by Alex Apostolov, USA
Protection Testing
cover story
20
We can not test
21st century IEDs with
20th century testing
technology.
As we can see from the History articles in the
magazine, different forms of protection have been used
in electric power systems for more than a century. With the
increase in the importance of protection, it has been necessary
to ensure that the relays are able to perform their functions
as expected, when necessary, and do not operate when not
required. Throughout the years different testing tools have
been used successfully with electromechanical, solid-state and
microprocessor based relays. Protection engineers and testing
specialists from around the world have developed confidence
in traditional methods such as “constant current” or “constant
voltage” that have served them well for over a century. These
methods were more or less the things you could do with the
test devices available through most of the twentieth century:
Fix the current at a specific level and then start reducing
the 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 advanced
protection IEDs. This brings us to the issue of Questions.
As with any human activity, before we start, it is important
first to ask ourselves some questions that will help us do a
better job, i.e. not only to get the job done, but also to do it in
the most efficient way possible.
Why are we testing?
This is the first question that we should ask. The reason
is, because the requirements for testing can be very different,
for example if a manufacturer is performing type testing
of a new relay or a user is considering the acceptance of a
multifunctional protection device, a certain set of things will
need to be tested:
Characteristics of every function
Performance of different functions under various
operating conditions
Performance of different functions under various
non-operating conditions
These tests can be performed also with settings of the
tested functions within the whole available range. There is
a significant difference if we are testing a multifunctional
protection IED during commissioning. In this case it will
be sufficient to do testing only of the used functions with
their application specific settings. Testing of the complete
characteristic is not necessary. Checking of a couple of points
just to ensure that the relay has the right settings and the
wiring between the test switch and the relay is correct, is
sufficient.
Things get more interesting when we are testing substation
automation systems. There is a difference in the requirements,
methods and tools if we are doing factory acceptance testing
versus site acceptance testing.
What are we testing?
This second question has a lot to do with the complexity
of the new world of protection, automation and control. And
there are different answers to it. The requirements for testing
tools and methods will depend on what we are testing:
Electromechanical relay
Multifunctional IED
Distributed protection scheme
Substation Automation System
Communications based protection scheme in the lab
Communications based protection scheme in the field
If we look at some of the examples above, testing
of an electromechanical relay can be performed using a
1 Quality assurance process
Market approval 1
Delivery and putting online
Project 1
Customer 1
Product realization
Project x
System test
Verification*
Customer x
Routine
test of
products
Prototype series
FAT*
of system
equipment
Site
commissioning
Site acceptance
test
Trial operation*
Project n
Type test
Customer n
Warranty
Customer's life cycle
Development
(EN ISO 9001)
Maintenance
Conformance
Test
* Optional
Decommissioning
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21
“constant current” or “constant voltage”, while the testing
of an advanced multifunctional IED may require the use of
electromagnetic 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 don’t already
have the answers to the first two questions. But this is not
sufficient. We need to also know very well the capabilities of
the test equipment available, as well as the functionality of the
available testing tools.
For example, if we are testing a high-burden
electromechanical ground overcurrent protection relay,
we will need to use a test set that can deliver the required
current at the necessary compliance voltage. But if we are
testing a relay using the IEC 61850 sampled values and
GOOSE messages – a test device with communications only
capabilities and support of the protocol may be used. Manual
testing may be OK if we are just verifying the connections
of 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 the
above questions and discuss some modern methods and tools
that can be used for twenty first century testing.
Quality assurance process
The requirements for testing that can be defined by a
quality assurance process used by a manufacturer, utility or
anyone else, can vary significantly from one entity to another
and depend on philosophy, experience, available tools and
other factors. One of the benefits from the introduction of
IEC 61850 is that it not only defines a new communications
protocol, but also describes in Part 4 of the standard a quality
assurance 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 and
manufacturing of individual devices and their acceptance
2
by the user, engineering, integration, commissioning and
maintenance of a substation protection, automation and
control system (SPACS). Each party –manufacturer, integrator
and customer – has a different role in the process that has to
be clearly defined for every stage of a project.
As can be seen from Figure 1, the manufacturer is
responsible for the development of devices according to ISO
9001, type testing and system testing of all IEDs that can be
integrated in a SPACS. After a product is made available on the
market, a vendor still needs to perform regularly, specific tests
to ensure the quality of the products delivered to customers.
Once a product is available on the market, the user makes
a decision if it can be applied in the system using acceptance
testing. In principle, acceptance testing is similar to type
testing – it is used to prove that the IED performs as stated
in its technical specification. What is included in the testing
is based on the acceptance criteria of the user. It may cover
every function in the tested device, but at least all features
to be used should be tested. In the case of microprocessor
based relays, this is the time when the user should test the full
characteristics – distance, inverse-time overcurrent, etc. This
is because digital algorithms do not deteriorate with time.
