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An Expert-System Based Power-System Restoration Prwedure H. Seifi, A. Sedighi

Abstract

The expert-system restoration procedure developed in this paper employs an algorithm in which by line swit- chings, the downloads are restored with regard to their priorities, while any line overloads and out-of-limit voltages are avoided. The solutions are presented to the user in an understandable way. Computer results on the IEEE-14-bus system are presented.

1 Introduction

A large-scale power system is continuously subject- ed to various types of disturbances, Electric utilities try, constantly, to maintain system stability, integrity and se- curity. Absolute security cannot, however, be guaran- teed and following cascading outages, blackout, system separation, out-of-limit “voltages, line flows, frequen- cy, etc.” and generators trip out, the system may attain one of the security states shown in Fig. 1 [ 11.

The restorative state is defined as a state where the demands are not fully met (partially down) while vari- ous voltage and line-flow constraints are still satisfied.

plications of expert systems in power systems are wide- spread [ 10- 121, however, Sakaguchi and Matsumoto [ 131 were the first who suggested that an expert system could be used for planning switching operations in a res- toration process. They used numerical as well as sym- bolic computations to restore the power supply, using switching operations, and without overloading any of transmission lines. Since then, this concept has been ex- panded, extended, generalized and new ideas have been developed [9, 14- 191.

The work referred to in this paper presents an expen- system based restorative procedure by using switching operations:

The control to re-energize the area where the power is down, is referred to as system restoration. In order to minimize the economic loss, the inconvenience to the

- The down loads are restored according to their prio- rities.

public and direct cost to the utility, the restoration pro- cess should be performed quickly and carefully. The op- erator must be attentive to many unusual voltage, line flow, stability and resource constraints.

The subject of power-system restoration has re- ceived much attention in the literature [2-51. Initially, the restoration process was performed mostly through manual operation. The solution method must have a ca- pability of treating combined nature of the problem, tak- ing care of physical laws and operating procedures [6-81. These are all based on written procedures and suffer from some basic disadvantages [9].

As it is not easy to formulate the restorative control problem within an analytical framework, expert systems may offer an attractive proposition in this area. The ap-

r 1 4 [[I Normal state

I 1-1

Emergency state

Fig, 1. Security states

- During the course of switchings, overloading of transmission lines as well as out-of-limit voltages are avoided.

- In order to keep the balance between generation and demand, the loads are dispatched most economical- ly among generating units using base points and par- ticipation factors [20].

The structure of the paper is as follows: Ch. 2 brief- ly reviews the basic elements of an expert system. The expert system-based power restoration procedure is de- veloped and outlined in Ch. 3. Computer results for a typical system are described in Ch. 4. The paper finally concludes with some general remarks.

2 Expert Systems, an Overview

An expert system is considered as one of the main branches of artificial intelligence. A schematic diagram is shown in Fig. 2. The knowledge base comprises knowledge that is specific to &he domain of application, including such things as simple facts about the domain, governing rules, and possibly, methods, heuristics and ideas for solving problems in the domain. The knowl- edge base may either be rule-based, frame-based, logic- based or a combination of them [2 1 ].

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Knowledge base

I interface I Fig. 2. Expert system

The inference engine actively uses the knowledge in the base to solve the problem. The user interface pro- vides communication between the user and the system. It also provides the user with an insight into the problem- solving process executed by the inference engine. It is convenient to view the inference engine and the inter- face as a module, usually called an expert-system shell.

3 Expert-System Based Restoration Procedure

As noted earlier, the algorithm developed aims at re- storing down loads, while their priorities are observed. In doing so, appropriate switchings are performed, eco- nomic dispatch is carried out and line overloads and out- of-limit voltages are avoided.

The expert system developed in this paper is based on turbo-Prolog (Programming in logic). Prolog was first developed by Alian Colmeruuer from Marseilles University (France) in 1970 [22,23]. It is an efficient and powerful language specially developed for logic pro- gramming.

