IEEE Transactions on Power Systems, Vol. 9, No. 1 , Februaiy 1994

DESIGN OF TELEMETERING CONFIGURATION FOR ENERGY MANAGEMENT SYSTEMS

N.D.R.Sarma. . V.Veera. Raju K.S.Prakasa Rao Student member, IEEE

'CMC Limited 115, Sarojini Devi Road Secunderabad 500 003 INDIA.

ABSTRACT

In modern power system control centers, operation and control of power systems are carried out with the help of real-time computers. Live data is captured by Remote Terminal Units (RTUs) located at various stations and transmitted over suitable communication media to the control center for display, monitoring and control.

Telemetering equipment forms a sizable component of the project cost. A methodology is presented in which RTUs are located at different stations to meet certain criteria such as observability of the system and absence of critical measurements. Additional reliability constraint of loss of information from a single RTU for the above two constraints is also imposed.

Keywords : Energy management systems (EMS), telemetering system, remote terminal units, observability, critical measurements.

INTRODUCTION

In modern power system control centers, also called as Energy Control Centers (ECC) , operation and control of power systems are carried out with the help of real-time computers. Live data is captured by Remote Terminal Units (RTUs) located at various stations and transmitted over suitable communication media to the control center computer system for display, monitoring and control. The data received at ECC are prone to errors arising from transducers, communication systems etc. A state estimator is used to provide a reliable and complete data base. In order to be able to estimate the state of the system, the measurement system should be sufficient .enough to estimate the magnitudes and phase angle of the voltages of all the buses in the system.

Bad data in the measurements should be detected and eliminated from the measurement set. Further, it is also well known that for reliable estimation of the system state, the measurement system should have sufficient

93 WM 187-5 PWRS by the IEEE Power System Engineering Committee of the IEEE Power Engineering Society for presentation at the IEEE/PES 1993 Winter Meeting, Columbus, OH, January 31 - February 5, 1993. Manuscript submitted January 7, 1992; made available for printing November 30, 1992.

A paper recommended and approved

Senior member, IEEE

Department of Elect. Indian Inst. of Tech., Delhi, Hauz Khas, NEW DELHI - 110 016 INDIA.

38 1

redundancy and uniform spread [ 13. Hence the location of RTUs plays an important role in an EMS project.

To support the security monitoring function in Energy Control Centers following alternatives are available with regard to placement of RTU:

a) Placement of RTUs at all the stations to gather the information of the network status (position of switches, breakers) and all the relevant measurements like MW,MVAR,KV etc.

b) Placement of RTUs at all the stations as in alternative (a) to gather the information on network status from all stations and gathering the information from measurements at only some selected stations.

c) Placement of RTUs at only some selected stations to obtain the network status and measurements also from the same stations. The remaining information with regard to network status is obtained manually.

The alternative (a) though desirable is most expensive and may not be practicable. Option (c) is the cheapest since the RTU placement and measurement systems are located at a selected number of stations only. Though the network status is not obtained in real- time, it can be updated whenever there is a change in the configuration. In the proposed method option (c) is considered.

In this paper a new method for the design of measurement system is presented wherein RTUs are placed only at some selected substations. The proposed method honors location of RTUs decided apriori and works from that point.

Handschin and Bongers [2] pointed out that local redundancy and the probability of detecting bad data are the most important factors when planning a measurement system. Their method consists of moving the measurements from the best to the worst part of the network, starting with an initial solution with almost designed redundancy. Roy and Villard [ 3 ] described a method in which different possible telemeasurement configurations are compared by off-li'ne simulations of state estimations. Koglin [4] used a general criterion to systematically eliminate Some of the measurements in the system to obtain an optimal set from various measurements. Phua and Dillon [5] developed a method based on entropy criterion. The problem is posed as non-linear programming problem which is

