and ease of use. The goal is to achieve maximum results with a
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minimum effort. That is why, the test configuration, execution efforts and analysis of the results
from a series of tests in most cases should be limited to a point-and-click action.
The testing of communication aided schemes should be performed in a way that as closely as
possible matches’ real life power system conditions. The sequence of steps in a test is different
as a function of the requirements for the specific scheme and system condition.
For example, if the test is for a Direct Transfer Trip scheme and the test conditions are normal
system with a noise triggered Carrier Receive signal, the sequence will include only three steps:
•
•
•
pre-fault with breaker in a closed position, nominal voltage and normal load current
receive of Direct Transfer Trip signal
post-trip condition with breaker opened, nominal voltage(assuming that bus voltage
is applied to the relay) and no current
If a more complex scheme is tested, the number of steps will increase accordingly. For example
if a Permissive Overreaching Scheme is tested, and a fault on an adjacent line with sequential
breaker opening is simulated, the test will have to include the following steps:
•
•
•
•
•
pre-fault with breaker in a closed position, nominal voltage and normal load current
initial fault condition with current flowing in reverse direction
receive of Permissive Trip signal
current reversal fault condition (simulating the opening of the breaker by the Zone 1
trip of the relay on the adjacent line)
post-fault condition with breaker closed, nominal voltage and normal load current
The test device is used to simulate both the analog and the digital signals received by the relay in
the field. At the same time its inputs are used to monitor the operation of different relay elements
as required by the scheme under test.
C. Test Results Analysis
The results from each test performed are automatically analyzed by the test software. The
analysis is based on an expert system comparing the operating time of a combination of
monitored protection elements that have picked-up during the test.
Fig. 9 Single-phase fault with current reversal simulation
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The operating time of the monitored protection elements is defined based on the protective relay
manufacturer’s technical specifications.
The results are displayed in a graphical format in the user interface and in detail in an
automatically generated test report.
The test system is used to simulate both the analog and the digital control signals received by the
relay in the field. At the same time its inputs are used to monitor the operation of different relay
elements as required by the scheme under test.
IV. END-TO-END TESTING
The next step in the Distance Protection Scheme testing is the extension of the same test cases
described above into the full operational protection system test. This is commonly referred to as
End-to-End Testing or System Testing.
These tests depend on two critical factors, first the coordination of the test cases for both ends of
the line to be tested must be the same and a mirror of the fault being simulated. In other words,
both ends must see the same Zone 1 fault so the scheme logic will respond accordingly. If the
local terminal “sees” a Zone 1 fault at 30% of line length, then the remote end must “see” it at
70% of the line length. But care has to be taken to make sure the test cases are structured
properly to provide the correct prefault conditions and prefault to fault transition to correctly
simulate the system condition of the test case. A simple step change transition can only test the
gross performance of the protection scheme; most modern distance relays utilize complex
algorithms to detect real system events. The step change may invoke additional logic that would
conflict with the scheme logic being tested, resulting in an undesired or unexpected response.
Second, playback synchronization accuracy is very important. A synchronization error of 1ms at
60 Hz produces a 21.6° error between two referenced vectors. For successful end-to-end testing,
synchronization has to be better than 10μs, which produces a phase error of 0.216°. The best test
equipment in the world today achieves a synchronization accuracy of 1μs, which is more than
adequate to test even traveling wave relays (a 0.0216° phase error). Today, GPS systems can
provide this level of timing to the test set; (Figure 10) one must ensure that the output of the test
set is as accurate and more importantly consistently repeatable.
In addition to the positive test cases where a response from the distance relay is expected, in endto-End testing it is even more important to have a suitable set of negative test cases where the
distance relay or communication system is verified it does not miss-operate. This requires an in
depth knowledge of the entire protection scheme and understanding of when it should not trip.
