System Electrical

Compatibility and Interoperability Evaluation of

All-digital Protection Systems Based on IEC

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Zarko Djekic, Student Member, IEEE, Levi Portillo and Mladen Kezunovic, Fellow, IEEE

important features of IEC 61850. Although many

interoperability tests have been performed at the bay level and

Abstract-- Recent development of electronic instrument

transformers and use of digital relays allow the development of an the IEC 61850-9-1 interoperability at the process level, the all-

all-digital protection system, where the traditional analog, digital protection system containing different electronic

hardwired, interface has been replaced with a digital

instrument transformers connected to different digital relays by

communication link (process bus) based on IEC 61850-9-2

an IEC 61850-9-2 process bus was not described in the

standard. An all-digital system should provide compatibility and

literature yet. Compatibility and interoperability evaluation of

interoperability so that different electronic instrument

the all-digital protection system assumes two kinds of test,

transformers can be connected to different digital relays (under a

multi-vendor connection). Since the novel all-digital system conformance and performance test. IEC 61850-10 gives

composed of IEDs from multiple vendors has never been guidance for the conformance tests of Intelligent Electronic

implemented and/or tested in practice so far, its performance

Devices (IEDs) interconnected in an all digital protection

needs to be evaluated. This paper presents a methodology for

systems [2]. The performance tests allow more extensive

performance and compatibility evaluation of an all-digital

assessment and can be used to determine the performance

protection system. The test results obtained using a digital

characteristics of the overall system [3]. Evaluation of the all-

simulator test bench and comparison of the compatibility of

systems provided by different manufactures are discussed. digital system performance is necessary in order to recognize

all possible conditions when protection system may miss-

Index Terms--compatibility, interoperability, IEC 61850,

operate, or operate with unacceptable performance (reduced

performance evaluation, process bus, protective relaying, relay

selectivity, increased operating time, etc). Identifying these

testing, optical transformers

abnormal situations is important for two reasons: a)

recognizing possible conditions for incorrect operation, b)

I. INTRODUCTION

proving that the novel implementation will not translate in

R ELAY testing is a very important issue when applying the

degrading protection system performance. In [4] authors

protective relays. Vendors need an evaluation tool to

propose a methodology of compatibility and interoperability

validate the design of the relay logic and communication.

evaluation for all-digital protection system through automatic

Utilities need a tool to compare the performance of different

implementation of application testing. This methodology is

relays, calibrate relay settings and perform troubleshooting.

now used as a base for compatibility and interoperability

The recent development of optical and Hall Effect instrument

testing of all digital protection systems composed using

transformers and the use of digital relays enable the

components from multiple vendors.

development of an all-digital protection system. Different

The paper is organized as follows. Section II introduces the

components of such system communicate using digital

compatibility and interoperability evaluation; Section III

communication protocol. The output of the electronic current

defines the evaluation methodology; Section IV discusses test

and voltage transformers are sampled (digital) signals, which

results and Section V conclusions.

after combining in merging units can be connected to digital

relays through an IEC 61850-9-2 digital process bus [1].

II. COMPATIBILITY AND INTEROPERABILITY

Compatibility and interoperability are one of the most

Compatibility means the ability of two or more IEDs to

perform requested functions (protection, control, metering,

This work was supported by PSerc project titled, Digital Protection etc) using IEC 61850 standard for communication and data

System Using Optical Instrument Transformers and Digital Relays

exchange. According to IEC 61850 interoperability is ability

Interconnected by an IEC 61850-9-2 Digital Process Bus

of IEDs or substation automation systems from different

Z. Djekic is currently with American Electric Power Inc. Columbus, OH (e-

vendors to execute bi-directional data exchange functions, in a

mail: zdjekic@ aep.com)

L. Portillo is with Dashiell Corporation, Houston, TX (e-mail: manner that allows them to operate effectively together. Unlike

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interoperability, the IEC 61850 standard was never intended to

M. Kezunovic is with the Department of Electrical and Computer

ensure interchangeability of IEDs [3]. However,

Engineering, Texas A&M University, College Station, TX (e-mail:

interchangeability of the transducer system (comprised of

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2

current and/or voltage sensors and merging units) and process E E E

PCIP1,P2= PPIP1-PPIP2

bus (made of copper or fiber wires and an Ethernet switch) is

not only a possibility but also a highly desirable feature of the

The average compatibility index of protection system P1 and

all-digital system, allowing utilities to choose between

different sensors and fast Ethernet switches without P2 is defined as:

restrictions.

