IEEE Transactions on Power Systems, Vol. 13, No. 4, November 1998

A Proposal of a Supporting Expert System for Outage Planning of Electric Power Facilities Retaining High Power Supply Reliability

Part I - Outline of a Supporting System and Outage Work Allocation Based on Indices -

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K. Kawahara H. Sasaki J. Kubokawa H.Asahara K.Sugiyama Member, IEEE Member, IEEE Member, IEEE Chugoku Electric Power Company

Department of Electrical Engineering

Fukuyama City, 729-02, Japan

Department ot Electrical Engineering,

Higashi-Hiroshima City, 739, Japan

Hiroshima City, 730, Japan Fukuyama University Hiroshima University,

Abstract: It is necessary for a power system to undertake maintenance works regularly to secure stable power supply, which inevitably cause outage of associated apparames. This problem, referred to as the outage planning of an electric power system in this paper, has been so far made based on the knowledge and experiences of planning engiricers. However, the scheduling made by the engineers is not necessarily optimal since there are no rational criterion to judge the results. Furthermore, this problem belongs to a class of combinatorial optimization problems. Therefore, we propose a supporting expert system for the outage planning retaining high power supply reliability. This paper presents some issues on the outage planning and an outline of the proposed supporting system. The concept of work unit is introduced and three security indices are utilized to properly merge several related outage work into the same group.

Keywords: power system maintenance, outage work planning, expert system, power supply reliability, object oriented database

I. IIVTRODUCTION

In order to meet ever increasing power demand securely, it is necessary to undertake continually a large number of maintenance and renewal works of power system facilities, which inevitably cause outages of power facilities in many dispersed locations. A single or a few such works, requested independently from each station, will be piled up to be an order of thousand per year in each regional control center. Even when such huge number of maintenance works are undertaken, power system security must be kept at a level higher than a specified value. To fulfill this objective, planning engineers must allocate a large number of required works in a concerted manner so as to satisfy many different kinds of iconstraints. Typical constraints include the number of workers, inventory of maintenance substances,

PE-952-PWRS-0-11-1997 A paper recommended and approved by the IEEE Power System Operations Committee of the IEEE Power Engineering Society for publication in the IEEE Transactions on Power Systems. Manuscript submitted December 31, 1996; made available for printing November 7, 1997.

season of extremely high demand, special holidays. In practice, conceivable countermeasures to realize this requirement are:

shifting the time and date of the outage works, merging several non-interacting outage works, network switching depending on situations, and to avoid outage works during heavy load and lightning seasons.

However, in consideration of a huge number of such maintenance works in a large power system, the planning tasks of arranging them appropriately have been tedious works even for experienced planning engineers. Nevertheless, it is true that most of this sort of planning tasks have so far resorted to sheer human powers of the engineers though some very minor parts have been computerized. It should be mentioned that even though plans thus made up by the engineers are feasible, no one knows whether or not the plans based on their knowledge and experiences are optimal. In other words, no rational criterion has not been adopted in the planning except the knowledge and past experience of the planning engineers in charge. The main cause of having to solve this problem by empirical approaches lies in that the problem is in its nature belonging to a class of combinatorial optimization problems. Therefore, conventional optimization algorithms such as the Newton method cannot be a generalized solver for this problem. Moreover, exact methods to handle this class of problems are essentially based on enumeration and hence it is powerless to large scale problems.

Throughout the paper, we shall call the problem of maintenance work allocation as “the outage work planning” since any of requested works cannot be accomplished without outages. In the below, a brief review on research works associated with the current status of outage work planning will be given.

Reflecting the fact that a large scale outage planning problem is exposed to combinatorial explosions and situations are greatly different from system to system, there are little research works up to now. S.D. Kaminaris, et al. proposed an AI approach for repair and maintenance scheduling of apparatuses in a substation [l]. Although this is the only literature appearing in the PES Transactions, the target is limited to devices in a substation and hence it has not treated very complicated matters of coordinating outage works among substations. Although references [2,3] discuss the effect of scheduled outages on the reliability calculation of a transmission system. their main purpose does not lie in outage work planning. Certainly, there are many literatures on thermal unit maintenance scheduling, but this problem has been formulated entirely different way compared

0885-8950/98/$10.00 0 1997 IEEE

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with the outage work planning. Typically, load is treated as gross and system configurations are not taken into account at all.

This paper proposes a supporting expert system for outage planning of electric power facilities, while it retains high power supply reliability during outage works; this paper is an extension of our former work [4]. The proposed system not only checks validity of outage work requests but also determines the outage planning and system configurations based on power system security indices. Since the outage work scheduling must take into account many ill-defined constraints that are almost impossible to formulate, the use of a knowledge based system is mandatory in order to rcalize the proposed system.

