An Expert System for Three-Phase Balancing of Distribution Feeders

1488 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 23, NO. 3, AUGUST 2008

An Expert System for Three-Phase Balancingof Distribution Feeders

Chia-Hung Lin, Member, IEEE, Chao-Shun Chen, Member, IEEE, Hui-Jen Chuang, Member, IEEE,Ming-Yang Huang, and Chia-Wen Huang

Abstract—In this paper, an expert system is designed to derivethe rephasing strategy of laterals and distribution transformersto enhance three-phase balancing of distribution systems. Theheuristic rules adopted by distribution engineers are incorporatedin the knowledge base of the expert system in the problem-solvingprocess. The neutral current reduction algorithm is developedto support the inference engine to derive the rephasing strategyto reduce the neutral current of distribution feeder so that thetripping of over-current relay can be prevented and the customerservice interruption cost and labor cost to execute the rephasingstrategy can be justified by the power loss reduction obtained.To demonstrate the effectiveness of the proposed expert systemto enhance three-phase balance, a practical distribution feederin Taiwan Power Company (Taipower) is selected for computersimulation. By minimizing the objective function subjected tothe rephasing rules, the rephasing strategy has been derived toidentify the laterals and distribution transformers for phasingadjustment. After executing the proposed rephasing strategy byTaipower engineers, the phase currents and neutral current oftest feeder has been collected by the SCADA system to verify thereduction of neutral current. The power loss reduction obtainedby three-phase balancing has been solved by three-phase loadflow analysis, which is then used to justify the customer serviceinterruption cost and labor cost for rephasing of test feeder.

Index Terms—Customer information system, expert system,outage management system.

I. INTRODUCTION

W ITH the dramatic growth in the number and size ofsingle-phase residential and commercial customers

served in the Taipower distribution system, the unbalance inthe phase currents leads to excessive neutral currents that maycause tripping of distribution feeders. Besides, the execution ofnon-interruptible load transfer between feeders for scheduledoutage and service restoration after fault contingency mayintroduce further tripping of supporting feeders due to theincrease of neutral current after load transfer. However, it is

Manuscript received October 29, 2007; revised April 19, 2008. Paper no.TPWRS-00773-2007.

C. H. Lin is with the Department of Electrical Engineering, National Kaoh-siung University of Applied Sciences, Kaohsiung 807, Taiwan, R.O.C. (e-mail:[email protected]).

C. S. Chen is with the Department of Electrical Engineering, I-Shou Univer-sity, Ta-Hsu Hsiang, Kaohsiung County, Taiwan, R.O.C.

H. J. Chuang is with the Department of Electrical Engineering, Kao YuanUniversity, Lu Chu, Taiwan, R.O.C.

M. Y. Huang is with the Department of Electrical Engineering, National SunYat-Sen University, Kaohsiung, Taiwan, R.O.C.

C. W. Huang is with the Power Research Institute, Taiwan Power Company,Taipei, Taiwan, R.O.C.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPWRS.2008.926472

Fig. 1. Connectivity of an OYD transformer to serve 1-� and 3-� loads.

very labor intensive and time-consuming for distribution engi-neers to improve three-phase balance of distribution feeders bythe conventional practice. Taipower has performed the phaseswapping of distribution laterals and transformers to improvephase loading balance by trial and error methods.

To serve the low voltage residential and commercial cus-tomers as well as the high voltage commercial and industrialcustomers, many 1- distribution transformers have been usedin Taipower distribution system. Due to the variation of cus-tomer load behaviors, it is difficult to maintain the three-phasebalance of distribution feeders for all time periods. Furthermore,the open-wye, open-delta (OYD) transformers with connec-tivity as shown in Fig. 1 have been widely applied to serve both1- and 3- loads simultaneously. It is noted that, only twounits of 1- transformers with different capacities are used andsupplied by two phases of the high voltage primary feeder. Thetransformer with larger capacity serves both 1- and 3- loadswhile the smaller one serves 3- loads only. Although one unitof 1- transformer has been saved, the three-phase unbalancewill be deteriorated due to the unsymmetrical configuration ofOYD transformer.