3.1 Knowledge Base

The knowledge base comprises the data of transmis- sion lines, generating units, loads, regulated buses, stat- ic capacitors and tap changing transformers. The data is stored in a manner as shown in Tab. 1.

3.2 Inference Engine

The inference engine uses the data provided in the knowledge base to solve the problem. The flow chart is shown in Fig. 3. The following points are worth men- tioning: I. The algorithm tries to restore the loads using line

switchings, keeping in mind that load priorities should be observed. No priorities are considered for line switchings.

11. More than one solution may be available. For in- stance, if two lines, L 1 and L2 are available, the pro- gram tries to restore the loads forthe following line- switching arrangements: -Switch L1, then restore the loads. - Switch L2, then restore the loads. -Switch L1, then restore the loads, partially (as

much as possible); switch L2, then restore the re- maining loads.

-Switch L2, then restore the loads, partially (as much as possible): switch L1, then restore the re- maining loads.

- Switch L1, then L2, then restore the loads. -Switch L2, then L1, then restore the loads. The process is considered to be successful provid- ed no line overloads andor-out-of limit voltages are observed. It is evident that more than one solu- tion may be available.

111. It may happen that a non-complete solution (in which all loads are restored according to their pri- orities) be available. The program still tries to re- store the loads without observing load priorities. However, the least amount of reordering (in terms of priorities) is used.

IV. All solutions, whether completely or partially suc- cessful, are provided to the user, so that heishe can decide on the best possible solution (see Ch. 3.3). To check for line overloads and out-of-limit voltag- es, an AC load flow is performed. The load flow is considered to be successful provided all constraints are met.

VI. Once a load is restored, the generation required is dis- tributed among the generating units (including slack unit) using base points and participation factors: - It is assumed that before the restoration proce-

dure, the units are in economic dispatch mode. -As the system loss is not known initially, the loss

is added to the generation of slack unit. -If a unit reaches its generation limit during the

course of allocating generations, its production is set at its limit and the remaining required genera- tion is redispatched among the remaining units.

VII. If all lines are used but still some loads unserved, the algorithm changes the order of the last unsuc- cessful re-energized load with the next one (to be restored) and retry the procedure.

V.

line( Nline , Fbus , Tbus , Shline , Pleft , Pright , FIMX , "State") mline( Nline , Fbus , Tbus , r , x , b , 'State") generator( Nbus , Nunit , RMX , Pold , Pf , Qnin , Pointer , Fi , 'State") bus( Nbus , V , Ang , Vmax , Vmin , 'Wpe , 'State") load( Nbus , Nload , P , Q , Imp , 'State") regulated-bus( Nbus , V , Qmin , W) static-c( Nbus , Susep) ltc-trans( Fbus , Tbus , Tap , Maxt , Mint)

Tab. 1. Knowledge base of the system under study

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+ Read one row of the off lines

User i n t e r f a c e H R e s u I t s m

I Select the load

with highest priority and solve ED

1 no

A

Q Data entry knowledge base + Form the list of down lines

(to be switched) I cl

L.

v

Any other off lines ?

Switch line i;

Fig. 3. Inference engine flow chart (ED: Economic Dispatch)

VIII. The program avoids automatically the case:

- where a load is to be energized but still its bus is not energized;

-where a line is to be switched, but still the con- necting buses are not energized;

-where a line is to be switched, but still one con- necting bus is not energized (it first energizes that bus).

3.3 User Interface

The user interface provides the results in an under- standable way to the user. As an example, consider the situation where there are five down loads with priorities

1 ,2 ,3 ,4 and 5 and two lines L1 and L2. The final out- put may look like:

Load 1 I 2 Line 1 Load2 Load3 t 4 Line2 Load4 Loads.

A slash line (1) followed by aload number shows that the restoration of that load has been unsuccessful. So for the situation shown, the description is: "First restore load 1, then energize line 1, then restore load 2, then restore load 3, then energize line 2, then restore load 4 and final- ly restore load 5".