As the proposed system must cover a wide scope of the outage planning tasks, it has been constructed by dividing it into the following three subsystems:

(1) support for preparing request forms for outage works, (2) generation of work units, and (3) allocation of outage works and the selection of suitable

The major function of subsystem 1 is to help maintenance technicians fill outage work request forms by identifying a group of devices to be affected by each particular outage work.

As the second step, all registered outage works need to be mapped onto the one-line diagram to evaluate power system security indices. However, simple security calculation does not make sense to obtain a better outage work scheduling. The concept of "work unit " has been introduced in subsystem 2, which signifies a symbolic outage work in the sense that i t can be deduced as a result of merging several outage works that could be undertaken simultaneously.

Subsystem 3 first calculates three security indices by using work units, that is, supply shortage index, overloading index and margin index. Then, based on the obtained security indices, the dates and time of outage works and system configurations are determined. Since no quantitative evaluations have not been made in the outage planning so far, the proposed system has a large possibility of making up a far better outage plan.

This research consists of two parts. The part I firstly summarizes the present situations and some issues on the outage work planning. Then, the concept and design philosophy of the proposed system will be described. This is followed by the explanation of how to use security indices in order to merge similar outage works and reschedule submitted work requests. Finally, the paper provides an algorithm for allocating and rescheduling outage works. The companion paper presents the knowledge processing for merging outage works, data representation and numerical simulations.

system configurations.

11. OUTAGE WORK PLANNING

A Pwsent Situations

Outage work planning consists of two kinds of planning schemes, that is, monthly and an annual schemes. In the former, the date and time of all requested outage works are determined, while in the latter those of only major outage works are rearranged due to inability of handling all the outage works in annual basis. The flow of the current way of adjusting outage works is shown in Fig. 1. First, a local control center submits request forms for outage works of power equipment in charge to the regional control center in the next hierarchy. In each request

form. the following information is specified: (a) the location. (b) class A. B or C depending on the xoltage level and the

(c) the contents. (d) the outage area to be caused, and (e) the date and time. In a regional control center, where a number of such request

forms are collected from local control centers under its commitment, only class C outage works is adjusted. Then, request forms after minor adjustments in regional control centers are accumulated to a local dispatching center in the next hierarchy, where class B outage works are adjusted. As the top of the hierarchy. the central dispatching center is taking in charge of scheduling of major outage works. In scheduling outage works in control and dispatching centers, the following items must be generally taken into consideration:

(a) the selection of an adequate power system configuration, (b) power supply reliability during outage works, (c) influence of service interruption on customers supplied

(d) effects on system operations, and (e) outage area caused by outage works. Because of fundamental difficulties inherent in this task, the

majority of work adjustments is just to sort request forms according to the date and time specified. Rearrangements of work contents are limited to cases where a selected system configuration for the originally planned outage works may cause supply shortages judging from past results. Since the number of requests for outage works at a regional dispatching center well exceeds 1.000 in a half year, it must be formidable task to schedule all the requests in a systematic manner. However, it is anticipated that the importance of obtaining the optimal outage work planning will be increasing more and more even under the current deregulated and competitive environments.

B. Insiiflcieiicies and Problems of Current Scheduling

degree of supply shortage.

with EHV,

f Central Didpatching Center \ - Adjustment works in the center

Reoional Control Center - Adjustment works In the center

works and its scheduling

Fig. 1 Flow of outage planning

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3 6uppor t for Preparing Reauests Forms for Outage Works - Display of the names of devices associatcd with an outage works - Identification of power facilities to be outaged - Estimation of work volume

=

The conventional oulage work scheduling consists of two consecutive facets: The l’irst is to make up outage work plans in control centers in consideration of necessary amount of materials, the number of availablc workers, etc., and the second is the overall scheduling of the submitted work requests by system planning engineers. In case where a part of the submittcd plans are unacceptable to planning engineers, they rearrange the relevant requests and examine whether or not a number of constraints posed are satisfied. However, this simple rescheduling of just shifting requested outage works results in a schedule which weighs more the standpoints of the maintenance engineers rather than those of the operation planning engineers. This signifies that the obtained schedule just satisfies the constraints imposed and there is a large possibility of obtaining a better schedule by merge and rearrangement of outage works. In addition, ex post facto modifications of already submitted work plans add further complications. Another important issue to be mentioned is that there is no explicit criterion to judge the quality of an obtained schedule. In summary, problems inherent to the current outage work planning may be itemized as follows:

The standpoints of operation planning engineers are rnade light of. There are many constraints to be considered i n the scheduling; most of them are ill-defined and hard to formulate. There are no clear indices for judging the validity of determined schedules. After the fact modification of outage work requests add further complications.