0885-8950/94/$04.00 0 1993 IEEE

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solved using a sequential linearly constrained minimization method. Mafaakher et a1 [6] have used the ability of bad-data detection of state estimation to design a metering system. Aam, Holten and Gjerda [7] provided a brief survey of the various methods of optimal meter placements highlighting the advantages and disadvantages of each method. They also presented a method by extending Koglin's method to obtain a more robust solution. Nabil Abbasy and Shahidehpour [8] proposed a mathematical programming problem model to identify redundant measurements from a given set of measurements. Young Moon Park et a1 [ 9 ] presented an algorithm of optimal meter placement for the state estimation, which minimizes the total investment subject to a prespecified accuracy of the estimated state. Hiroyki Mori and Yasuo Tamura [lo] compared various approaches of meter placement in power system static state estimation and proposed a method based on stochastic load flow model.

In all the above methods the emphasis was on the design of a measurement system at the meter level only. An RTU is required to be located at a station whether one or more quantities are to be measured at this station. Current trend is towards building transducers at lower costs. Hence it is logical to gather maximum possible information by the located RTU in a station. Thus the design of measurement system at the RTU level becomes most relevant.

In this paper a new methodology for design of telemetering configuration is presented in which RTUs are located at different stations to meet certain criteria such as observability of the system, absence of critical measurements. Additional reliability constraint of loss of information from a single RTU for the above two considerations is also imposed. The locations where RTUs are placed for SCADA purposes as desired by the utilities are honored. The proposed method is tested on standard IEEE systems and on a practical system. The results are presented and discussed.

THEORY OBSERVABILITY CRITICAL MEASUREMENTS

System observability and absence of critical measurements are important criteria for the design of telemetering configuration. The concepts of observability and critical measurements are explained below.

Observability L

A power system is said to be 'observable, in the sense of state estimation with respect to a given measurement set M, if the bus voltage magnitudes and angles throughout the system can be determined by processing the measurements in M by a state estimator. Otherwise the power system is said to be 'unobservable' with respect to M. It was proved in [I13 that if measurements in the system form a spanning tree connecting all the buses then the system is observable. Clements [12] has explained various

aspects of observability and reviewed some methods of meter placement.

Critical Measurements L

A measurement is said to have detectable error residual if an error in the measurement shows up in the measurement residual, residual being the measured value minus calculated value [13]. It was proved that there may be some measurements in which an error in the measurement will not be reflected in the residual. The problem of determining which measurements have detectable error residuals is solved by identifying a class of measurements, called 'critical measurements' and showing that only non-critical measurements have detectable error residuals. Since only non-critical measurements have detectable error residuals, a bad data in those measurements can be detected. But in the case of critical measurements, a bad data is not reflected in the error residual and hence cannot be detected. A critical measurement is defined as that measurement, which when not available makes the system unobservable.

Thus critical measurements imply the following :

. loss of critical measurements would make the system unobservable.

. an error in the critical measurement cannot be detected.

Hence it is important to see that a system would not have any critical measurements and be designed accordingly.

DESIGN TELEHETERING CONFIGURATION

In this section a new design methodology of the telemetering configuration at the RTU level is proposed.

Various inputs and outputs of 'Telemeterinq Confiqurator' are shown in Fig.1. It is assumed that all possible MW and W A R flows in the lines and transformers are acquired by the RTU at the sub-station so that maximum information is gathered from the substation ( s f s ) . It is assumed that voltages are measured at all the buses in the s f s where RTUs are placed.

The complete procedure of designing the telemetering configuration is divided into

System Specified Desired List of zero topology RTU bus bus injection

locations injection measurements measurements

TELEMETERING CONFIGURATOR 4 Location of RTUs List of measurements

to be measured by RTUs.

Fig.1 Block diagram of telemetering configurator.