In this case, it is evident that all loads have been re- stored according to their priorities. There well may be the cases that the restoration procedure is not successful. Other situations will be shown in the case study present- ed in Ch. 4. Also, new generations are calculated and provided to the user (see Ch. 4).

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Fig. 4. IEEE- 14-bus test system

4 Computer Results

To assess and appreciate the algorithm developed, the IEEE-1Cbus test system is considered (see Fig. 4). The case study is as follows:

Lines 7 and 16 are out, and they may be re-ener- gized. Also the down loads are 3.6, 10, 12, and 13 with priorities [6, 3, 10, 12, 131. The expert-system proce- dure has restored the loads. The results are shown in Tab. 2. With regard to this table, the following points are worth mentioning:

I.

11.

The 1st column of part a) shows the number of trials. A ** sign shows the case where all ioads have been successfully restored according to their priorities. A *sign shows the case where all loads are still restored but their priorities have not been observed. No sign shows the case where the loads are partially restored.

The 2nd column of part a) shows total restored loads.

~

For load 6

Bus Unit Pg[old] %[new] Increase (in MW) (in MW) (in MW) no. no.

2 3 10.00 11.00 I .00 2 2 15.00 16.99 I .99 2 1 15.00 16.99 1.99 1 1 88.70 95.20 6.50

For load I0 BUS Unit Pg[old] Pg[new] Increase

(in MW) (in MW) (in MW) no. no.

2 I 16.99 18.59 1.60 2 2 16.99 18.59 1.60 2 3 11.00 11.80 0.80 I I 95.20 100.60 5.40

For load 1.2 Bus Unit Pg[old] Pg[new] Increase no. no. (in MW) (in MW) (in MW)

2 3 11.80 12.34 0.54 2 2 18.59 19.68 1.08 2 I 18.59 19.68 1.08 I 1 100.60 103.90 3.30

Tab. 3. Generation changes for row 2

111. The 3rd column of part a) shows the number of switchings required for restoring loads in that row. Re-energizing a line is considered to have two switching operations and re-energizing a load is .considered to have one switching operation.

IV. The 4th column of part a) shows the restored loads and their orders of re-energizing.

V. Part b) of this table shows how the restoration pro- cedure should be performed.

Based on the descriptions provided in Ch. 3.3, as ex- amples, for the following two cases:

a) Total restored MMbar of Reetorad load6

Row loads (in MW) switchings order

1 120.50 6 [6.3.10,121 2 26.30 5 [6,10,121 3 *- 134.00 9 [6,3,10,12,13 1 4' 134.00 9 [6,10,3,12.131 5" 134.00 9 [6,3,10,12,131 6" 134.00 9 [6,3,10,12.131

Description

Load->6/3 line->7 Load->3 Load->lO Load->12/13 LNS->13 Load->6/3 line->16/3 Load->10/3 Load->12/3/13 LNS->3 LNS->13 Load->6/3 line->7 Load->) Load->lO Load->12/13 line->16 Load->13 * All loads in * Load->6/3 line->16/3 Load->10/3 line->7 Load->3 Load-sl2 Load->13 * ALL loads in Load->6/3 line->7 Load->16 Load->3 Load->lO Load->l2 Load->13 * All loads in *

Load->6/3 line->16 line->7 Load->3 Load->lO Load->l2 Load->13 - All loads in * ~

* : all loads restored(pri0rities not observed) Hint: 'LNS' means 'Load Not Served' *': all loads restoredcpriorities observed) Li6es Off: [7,16]; Total initial loads Off: [6,3,10,12.131 Total initial loads Off: 134 MW

~

Tab. 2. User interface output for case study a), and description of output b)

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For load 6

Bus Unit Pg[old] Pg[new] Increase no. no. (in MW) (in MW) (in MW) 2 3 10.00 11.00 1 .oo 2 2 15.00 16.99 I .99 2 I 15.00 16.99 1.99 I I 88.70 95.20 6.50