Due to the reasons mentioned above, we have introduced numerical indices reflecting the power system security by which

(Central Dispatching Center) Tmporary Database The Past records

- Facilities data - Operation data - Outage request forms - System data

- Work volume - Expansion area - Personnel management ...

the quality of a proposed outage work plan can be evaluated quantitatively. In order to handle a large number of ill-defined constraints and to avoid combinatorial explosions, a knowledge based approach has been adopted with the objective of using the knowledge and skills of both of experienced operation planning and maintenance planning engineers.

I

111. CONFIGURATION OF THE PROPOSED SYSTEM

Object Oriented Database

... /

The proposed expert system that supports the outage work scheduling must cover a wide range of tasks and functions. After careful analysis of necessary tasks for assisting the scheduling, it is decided to construct the proposed system by dividing into three subsystems as shown in Fig. 2. In brief, subsystem 1 is to assist in making up request forms for outage works in local control centers which are located geographically apart and hence it should be constructed as a distributed system by means of C language with powerful database accessing functions. This subsystem has another important objective of forming the data base for outage works in each local control center. On the other hand, subsystems 2 and 3 are to make adjustments among submitted work requests and make up near optimal outage work schedules by means of knowledge processing and hence are implemented as expert systems. The prototype system has been developed on the ART-IM of the Inference Corp. Ltd., a supporting tool for building up an expert system. Numerical programs such as DC load flow and security evaluations are coded in FORTRAN; an object oriented database (OODB) is constructed by VARSANT, a powerful OODB management system in the market.

I I I I

1 Subsystem 1

Knowledge Base Tmporary Database I (Generation of Work Un& - Identification of outage area - Merge of outage works

’ - Main and appurtenant work -Records of work units = - Work force constraints .I - Results of security indicies - Outage area for each work -System data

/ \ ... I - Data Row <Specification of order of commencing outage w o r y ...

- Arrangement strategy I - - System configurations - The past cases

I - Determination of the date and time Analysis Program - DC load flows - Security calculation

Fig.2 Configuration of thc proposed system.

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A. Support f o r preparing request forms f o r outage works Subsystem 1

An electric utility has its own standards and procedures for executing outage works which have been determined so that any adverse effect on power system operation will not occur during outage works. To complete a request form of an outage work according to the rules and procedures, more than 30 different items must be specified. The following functions will be provided with to reduce the time required for document filling as well as to facilitate database management.

1. Dimlay of the device names It is generally difficult to correctly identify and retrieve the

names and data of devices related to each outage work out of the associated database. This function is to display on the CRT screen the names of devices which will be affected by a particular outage work. The name of each device is displayed in the appropriate level of hierarchy through the graphical user interface (GUI). If one particular substation is specified, the names of apparatuses in the substation and the connection status of buses, CBs, etc. are displayed.

2 . Estimation of work volume For each outage work request, the system estimates the work

volume needed based on past records in the database to check if the duration of the work is properly set. If such past data are not available, the necessary period of works is evaluated according to the standards about work volume.

3. Retrieve of the Dast work records Past work records similar to each requested work are retrieved

from the database for references

B. Generation of Work Units - Subsystem 2

The prime function of subsystem 2 is to generate a work unit efficiently. From the standpoint of the both system security and work efficiency, it is desirable to undertake simultaneously as many outage works as possible under the same outage area. As is already mentioned, the concept of work unit has been introduced. A work unit is defined as a merge of several outage works that need the same outage area and hence it can be mapped onto a line or a bus on the one-line diagram. Thus, it is possible to evaluate the system security during a group of outage works. In order to generate a work unit efficiently, the following functions have been embedded in this subsystem:

a. Identification of outage area Outage area to be caused by each outage work is identified to

avoid any unwanted accidents. b. Merge of outage works Several outage works which can be undertaken under the same

outage area are merged while taking into consideration work force constraints.

c. Specification of order of commencing vutage works It is likely that after finishing a major outage work a few related

minor works must be executed in succession. Hence, the order of commencing outage works may be specified.

d. Assignment of priority to outage works Prior to the determination of outage work combinations, the

proposed system assigns a priority to each outage work according to two factors: security indices evaluated for each work unit and the amount of estimated work volume.