1 E E

Performing compatibility and interoperability tests gives PPIP1-PPIP2

PCI =

P1,P2 N

ability to make conclusion about possible interchangeability

E EB

between protection systems components made by different

The protection system includes the transducer system

vendors. As mentioned, compatibility and interoperability

(sensor and merging unit), the process bus (the Ethernet LAN)

evaluation of the all digital protection system requires two

and the protective relay. By definition, the smaller the PCI, the

kinds of test, namely conformance and performance test.

better compatibility and interoperability.

Conformance tests belong to certification tests which aim at

verifying whether an IED satisfies the criteria specified by

III. EVALUATION METHODOLOGY

certain standard or authority. These tests are performed at the

vendor s laboratories or at independent test institutes. Criteria The compatibility indices, defined in the previous section,

for performance evaluation of the protection system are not a are calculated by analyzing output signals of IEDs from

new topic and have been investigated in different research different manufacturers combined into a test system. Three

efforts [5], [6]. Although they have proven to be effective to transducer sets (composed from current sensors and merging

evaluate the performance of conventional protection system, units), two 100Mbit/sec Ethernet switches and one digital relay

they need to be extended to be applicable for all-digital where available for testing. Generic evaluation systems

systems. Performance tests belong to application tests which diagram is as shown on Fig 1.

aim at verifying the behavior of the protection system, the

accuracy and operating times under various conditions. For the

all-digital protection system, the interests for performance tests

are the trip/no trip decisions and the operating times.

In [4] authors propose how the compatibility and

interoperability evaluation of all-digital protection system is

performed through protection system performance tests.

Fig. 1. Generic test evaluation setup

A. Performance Indices

This paper follows selection of the performance indices Simulation scenarios define the power system events to be

created and replayed into the modeled referent protection

adopted in [4] to meet the needs of the all-digital protection

systems and the all-digital protection system assembled in the

system. Two kinds of indices are used for performance

lab. These events are simulated using a sequence of circuit

evaluation:

breaker switching corresponding to various power system

The performance index of protection system P when fed by

conditions. Any particular scenario is defined by two

E

exposure E is denoted by PPIP. The average performance parameters: Time at which the event starts and finishes and

scenarios shown in table I.

index of protection system P is defined as:

1 E

PN

PPI = PPIP where N is the number of exposures TABLE I. SIMULATION SCENARIO, OVERCURRENT PROTECTION

E EB

There are two types of protection performance indices

calculation methods, namely the trip decision method and trip

time method respectively. For the trip decision method:

N1 + N 0

E

PPI P = s = where N and, N denotes number

1 0

N Simulated scenarios are selected to create those power

system conditions in which correct operation of the protection

of correct trip/restraints and N is the total number of exposures

system is critical [7]. Voltage sensors were not available from

For the trip time method: all vendors so scenarios were limited to those that do not

t require directionality (forward zone 20% and 70%).

PPI P = Dt

E

where D stand for the trip time of

Overcurrent protection is expected to operate (issuing a trip

the tested protection system. command) for faults in the forward zone of protection.

Features of the tested overcurrent relay function are:

B. Compatibility Index Phase time overcurrent protection as backup protection

Residual time overcurrent protection

The compatibility index of protection system P1 and P2

Settings of the relays are:

when fed by the same test signal E is defined as:

Nominal input current of relay model is In=5A

3

Pickup current is set to 1.5 times the nominal value III and IV are obtained following methodology presented in

Very inverse time-current characteristic was used. previous section.