For load 3 Bus Unit Pg[old] Pg[new] Increase no. no. (in MW) ( in MW) (in MW) 2 I 16.99 33.74 16.75 2 2 16.99 33.74 16.75 2 3 11.00 19.37 8.37 I 1 95.20 152.00 56.80

For load 10 Bus Unit Pg[old] Pg[new] Increase no. no. (in MW) (in MW) (in MW) 2 3 19.37 20.17 0.80 2 2 33.74 35.34 1 .ti0 2 1 33.74 35.34 I .60 I I 152.00 157.60 5.60

For load I2 Bus Unit Pg [old] Pg [new] Increase no. no. (in MW) (in MW) (in MW) 2 1 35.34 36.42 I .08 2 2 35.34 36.42 1.08

loads by line switchings while various constraints are met. The authors are in the process of augmenting the procedure by considering other constraints such as fre- quency limits. The use of tap changing transformers as- well as off units (the units which may be re-energized) are also under investigation.

6 Nomenclature

NI i ne Fbus, Tbus Shline Pleft, Pright

Nbus Type

Pf

Fi

Maxt, Mint State

Imp

References [I1

2 3 20.17 20.7 I 0.54 I I 157.60 161.10 3.50

[21

Bus Unit Pg[old] Pg[new] increase no. no. (in MW) (in MW) (in MW) PI

For load 13

2 3 20.7 I 21.91 I .20 2 2 36.42 38.82 2.40 2 1 36.42 38.82 2.40 I I 161.10 169.70 8.80

[41

[51 Tab. 4. Generation changes for row 5

[61 - Row 2: Restore load 6, then switch line 16, then re- store load 10, then restore load 12 (but unsuccessful in restoring loads 3 and 13),

- Row 5: Restore load 6, then switch line 7, then switch line 16, then restore load 3, then restore load 10, then restore load 12, then restore load 13,

based on base points and participation factors, the re-

[71

[81

quired changesin units geieratiok are shown in Tab. 3 and 4, respectively. The results are self-explanatory.

The results shown demonstrate that the algorithm shows a number of ways to the operator so rhat he can easily select an option. The option may be the solution with the least number of switchings or perhaps the first solution in which all loads have been restored success- fully (i. e. the solution with **). In any case, the required changes in units generations are also demonstrated.

I

91

line number numbers of connecting buses for a line parallel line number power transmitted from both sides of a line bus number bus type (0: PQ bus , 1 : PV bus , 2: slack bus) power generated by a generator in oper- ating conditions inverse of participation factor load priority maximum and minimum values of tap state (“On” or “Off ’)

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5 Conclusions

An expert-system restoration procedure was devel- oped in this paper. The algorithm restores the down pp. 320-326

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Manuscript received on April 4, 1996

The Authors Dr. Hossein Seifi (1957) received his B.Sc. from Shiraz University (Shi- raz/lran) in 1980 and his M.Sc. and Ph.D. both From UMIST (U.K.) in 1986 and 1989, respectively. He spent a year of sabbatical leave in ABB Netcom (Switzerland) in 1995. He is currently an Associate Professor with Tarbiat- Modarres University. His research interests are power-system dynamics, operation and control. (Dept. of Electr.

Engng., Tarbiat-Modarres University, F!O.Box 141554838. Tehran/ Iran, Phone +982 I I8CQ5035, Fax +98 2 1 I8 0065 44, E-mail: seifi-ho@netlcs.modares.ac.ir)

A. Sedighi (1967) received his B.Sc from Isfahan University of Technolo- gy (Isfahadran) and MSc. from Tar- biat-Mdarres University (Tehran/ Iran) in 1991 and 1994, respectively. His research interests are power system control and operation. He is currently with Yazd University (Dept. of Electr. Engng.,Yazd University, Yazd Iran, Phone: +98351/45021, Fax: +98 3 5 1 /47 1 00)

138 ETEP Vol. 8, No. 2, MarcWApnl 1998