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buses of the first linking first & i so la ted bus second, find the

four phases. Each of these phases is explained below :

Phase 1 L Placement of RT.Us till the redundancy atleast e m a l to 1.0 :

In general generating stations and some substations are normally considered as very important by the utilities from the point of view of data acquisition. These stations are treated as specified locations for initial placement of RTUs. With this initial measurement set the redundancy is computed. Redundancy is defined as the ratio of number of known measurements to the number of unknowns (state variables). If the redundancy is less than 1.0 it implies that state variables cannot be calculated by the state estimator. If the redundancy is less than one, location of RTUs is continued at other locations till the redundancy is atleast equal to 1.0. When the redundancy is atleast equal to 1.0, the next phaseistomake the system observable. At any stage the criterion for placing an RTU at a location other than specified locations is to pick up that bus at which the maximum number of lines/transformers are incident at the corresponding s / s . The details of this phase are shown in the form of a flow chart in Fig. 2.

Phase 2 i Placement of R T U s until the system observable i

With the measurement set obtained after the placement of RTUs in phase 1 the system is tested for observability. The test for observability is explained in the previous section. If the system is not observable, enhancement of the measurement system is done by placing additional RTUs at appropriate locations. ( If the system is not observable, then it is divided into different groups of buses and/or isolated buses with respect to

'Arrange these buses so on in the decreasing order of number of 1inesfTr incident at the correspond- ing s / ~ .

[Place RTUs at specified Locationsj s./

Calculate the redundancy

greater than or phase 2

order of number of lines\tr incident

an RTU in that next bus with highest

Place a RTU at this location!

Fig. 2 . Flow chart for phase 1.

the measurement system ) . The criteria for placement of additional RTUs is shown in Fig. 3 as a flow chart.

Phase 3 i Placement of R T U s to cover critical measurements i

In this phase additional measurements are added to the measurement set by placing additional RTUs to make the resultant measurement system free of critical measurements.

Critical measurements ( line flows and bus injections ) are first identified using the complete set of measurements available through the RTUs at the end of phase 2. (Since zero bus injection measurements are not physically metered, they need not be considered). These are called first order critical measurements.

1. To start with, the first critical measurement from the above measurement is suppressed. The system becomes unobservable-Then one or more additional RTUs are to be placed as per the procedure explained in phase 2 to make the system observable under this condition. This will lead to a new set of critical measurements ( called second order critical measurements ) . This procedure of finding second order critical measurements is repeated sequentially for all the first order critical measurements considering one at a time.

Get the measurement list from phase 1. I

observable?

Find the buses which have lines linking first and

Are there any isolated buses

4- P e l e J t the ' Is there already an RTU placed at this bus ? bus with

next highest

lines/Tr incident

Fig. 3 . Flow chart for phase 2.

384

results in more no. of critical measurements.

2. The first order critical measurement which results in the least number of second order critical measurements is chosen for consideration of placement of additional RTUs. If, at this stage, the number of second order critical measurements happens to be same for two or more first order critical measurements, the choice of the bus (es) for the placement of additional RTU(s) is made by the criterion of the maximum number lines/transformers incident at a bus(es).The critical measurements in the system at this stage are treated as first order critical measurements.

The procedure of steps 1 and 2 above is now repeated successively till the system is completely free of any critical measurements. The detail description of this phase is shown in Fig. 4.

Phase 3 Placement of RTUs under continsent loss of information L

It is sometimes possible that either the RTU or the communication from an RTU may fail and hence the information from an RTU may not be available in the measurement set. One way to overcome such situations is by using pseudo-measurements. But if utilities would like to enhance their measurement system further for SCADA purposes and/or to make the

results in more no. of groups. If there is a tie

Get the measurement list from phase 2

Find the critical measurements (line flow & bus injection measurements)

4

Is the no. of c:itical measurements = O?

of RTU whose loss

<

1

of RTU whose loss

among these cases select that case which has more no. of 1inesfTr. covered by the RTU .

I Consider first critical measurement 1 .L

Supress this measurement from the list. The system is unobservable

J, Place an RTU to make the system observable. Find the number of critical measurements and store them. Also store the number(s) of bus(es) along with the number of 1inesfTr. at this bus(es) where an RTU is placed to make the system observable.