C. Arrangement of Outage Works - Subsystem 3

In this subsystem. three security indices are evaluated for each work unit to properly arrange work units, the details of which will be explained in Section I’r! The main tasks to be performed in Subsystem 3 may be summarized as the following:

a. Calculation of securitv indices For each work unit, the security indices are calculated under

a huge number of assumed contingencies. A DC load flow program written in FORTRAN is used to alleviate the computational burden without sacrificing the accuracy. The security calculation is carried out by removing two lines from the one-line diagram, corresponding to a particular work unit and a contingency.

b. Determination of the date and time As the knowledge of arranging outage works, the following

standards are prescribed in some electric utility. In this study, we propose a procedure which mainly takes into account item 1 in the below in the arrangements while items 2 and 3 are incorporated as the constraints. Item 4 is dealt with as the dates and time of outage works are specified.

1 . To avoid outage works on Sunday as much as possible. 2. Service interruption to customers should be at most twice

a month. 3. A period between outage works should be distributed

evenly as much as possible. 4. For an outage work causing supply shortage to a high-

priority customer, the date and time are prespecified. c. Determination of system configurations In order to make feasible outage work plans while avoiding

combinatorial explosions, a power system configuration corresponding to each outage work is retrieved out of power system patterns stored in the database.

IV. THE PROCEDURE OF OUTAGE WORK PLANNING

A. Securic Indices

It is a matter of fact requirement to maintain a high level of security during outage works. That is, an adequate power system configuration should be selected so that any single contingency does not cause supply shortage. In the proposed method, three indices are jointly used to evaluate the relative security level of selected configurations. The indices have already been used in the static security assessment, and hence are inherently deterministic.

1 . SuDplv shortage index This index is defined as a sum of loads not supplied due to

the separation of loading buses and is given by

where L : load at bus i,

0 without supply shortage 1 with supply shortage

M C : the number of assumed contingencies, M B : the total number of buses.

2. Overloading index Here. overloading on a transmission line means the power

flow in excess of its upper limit. However, a simple sum of overloading over all overloaded lines may mask the significance

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of a few heavily overloa'ded lines. Accordingly, the squared sum of overloading is adopted in this study as thc following:

where

( 3 )

emax : maximum power flow of line i El : power flow oker line i under contingency on line j M L : the total number of lines

3. Margin index A line without overloading has a margin that is defined as a

difference between its upper limit and an actual power flow. Hence, an index reflecting the overall margins can be defined as:

where w,is as in (4). The importance of the above indices decreases according to

the order of being defined since it is most important to avoid supply shortage. Note that the indices are used to judge the relative security level of candidate system configurations and also in the scheduling process to be explained in section V.

B. Conceptual Structure of the Proposed Method

The conceptual structure of the proposed outage work scheduling scheme is depicted in Fig. 3. In the first place, work request forms are input to the system in which the dates and times are not specified except those with strict requiremenis on them. This is to make the proposed system search a better scheduling by shifting outage works.

In the second place, each requested outage work is mapped onto each of the N prepared system configurations, which is followed by the security indices calculations. Then, the combination of outage works is determined so that some prescribed criterion be minimized. For example, it is the reduction of the number of work days or balancing work volumes through a planning period. Planning engineers should specify an appropriate criterion according to the system environments.

V. PROCEDURES OF DETERMINING WORK ALLOCATION AND SYSTEM CONFIGURATION

A. Evaluation of Power Supply Reliability

It is true that the selection of suitable system configurations through the power supply reliability by using the above indices is by no means straight forward. The flow of evaluation procedure is shown in Fig. 4. Firstly, it is checked where or not there is supply shortage. If there is only one system configuration without shortage or with shortage clearly less than that of the next best system (say, by 5 %), this is chosen as the best. Otherwise, power supply reliability is evaluated by the second important index, the overloading index. If we can identify a unique system configuration referring to this index, it is chosen as suitable system. If it is not possible to identify a unique configuration even at this stage, the third index is used to judge a suitable system configuration.

B. Allotment of the Order of Priority to Work Units

To realize efficient scheduling, the system allots the order of priority to each work unit. The priority order is determined by first considering the length of work periods and then the number of workers. For work units with the same period, the number of necessary workers determines the priority order. This is because work units requiring long work periods and many workers affect

System Configuration N

/

J System Configuration 1 3r Fault

f Calculations

- Maximum Electric Power Demand - Maximum Generated Power >

>

Work Request 1 >Work Request 2

Power System Security Indices - Supply Shortage Index - Overloading Index - Margin Index

Work Requixt I Work Requcst 2 , ill, , _ _

(Use of Expert System) - Arrangement of Outage Works

Flatting work volume, Reducing the number of working days - Determination of System Configuration5

Calculation of Supply Shortage I A

Are there any configuration: - without shortage? or - with a few percent of difference

for the minimum value? <

A

Fig. 4 Procedures for powcr supply reliability evaluation. Fig.? A conceptual structure of the proposed outage work scheduling scheme

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much the security of a power system.