Once we know that the characteristics are within the stated
tolerances, we do not need to check the full characteristic, just
a 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. This
requires interoperability and integration testing.
Factory and site acceptance testing play a key role in
ensuring that the system will operate correctly under all
possible conditions. The system hierarchy and distribution
of functions between multiple devices require the use of
new methods and tools that become more complex when
communications 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 the
design of the system. The description of the functionality of
Transmission line protection block diagram a protection, automation and control scheme should come
with a specification of the tests to be performed to verify
that it performs as designed. Once the SPACS is in service,
we need to make sure that all components of the system
Distance Protection
are operating properly at any moment in time. The quality
Relay
of IED based systems can be ensured successfully using the
Outputs
Waveform
Module
advanced monitoring tools that are built into most of these
Recording
devices. Analysis of the monitoring functions can help the
user determine which components’ states are not know in
Analog
V, I,
Distance
order to define the requirements for maintenance related
Inputs
Protection
V0, I0,
testing. Once we understand the quality assurance process,
Module
V2, I2 I Module
we can start looking into how it can be implemented. This is
Data
easier to achieve by using a specific example – multifunctional
Bus
transmission line protection.
Opto
Protection
Transmission Line Protection Functions
Inputs
Scheme
We can start our analysis by looking into what we are
Module
Module
testing. The main purpose of any multifunctional distance
protection IED is to detect and clear as quickly as possible
Alex Apostolov
received MSEE,
MSAM and Ph.D.
degrees from the
Technical University
in Sofia, Bulgaria.
He has more than
30 years experience in protection,
automation and
communications.
He is presently
Principal Engineer at
OMICRON
electronics in Los
Angeles, CA. He
is IEEE Fellow and
Member of the
Power Systems
Relaying Committee
and Substations C0
Subcommittee. He
serves on many IEEE
PES working groups
and is Chairman
of Working Group
C9. He is Member
of CIGRE and is
Convener of CIGRE
WG B5.27. He is US
representative in
IEC TC 57 WG 10,
17, 18. He holds
three patents and
has authored and
presented more
than 280 technical
papers.
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Protection Testing
cover story
22
Advanced
protection
functions
based on
superimposed
components
require the use
of different
testing
methods
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. The functions in the transmission
line protection relay have a hierarchy that needs to be
considered for the testing of the device (Figure 2). The IED
actually works with an image of the electric power system
currents and voltages provided through several conversions
– the instrument transformers and secondary circuits in
the substation, as well as the analog inputs and internal
processing in the device. 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 are 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.
The multifunctional distance protection relays also
perform automatic functions such as multi-shot reclosing and
local backup protection such as breaker failure protection.
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 also 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.
Last, but not least, the relays are also used as the first level
in the hierarchy of a substation automation system, event
recording and analysis functions. 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.
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 relays. For
example, a directional ground overcurrent protection in a
microprocessor-based relay is achieved as a combination of
overcurrent and directional elements.
3 Superimposed components calculation
4 Superimposed components element
Prefault
The testing methods
operation during power swing
Fault
compared
to electromechanical
i rly
= i mem
+
Current
or solid state
= Superimposed
ir
Power
Swing
Start
Power
Swing
Detection
relays.
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and t
ethods
23
The requirements for testing of
protection IEDs depend on the purpose of
the test.
and tools should eliminate the need for
'natural' testing.
Failures of
different types
of substation or
system equipment
result in 'natural'
testing of
Advanced functions
protection devices.
The technology for protection
of transmission lines has changed
significantly in the last two decades due to the advancements
of microprocessor based hardware and new algorithms
implemented in the relay software. We can not cover a lot
in this article due to the limited space, so we will just use as
an example some protection or protection related functions
based on superimposed components - fault, directional and
power swing detection, as well as faulted phase selection.
When a fault, such as a short circuit, occurs in the electric
power system, it leads to a dynamic transition from the
normal system condition to a fault system condition. The
currents and voltages measured by the relay will change
as a function of the pre-fault system configuration and
the parameters of the fault - fault type, fault location, fault
resistance, etc.
Superimposed components can be used for system
analysis if the fault system condition is caused by a single
event (the fault inception) and no other simultaneous event
has occurred. In this case the faulted network state can be
considered as the result of the superposition of the pre-fault
and the fault generated quantities (Figure 3).
There are different approaches to the derivation of the
superimposed components. The general aim is to estimate
what the expected no-fault current or voltage sample should
be at this moment and then subtract that from the latest
sample captured.
Once the superimposed components of the currents and
voltages have been calculated, the relay can run in parallel the
different functions based on these quantities:
Fault detection
Faulted phase selection
Directional detection
Power swing detection
Testing Requirements
Understanding the algorithms of the devices we are
testing answers the question what we are testing. This can
be used to determine how we are going to perform the tests.
