System Test
Resume:
Compatibility and Interoperability Evaluation
for All-digital Protection System
through Automatic Application Test
Peichao Zhang, Levi Portillo and Mladen Kezunovic, Fellow, IEEE
conformance and performance test. IEC 61850-10 gives guid-
Abstract This paper proposes a methodology of compatibility
and interoperability evaluation for all-digital protection system ance for the conformance tests [2]. The conformance tests are
through automatic application testing. The paper starts with only the rst step to verify the interoperability. Compared with
the introducing of compatibility and interoperability evaluation
conformance tests, performance tests allow more extensive
issues, explaining the importance of application tests. The paper
assessment and can be used to determine the performance
then de nes the performance indices and compatibility indices
characteristics of the overall system. Thus, performance tests
as well as the evaluation methodology. Making full use of the
advantages of IEC 61850, a fully networked and automated will be as important if not even more so, as than conformance
application test solution for all-digital protection system is then tests [3].
proposed to calculate these indices. The related IEC 61850
Performance testing of the protection system by itself is
information models and ACSI (Abstract Communication Service
not a new topic [4], [5], [6]. All these papers are within
Interface) services which contribute to the automatic testing
the scope of conventional protection system. The evaluation
are described. Finally, the hardware architecture and software
implementation are proposed in this paper. The framework criteria and methodology proposed in these papers are purely
technology is used to obtain an open software architecture, so for performance evaluation purpose, not for compatibility and
that different power network models, transducer models as well
interoperability evaluation purpose.
as multiple performance evaluation criteria can be added to
This paper is aimed at proposing a methodology of com-
the system, without altering the overall software structure and
patibility and interoperability evaluation for all-digital pro-
control ow of the automatic testing system.
tection system through automatic application testing. This
Index Terms compatibility, interoperability, IEC 61850, per-
methodology is now being realized in a PSerc [7] project.
formance evaluation, process bus, protective relaying, relay test-
ing. The paper is organized as follows. Section II introduces the
compatibility and interoperability evaluation issues; Section III
de nes the performance evaluation criteria and methodology;
I. I NTRODUCTION
Section IV discusses the automatic application test; Section
The recent development of optical instrument transformers
V describes the hardware architecture of the application test
and the spread of digital relays permit the development of
system; Section VI describes the software implementation.
an all-digital protection system. In such system, the output of
both the optical current and voltage transformers is a digital
II. C OMPATIBILITY AND I NTEROPERABILITY E VALUATION
signal, which can be connected to digital relays through an
IEC 61850-9-2 digital process bus [1]. A. Compatibility and Interoperability
Compatibility and interoperability are among the main
Compatibility is often understood as the ability of two or
driving forces behind the creation of IEC 61850. Many
more components or devices to perform their functions while
interoperability tests have been performed in the past few
sharing a common environment. In the context of 61850,
years. Through these tests, the bay level interoperability and
compatibility means the ability of two or more IEDs to
the IEC 61850-9-1 interoperability at the process level have
perform their intended functions (protection, control, metering,
been veri ed. But the all-digital protection system containing
etc) while sharing the IEC 61850 common communication
different optical instrument transformers connected to different
standard. Interoperability according to IEC 61850 means the
digital relays by an IEC 61850-9-2 process bus was not
ability of IEDs or substation automation systems from differ-
described in details in the literature yet.
ent vendors to execute bi-directional data exchange functions,
Compatibility and interoperability evaluation of the all-
in a manner that allows them to operate effectively together.
digital protection system requires two kinds of test, namely
This is achieved by standardizing the access to data stored in
different logical nodes (LN) through the Abstract Communi-
This work was supported by PSerc project titled, Digital Protection System
Using Optical Instrument Transformers and Digital Relays Interconnected by cation Service Interface (ACSI) [8].
an IEC 61850-9-2 Digital Process Bus
As a standard of the communication networks and systems
P. Zhang is with the Department of Electrical Engineering, Shanghai
for substations, IEC 61850 does not try to specify the actual
Jiaotong University, Shanghai, China (e-mails: *******@****.***.**). He is
now working as a visiting scholar in Texas A&M University. L. Portillo implementation of a function. Vendors will still use their
and M. Kezunovic are with the Department of Electrical and Computer
individual algorithms, which they regard as best suited to per-
Engineering, Texas A&M University, College Station, TX 77843-3128, USA
form a certain task. So unlike interoperability, the IEC 61850
(************@****.***, *******@***.****.***).