C. Procedures in Allocating Outage Works and Determinirig System Configuration

The proposed scheduling procedure is shown in Fig. 5 . Note that a planning period is divided into subperiods consisting of 7 days and allocations of work units to Sundays are avoided as much as possible. [Step 11 Prioritizing work units

For work units obtained from requests of outage works. the priority is determined according to work periods and the number of workers.

[Step 21 Initial scheduling of work units Work units are allocated one by one from the highest to lower

order of priorities to subperiods with the minimum to larger power demands, while trying to exclude Sundays. In this allocation, an overlap of work units on any day is not allowed. which is referred to as the first layer allocation.

Set k = 0, a counter expressing the layer number for allocation.

[Step 31 Determination of system configurations I Reliability indices of the N prepared system configurations

are calculated with respect to the work units allocated in step 2 to select the most suitable configuration.

If there are no work units left unscheduled, terminate the procedure. Otherwise, go to the next step.

[Step 41 Second phase scheduling of work units Increment counter k (k+l + k) Among the remaining work units, one with the highest priority

is selected and the most reliable combination with units which have already been scheduled by the k- l layers is sought based on the values of the security indices. That is, this particular work unit is allocated in the k-th layer. Then, a work unit with the next highest priority is allocated similarly. This is repeated until unscheduled work units are uniformly allocated in the k-th layer.

[Step 51 Determination of system configurations I1 As a result of new allocation of work unit in Step 4; the system

configurations already determined by the previous allocation may become not necessarily optimal. Hence; it is reexamined and more reliable system configuration is sought in the same way as in Step 3 .

If there are no work units left unscheduled, then stop the procedure. Otherwise, go back to Step 4.

VI. CONCLUSIONS

This paper has proposed a supporting expert system for outage work planning of electric power facilities to ensure high power supply reliability. The main advantages of the proposed system are:

(i) outage works requesters do not need to specify the date and time of outage works,

(ii) the system can assist technicians for preparing request forms for outage works,

(iii) the combination of outage works is determined so as to avoid works on Sundays,

(iv) the three security indices are introduced to enable a qualitative evaluation of the obtained outage work plans,

(iv) suitable system configurations are selected out of the

Are there work units left unscheduled? & (END)

Fip 5 A flow of allocating outase works

prepared configurations based on the indices to avoid combinatorial explosions, and

0) the knonledge associated with the adjustment of outage planning is incorporated as production rules.

VII. REFERENCES

S.D Kaminans. A.V.Machias, and B C.Papadias, ”An Intelligent Tool for Distribution Substations Troubleshooting and Maintenance Scheduling.” IEEE Trans on Power Delivery, pp.1038-1044, July 1991 L Wang. “The Effrcts of Scheduled Outages in Transmission System Reliabilit? E\ aluarion.“ IEEE Trans on Power Apparatus and Systems. Vol.PAS-97. Yo 6. 1978. pp 2346-2353 D.Elmakis and P.Levy. ”Reliabil i ty Calculation under Planned hlaintcnance.“ IEEE Trans. on Power Systems, Vol.PWRS-2, No.1, 1987,

K.Katbahara. H Sasaki. and H.Asahara, “Development of a Supporting Expert S>stem for Outage Planning of Electric Power Facilities,” Proceedings of ESAP‘93, 1993, pp.39-44, Melbourne, Australia EEE Comrruttee Report, “Reliability Indices for Use in Bulk Power Supply Adequacl E\ aluation.” IEEE Trans. on Power Apparatus and Systems, Vol.PAS-97. No 4. 1978,pp.1097-1103 K..4. Clements. B.P. Lam. D.J. Lawrence, and N.D. Reppen,“Computation of Upper and Lower Bounds on Reliability Indices for Bulk Power SJstenx.” EEE Trans. on Power Apparatus and Systems, Vol.PAS-103, S0.8. 1984. pp 318-2325 R ASchluetcr. J.E.Sekerke, and A.G.Costi, “Improved Contingency Measures for Operation and Planning Applications,” IEEE Trans. on Power Systems. Val 1. So.4, Oct. 1989, pp.1430-I437 IEEE Committee Report. “Bulk System Reliability - Measurement and Indices.” IEEE Trans Power Systems, Vo1.4, No.3, Aug 1989, pp.829- 835

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