This characteristic is defined as:

TABLE II. OVERCURRENT PERFORMANCE RESULTS, TEST SETUP NO1

13.5 k

t = k was chosen as: k=0.025

operate I -1

n

A total of 120 different exposures (1200 tests since each

exposure is replayed 10 times) are generated for the

overcurrent protection testing.

Three complete all digital protection systems are composed

by interchanging available sensors and Ethernet switches.

Electronic transducer sets from NxtPhase, AREVA and

Siemens, Ethernet switches from GE Multilin and

RuggedSwitch and relay AREVA Micom P441 are used. Test

set configuration are shown on Fig 2.

TABLE III. OVERCURRENT PERFORMANCE RESULTS, TEST SETUP NO2

TABLE IV. OVERCURRENT PERFORMANCE RESULTS, TEST SETUP NO3

Fig. 2. Test Setup configurations

During the tests all three MUs have been connected to

Ethernet network but only one at a time with destination The following conclusions can be made, based on the

address set that matches relay address. This is used to simulate presented results:

real conditions when multiple IEDs share the same Ethernet Selectivity of overcurrent protection function for the tested

network. In addition, random data packets are generated using all-digital protection systems is as expected.

PC connected to the same network at the rate of thousand A comparison of the average tripping times shown in Tables

1500Byte packets per second. This is used to simulate higher II through IV demonstrates that for all simulated fault types

network traffic on the process bus. the reaction times of the tested systems are very close to

each other.

IV. TEST RESULTS Average values for the standard deviation show that there is

a high degree of certainty that the tested digital protection

This section presents application of the evaluation

system s operating time for any given fault will be

methodology. Results are obtained by using simulation and test

consistent.

procedure detailed in the previous chapter. Performance

indices for tested all digital protection systems are presented in

the form of average values. The test system for performance B. Interoperability results

testing was described in previous section. Interoperability of

Compatibility indices which describe interoperability between

protection system modules is tested and results are presented.

all three tested protection systems are presented. Indices are

A. Performance results calculated as it is described in previous sections. Results for all

possible combinations of the tested setups are given in Table

Absolute performance indices for all three test setups are

V.

presented. All performance indices results shown in Table II,

4

Small interoperability indices show that tested protection

TABLE V. COMPATIBILITY INDICES, TEST SETUP NO3 systems are compatible and can be interchanged without

1st Test Setup 2nd Test Setup Avg. significant effect on protection system performance. Sensors

Tests (CT+MU - Eth. (CT+MU - Eth. trip PCI

and merging units interchanged during these tests have very

Switch - Relay) Switch - Relay) time

similar performance characteristics.

I - II AREVA NxtPhase 0.1241 0.0024

RUGGEDCOM RUGGEDCOM

AREVA AREVA VI. ACKNOWLEDGMENT

II - III NxtPhase Siemens GE 0.1212 0.0076

This research was funded by the Power Systems

RUGGEDCOM Multilin

AREVA AREVA Engineering Research Center under the project titled: Digital

I - III AREVA Siemens GE 0.1226 0.0065 Protection System Using Optical Instrument Transformers and

RUGGEDCOM Multilin

Digital Relays Interconnected by an IEC 61850-9-2 Digital

AREVA AREVA

Process Bus. The equipment and support for this project also

comes from AREVA, GE Multilin, RuggedCom and

Compatibility indices are calculated following definition

NxtPhase.

shown in section III. The following conclusions can be made,

based on the results:

VII. REFERENCES

Comparison between average tripping times for all systems

shows that they are very close to each other. [1] Communication networks and systems in substation-Part 9-2: Specific

Compatibility indices which describes performance communication service mapping (SCSM)- Sampled analogue values

over ISO 8802-3, IEC Std. 61850.

difference between given systems are relatively small

[2] Communication networks and systems in substation-Part 10:

Parts of the tested systems can be interchanged without Conformance testing, IEC Std. 61850.