Many utilities are moving to system testing as their normal maintenance test procedure for many
reasons. A few of these are:
•
•
•
Understanding and verification of the entire protection system is critical to utility
operations
A digital relay’s reliability, once functionally verified, is seldom enhanced with
additional element testing except in the case of settings or firmware changes
System testing reveals more in depth data and pinpoints problems faster with proper
test cases
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•
•
Costs for system testing can actually be less, with better trained personnel, as
compared to conventional routine testing
System testing requires development of good testing habits resulting in more
consistent testing
Fig. 10 Testing of the Communication based Power System Protection Schemes using GPS
satellite system for the test synchronization
V. CONCLUSIONS
Testing of multifunctional distance protection relays requires complete understanding of their
functionality, operating principles of the individual elements and different distance protection
schemes.
Software and hardware tools to appropriately simulate the test conditions and the permissive,
blocking or other status signals received from the relay under test are essential for successful
logic scheme performance evaluation.
The testing should follow the functional hierarchy of the distance protection relay. It should start
with testing of the analog signal processing and measurements, followed by individual protection
or other elements and finish with the distance protection logic schemes.
Testing of complex distance characteristics using different methods for single-phase-to-ground,
phase-to-phase or three phase faults is an important step in the overall testing process of distance
relays. The operating principles and algorithms implemented in the relay should be taken into
consideration when selecting the fault simulation method.
Testing for internal and external faults, as well as faults on a parallel circuit (if an application on
a double circuit line is tested) should be included in the test cases.
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Automatic rules based expert system relay performance analysis and test report generation
significantly improves the efficiency and further simplifies the overall testing process.
V. REFERENCES
Automated Testing of Communications Based Schemes in Transmission Line Protection Relays,
A. Apostolov, B. Vandiver Power Industry Computer Applications PICA 2001, Sydney,
Australia, May 2001
VI. BIOGRAPHIES
Alexander Apostolov received MS degree in Electrical Engineering, MS in
Applied Mathematics and Ph.D. from the Technical University in Sofia, Bulgaria.
He has worked for fourteen years in the Protection & Control Section of
Energoproject Research and Design Institute, Sofia, Bulgaria.
From 1990-94 he was Lead Engineer in the Protection Engineering Group, New
York State Electric & Gas. 1994-95 he was Manager of Relay Applications
Engineering at Rochester - Integrated Systems Division. 1995-96 he was Principal Engineer at
Tasnet. !996-2006 he was Marketing Segment Leader and Principal Application Engineer for
AREVA T&D Automation
He is presently Principal Engineer for OMICRON electronics in Los Angeles, CA. He is a
Senior Member of IEEE and Member of the Power Systems Relaying Committee and
Substations C0 Subcommittee. He is Chairman of the Relay Communications Subcommittee,
serves on several IEEE PES Working Groups. He is Chairman of Working Group C9: Guide for
Abnormal Frequency Load Shedding and Restoration.
He is member of IEC TC57 WG 10, 17, 18, 19 and CIGRE WG B5.7, B5.11.and Convener
of WG B5.13. He is Chairman of the Technical Publications Subcommittee of the UCA
International Users Group, holds three patents and has authored and presented more than 200
technical papers.
Benton Vandiver III received BSEE from the University of Houston in 1979.
He began his career with the Substation Division of Houston Lighting &
Power, in 1978 engineering relay protection systems for all levels of
transmission, distribution, and generation. His main interests were in
computer design automation of protection schemes and substation projects.
He developed extensive knowledge in the application, setting, testing,
modeling, and design of traditional and digital relaying systems used in all types of power
system protection, control, and monitoring. In 1991 he joined Multilin Corp. as a Project
Manager on a team responsible for designing and developing the hardware and software for a
new family of utility grade digital relays. In 1995 he joined OMICRON electronics as a Sales &
Application Engineer with primary responsibilities of sales, training, and promotion of the
revolutionary CMC Universal Test Set to North & South America.
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He is currently Technical Director for OMICRON Electronics Corp. USA in Houston, TX.
He is a long time member of IEEE and is Chairman of Working Group H5-C Common Data
Format for IED Sampled Data. He holds a US Patent and has authored or co-authored numerous
technical papers for various conferences in North America.