Are all critical measurements covered ?

&

cr itica 1 measurements, L

~

Now consider that case which makes the system to have less number of critical measurements after placing the RTU to cover this case. If there is a tie in number of critical measurements, select that case where the number of 1inesfTr. incident are more at the bus where an RTU is placed to cover this case. Place an RTU at the selected location to cover khis case. 1

Fig. 4 . Flow chart for phase 3 .

system observable and-free from critical measurements (to the extent possible ) under such situations, additional RTUs are required to be placed. So in this phase additional RTUs are placed to consider the l o s s of information from an RTU. It may be noted that sometimes the system may not become free of critical measurements under this contingency. This has to be repeated for each of the RTUs located in the system at the end of phase 3 . The details of this phase are given in Fig. 5.

RESULTS AND DISCUSSION

The proposed method is tested On standard IEEE systems, viz., IEEE 14-bus, 30- bus, 57-bus systems and on a practical system.

Figs. 6-9 give the measurement systems at the end of phase 1, 2, 3 and 4 for IEEE 14-bus system respectively. It can be seen that the IEEE 14 bus system has total number of 11 substations. It is assumed that the

]Get the measurement set from phase 3 1

lthere in the list of RTUs. , I

& Go to phase 2 - placement of RTUs to make the system observable.

Fig. 5 . Flow chart for phase 4 .

385

R *Bus-lnject ion p R T U measurement o, Zero - Bus-

in jection tuiearu r e me nt Mea su r a m en t

+ Line -Flow

F i g . 6 Measurement System a t t h e end of Phase 1 f o r IEEE 1 4 Bus system.

R

R R

Fig .7 Measurement System a t t h e end o f Phase 2 f o r I E E E 1 4 bus system.

F ig .8 Measurement System a t t h e end of F ig .9 Measurement System a t t h e end of Phasa 3 f o r IEEE 1 4 bus system. a l l phases f o r IEEE 1 4 bus system.

R 50

all phases f o r IEEE 30 bus system. F i g . 1 0 Measurement System a t t h e end of

386

specified locations of RTUs are substations consisting of buses 1,2,8 and (5,6). The redundancy with this measurement system is found to be greater than s.0.. So at the end of phase 1 there are 4 RTUs in the system ( Fig. 6 ) . Now in order .to make the system observable, one additi'onal RTU is required at the substation consisting of buses 4,7 and 9. (Fig.. 7 ) . Further to make the system free of critical measurements, two more RTUs are required at the substations containing buses 11 and 13 respectively (Fig. 8). Now in order to consider the criterion of loss of information from any single RTU, one more RTU is placed at the substation containing bus 10. However it is not possible to make the system free of critical measurements due to loss of information from an RTU because of the topology of the IEEE 14 bus system. Thus at the end of all phases there are total number of 8 RTUs for the IEEE 14 bus system as shown in Fig. 9.

Fig. 10 gives the measurement system for IEEE 30-bus system at the end of all phases.

In the case of a practical system considered for the study which has 65 buses and 99 lines, it is found that 24 no. of RTUs are required to make system fully observable. If it is required to make the system free of critical measurements it is found that it needs seven additional RTUs which implies additional costs. Thus telemetering system can be designed keeping in view the objectives outlined in this paper and alsa' the financial considerations. The measurement system arrived at this method gives a good spread of measurements since it is designed based on observability of the system and absence of critical measurements.

The summary output for the various test systems studied is given in Fig. 11. In all the cases it is found that it is not possible to make the system free of critical measurements due to loss of information from an RTU. However loss of information from an RTU keeps the system observable.