significant effect to system performance. [3] D. Dolezilek. (2005) IEC 61850: What you need to know about

functionality and practical implementation. [Online]. Available:

http://www.selinc.com/techpprs/SEL Dolezilek IEC61850 6170.pdf

V. CONCLUSIONS

[4] P. Zhang, L. Portillo, M. Kezunovic, "Compatibility and Interoperability

Methodology and results from the compatibility and Evaluation for All-Digital protection System through Automatic

Application Test," IEEE PES General Meeting, Montreal, 2006.

interoperability evaluation of an all-digital protection system

[5] M. Kezunovic and T. Popovic, Assessing application features of

based on an IEC-61850-9-2 process bus are presented. Testing

protective relays and systems through automated testing using fault

is performed on three different all digital protection systems transients, in Proceedings of the IEEE Power Engineering Society

assembled in Texas A&M Protection and Control Lab. Results Transmission and Distribution Conference, vol. 3, Yokohama, Japan,

Oct. 2002, pp. 1742 1767.

are definitely helpful in gaining understanding on what level of

[6] M. Kezunovic, T. Popovic, S. Donald, and D. Hyder, Transient testing

performance and compatibility between systems can be

of protection relays: Results, methodology and tools, in International

expected from the novel systems, how does the measured Conference on Power Systems Transients (IPST), New Orleans, 2003.

performance compares to each other, what elements of the [7] D. Ristanovic, S. Vasilic, M. Kezunovic, "Design and implementation

novel system contribute to problematic performance and under of scenarios for evaluating and testing distance relays", Proceedings of

the 33rd North American Power Symposium, Vol. 1, pp. 470-475,

what conditions. It was concluded that:

October 2001.

Performance of the novel system can be regarded as

VIII. BIOGRAPHIES

excellent when considering test results for the directional

Zarko Djekic (S 03) received his Dipl. Ing. Degree in electrical

overcurrent protection function. Relevance of this result lies

engineering from the University of Novi Sad, Serbia, in 2004 and M.S. degree

in the fact that these two principles (comparison of the from Department of Electrical Engineering, Texas A&M University, College

measured quantity versus a threshold and distinction of Station in 2007. He is currently with the American Electric Power Inc. His

current flow) are the basis for many other protection research interests include power system monitoring, protective relaying, relay

testing and substation automation.

functions

Average values for the standard deviation show that there is Levi Portillo was born in 1979 in the state of Zulia, Venezuela. He

a high degree of certainty that the tested protection system s received his B.S. in Electrical Engineering from Zulia University in 2000 and

operating time for any given fault will consistently follow M.S. degree from Department of Electrical Engineering, Texas A&M

University, College Station, in 2006. He is currently with the Dashiell

the operating time-current characteristic with almost a

Corporation.

negligible level of dispersion from the mean trip time - 2 ms.

Overall protection system performance in not affected by Mladen Kezunovic (S 77-M 80 SM 85 F 99) received the Dipl. Ing.,

interchange of Ethernet switches. For the present level of M.S. and Ph.D. degrees in electrical engineering in 1974, 1977 and 1980,

respectively. Currently, he is the Eugene E. Webb Professor and Site Director

traffic load on the process bus and low level of EMI in the

of Power Engineering Research Center (PSerc), an NSF I/UCRC at Texas

lab difference in performance indices are negligible. A&M University He worked for Westinghouse Electric Corp., Pittsburgh, PA,

Ethernet switch interoperability should be further tested in 1979-1980 and the Energoinvest Company, in Europe 1980-1986. He was

harsh environment with the high level of traffic load. also a Visiting Associate Professor at Washington State University, Pullman,

1986-1987. His main research interests are digital simulators and simulation

Overcurrent performance indices for systems composed by

methods for relay testing as well as application of intelligent methods to

interchanging sensors and merging units are very similar.

power system monitoring, control, and protection. Dr. Kezunovic is a member

Testing based on the same input signals and relay settings of CIGRE and Registered Professional Engineer in Texas.

shows that there is no significant difference in protection

system performance.

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