2
standard was never intended to ensure interchangeability of
IEDs [3]. However, interchangeability of the transducer system
(comprised of current and/or voltage sensors and merging
units) and process bus (also called process LAN, usually made
of copper or ber wires and an Ethernet switch) is not only a
possibility but also a highly desirable feature of the all-digital
system, allowing utilities to choose between different sensors
and fast Ethernet switches without restrictions.
The assessment of the compatibility and interoperability of
different IEDs or systems will enhance the practical applica-
tion of all-digital protection system and make a signi cant
contribution to industry s understanding of the novel system.
B. Conformance Tests and Performance Tests
Compatibility and interoperability evaluation of the all- Fig. 1. Compatibility and interoperability evaluation
digital protection system requires two kinds of test, namely
conformance and performance test.
can be tested. So performance tests will be as important as the
Conformance tests belong to certi cation tests which aim
conformance tests in the future.
at verifying whether an IED satis es the criteria speci ed
As a result, both conformance and performance tests play
by certain standard or authority. These tests are performed at
an important role in improving the compatibility and interop-
the vendor s laboratories or at independent test institutes. IEC
erability of the all-digital protection system and minimizing
61850-10 [2] gives guidance on how these conformance tests
the chance of inter-working problems. Fig. 1 summarizes
have to be performed and UCA R International Users Group
these tests. This paper will suggest how to evaluate the com-
[9] has authorized KEMA to perform IEC 61850 conformance
patibility and interoperability of all-digital protection system
tests in accordance with the Users Group Assurance Testing
through performance tests. To quantitatively determine the
Program Procedures.
compatibility and interoperability, the evaluation criteria and
Performance tests belong to application tests which aim at
methodology are de ned in the next section.
verifying the behavior of the protection system, the accuracy
and operating times under various power network conditions
(both faulted and unfaulted conditions) and computer network III. E VALUATION CRITERIA AND METHODOLOGY
conditions [10]. When compared with conformance tests, A. Referent Models
performance tests concentrate on IEDs operation as part of
The purpose of the introduction of referent models is to
a complete application system, verifying that the performance
evaluate the performance of the transducer system and the
for a given application is adequate and ensuring that the whole
overall protection system automatically. There are two kinds
system is as robust and reliable as feasible. For the all-digital
of referent models, namely the transducer system model and
protection system, the interests for performance tests are the
the protective relay model.
trip/no trip decisions and the operating times.
The referent transducer system model in this paper is
These two types of tests have a stronger bind for the all-
regarded as ideal, and therefore, delivers exact signals from
digital protection system than the conventional one. On the one
the primary side.
hand, conformance tests are the basis of performance tests. It
The referent protective relay model is a software simulation
means only IEDs which have passed the conformance tests
model of a protective relay. Several protective relay models
may be connected together to build an application system for
with different relay principles such as over-current and im-
performance tests. But it should be noted that the passing of
pedance have been realized [11], [12].
conformance tests does not imply the passing of performance
tests. Because of the unanticipated network performance, the
B. Performance Indices
complexities of IEC 61850 standard, the multifunction of
IEDs, the mission sensitive and time critical features of the A number of performance indices for evaluation, design
protection and control functions, it may happen that IEDs did and setting optimization of measuring algorithms, operating
pass the conformance tests but did not perform the necessary principles, complete relays and protective systems are de ned
functions when connected together. So passing conformance in details in [5]. This paper adapts some of the performance
tests is just a necessary but not a suf cient condition for indices to meet the needs of the all-digital protection system.
