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An Expert System for Three-Phase Balancing of 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 derive the rephasing strategy of laterals and distribution transformers to enhance three-phase balancing of distribution systems. The heuristic rules adopted by distribution engineers are incorporated in the knowledge base of the expert system in the problem-solving process. The neutral current reduction algorithm is developed to support the inference engine to derive the rephasing strategy to reduce the neutral current of distribution feeder so that the tripping of over-current relay can be prevented and the customer service interruption cost and labor cost to execute the rephasing strategy can be justified by the power loss reduction obtained. To demonstrate the effectiveness of the proposed expert system to enhance three-phase balance, a practical distribution feeder in Taiwan Power Company (Taipower) is selected for computer simulation. By minimizing the objective function subjected to the rephasing rules, the rephasing strategy has been derived to identify the laterals and distribution transformers for phasing adjustment. After executing the proposed rephasing strategy by Taipower engineers, the phase currents and neutral current of test feeder has been collected by the SCADA system to verify the reduction of neutral current. The power loss reduction obtained by three-phase balancing has been solved by three-phase load flow analysis, which is then used to justify the customer service interruption cost and labor cost for rephasing of test feeder. Index Terms—Customer information system, expert system, outage management system.
I. INTRODUCTION
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ITH the dramatic growth in the number and size of single-phase residential and commercial customers served in the Taipower distribution system, the unbalance in the phase currents leads to excessive neutral currents that may cause tripping of distribution feeders. Besides, the execution of non-interruptible load transfer between feeders for scheduled outage and service restoration after fault contingency may introduce further tripping of supporting feeders due to the increase 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 Kaohsiung 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 University, Ta-Hsu Hsiang, Kaohsiung County, Taiwan, R.O.C. H. J. Chuang is with the Department of Electrical Engineering, Kao Yuan University, Lu Chu, Taiwan, R.O.C. M. Y. Huang is with the Department of Electrical Engineering, National Sun Yat-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 online at 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 engineers to improve three-phase balance of distribution feeders by the conventional practice. Taipower has performed the phase swapping of distribution laterals and transformers to improve phase loading balance by trial and error methods. To serve the low voltage residential and commercial customers as well as the high voltage commercial and industrial customers, many 1- distribution transformers have been used in Taipower distribution system. Due to the variation of customer load behaviors, it is difficult to maintain the three-phase balance of distribution feeders for all time periods. Furthermore, the open-wye, open-delta (OYD) transformers with connectivity as shown in Fig. 1 have been widely applied to serve both 1- and 3- loads simultaneously. It is noted that, only two units of 1- transformers with different capacities are used and supplied by two phases of the high voltage primary feeder. The transformer with larger capacity serves both 1- and 3- loads while the smaller one serves 3- loads only. Although one unit of 1- transformer has been saved, the three-phase unbalance will be deteriorated due to the unsymmetrical configuration of OYD transformer. According to the operation and computer simulation of distribution feeders in Taipower, the severe three-phase unbalance of distribution system has caused the increase of power loss, induced communication interference, and customer service interruption due to unexpected tripping of low energy over current relay (LCO) [1]. To solve the three-phase unbalance of distribution system, the authors in [2] have formulated the phase balancing problem as a linear objective function with mixed-integer programming. However, the decision-making criteria for rephasing strategies of distribution systems may not be represented as a linear function very well. For instance, the system loss minimization problem is itself a nonlinear integer objective function and will be very difficult to be solved analytically. The simulated annealing (SA) method has been applied to solve the phase swapping problem in [3] as a large-scale nonlinear integer programming 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
algorithm to optimize the phase arrangement of distribution transformers connected to the primary feeder. However, these approaches do not incorporate the heuristic rules of system planning in the problem formulation. Besides, the labor cost and the customer service interruption cost caused by the scheduled power outages for the execution of rephasing strategy have not been considered according to the number of customers affected, the amount of loading disconnected and the outage duration time. To justify the cost effectiveness of rephasing strategies, the objective function of rephasing strategy for distribution systems should be derived in a more practical way by including all of the related cost and the reduction of neutral current and system power loss. In this paper, a knowledge-based expert system [5], [6] is developed to derive the rephasing strategies for distribution feeders to enhance the three-phase balancing by reducing the magnitude of neutral