2004 lntemationaf Conference on Power System Technology - POWERCON 2004 Slngapore, 2f-24 November 2004

The Automatic Routing System of Urban Mid- Voltage Distribution Network Based on Spatial GIS

Chengshan Wang and Saiyi Wang

Absrruct- Considering the features of spatial data structure in mid-voltage distribution network, the CIS spatial database and planning model suitable to distribution network planning are designed. On this basis and according to the property that the distribution lines are arranged along the streets, the line structure can be represented by length-varying character chromosome encoding strategy and the preIiminary network is formed by the shortest path method. Then, the preliminary network is optimized by genetic tabu hybrid aIgorithm. In the routing process, different methods are designed adapt to several familiar connection modes such as radial, multi-sectioned and multi-linked, and switching station network. By means of the analysis on actual calculation example, a practical method for distribution network planning is discovered in which the spatial CIS is used for phtform, the distribution network planning is tightly correlated with geographical environment and the searching process is guided by optimization algorithms.

Itider Terms- distribution network planning; CIS spatial database; character chromosome encoding strategy; shortest path method; genetic algorithm; tabu search; connection mode

I. INTRODUCTION RBAN mid-voltage distribution network planning is a

U c o m p I e x NP-hard opthimtion problem full of characteristics such as multi&jects, uncertain, non-linear, and multi-stages. In recent years, many papers put forward intelligent algorithms and geographical information system (GIS) to solve this complex problem, such as [1]-[3]. And these successful applications give some usefbl ideas to this paper. htelligent urban mid-voltage distribution network planning, one of the research hotspots at present, should complete routing of M V lines automatically and form the complex network structure including manifold connection modes with both overhead lines and cables. And this network should accommodate to the development of city consbvction and the increase of loads, and also satisfy with the demands of customers to power quality and supply reliability.

The automatic routing system introduced in this paper adopts the idea of designing GIs database and combines routing optimization with geographic conditions in order to solve distribution network restriction problem in condition of graphical interfaces. The core algorithm of system is genetic

Chengshan Wang is with the School of Electrical Engineering and Automation, Eanjin University, Tianjin 300072, People's Republic of China (e-mail: cswan@tiu.edu.cn).

Saiyi Wang is with the School of Electrical Engineering and Automation, Tianjin University, Tianjin 300072, People's Republic of China (smail: -.

tabu hybrid algorithm (GATS), which combines the global searching trait of genetic algorithm (GA) and the strong local searching trait of tabu search algorithm (TS). Because of the asymmetry of load densities and the different demands of actual system, the actual M V distribution network is usually a complex network including several connection modes with both overhead lines and cables, such as radial, multi-sectioned and multi-linked, and switching station network. In this paper, three steps are taken to complete the automatic routing of these connection modes. After this, the flexible automatic routing method of manifold connection modes based on different load densities is proposed. Lastly, the method introduced in this paper is proved to be feasible by analyzing the actual example.

11. GIs SPATIAL. DATABASE OF DISTRIBVTION NETWORK

A. Why UseGIS In power system, transmission and distribution networks,

substations and facilities, consumers and loads are all oriented to geography [3]. In order to "age and use these data information, we should store these data with spatial data structure by combining with geographical conditions.

GIs is a computer system, which can gather, save, manage, analyze and reproduce the information related to geographical spatial data. The most important portion of GIs is spatial data, which effectively expresses the spatial locations and attributive information. GIS spatial database is a system that can save these spatial data mare availably. Based on this, the spatial database management system can provide spatial query, spatial anaIysis, and also credible assistant design method for urban mid-voltage distribution network planning.

B. GIs Spatial Database Design Based on analyzing the data structure models of some

successful GIs sofhvare home and abroad, and considering the speciatties of distribution network spatial data structure, this paper designs GIs spatial database suitable to distribution network planning.

Every street segment is a beeline Without any crossing or barrier points after the street i s intersected. That is, the linear street segment can be a segment between two corners or barriers of the street. Fig. I is the sketch map of segments of polygonal streets. Tables I and II express the spatial data structure of street and street segment with linear topology structure.

o-7ao3-~610-81041$20.00 o 2004 IEEE 1827

Fig. 1. Segments ofpolygonal streets.