The cost particulars for different systems for various alternatives are given in Fig.12. It is assumed that the cost of an RTU is Rs. 0.2 million and cost of MW and W A R transducers is Rs. 9000 each. The cost of monitoring the status is assumed'to be Rs. 500 and the cost of voltage transducer is taken as Rs. 2000. It can be easily seen that the proposed scheme has the lowest cost and meets the required criteria.

Fig. 13 gives the execution times on VAX-11/750 of Digital Equipment Corporation, USA, ( without floating point accelerator ) for first three phases and for all the phases separately for each of the system studied. With regard to the practical system it has taken about 7 hours to execute first three phases. For the purpose of comparison of time taken for design of measurement system by a different method is referred to here, as available in published literature. Aam Holten and Gjerda [7] state that it took 13 hours on a 32-bit Nord-500 mini-computer [which is faster than VAX 11/750 used in our study] for a system consisting of 45

stations, 52 transmission lines and 6 transformers [a system smaller than a practical system considered here]. It can be very easily seen that the proposed method is more systematic and hence takes much smaller time than that of Aam et a1 [7]. Such functions usually take longer times and are required only in planning stages.

............................................. Phase No. of RTUs for various systems

(Figure in brackets is redundancy) IEEE IEEE IEEE Practical

14 bus 30 bus 57 bus system (61 buses) .............................................

No. of s f s in the system 11 26 42 47

PHASE 1 4(1.49) 7(1.31) 7(1.12) 12(1.30) PHASE 2 5(2.44) 13(2.39) 20(2.56) 24(2.75) PHASE 3 7(2.89) 14(2.47) 23(2.67) 33(3.26) PHASE 4 8(3.07) 17(2.73) 24(2.72) - ............................................. Fig.11 Summary output for test systems

s1. System Cost in million Rupees. No. option(a) option(b) option(c)

Proposed method .............................................

1 IEEE 14 bus 2.968 2.854 2.251 2. IEEE 30 bus 6.851 6.547 4.939 3. IEEE 57 bus 11.363 10.569 6.748 4. Practical 13.148 12.508 9.691

system ____________________------------------------- Fig.12 Cost particulars for various

alternatives. (options are as described in 'Introduction')

IEEE 14-bus system All the phases executed 00 00 32.20 First three phases 00 00 13.62 executed IEEE 30-bus system All the phases executed 00 19 44.64

executed IEEE 57-bus system All the phases executed 02 13 18.38

executed Practical system First three phases , 06 59 00.21 executed

First three phases 00 01 09.44

First three phases 00 30 42.68

CONCLUSIONS

A new design methodology has been proposed for the telemetering' configuration of an Energy Management Systems based on a systematic approach. The methodology ensures at planning stage

* complete observability of the system

387 [8]. Nabil Abbasy and S.M.Shahidehpor, "An Optimal set of measurements for the Estimation of Systems States in Large-scale Power Networks*I, Journal of Electrical Machines and Power Systems, Vol 15, No.4-5, pp 311-332, 1988.

[9]. Young Moon Park, Young Ayun Moon, Jin Boo Choo and Tae Won Kwon, "Design of Reliable Measurement Systems for State EstimationBt, IEEE Trans. on Power systems, V01.3 No.4, pp 830-836, Aug. 1988.

[lo]. Hiroyuk Mori, Yasuo Tamura, IIComparison of Approaches to meter placement in Power System Static-State Estimation" , International Journal of Energy Systems, VO1.8, No.3, pp 139-163, 1988.

[ll]. G.R.Krumpholz, K.A.Clements, P.W.Davis, 'Power System Observability : A Practical Algorithm Using Network Topology', IEEE Trans. on Power Apparatus and Systems, Vol. PAS-99, No. 4, pp 1534-1542, July/Aug. 1980.

[12]. K.A.Clements, 'Observability method and optimal meter placement', Electrical Power and Energy Systems, Vol. 12, No.2, pp 88-93, April 1990.

[13]. K.A.Clements, G.R. Krumphoiz and P.W.Davis, 'Power System State Estimation Residual Analysis : An Algorithm using Network Topology', IEEE Trans. on Power Apparatus and Systems, Vol.PAS-100, pp 1779- 1787, April 1981.