current. It is designed by including the heuristic rules used by Taipower engineers to execute the feeder rephasing work. With the heuristic rules imbedded in the knowledge base of expert system, an inference engine is established to derive an optimal rephasing strategy of laterals and distribution transformers using the system data and heuristic rules as the basis of reasoning. To achieve the reduction of neutral current and distribution system power loss, a two-phase rephasing algorithm is also developed to support the inference engine. The objective of phase I algorithm is to derive the rephasing strategy of laterals and distribution transformers so that the neutral current of whole feeder can be reduced to of LCO protective relay, be less than the setting limit which is 70 A in Taipower. After that, the phase II algorithm is applied to solve the rephasing strategy for further enhancement of three-phase balance to achieve the minimization of objective function, which is formulated by considering the total system power loss cost, the customer service interruption cost and the labor cost required for the execution of rephasing strategy. By this way, the related cost involved in the execution of feeder rephasing can therefore be justified by the reductions of neutral current 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 distribution feeders in Taipower have been collected by the Supervisory Control and Data Acquisition System (SCADA) of Distribution Dispatch Control Center (DDCC). Figs. 2 and 3 show the daily current profiles of test feeder BD31 for the summer season and winter season, respectively, in 2006. Feeder BD31 serves the loads of 1329 residential customers and 87 commercial customers as well as one high voltage customers in urban area of FengShan. There are 182 OYD transformers and 80 1- transformers applied in this feeder. From Fig. 2, it is found that severe three-phase unbalance has been introduced because of the dramatic difference among three-phase currents. With phase curand , the neutral current of In berent much less than comes larger than the LCO setting of 70 A during the time periods of 11 AM to 1 AM and it reaches the peak value of 95 A at 2 PM for the summer season. During the winter season, the
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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 time periods of 12 PM to 2 PM and 6 PM to 10 PM and it reaches the peak value of 85 A at 7 PM. The hourly neutral current, which is formulated as the phasor summation of three-phase currents, is even larger than the phase current . The service reliability and operation efficiency of test feeder BD31 have been deteriorated because of LCO relay tripping and increase of power loss due to three-phase unbalance. III. PROCESS OF REPHASING STRATEGY FOR DISTRIBUTION FEEDERS To derive the rephasing strategy of distribution transformers and laterals to improve three-phase balance of distribution feeders, the phase currents and neutral current of all primary trunk line sections, laterals and transformers have been simulated by three-phase load flow analysis according to the feeder phase currents and neutral current collected by SCADA system. The attributes of distribution components such as line segments, distribution transformers, etc., have been retrieved from the facility database of outage management system (OMS) in Taipower [7], [8]. The network configuration of distribution feeder is then identified after performing the topology process according to the connectivity attributes of distribution components. To represent the load behavior at each bus more accurately for load flow analysis, the daily load patterns of customer classes, which have been derived by load survey study [9]–[11], and the monthly energy consumption of customers in the database of customer information system (CIS) are used to solve the power demand of each customer. The hourly loading
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Fig. 5. Typical load patterns of residential, commercial, and industrial customers.
C. Phase Loading Evaluations of Load Buses Fig. 4. Flowchart of rephasing strategy of distribution feeders.
of each distribution transformer is then obtained by integrating the power profiles of all customers served. The three-phase currents and neutral current of each primary trunk line section and each lateral are therefore derived by considering the mutual effect between phase conductors in the three-phase load flow analysis. The objective function for rephasing strategy of distribution transformers and laterals is then formulated by including the number of customers affected, the total load demand interruption and the time duration to complete the rephasing works by distribution engineers. Fig. 4 shows the overall process to derive the optimal rephasing strategy to enhance 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 load flow analysis by using the conventional paper maps and facility data files. To support load flow analysis for phase balancing study more effectively, the facility database of OMS system in Taipower is applied. The OMS database provides the capability to integrate the graphic representation of components with spatial relationship and information management. B. Topology Process of Distribution Network After retrieving the attributes of distribution components such as line sections, distribution transformers, etc., the topology process is executed to identify the network configuration of distribution feeders based on the attributes of network connectivity model and the dynamic switch statuses in the OMS database. By tracing the FROM and TO fields of connectivity table of each component, which points to its upstream device and downstream device, respectively, the system network is determined and updated according to the operation of line switches.