According to the operation and computer simulation of dis-tribution feeders in Taipower, the severe three-phase unbalanceof distribution system has caused the increase of power loss, in-duced communication interference, and customer service inter-ruption due to unexpected tripping of low energy over currentrelay (LCO) [1].

To solve the three-phase unbalance of distribution system,the authors in [2] have formulated the phase balancing problemas a linear objective function with mixed-integer programming.However, the decision-making criteria for rephasing strategiesof distribution systems may not be represented as a linearfunction very well. For instance, the system loss minimizationproblem is itself a nonlinear integer objective function andwill be very difficult to be solved analytically. The simulatedannealing (SA) method has been applied to solve the phaseswapping problem in [3] as a large-scale nonlinear integerprogramming problems. In [4], Chen et al. presented a genetic

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LIN et al.: EXPERT SYSTEM FOR THREE-PHASE BALANCING OF DISTRIBUTION FEEDERS 1489

algorithm to optimize the phase arrangement of distributiontransformers connected to the primary feeder. However, theseapproaches do not incorporate the heuristic rules of systemplanning in the problem formulation. Besides, the labor cost andthe customer service interruption cost caused by the scheduledpower outages for the execution of rephasing strategy have notbeen considered according to the number of customers affected,the amount of loading disconnected and the outage durationtime. To justify the cost effectiveness of rephasing strategies,the objective function of rephasing strategy for distributionsystems should be derived in a more practical way by includingall of the related cost and the reduction of neutral current andsystem power loss.

In this paper, a knowledge-based expert system [5], [6] isdeveloped to derive the rephasing strategies for distributionfeeders to enhance the three-phase balancing by reducing themagnitude of neutral current. It is designed by including theheuristic rules used by Taipower engineers to execute thefeeder rephasing work. With the heuristic rules imbedded inthe knowledge base of expert system, an inference engine is es-tablished to derive an optimal rephasing strategy of laterals anddistribution transformers using the system data and heuristicrules as the basis of reasoning. To achieve the reduction ofneutral current and distribution system power loss, a two-phaserephasing algorithm is also developed to support the inferenceengine. The objective of phase I algorithm is to derive therephasing strategy of laterals and distribution transformersso that the neutral current of whole feeder can be reduced tobe less than the setting limit of LCO protective relay,which is 70 A in Taipower. After that, the phase II algorithm isapplied to solve the rephasing strategy for further enhancementof three-phase balance to achieve the minimization of objectivefunction, which is formulated by considering the total systempower loss cost, the customer service interruption cost and thelabor cost required for the execution of rephasing strategy. Bythis way, the related cost involved in the execution of feederrephasing can therefore be justified by the reductions of neutralcurrent and system power loss.

II. ANALYSIS OF THREE-PHASE UNBALANCE

FOR DISTRIBUTION FEEDER

To analyze the three-phase unbalance of distribution systems,the hourly phase currents and neutral currents of distributionfeeders in Taipower have been collected by the SupervisoryControl and Data Acquisition System (SCADA) of Distribu-tion Dispatch Control Center (DDCC). Figs. 2 and 3 show thedaily current profiles of test feeder BD31 for the summer seasonand winter season, respectively, in 2006. Feeder BD31 servesthe loads of 1329 residential customers and 87 commercial cus-tomers as well as one high voltage customers in urban area ofFengShan. There are 182 OYD transformers and 80 1- trans-formers applied in this feeder. From Fig. 2, it is found that severethree-phase unbalance has been introduced because of the dra-matic difference among three-phase currents. With phase cur-rent much less than and , the neutral current of In be-comes larger than the LCO setting of 70 A during the time pe-riods of 11 AM to 1 AM and it reaches the peak value of 95 Aat 2 PM for the summer season. During the winter season, the

Fig. 2. Three-phase currents and neutral current of Feeder BD31 (summer).