* absence of critical measurements (hence ability to detect bad data in all the measurements )

* the observability' of' the system in the case of loss of information for any single , RTU and absence of critical measurements, if possible.

* a good spread of the measurements in the system.

The proposed method has been tested on standard IEEE system viz., IEEE 14-busI 30- bus, 57-bus systems. It is also tested on a practical utility system and the results are presented and discussed.

ACKNOWLEDGMENTS

The authors thank CMC Limited for providing facilities to carry out this work and for the support and encouragement given in bringing out this paper. The authors also would like to acknowledge the reviewers for their valuable comments and suggestions in improving this paper.

REFERENCES

[l]. A.Bose and K.A.Clements,'Real-Time Modeling of Power Networks', proceedings of IEEE, Vol. 75, NO. 12, pp 1607-1622, Dec. 1987.

[2]. E.Handschin and C.Bongers, 'Theoretical and Practical Considerations in the Design of State Estimators for Electric Power System', Proceedings of the International symposium, 'Computerized Operation of Power Systems (COPOS '75'), '18-20 Aug 1975 Brazil, edited by Savu Crivat Savulescu pp 104-136.

[3]. A.Le Roy and P. Villiard, 'Application of State Estimation Methods to the Evaluation of a Telemeasurement Configuration for Energy Power Systems', Proceedings of the International Symposium, 'Computerised Operation of Power Systems (COPOS '75'); 18- 20 Aug.1975, Brazil, edited by Savu Crivat Savulescu pp 156-175.

[4]. H.J. Koglin, 'Optimal Measuring system for State Estimation', Proceedings, PSCC Conference Paper 2.3112, Cambridge, Sept. 1975.

[5]. K . Phua, T.S. Dillon, 'Optimal Choice of Measurements for State Estimation', Power Industry and Computer Applications Conference, Torento, Canada, 24-27, pp 431- 441, May 1977.

[6]. F.Mafaakher, A Brameller and J . F . Bermudez, tlOptimum Metering design using fast-decoupled estimator", Proceedings, IEE, Vol 126, No.1, pp 62-68, January 1979.

[7]. S. Aam, L.Holten and O.GJerda, 'Design of the Measurement System for State Estimation in the Norwegian High-voltage Transmission Network', IEEE Transaction on Power Apparatus and Systems, Vol.PAS-102, No.12, pp 3769-3777, Dec. 1983.

N. D.R.Sarma (S"81-Mf87-S1189) was born in Kuchipudi, Andhra Pradesh, India on 7th Oct. 1961. He received the B.Tech (Elect.) and M.Tech. (Elect.) with Specialization in Power System Engineering from Regional Engineering College, Warangal, India in 1983 and

1986 respectively. Presently he is working at CMC Limited, Secunderabad. He has registered for Ph.D degree in the Dept. of Elect. Engg. at IIT Delhi, India. His areas of interest include Real Time control of Transmission and Distribution Systems.

V.Veera was born in Andhra Pradesh, India in 1956. He received B.E. (Elect.) from Osmania University and M.Tech.

with (Electrical) specialization in Power system Engineerinq from I.I.T., Madras.He is working as systems Specialist at CMC Limited,

Secunderabad since 1982. His areas of interests are Energy Management Systems, Security Analysis, Power System Restoration.

K.S.Prakasa Rae (S'70-M'74-SM'81) was born in Prakkilanka,A.P,India on July 15th , 1942. He received his B.E. (Elect. ) , M.E. (Power Systems) from Osmania Univ., in 1964 and 1966 respectively. He obtained his Ph. D degree from the Indian Inst. of Tech. Kanpur, India in 1974. He is presently a professor in the Elect. Engg. Dept. of 1.I.T Delhi, India. His fields of interest are Power System Planning, Operation and Reliability.

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