Record Name street name

street id

TABLE I SPATIAL DATA STRUCTURE OF STREIZT

Data Type DeSCriptiOn character name of street integer unique id of each street

topologic type start-point id

m-point coordinates

Character topologic type: line integer unique id of street start-point float start-point coordinates of s a e t

end- oint id uni e id of street end- oint

total leu& the total len of sweet segment numbex ink er the number of meet se mts

e n d - p o i n ~ m r d i n a t e s ~ end- iat c00rdiaates of SUM

TABLE n SPATIAL DATA STRUCTURE OF STMET SEGMENT

RecordName I Datame 1 Description street id I integer I the street id matched with Table I

There are only basic fields in Table I because of the limit space of the paper. The other spatial data structures can be designed according to the above method. After this, database management system (DBMS) i s used to manage all these spatial data tables, and the integrated spatial GIs is formed with the graph system.

m. SYSTEMMODEL

The model used in this paper is the distribution network planning model which takes the economy index as object function and takes the reliability index, equipment capability, and voltage drop as constraints. The economy object can be represented as follows.

minF= FNs t Fm + Fm (1)

where F is the object function. FNs, Fm and Fm are the annual investment and operation costs of HV substations, M V lines and switches. The annual investment cost has been converted into the cost shared alike in each year by the economic method represented as follows.

where Fm is the annual investment and operation costs of each equipment, and i can be S, L, or K. Zi and Yi are original investment cost and annual operation cost of each equipment including annual loss and maintenance costs. n, is the life period of each equipment and ro is the interest rate of power system.

After the economy object is up-built, some constraints correlated with the practical operation trait of urban M V distribution network should be considered as follows. a) Reliability constraint

RAm, average supply availability index (ASAI), is used to evaluate the reliability index in order to reflect the ponderanix and importance of system interruptions as follows.

N,, x 8 7 6 0 - x U j N i , R = (3)

where Nto,d is the total customers of the system. is the average annual interruption duration and NJ is the outage customers. 8760 i s the total hours in one year and R- is the lower limit of reliability index. b) Capability constraint

P, I e",I = l ; - . , N , (4)

where P, and 4- are the totd loads and the upper limit loads of the line I. NI is the total number of lines of H V substations. c) Voltage drop constraint

vk- sv, I v k - , k = l , - . * , N , (5 )

where Vpm, Vkm and Vk are the lower limit, upper limit and normal operation voltage of the beginning or end p i n t of the line sections. N, is the total number of the b e w i n g or end points of the line sections,

N. SYSTEM COREhGORlTHh4

A. Genetic Tabu Hybrid Algorithm Modern hemistic methods such as genetic algorithm (GA)

and tabu search algorithm (TS) provide feasibility to solve the large NP-bard problem because of their proper strong global searchhg ability. Nevertheless, GA is easy to get into local optimization and its convergence speed is lower. In the other hand, the local searching ability of TS is much stronger. So it will get satisfying solutions if combining these two algorithms and applying this hybrid algorithm in the distribution network planning. By analyzing the theory of GA, it is obvious that the aim of mutation operation is to improve the local searching ability of GA, maintain the variety of populations, and avoid appearing the earliness. What's more, the effects of mutation operation are equal to 'move' operation of TS, which this operation of searching from one point to the other point is applying to improve the structures of solutions. So genetic

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tabu hybrid algorithm (GATS) with TS mutation operation will improve the local searching ability of GA. In th is paper, the automatic routing system of distribution network adopts this hybrid aIgorithm as system core algorithm.

B. Encoding Strutegv It is very important to select a good encoding strategy while

solving the distribution network planning using GATS. The prevalent method is gathering all possible feeder sections as variables muster of the planning problem. But this method will make the planning process not so effective and need long time to get the satisfying solutions because there are usuaIly plenty of ill-suited solutions in the iterative process. So a better encoding strategy should make each test solution as radial structure in order to improve the efficiency of the searching process and shorten the process period.

Length-varying character chromosome encoding strategy [4] is used in this paper to make the lOkV main lines up of Si (the serial numbet of substations), Ti (the serial number of substation outgoing points), and Ri (the serial number of main street segment points). This encoding strategy can express the locations of lines in the urban geographical map. There maybe exist Fi (the serial number of sectioned switches) and Bi (the serial number of outgoing points o f the first ramose lines) between different Ti and Ri.