Fig. 3. Three-phase currents and neutral current of Feeder BD31 (winter).

neutral current of In becomes larger than 70 A during the timeperiods of 12 PM to 2 PM and 6 PM to 10 PM and it reaches thepeak value of 85 A at 7 PM. The hourly neutral current, which isformulated as the phasor summation of three-phase currents, iseven larger than the phase current . The service reliability andoperation efficiency of test feeder BD31 have been deterioratedbecause of LCO relay tripping and increase of power loss dueto three-phase unbalance.

III. PROCESS OF REPHASING STRATEGY

FOR DISTRIBUTION FEEDERS

To derive the rephasing strategy of distribution transformersand laterals to improve three-phase balance of distributionfeeders, the phase currents and neutral current of all primarytrunk line sections, laterals and transformers have been sim-ulated by three-phase load flow analysis according to thefeeder phase currents and neutral current collected by SCADAsystem. The attributes of distribution components such as linesegments, distribution transformers, etc., have been retrievedfrom the facility database of outage management system (OMS)in Taipower [7], [8]. The network configuration of distribu-tion feeder is then identified after performing the topologyprocess according to the connectivity attributes of distributioncomponents. To represent the load behavior at each bus moreaccurately for load flow analysis, the daily load patterns ofcustomer classes, which have been derived by load survey study[9]–[11], and the monthly energy consumption of customers inthe database of customer information system (CIS) are used tosolve the power demand of each customer. The hourly loading


Fig. 4. Flowchart of rephasing strategy of distribution feeders.

of each distribution transformer is then obtained by integratingthe power profiles of all customers served. The three-phasecurrents and neutral current of each primary trunk line sectionand each lateral are therefore derived by considering the mu-tual effect between phase conductors in the three-phase loadflow analysis. The objective function for rephasing strategyof distribution transformers and laterals is then formulatedby including the number of customers affected, the total loaddemand interruption and the time duration to complete therephasing works by distribution engineers. Fig. 4 shows theoverall process to derive the optimal rephasing strategy toenhance the three-phase balance of distribution feeders.

A. Facility Database of Outage Management System (OMS)

With voluminous facilities involved for a distribution feeder,it is very tedious to prepare the input data for three-phase loadflow analysis by using the conventional paper maps and facilitydata files. To support load flow analysis for phase balancingstudy more effectively, the facility database of OMS system inTaipower is applied. The OMS database provides the capabilityto integrate the graphic representation of components with spa-tial relationship and information management.

B. Topology Process of Distribution Network

After retrieving the attributes of distribution componentssuch as line sections, distribution transformers, etc., thetopology process is executed to identify the network configura-tion of distribution feeders based on the attributes of networkconnectivity model and the dynamic switch statuses in the OMSdatabase. By tracing the FROM and TO fields of connectivitytable of each component, which points to its upstream deviceand downstream device, respectively, the system network isdetermined and updated according to the operation of lineswitches.

Fig. 5. Typical load patterns of residential, commercial, and industrial cus-tomers.

C. Phase Loading Evaluations of Load Buses

To execute the three-phase load flow analysis for a distribu-tion feeder, the loading of each load bus has to be evaluated.With the stochastic variation of customer load characteristicsand load composition, the phase loadings of each distributiontransformer and each high voltage customer will be difficult tosolve. In this paper, the typical daily load patterns of variouscustomer classes, which have been derived by the load surveystudy, are used to represent the load behaviors of all customersserved by the distribution feeder. Fig. 5 shows the typical loadpatterns of residential, commercial and industrial customers inTaipower, which have been derived by statistic analysis of actualhourly power consumption of customers which have been se-lected by stratified sampling method for load survey study. Afteridentify the customers served by each distribution transformerby executing the customer-to-transformer mapping process, themonthly energy consumption of each customer served by thetransformer is then retrieved from the Customer InformationSystem (CIS) database. With the customer load patterns andmonthly energy consumption, the power loading of each trans-former and high voltage customer is evaluated to represent thephase loading of each load bus.