To execute the three-phase load flow analysis for a distribution feeder, the loading of each load bus has to be evaluated. With the stochastic variation of customer load characteristics and load composition, the phase loadings of each distribution transformer and each high voltage customer will be difficult to solve. In this paper, the typical daily load patterns of various customer classes, which have been derived by the load survey study, are used to represent the load behaviors of all customers served by the distribution feeder. Fig. 5 shows the typical load patterns of residential, commercial and industrial customers in Taipower, which have been derived by statistic analysis of actual hourly power consumption of customers which have been selected by stratified sampling method for load survey study. After identify the customers served by each distribution transformer by executing the customer-to-transformer mapping process, the monthly energy consumption of each customer served by the transformer is then retrieved from the Customer Information System (CIS) database. With the customer load patterns and monthly energy consumption, the power loading of each transformer and high voltage customer is evaluated to represent the phase loading of each load bus.
D. Phasing Arrangement For the study of rephasing strategy to enhance three-phase balance of distribution feeders, the notation (X, Y, Z) is used in this paper to represent the phasing arrangement of laterals and distribution transformers. The possible connection schemes for various types of phasing arrangement are listed in Table I for the 3- laterals and transformers as well as 1- and OYD transformers. It is important that same phase sequence (positive or negative) has to be maintained in the derivation of rephasing strategy for laterals and distribution transformers to prevent customer damage due to reverse operation of three-phase rotating loads after rephasing. For instance, an OYD transformer with 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 A phases (B,*,A) to ensure same phase sequence for 3- motor loads connected at the secondary side of the transformer. By
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TABLE I VALID REPHASING SCHEMES FOR LATERALS AND TRANSFORMERS
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loss over one year period, which will be used to justify the customer service interruption cost and labor cost for the rephasing strategy. A. Heuristic Rules for Rephasing Strategy of Laterals and Distribution Transformers After discussion with distribution engineers who are in charge of three-phase balancing in Taipower, the following heuristic rules for rephasing of distribution feeders are determined as follows. Rule 1) For any distribution feeder with alarm events activated due to more than ten times a month, a over neutral current of rephasing strategy has to be derived. Rule 2) Only the laterals which are connected to the primary trunk sections and the distribution transformers are considered as the rephasing candidates.
Fig. 6. Expert system structure for rephasing strategy of distribution systems.
the same way, a 3- lateral with original phasing (A,B,C) can be rephasing either as (C,A,B) or (B,C,A) only. IV. EXPERT SYSTEM FOR REPHASING A proper rephasing strategy of laterals and distribution transformers to enhance three-phase balancing of distribution feeders has to comply with the following constraints. 1) The heuristic rules of system planning and operation must be complied. 2) The neutral current of distribution feeder has to be less than LCO relay setting after rephasing. 3) The customer service interruption cost due to rephasing work and the labor cost to perform the rephasing must be justified by the reduction of system power loss. As shown in Fig. 6, the expert system is designed by performing the interview with distribution engineers to identify the heuristic rules currently used for three-phase balancing of distribution systems in Taipower. After the heuristic rules for three-phase balancing being embedded in the knowledge base, a two-phase module of neutral current reduction algorithm is then developed to support the inference engine to derive the optimal rephasing strategy by minimizing the objective function. The module will calculate the reduction of neutral current and the reduction of power loss of distribution feeder for each rephasing strategy. According to the practice of distribution systems in Taipower, the rephasing work can only be executed once a year for each distribution feeder to reduce the customer service interruption for system reliability concern. Therefore, the expert system will determine the rephasing strategy based on the heuristic rules and system data to achieve the reduction of neutral current and system power
Rule 3) Same phase sequence has to be maintained after rephasing of laterals and OYD transformers to prevent the possible damage of three-phase motor loads due to reverse operation. Rule 4) The phase sequence of open-tie switch at the ending point of the lateral to be rephased has to be adjusted according to the rephasing of the lateral to prevent the phasing inconsistency for non-interruptible load transfer. B. Overall Procedure to Solve the Rephasing Strategy The following steps are executed to solve the optimal rephasing strategy of distribution feeders in this paper. Step 1) Solve the three-phase currents and neutral current of each primary trunk line section and each lateral by three-phase load flow 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 transformers for neutral current reduction. For the candidate lateral which will result in the largest reduction of neutral current with will be selected for rephasing. The phase currents and neutral current In of the upstream trunk sections and the feeder outlet are then updated according to the rephasing of the lateral. The process of lateral rephasing is continued until neutral current of In becomes less than 70 A. Otherwise, the rephasing of distribution transformers is executed to achieve further neutral current reduction. Step 3) Execute the neutral current reduction algorithm (Phase II).