It is flexible to define the structures of radial lines using this length-varying encoding strategy. For example, ZI and I? are possible radial l i e s of substations SI and SI, so the structures can be ‘&-Ti-R1-R7&-&’ as the radial line I1 and ‘ S 2 - T 3 - R r R s R ~ R e ’ as the radial line 12. Then the code cluster ‘S1-Tl-R1-R2-R3-&-RgRrR~R5 -T3a2’ forms the l i e d connection mode of different substations SI and S2, and ‘R4-Re’ expresses the linked line between different lines 11 and 12.

A chromosome, network scheme of an urbm MV distribution system, is made up of several HV substation sub-individuals and one linked lines sub-individual. Among these sub-individuals, the HV substation sub-individual includes some radial lines and the linked lines sub-individual includes linked lines between all radial Iines, which are also made up of street segment points.

C. Fig. 2 is the flow chart of the automatic routing system

based on GATS. There are two iterative processes in the search. One is the genetic optimization main-process in order that the solutions are met with gfobal convergence, the other one is TS mutation optimization sub-process in order that the populations are various and the solutions are met with local convergence.

The Process ofAutomatic Routing System

e-, calculation

t

Solutian output by GIS

U I

manage and

calculation

TS mutation

The initialized populations including radial network, multi-sectioned and multi-linked network, switching station network and their combinations are generated by the method proposed in Section V.

The new solutions are generated through selection, crossover, and mutation operations to the last population. As inkoduced ahead, the objects of crossover and mutation operations are Ti and R,. The information of new chromosomes is changed aRer the operations are completed.

D. Select and Cmsmver Operations The selection operation adopts elitist preservation strategy

[4]. That is, some individuals with good fitness of the cment population don’t participate in crossover and mutation operations. What’s more, the same number of individuals generated by crossover and mutation operations are replaced with these individuals.

The crossover operation adopts edge recombination (ER) operator [4]. The most important step is to build J~dge(x) (the relation adjacency muster) between different Ti and Ri in the lines of two parent individuals, where x are and R j selected in the last two parent individuals in crossover operation. The lines of children populations are made up of relevant Ri selected fkom the built Je+(x) through the relation adjacency. Here, it is notified that Ri appeared in the two parent individuals simultaneously are selected f d y .

E. TS Mutation Operation The idea of tabu search is making the current solution to

generate a set of neighbor test solutions by ‘move’ in the optimization process [5]. Among the generated neighbor solutions, the one that improves the fitness best is selected to be the new current solution as the best solution up to the present in the optimization process. Repeat this tabu searching sub-process until the stop criterion is satisfied.

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As introduced ahead, each line is made. up of RI by length-varying character chromosome encoding strategy. The current solution is not the best one if some lines are routed with unreason. The unreasonable network structure is formed that the loaded rates of these Lines are lower OT these circuitous lines are too long. So the Ri md loads of these lines need to be changed in order to optimize the network The simplest defying criterion is used in this paper in order to prevent the loss of excellent solutions [5]. If the fitness of some solution is better than the c m n t best solution, then the solution will replace the current best solution and the current solution without considering its tabu property.

F. Fitness Calculation Fitness is correlated to each chromosome [4]-[5]. In the

paper the fitness is the total object F introduced in Section KU including annual investment and operation costs and reliability index, etc. The individual with best fitness is the best classic one. In the other hand, the individuals with bad fitness are not feasible and will be washed out in the whole optimization process.

Punished function method is used to deal with the individuals after each index is calculated. If the constraints are not satisfied, the individuals are difficult to exist and are washed out after their fitness is evaluated to be very large as follows.

F. G are satisfied M,, G aren't satisjkd

where F' is the filness of individual and F is economy index calculated by (1). G are the constraints a, b, and c proposed in Section IlI and M,, i s a very large positive number.

G. Stop Criterion The optimization process is over if one of the two stop

criterions used in the search is satisfied. The result returns to the spatial GIs of distribution network if the GA stop criterion is satisfied, and if the TS stop criterion is satisfied the TS mutation operation i s over. a. The difference of the fitness of best solutions, gained

&om the continuous populations in the optimization process, is in the range bounded by E (some random small positive number).

where F L and F& are the best solution fitness of the new and last populations. The stop iterative number of GA or TS is satisfied.

V. FLEXIBLE ROUTING " E l O D OF MANIFOLD CONNECTION MODES BASED ON DIFFERENT LOAD DENSKIES

In actual urban M V distribution system, the load density of each area is usually different so that the economy and reliability demands of power supplying are also different to each area. So, there need to be different connection modes to

b.

satisfy with the power supplying demands of different areas. In the condition of different load densities, economy and reliability indexes of several familiar connection modes with overhead lines and cables are calculated and analyzed to get some useful proposed advices in [SI. These advices are usually related with the whole planning of the city. That is, except that the load forecasting is accurately carried out, some practical demands are also considered in order to get the actual urban MV distribution system of manifold connection modes.