D. Phasing Arrangement

For the study of rephasing strategy to enhance three-phasebalance of distribution feeders, the notation (X, Y, Z) is used inthis paper to represent the phasing arrangement of laterals anddistribution transformers. The possible connection schemesfor various types of phasing arrangement are listed in Table Ifor the 3- laterals and transformers as well as 1- and OYDtransformers. It is important that same phase sequence (positiveor negative) has to be maintained in the derivation of rephasingstrategy for laterals and distribution transformers to preventcustomer damage due to reverse operation of three-phase ro-tating loads after rephasing. For instance, an OYD transformerwith the primary side connected to A and B phases (A,B,*)can only be rephased as B and C phases (*,A,B) or C and Aphases (B,*,A) to ensure same phase sequence for 3- motorloads connected at the secondary side of the transformer. By


TABLE IVALID REPHASING SCHEMES FOR LATERALS AND TRANSFORMERS

Fig. 6. Expert system structure for rephasing strategy of distribution systems.

the same way, a 3- lateral with original phasing (A,B,C) canbe rephasing either as (C,A,B) or (B,C,A) only.

IV. EXPERT SYSTEM FOR REPHASING

A proper rephasing strategy of laterals and distribution trans-formers to enhance three-phase balancing of distribution feedershas to comply with the following constraints.

1) The heuristic rules of system planning and operation mustbe complied.

2) The neutral current of distribution feeder has to be less thanLCO relay setting after rephasing.

3) The customer service interruption cost due to rephasingwork and the labor cost to perform the rephasing must bejustified by the reduction of system power loss.

As shown in Fig. 6, the expert system is designed by per-forming the interview with distribution engineers to identify theheuristic rules currently used for three-phase balancing of dis-tribution systems in Taipower.

After the heuristic rules for three-phase balancing being em-bedded in the knowledge base, a two-phase module of neutralcurrent reduction algorithm is then developed to support the in-ference engine to derive the optimal rephasing strategy by min-imizing the objective function. The module will calculate thereduction of neutral current and the reduction of power loss ofdistribution feeder for each rephasing strategy. According to thepractice of distribution systems in Taipower, the rephasing workcan only be executed once a year for each distribution feederto reduce the customer service interruption for system relia-bility concern. Therefore, the expert system will determine therephasing strategy based on the heuristic rules and system datato achieve the reduction of neutral current and system power

loss over one year period, which will be used to justify the cus-tomer service interruption cost and labor cost for the rephasingstrategy.

A. Heuristic Rules for Rephasing Strategy of Laterals andDistribution Transformers

After discussion with distribution engineers who are in chargeof three-phase balancing in Taipower, the following heuristicrules for rephasing of distribution feeders are determined as fol-lows.

Rule 1)For any distribution feeder with alarm events activated due toover neutral current of more than ten times a month, arephasing strategy has to be derived.

Rule 2)Only the laterals which are connected to the primary trunksections and the distribution transformers are considered as therephasing candidates.

Rule 3)Same phase sequence has to be maintained after rephasing oflaterals and OYD transformers to prevent the possible damageof three-phase motor loads due to reverse operation.

Rule 4)The phase sequence of open-tie switch at the ending point ofthe lateral to be rephased has to be adjusted according to therephasing of the lateral to prevent the phasing inconsistency fornon-interruptible load transfer.

B. Overall Procedure to Solve the Rephasing Strategy

The following steps are executed to solve the optimalrephasing strategy of distribution feeders in this paper.

Step 1)Solve the three-phase currents and neutral current of eachprimary trunk line section and each lateral by three-phase loadflow analysis.