performance tests success. On the other hand, the passing of De nition 1: A single exposure E is a disturbance which
performance tests will verify the conformance tests to a great triggers a protection system P to perform certain operations or
extent. As compared with conformance tests, performance tests other signals if called upon [5]. The exposures database EB
allow more extensive assessment and can be used to determine is a database of exposures collected from the actual system
or using simulators. Signal S t, S r denote the digital output
the performance characteristics of the overall system. Thus,
the feasibility, applicability and ef ciency of the new standard of the tested and the referent transducer system (with merging
3
unit) respectively. Decision D t, D r denote the decision of the
tested and the referent protection system respectively.
De nition 2: The performance index of transducer T
E
when fed by exposure E is denoted by T P I T, E =
{e1, e2, e3, en }. The average performance index of trans-
ducer T is de ned as:
1 E
T P IT = T P IT (1)
N
E EB
where N is the number of exposures in the database.
E E
There are two primary types of transducer performance T1 T2
indices calculation methods, namely the time domain method
and frequency domain method respectively. For the time E
E
domain: P2
P1
n n
E t r r
Si )2 / (Si )2
T P IT = (Si (2)
Fig. 3. Compatibility indices calculation
i=1 i=1
For the frequency domain:
where:
1
m m if relay trips
Dt, Dr =
E
(Fjt Fjr )2 / (Fjr )2
T P IT = 0
(3) otherwise
j =1 j =1
For the trip time method:
Fjt, Fjr t r
Si, Si
where stand for the FFT coef cients of P P IP = Dt Dr
E
(6)
respectively.
where D t, D r stand for the trip time of the tested and the
De nition 3: The performance index of protection system
E
P when fed by exposure E is denoted by P P I P . The average referent protection system respectively.
performance index of protection system P is de ned as:
1 C. Compatibility Indices
E
P P IP = P P IP (4)
N The evaluation methodology is described in Fig. 3. It will
E EB
be explained by the following de nitions.
where N is the number of exposures in the database. De nition 4: The compatibility index of transducer T 1 and
T 2 when fed by the same test signal E is de ned as:
There are two types of protection performance indices
calculation methods, namely the trip decision method and trip E E E
T CIT 1,T 2 = T P IT 1 T P IT 2 (7)
time method respectively. For the trip decision method:
The average compatibility index of transducer T 1 and T 2 is
P P IP = Dt Dr
E
(5)
de ned as:
1 E E
T CIT 1,T 2 = T P IT 1 T P IT 2 (8)
N
E EB
In this paper, the transducer system includes the optical
sensors and merging unit. By the de nition, the smaller TCI,
the better compatibility and interoperability.
De nition 5: The compatibility index of protection system
P 1 and P 2 when fed by the same test signal E is de ned as:
E E E
P CIP 1,P 2 = P P IP 1 P P IP 2 (9)
The average compatibility index of protection system P 1 and
P 2 is de ned as:
1 E E
t r
P CIP 1,P 2 = P P IP 1 P P IP 2 (10)
N
E EB
In this paper, the protection system includes the transducer
t r
system, the process bus (the Ethernet LAN) and the protective
relay. By the de nition, the smaller PCI, the better compati-
bility and interoperability.
In table I, The symbol T, B and P stand for the transducer
Fig. 2. Performance indices calculation system, the process bus and the protective relay respectively.
4
TABLE I
meaning but they are usually originated in proprietary
T EST CASES AND COMBINATIONS OF PROTECTION SYSTEMS
form.
4) A third party digital fault recorder or high precision
Case P1 P2 oscilloscope may be needed to record the disturbance
T B R T B R data. Only the possibility of recording disturbance data
1 T1 B1 R1 T2 B1 R1 in COMTRADE les brings some form of standardiza-
T1 B1 R2 T2 B1 R2 tion. However, only IEC 61850, because of its nature and
2 T1 B1 R1 T1 B1 R2 the way it has been conceptualized, could offer complete
T2 B1 R1 T2 B1 R2 real time-data acquisition standardization.