Conventional methods for testing of the main functions in
transmission line protection relays – the distance and the
directional – have been used for many years based on the
requirements for testing of electromechanical or solid state
relays. They are also influenced by the technology available
at the time. As a result the Constant Current and Constant
Voltage methods are the ones usually applied. The main
characteristic of these methods is that one of the parameters is
fixed at a pre-selected value, and then the second parameter is
changed until an operation of the tested elements is detected.
From the description of the superimposed components
based functions in transmission line protection relays, it is
clear that using these methods is not going to work for the
testing of such devices. This is due to the simple fact that the
relays are designed to detect faults in real life conditions, i.e.
when there is simultaneous change in the magnitude and
angle of both the faulted phases currents and voltages.
The requirements for testing of such advanced functions
clearly point towards dynamic testing. 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.
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Protection Testing
cover story
24
The testing process
should follow the functional
hierarchy of the tested
device.
T e s ting of Multifunc tional dis tance
protection relays
When we analyze the complexit y 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.
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.
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. The
test device needs to be able to properly simulate the distance
protection environment from Figure 2, as well as to monitor
the operation of the relay under the simulated conditions.
Testing of the analog signal processing and
measurement functions
The analog signal processing is the first critical step in the
testing of a transmission line protection relay because if any
5 Test system block diagram
The test
system includes
Network
Simulator
Waveform
Record
Comtrade file
Test
Sequence
different
Test
Computer
simulation,
V, I, binary
signals
performance
Test
Device
evaluation and
V
I
52a
documentation
Multifunctional
Protection IED
tools.
Trip
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, 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 and recording functions.
The testing of the measurement functions of the relay is
the next step. It can use the same setup as described above, at
least as the initial measurements test condition. 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).
Testing of the main protection functions
When testing the individual protection elements in a
conventional fashion, 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
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.
Constant voltage and constant current methods may be
used for the distance characteristics testing. This is acceptable
for some microprocessor based relays that use distance
elements based on the relationship of current and voltage
phasors. While such 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 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. Electromagnetic transient simulation is the best
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25
way to generate the signals used for the performance testing
of the distance element. Evaluation of the distance element
operation for multiple points on the selected characteristic is
typically required. Figure 7 shows the configuration for the
testing of a distance relay with a complex characteristic.
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.
Testing Of Distance Protection Schemes
The testing of distance protection schemes is the final
step in the testing of a distance relay and 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.
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.
The next step in the Distance Protection Scheme testing is
the extension of the same test cases into the full operational
protection system test. This is commonly referred to as
End-to-End Testing or System Testing.
IEC 61850 can also be an integrated function in
transmission line protection relays. It impacts the testing
process by requiring the test system to be configurable using
the Substation Configuration Language, as well as to be able to
simulate GOOSE and sampled values, as well as to subscribe
to and process GOOSE messages from the tested relay.
Transient simulation based testing
When we analyze the complexit y of modern
multifunctional distance protection devices, it is clear that
their testing requires the use of advanced tools that can
simulate the different system conditions and status of primary
substation equipment and other multifunctional devices.
The test system should be able to replay COMTRADE
files from disturbance recorders or produced from
electromagnetic transient analysis programs. The focus at
this time is determining the performance of the device under
realistic system conditions as required by the application. If
necessary, the test cases should also include synchronous or
asynchronous out of step conditions simulation to test the
power swing blocking or tripping functions. Performance of
the tested relay when a fault occurs during a power swing
should also be included in the test plan.
The testing tools should allow easy configuration and
execution of such transient simulations as part of the testing
process, as well as proper evaluation and reporting of the
operation of the tested device (Figure 6).
The protection of double circuit or other parallel line
configurations need to be able to operate under different
system conditions, evolving and cross-country faults,
sequential tripping conditions and under the influence of
mutual coupling. All of the above needs to be considered
in the testing of such protection relays or communications
based schemes.
If we are testing a relay used on a double circuit line, it
is important to properly model not only the impedances
of the two circuits, but also the mutual coupling between
them. Generic programs such as EMTP or ATP can be
used to produce such files. They require good knowledge
of the software and proper configuration of the model and
simulation. Specialized testing tools make this easier by
providing a template for the transient simulation of different
fault conditions on mutually coupled double circuit line
(Figure 6).
This offers significant advantages over the testing based
on a sequence of steps programmed in the software by
manually entering voltage and current phasors calculated by
a steady state fault analysis software which do not properly
simulate the dynamic transition from one state to another.
6 Double circuit line transient simulation
7 Distance characteristic test configuration
Transient
simulation
should be
used for the
testing of the
performance
of advanced
protection
functions
that operate
based on
simultaneous
changes of
currents and
voltages.
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