Step 2)Execute the neutral current reduction algorithm (Phase I).

For the distribution feeder with neutral current greater than, the phase I algorithm as shown in Fig. 7 is applied to

solve the rephasing strategy of laterals and distribution trans-formers for neutral current reduction. For the candidate lateralwhich will result in the largest reduction of neutral current with

will be selected for rephasing. The phase currentsand neutral current In of the upstream trunk sections and

the feeder outlet are then updated according to the rephasing ofthe lateral. The process of lateral rephasing is continued untilneutral current of In becomes less than 70 A. Otherwise, therephasing of distribution transformers is executed to achieve fur-ther neutral current reduction.

Step 3)Execute the neutral current reduction algorithm (Phase II).


Fig. 7. Flow chart of neutral current reduction algorithm (Phase I).

After rephasing of laterals and distribution transformers toachieve the reduction of neutral current to be less than the LCOsetting value of , further rephasing of laterals and dis-tribution transformers are considered in Phase II for power lossreduction. The objective function is formulated as (1) by consid-ering the power loss cost over one year, the customer service in-terruption cost and the labor cost to perform the rephasing of lat-erals and distribution transformers. The priority list of candidatelaterals and distribution transformers is then built according tothe reduction of feeder neutral current after rephasing. The lat-eral or the distribution transformer with high priority and com-plies with the heuristic rules will be selected for rephasing

Subject to (1)

where

rephasing strategy;

TLC total system power loss cost over one year byapplying rephasing strategy ;

CIC customer service interruption cost;

LC labor cost;

TABLE IIOUTAGE DURATION TIME OF REPHASING WORK

voltage drop at load point ;

maximum voltage drop allowed (5% in Taipower).

1) System Power Loss Cost: After solving the rephasingstrategy by the proposed expert system, the network configu-ration of distribution feeder is then updated. By executing thethree-phase load flow analysis, the hourly system power lossis then obtained for the calculation of power loss cost. In thispaper, total system power loss cost consists of both energy losscost and demand loss cost. Energy loss (kWh) is determined bycalculating the feeder loss over one year based on the hourlyfeeder loading. Demand loss (kW) represents the power loss ofdistribution feeder for the annual peak loading. The total losscost (TLC) can be formulated as follows:

(2)

where

unit annual demand cost ($112.5/kW-year);

unit energy loss cost ($0.07/kWh);

peak power loss of test feeder;

total annual energy loss of test feeder.

2) Customer Service Interruption Cost (CIC): The CIC rep-resents the customer service interruption cost introduced by thepower service outage due to rephasing works of laterals and dis-tribution transformers as expressed in the following:

(3)

where

total number of nodes affected by rephasing workat node ;

total interruption cost of customers at node due torephasing work at node ;

unit interruption cost of node ($/kW);

outage duration time to complete the rephasing workat node ;

total load demand of node .

In Taipower, the outage duration time of rephasing work for alateral, an OYD transformer, and a 1- transformer is illustratedin Table II.

The unit service interruption costs derived in [12] for the res-idential, commercial, and industrial are adopted in this paperto represent in (3). Besides, three different categories ofkey customers with high service priority levels in Table III are


TABLE IIICATEGORIES OF KEY CUSTOMERS

TABLE IVLABOR COST OF REPHASING WORK

considered too. The rephasing scheme which involves key cus-tomers with higher service priority will be issued a higher inter-ruption cost in the objective function [7].

The customer interruption cost at node by including the keycustomers is represented in the following:

(4)

where

Res, Com, Ind,Pri

load percentage of residential,commercial, industrial, and keycustomers at node ;

, , , interruption cost function of residential,commercial, industrial, and keycustomers;

priority level of key customers.

3) Labor Cost: The labor cost to execute the rephasing of lat-erals and distribution transformers is estimated based on the manpower required and the time duration to complete the rephasingwork in Taipower as shown in Table IV.