3 T1 B1 R1 T1 B2 R1
With the introduction of IEC 61850, the data acquisition
and description methods are standardized. Both the application
system and the application test system will bene t from the
standardization. From the application test perspective, the
To calculate the PCI, we combine the T, B and P in different
standard will provide the possibility of automatic testing.
ways to assemble different protection systems.
The automatic testing feature and technology for all-digital
There are three test cases listed in the table.
protection system are described next.
1) In case 1, same protective relay but different transducers
are used to assemble a protection system. The purpose
is to evaluate both the interoperability between the A. Automatic Test Scenario Con guring
transducers and IEDs and the interchangeability between
IEC 61850 uses SCL (Substation Con guration description
the transducers.
Language) which is based on XML to describe both the IED
2) In case 2, same transducer but different protective relays
capabilities and the system con guration. Besides, it pro-
are used to assemble a protection system. Because of the
vides some ACSI (Abstract Communication Service Interface)
fact of different operation characteristics of relays, only
services to extract con guration information from IEDs at
the interoperability between the transducers and relays
runtime. Thus, the application test software can use the SCL
is evaluated.
le as a test speci cation for automatic testing.
3) In case 3, same transducer and relay but different Eth-
From the application test perspective, the IEDs capabilities
ernet LANs are used to assemble a protection system.
and con guration of interest involve the types, zones and
Ethernet switches with different traf c load will be sim-
operating characteristics of relays, the services the relays
ulated to evaluate the performance of the all-protection
support and the communication links between the optical
system during network overload and the reaction of all-
transducers and relays. The application test software will then
digital protection system during Ethernet failure will be
automatically con gure the test cases and scenarios according
inspected.
to the tested protection system. The test cases and scenarios
Since there is a large number of tests to be performed, the
for relay testing used in this paper are described in details in
automatic application testing is indeed necessary. The intro-
[4], [6].
duction of IEC 61850 makes the automatic testing possible,
which will be demonstrated in the next section.
B. Automatic Con guring of a Relay
IV. AUTOMATIC A PPLICATION T ESTING
The con guration of the protection system being tested is
For the conventional protection system, the application test a critical step in the application test. The main problem with
cannot be automated for the following reasons: the advancements in the functionality of modern protective
relays is the increased complexity of the setting of the IEDs.
1) The IEDs have no self-description capability. The tech-
The protection engineer has to con gure hundreds of settings.
nical manuals from different vendors are in free format,
Another problem is that when using devices from different
which prevents the application test software from se-
vendors, models or types the engineers have to usually con-
lecting the proper test cases and scenarios automatically
gure them using different software tools. IEC 61850 makes
according to the system con guration and IEDs capabil-
a signi cant improvement in that process, de ning standard
ities.
object models and a con guration language that will allow
2) There are no common con guration tools for the IEDs.
automatic processing. Ongoing work in IEC and IEEE working
The IEDs from different vendors provide their con-
groups will result in a standard IED con guration le format
guration data in their own proprietary format and
based on the object models and SCL (Substation Con guration
use multiple standardized or proprietary communication
Language) de ned in IEC 61850 [13], which will further
protocols. This prevents the application test software
simply the con guration tasks.
from con guring the IEDs automatically according to
the selected test cases. The SCL concept promises many advantages, such as con-
3) Hard wired tripping signals do not carry with them any sistent description of the device and substation con guration
information related to their origin and meaning. Event and the possibility to use vendor independent con guration
reports generated by protection devices do provide the tools.
5
From the application test perspective, the main con guration
tasks involve relay setting and GOOSE (Generic Object Ori-
ented Substation Event) con guration. For the relay setting,
Status
IEC 61850 provides SGCB (Setting Group Control Block)
Settings
services to ful ll the task. For the GOOSE con guration, IEC
61850 provides GoCB (GOOSE Control Block) services to
ful ll the tasks.