It is commonly difficult to get the network structure automatically of manifold connection modes with overhead h e s and cables by programming. Based on applying the repository theory of expert system, a simple method with a c b I effects is used before the automatic routing is to be done. Firstly, one or two main connection modes of the planning city are cordinned by the experts based on the different demands of areas and the advices proposed in [6]. Secondly, some schemes are gained afier the automatic routing of M V main lines is completed according to the dissimilar combinations of the confirmed main connection modes and some rules introduced below. Lastly, the final scheme i s c o h e d based on the analysis of economy and reliability indexes of these schemes.

RULES-There are some rules that can be used in the automatic routing of several familiar connection modes as follows. a. For radial network, routing operation can be carried out

by the method proposed in Part A of Section V and in [7]. b. For multi-sectioned and multi-linked network, muting

operation should be carried out based on the actual landforms of the city. Detailed method sees in Part B of Section V and in IS]. For switching station network, routhg operation can be carried out by the method proposed in Part C of Section V and in [9]. It should be notified that the lOkV switching stations are b d t when the loads of some area are dense and far from the HV substations or the outgoing lines of the HV substations are not enough.

d. The proportion of overhead lines and cables in the network is c o n h e d based on the practical needs of distribution system planning.

Automatic Rowing of Radial Network

c.

A. The automatic routing methods of each connection mode

described in this paper are all carried out by the same idea of optimizing the initialized network with the core algorithm proposed in Section IV.

The main idea of radial network initialization algorithm is to route the radial line one by one, which is to complete a radial line routing by the shortest path method in each time. The source point is the substation, while the end point is the farthest load far fiom the substation in the unsupplied loads muster. In the process of each radial Iine routing, it is important to form the correlated muster of street segment points through the relations of street segments. Then some disposals are used to the muster. For example, the street segment points existing already in the line are deleted in order to avoid getting a circle. And it is necessary to judge whether

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the constraint of the maximal number of lines in the street is reached, naturalness, the street segment points dissatisfied this constraint are also deleted. In the next step, one street segment point is selected randomly in the disposed muster to be a tending towards of the line. At the same time, the loads are selected in the order of the load values in the load areas beside the confirmed street segment. If the load is in the supplying area of the correIated substation and main line, the load is selected. Otherwise, the load is not selected. The iterative searching process is kept up until the constraint of the maximal allowed power of the line is reached. Thus, the automatic routing of this radial line is completed.

B. Automatic Routing of Multi-sectioned and Muiti-linked Network

The multi-sectioned and single-linked network is the most familiar and simplest one of multi-sectioned and multi-linked connection modes. The automatic routing of multi-sectioned and single-linked network proposed in the paper is to search both radial lines and linked fines at the same time. The routing process is made up of the routing of normal and reverse radial lines in one time. The initialized routing of normal radial line is carried out based on the general method introduced in Part A of Section V, which the source point is the substation and the end point is the farthest load far €rom the substation in the unsupplied loads muster. The routing method of reverse radial line is in the other way round the normal radial line, which the source point i s the supplied load and the end point is the substation of the supplied load. The meaning of the substation of the supplied load is that the load is supplied through the substation. The supplied load is selected randomly near the end of the normal line and confirms the substation. Then, the reverse radial line is routed in the supplpg area of the confirmed substation, Thereby, the routing of the n o m 1 and reverse radial lines is related through the idea of global searching so that a multi-sectioned and single-linked connection unit is formed.

Based on the automatic routing method of single-linked connection mode, the general automatic routing method of mutti-sectioned and multi-linked network is proposed as follows.

The maximal allowed power of each radial h e is confirmed by the linked connection mode in the normal operation BS (8).

where P--id and Pm4k are the maximal load and allowed power of the line. m is the number of linked points of each radial line at most and thus the connection mode is multi-sectioned and m-linked network, Then, the routing of the radial lines and linked lines at end-point is carried out based on the automatic routing idea of multi-sectioned and single-liked network introduced above. The sectioned switches and other linked points are also confirmed in this step. Finally, the optimal path routing between other linked

points is carried out by the path search between the discretional two points in the condition of some relations.