V. NUMERICAL RESULTS

To demonstrate the effectiveness of the optimal rephasingstrategy proposed by the expert system to enhance three-phasebalance of distribution systems, the test feeder BD31 in Feng-shan District of Taipower, which is illustrated in Fig. 8, has beenselected for computer simulation. It is a 11-kV overhead feederwith total length of 21.8 km to serve the mixture loading ofresidential and commercial customers. It consists of three ser-vice zones (T1,T2,T3), two laterals (L1,L2) with 182 units ofOYD transformers and 80 units of 1- transformers to providepower service to more than 1416 low voltage customers and onehigh voltage customers. Because of the usage of so many 1-and OYD transformers in this feeder, very serious three-phaseunbalance has been introduced as described in Section II. The

rephasing strategy of laterals and distribution transformers hasto be derived so that the neutral current can be reduced to be lessthan the LCO relay setting to prevent the feeder from unbalancetripping.

Based on the actual phase currents and neutral current of testfeeder in Fig. 2, the phase currents of the primary trunk sectionsand laterals have been calculated by executing the three-phaseload flow analysis. The three-phase currents and neutral currentof lateral L1 at peak period of 8 PM have been solved as

, , and as show in Fig. 9.It is found that the current loading of phase C, , is much lessthan those of and . The neutral current In is even largerthan , which illustrates the severity of three-phase unbalancefor the lateral.

A. Rephasing Strategy of Test Feeder

Table V shows the rephasing strategies of laterals and distri-bution transformers solved by executing the two-phase neutralcurrent reduction algorithm. Fig. 10 shows the phase currentsand neutral currents of the test feeder before and after phaseI rephasing, which have been solved by three-phase load flowanalysis based on the feeder loading in Fig. 2. By rephasing oflateral L1 from (A,B,C) to (C,A,B), the neutral current has beenreduced from 95 (A) to 67 (A), which implies that the problemof feeder tripping by LCO protective relay has been solved suc-cessfully. By comparing to the phase currents in Fig. 2 beforephase I rephasing, the hourly magnitudes of phase currenthave been reduced while the magnitudes of phase currents,and , have been increased, which illustrates the effectivenessof neutral current reduction by rephasing of lateral L1.

To achieve the power loss reduction by further enhancing thethree-phase balance of test feeder in Fig. 8, Phase II algorithmfor neutral current reduction has been applied to identify thelaterals and distribution transformers for rephasing. It is foundthat lateral L2 should be selected for rephasing from (A,B,C)to (C,A,B) and the OYD transformer at node N87 should berephasing from (A,*,C) to (*,C,A). By executing the loadflow analysis for the test feeder with new configuration afterrephasing, the hourly three-phase currents and neutral currentat feeder outlet have been solved as shown in Fig. 11. It is foundthat the three-phase unbalance has been improved and the peakvalue of neutral current at 2 PM has been reduced from 67 Ato 36 A.

After solving the rephasing strategy for the test feeder inTable V, Taipower engineers have completed the field worksfor rephasing of laterals L1, L2 and the OYD transformer atlocation of N87. To verify the effectiveness of the rephasingstrategies derived by the proposed expert system to enhancethree-phase balance of distribution systems, the actual hourlyneutral currents of the test feeder have been collected by theSCADA system. Fig. 12 shows the hourly neutral currents oftest feeder before and after executing the proposed rephasingstrategy. It is found that the peak value of neutral current hasbeen reduced from 95 A to 38 A and the average value of dailyneutral current has also been improved from 72 A to 28 A.Therefore the proposed rephasing strategy solved by the expertsystem can therefore reduce the neutral current to enhance thethree-phase balance of distribution feeders.


Fig. 8. One-line diagram of Feeder BD31 in Taipower.

Fig. 9. Hourly phase currents and neutral current of lateral L1.