C. Automatic Processing of a Tripping Message
GOOSE is used to replace the hard wired control signal
exchange between IED s for interlocking and protection pur-
poses [8]. Status information like trip commands is transmitted
via GOOSE messages. The GOOSE messages de ned in IEC
61850 allow the IEDs to multicast change of state information
to other IEDs connected to the substation LAN.
The test device has to monitor the outputs of the relay under
test in order to change the states of the simulation and to
evaluate the performance of the relay under test. In the all-
digital protection test system, the test device can subscribe
Fig. 4. LNs and related ACSI services used in application test system
and analyze the GOOSE message sent by the relay to get the
trip decision and trip time of the relay.
SCD Test Computer ProcessBus IT & MU Relay
IED Capabilities
D. Automatic Recording of Disturbance Simulation
The disturbance data should be recorded to calculate the
Setting Setting
transducer performance indices and compatibility indices. In
the all-digital protection system, the merging unit will multi- Test Signal
cast the samples of currents and voltages over the IEC 61850- Multicast SV
9-2 process bus. The multi-use of these data will serve not SV SV Decision
only the protection but also the application test device. The test Repeat N
GOOSE GOOSE
times
device will subscribe to the sampled value using the Sampled
Value service provided by IEC 61850.
Performance
Evaluation
E. Overall Information Model Compatibility
Final Report Evaluation
As described in the above, IEC 61850 de nes standardized
access to all data and resources, which makes the fully Fig. 5. Test sequence of the automatic application test system
networked, automatic relay testing possible. IEC 61850 de nes
not only the object models of IEDs and functions in a
substation automation system, but also the semantics of the V. H ARDWARE A RCHITECTURE
communications between the components of the system and
The hardware architecture of the automatic application test
the different system requirements.
system is described in Fig. 6.
The basic functional elements de ned in IED 61850 are
The simulation computer will generate the exposures ac-
the Logical Nodes. A Logical Node is the smallest part of a
cording to the selected test scenarios which will be fed to the
function that exchanges data . The information model of the
optical transducers. A test switch is used to select the active
test system is showed in Fig. 4. The related data model and
transducer system. The merging unit will then multicast the
services are summarized in table II.
sampled values over an IEC 61850-9-2 process bus.
In the new generation test system, the test devices will act
When the protective relay with the tested function oper-
as IEC 61850 clients, which will be connected with the IEDs
ates, it will send GOOSE messages. The test computer will
being tested which act as IEC 61850 servers.
subscribe and capture these messages to get the trip decision
and trip time of the relay. Meanwhile, the test computer
will subscribe and capture the sampled values to get the
F. Procedure of Application Test
disturbance fed to the relay. The use of modern Ethernet switch
Fig. 5 is the sequence diagram of the application test system. technology allows the merging of the station bus (IEC 61850-8
for GOOSE and con guration services) and process bus (IEC
6
TABLE II
D ATA MODEL AND ACSI SERVICES USED IN TEST SYSTEM
Catalog Item Description
LN LLN0 LOGICAL-NODE-
ZERO
PDIS, PTOC, PTRC LNs for protection
functions
TCTR, TVTR LNs for transducer
functions
Control SGCB Setting Group Control
Block Block
GoCB GOOSE Control Block
MsvCB Multicast Sampled
Value Control Block
ACSI SelectActiveSG, setting related services
SelectEditSG,
SelectSGValues,
Con rmEditSGValues,
GetSGValues,
GetSGCBValues
SendMSVMessage, sampled value related
GetMSVCBValues, services Fig. 6. Hardware architecture of fully networked test system
SetMSVCBValues
SendGOOSEMessage, GOOSE related ser-
GetGoCBValues, vices
To ful ll the automatic application test, the software system
GetGOCBValues, ...
should have the ability to be varied with different models
and criteria as described in the above. The framework design
technology is used to obtain an open software architecture, so
61850-9-2 for sampled value service) to one communication
that different models and criteria can be added to the system,
network, without affecting the performance of the whole
without altering the overall software structure and control ow
system.
of the automatic test system.