Automatic Routing ofswitching Sfation Nefwork C. The switching station is made up of some mother lines and

switches. When the loads are far fiom the substation so that the Line length is much longer using the direct supplying modes, a switching station is necessary to build up near these loads or in the allowable places considering the demands of surroundings effects. Then these loads can be supplied naturally through the outgoing lines of the built switching station. Though the investment cost of the whole distribution system scheme increases because of the investment of the switching stations, the probability of failures is lower as the lengths of switching station outgoing lines are much shorter than the direct supplying modes [6]. So the reliability of the switching station network is much improved so that it is much more wed in the areas where high reliability is needed such as urban flourishing.

Though this connection mode is very applied, the automatic routing method is more difficult. The steps of the algorithm are proposed as follows.

The best number and locations of switching stations are confumed based on spatial GIs of distribution network and the method of substation locating and capacity calculating. Then, the automatic routing of incoming lines is put forward as searching the shortest paths between the switching stations and the HV substations through the main streets. Finally, the outgoing lines are automatically routed based on the automatic routing method of multi-sectioned and multi-linked network introduced in Part B of Section V.

W. CASE STUDY AND hIALYSlS

Tbe case used in this paper is M V distribution network planning of Pujiang centre town in Shanghai of P. EL China, which has ten 3*20MVA 35kV HV substations. The loads in the middle and south parts are denser than the north part but the supply areas of 35kV substations in the north part are larger than the others. By the analysis of the load distributions ofthe town, many situations are suitable to build the lOkV switching stations. Several feasible schemes are gained by using the method proposed in the paper. And the final distribution network of the town is confirmed after the economy and reliability indexes of these feasible schemes am calculated and analyzed. The total annual investment and operation costs are 51.685 million yuan and the reliability index is 0.99993087.

Because of the limit space of the paper, the planning solution of the south part is introduced mainly in the paper. Fig. 3 is the scheme of final solution. Thick lines are the 35kV substations direct supplying modes with two-linked or single-linked hybrid connection modes using YJV-3 *400mmz cables or LGJ-3*240mz overhead lines. Thin lines are the outgoing lines of lOkV switching stations with loop connection mode using YJV-3*120mm2 cables. In order to

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show clearer, the sectioned and linked switches are not aaoeared in Fig. 3.

* 3s kv substatiOn A10 W sgritching station I 2 ' 8 O O kVA MV svbstatm I3 2.630 kVA Mv substation

Fig. 3. Final scheme of MV disiribution network planning in south of Pujiang center town.

Fig. 4 is the scheme map of incoming lines of lOkV switching stations in the town with double sources connection mode usinrr YJV-3*400mm2 cables.

Fig. 4. Sketch map of incoming Lines of switching stations in south of hj iang center town.

It is obvious from the figures that the solution is absolutely according with the actual urban MV distribution network full of multi-sectioned and multi-linked, and switching station connection modes. And it is shown ftom the analysis of the case that the method proposed in the paper is feasible.

WI. CONCLUSIONS

The automatic routing system of urban mid-voltage distribution network needs plenty o f data analysis. And GIS can provide data management to the system strongly so that the solution will be more practical. In addition, this intelligentized distribution network planning is a complex

NP-hard optimization problem full of characteristics such as multkbjects, uncertain, non-linear, and multi-stages. Considering the merits of heuristic methods and the traits of distribution network planning, genetic tabu hybrid algorithm (GATS) is used to optimize the planning problem. By using the spatial GIs and artificial intelligent optimization algorithms, three steps are taken to complete the automatic routing, such as automatic routing of radial, multi-sectioned and multi-linked, and switching station network. By means of the analysis on actual example, a practical method for distribution network planning is discovered in which the spatial GIs is used for platform, the distribution network planning is tightly correlated with geographical environment and the searching process is guided by optimization algorithms.

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M. Bimwms Chengshan Wang was born in Tanjin of P. R Cha, 111 November 1962. And now he is a professor of the School of Electrical Eogirreering aad Automation in liaajin University. His research interests are distribution system o p t i o n and planning, and power system stability.

Saiyi Wang was born in Ningbo, Zhqiang Rovince of P. R. China, on January 30, 1978. He received his BS and MS degrees io electrical engineering in Tianjin University. And he is now a doctor student majored in the same university. His research interests include the applications of GIS and artificial intelligence in distribution sytem planning.

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