To investigate the reduction of feeder power loss by the pro-posed rephasing strategy, the three-phase load flow analysis hasbeen executed for the test feeder before and after rephasing oflaterals and distribution transformers. Fig. 13 shows the loss per-centage which is defined as the ratio of feeder power loss withrespect to feeder power loading. By executing the rephasingstrategy for the test feeder, the peak power loss has been reducedfrom 5.3% to 4.2% at 8 PM and the daily power loss has been im-proved from 4% to 2.8% after the enhancement of three-phasebalance by rephasing two laterals and one OYD transformers. Toillustrate the cost benefit of rephasing strategy, the total powerloss cost reduction, the customer service interruption cost and

Fig. 10. Three-phase currents and neutral current of Feeder BD31 after PhaseI rephasing.

TABLE VPROPOSED REPHASING STRATEGIES FOR FEEDER BD31

the labor cost to perform the rephasing work for the test feederhave been solved as shown in Table VI. The total power loss cost


Fig. 11. Three-phase currents and neutral current of Feeder BD31 after PhaseII rephasing.

Fig. 12. Neutral current of test feeder before and after rephasing.

Fig. 13. Power loss percentage of test feeder before and after rephasing.

TABLE VIREDUCTION OF POWER LOSS COST, CUSTOMER SERVICE INTERRUPTION

COST, AND LABOR COST OF THE PROPOSED REPHASING STRATEGY

over one year period has been reduced by $14 000 and the cus-tomer service interruption cost and the labor cost are $6407 and$469, respectively, for the executions of the proposed rephasingstrategy.

TABLE VIINEUTRAL CURRENTS OF FEEDER BD31 AND FEEDER BD32 BEFORE

AND AFTER LOAD TRANSFER FOR SERVICE RESTORATION

B. Enhancement of Load Transfer Capabilityby Rephasing Scheme

Besides the reduction of neutral current, the rephasingstrategy can also enhance the capability of load transfer forservice restoration of distribution systems after fault contin-gency [5], [6], [8]. For the distribution system configurationof test feeder BD31 in Fig. 8, there is an open-tie switch atnode N202 for load transfer between Feeder BD31 and FeederBD32. When a fault occurs at the location of feeder outlet, thefaulted zone is isolated first by opening the circuit breaker ofFeeder BD31. The unfaulted but out of service sections are thenrestored by closing the open tie switch at N202 to completethe load transfer from Feeder BD31 to Feeder BD32. Table VIIshows the neutral currents of both feeders before and after loadtransfer. Without performing the rephasing scheme for FeederBD31, the neutral current of Feeder BD32 after load transferwill be increased from 57 A to 149 A, which will activate theLCO relay to cause service interruption of Feeder BD32. Whenthe rephasing scheme for both feeders are applied to enhancethree-phase balance in advance, the neutral currents of FeederBD31 and BD32 are reduced to be 36 A and 23 A, respectively.To perform the load transfer for the same fault contingency, theneutral current of Feeder BD32 after load transfer for servicerestoration has been reduced to 55 A and the over neutralcurrent problem can be prevented effectively.

VI. CONCLUSIONS

To solve the three-phase unbalance problem for distributionfeeders, the optimal rephasing strategy of laterals and distri-bution transformers has been proposed by applying the expertsystem in this paper. The hourly loading of each distributiontransformer and each high voltage customer has been solved ac-cording to the typical load patterns of customer classes and theenergy consumption of customers served. The attributes of dis-tribution components are retrieved from the database of outagemanagement system in Taipower to determine the feeder net-work topology and to prepare the input data file for computersimulation. By executing the three-phase load flow analysis, thephase currents and neutral currents of all service zones, lateralsand primary trunk sections have been derived.