A GPS receiver is used to properly synchronize the different
devices. This will ensure that all events or disturbance records
are time-stamped with suf cient accuracy.
B. Framework
Multiple merging units and protective relays can be inter-
connected by a single IEC 61850-9-2 process bus. The com- Framework is a set of cooperating classes that makes up
munication links between the merging units and the relays can a reusable design for a particular kind of application [14]. A
be recon gured. Thus, the compatibility and interoperability framework provides architectural guidance by partitioning the
of the all-digital protection system can be evaluated. design into abstract classes and de ning their responsibilities
Compared with the conventional test system, the hardware and collaborations. The basic properties of a framework in-
architecture described in the above is a fully networked test volve modularity, reusability, extensibility and ease of change.
system, which eliminates the hard-wired interface. One import idea in the framework design is to identify
commonality and variability among a particular kind of ap-
VI. S OFTWARE I MPLEMENTATION plications. Once identi ed, the variant aspects of the system
will be encapsulated and localized in so called hotspots or
A. Variability in the Application Test System
extension points . Thus, all the variants in the system will be
The automatic application test system should be designed
predicted, planned and controlled.
for general purpose, not for speci c IEDs from speci c
vendors. From the software implementation perspective, the
variability in the requirement is a challenge to the software
C. Class Design
design. The main variants contained in this system involve:
Fig. 7 is the UML (Uni ed Modeling Language) [15]
1) Power system models. Different power system models
classes of the simulation subsystem. Fig. 8 shows the UML
should be provided for different test cases and scenarios;
classes of the test subsystem.
2) Referent models. Different referent models (including
transducer models and protective relay models) should In the class diagrams, the classes in white box model the
be provided to simulate devices with different operation common parts of the system, whereas the classes in grey box
characteristics; model the variants in the system. The common parts and the
3) Evaluation criteria. Different evaluation algorithms (in- variable parts of the system are divided by the classes marked
cluding performance evaluation and compatibility evalu- as abstract . As described in the above, the classes in grey
ation algorithms) should be provided to support different box are the hotspots where we can extend or change the
evaluation criteria. system.
7
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Peichao Zhang was born in 1970 in Jiangsu province, China. He received
VII. C ONCLUSION
his M.S. and Ph.D. degrees from Shanghai Jiao Tong University, all in
electrical engineering, in 1996 and 2004, respectively. Dr. Peichao ZHANG
This paper is aimed at proposing a methodology for compat-
is an associate professor in Shanghai Jiao Tong University. His main research
ibility and interoperability evaluation of all-digital protection interests are power system protection, system-wide disturbances, as well as
system through automatic application testing. The proposed signal processing and arti cial intelligence applications in power systems.
He is now working as a visiting scholar in Texas A&M University. Email:
methodology is now being realized in a PSerc project.
*******@****.***.**.
The importance of application test for the compatibility Levi Portillo was born in 1979 in the state of Zulia, Venezuela. He received
and interoperability evaluation is discussed. A number of his B.S. in Electrical Engineering from Zulia University in 2000. He is cur-
rently pursuing the M.S. degree in the Department of Electrical Engineering,
performance indices and compatibility indices are de ned to
Texas A &M University, College Station. Email: ************@****.***.
ful l the evaluation. Mladen Kezunovic (S 77, M 80, SM 85, F 99) has been with Texas A&M
The necessity and possibility of automatic application test- University since 1987 where he is the Eugene E. Webb Professor and Director
of Electric Power and Power Electronics Institute. His main research interests
ing for an all-digital protection system are discussed. The are digital simulators as well as application of intelligent methods to control,
hardware architecture as well as the software implementation protection and monitoring. Dr. Kezunovic is a registered professional engineer
are proposed. in Texas, and a Fellow of the IEEE.
it.dvi
Contact this candidate