To derive the optimal rephasing strategy to enhance the three-phase balance of distribution feeders, a two-phase neutral cur-rent reduction algorithm has been developed by including thereductions of neutral current, the feeder power loss cost, thecustomer interruption cost, and the labor cost to perform the


rephasing of laterals and distribution transformers. To demon-strate the effectiveness of the expert system to enhance three-phase balance of distribution systems, a distribution feeder ofTaipower has been selected for computer simulation to derivethe rephasing strategy. By Phase I algorithm, the over neutralcurrent problem has been solved successfully after rephasing ofone lateral proposed. By Phase II algorithm, the neutral currenthas been further reduced by rephasing of another lateral and onedistribution transformer to achieve the system power loss reduc-tion. After executing the rephasing strategy by Taipower engi-neers, the phase currents and neutral currents of the test feederhave been collected by the SCADA system. By comparing to thephase currents and neutral current before rephasing, the three-phase balance has been improved significantly by the proposedrephasing strategy. Besides solving the problem of over neu-tral current, the customer service interruption cost and the laborcost to perform the rephasing work can also be justified by thereduction of system power loss. Moreover, the capability ofload transfer between distribution feeders for service restora-tion after fault contingency has also been enhanced by applyingthe rephasing scheme to improve three-phase balance of distri-bution systems.

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Chia-Hung Lin (S’95-M’98) received the B.S. degree from National TaiwanInstitute of Technology, Taipei, Taiwan, R.O.C., in 1991, the M.S. degree fromUniversity of Pittsburgh, Pittsburgh, PA, in 1993, and the Ph.D. degree in elec-trical engineering from University of Texas at Arlington in 1997.

He is presently a full Professor at National Kaohsiung University of AppliedSciences, Kaohsiung, Taiwan. His area of interest is distribution automation andcomputer applications to power systems.

Chao-Shun Chen (S’81-M’84) received the B.S. degree from National TaiwanUniversity, Taipei, Taiwan, R.O.C., in 1976 and the M.S. and Ph.D. degrees inelectrical engineering from the University of Texas at Arlington in 1981 and1984, respectively.

From 1984 to 1994, he was a Professor in the Electrical Engineering De-partment at National Sun Yat-Sen University, Kaohsiung, Taiwan. From 1989to 1990, he was on sabbatical at Empros Systems International. Since October1994, he has been working as the Deputy Director General of Department ofKaohsiung Mass Rapid Transit. From February 1997 to July 1998, he was withthe National Taiwan University of Science and Technology as a Professor. FromAugust 1998 to January 2008, he was with the National Sun Yat-Sen Univer-sity as a Professor. Since February 2008, he has been with I-Shou University,Ta-Hsu Hsiang, Taiwan, R.O.C., as a full Professor. His majors are computercontrol of power systems, electrical, and mechanical system integration of massrapid transit systems.

Hui-Jen Chuang (S’98-M’02) received the B.S. and M.S. degrees in electricalengineering from National Taiwan University of Science and Technology,Taipei, Taiwan, R.O.C., in 1990 and 1992, respectively, and the Ph.D. degreein electrical engineering from National Sun Yat-Sen University, Kaohsiung,Taiwan, in 2002.

He is presently an Associate Professor at Kao Yuan University, Lu Chu,Taiwan. His research interest is in the area of load flow and power systemanalysis of mass rapid system.

Ming-Yang Huang received the M.S. degree in electrical engineering from Na-tional Cheng Kung University, Tainan, Taiwan, R.O.C., in 1993. He is currentlypursuing the Ph.D. degree in electrical engineering of National Sun Yat-SenUniversity, Kaohsiung, Taiwan.

Chia-Wen Huang received the B.S. degree in electronic engineering from Na-tional Taiwan Ocean University, Keelung, Taiwan, R.O.C., in 1972.

He is a Senior Research Engineer of the Power Research Institute of Taipower,Taipei, Taiwan, and works as the project leader of the Taipower system loadsurvey and development of master plans for demand-side manager and inte-grated resource planning.

Documents

An Expert System for Three-Phase Balancing of Distribution Feeders