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TABLE II OUTAGE 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 rephasing strategy by the proposed expert system, the network configuration of distribution feeder is then updated. By executing the three-phase load flow analysis, the hourly system power loss is then obtained for the calculation of power loss cost. In this paper, total system power loss cost consists of both energy loss cost and demand loss cost. Energy loss (kWh) is determined by calculating the feeder loss over one year based on the hourly feeder loading. Demand loss (kW) represents the power loss of distribution feeder for the annual peak loading. The total loss cost (TLC) can be formulated as follows: (2) where unit annual demand cost ($112.5/kW-year); unit energy loss cost ($0.07/kWh); peak power loss of test feeder; total annual energy loss of test feeder.
Fig. 7. Flow chart of neutral current reduction algorithm (Phase I).
After rephasing of laterals and distribution transformers to achieve the reduction of neutral current to be less than the LCO , further rephasing of laterals and dissetting value of tribution transformers are considered in Phase II for power loss reduction. The objective function is formulated as (1) by considering the power loss cost over one year, the customer service interruption cost and the labor cost to perform the rephasing of laterals and distribution transformers. The priority list of candidate laterals and distribution transformers is then built according to the reduction of feeder neutral current after rephasing. The lateral or the distribution transformer with high priority and complies with the heuristic rules will be selected for rephasing
2) Customer Service Interruption Cost (CIC): The CIC represents the customer service interruption cost introduced by the power service outage due to rephasing works of laterals and distribution transformers as expressed in the following:
(3) where total number of nodes affected by rephasing work at node ; total interruption cost of customers at node due to rephasing work at node ; unit interruption cost of node
Subject to
(1)
($/kW);
outage duration time to complete the rephasing work at node ; total load demand of node .
where rephasing strategy; TLC
total system power loss cost over one year by applying rephasing strategy ;
CIC
customer service interruption cost;
LC
labor cost;
In Taipower, the outage duration time of rephasing work for a lateral, an OYD transformer, and a 1- transformer is illustrated in Table II. The unit service interruption costs derived in [12] for the residential, commercial, and industrial are adopted in this paper in (3). Besides, three different categories of to represent key customers with high service priority levels in Table III are
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rephasing strategy of laterals and distribution transformers has to be derived so that the neutral current can be reduced to be less than the LCO relay setting to prevent the feeder from unbalance tripping. Based on the actual phase currents and neutral current of test feeder in Fig. 2, the phase currents of the primary trunk sections and laterals have been calculated by executing the three-phase load flow analysis. The three-phase currents and neutral current of 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 less than those of and . The neutral current In is even larger than , which illustrates the severity of three-phase unbalance for the lateral.
TABLE III CATEGORIES OF KEY CUSTOMERS
TABLE IV LABOR COST OF REPHASING WORK
A. Rephasing Strategy of Test Feeder considered too. The rephasing scheme which involves key customers with higher service priority will be issued a higher interruption cost in the objective function [7]. The customer interruption cost at node by including the key customers is represented in the following:
(4) where Res, Com, Ind, Pri ,
,
,
load percentage of residential, commercial, industrial, and key customers at node ; interruption cost function of residential, commercial, industrial, and key customers; priority level of key customers.
3) Labor Cost: The labor cost to execute the rephasing of laterals and distribution transformers is estimated based on the man power required and the time duration to complete the rephasing work in Taipower as shown in Table IV. V. NUMERICAL RESULTS To demonstrate the effectiveness of the optimal rephasing strategy proposed by the expert system to enhance three-phase balance of distribution systems, the test feeder BD31 in Fengshan District of Taipower, which is illustrated in Fig. 8, has been selected for computer simulation. It is a 11-kV overhead feeder with total length of 21.8 km to serve the mixture loading of residential and commercial customers. It consists of three service zones (T1,T2,T3), two laterals (L1,L2) with 182 units of OYD transformers and 80 units of 1- transformers to provide power service to more than 1416 low voltage customers and one high voltage customers. Because of the usage of so many 1and OYD transformers in this feeder, very serious three-phase unbalance has been introduced as described in Section II. The
Table V shows the rephasing strategies of laterals and distribution transformers solved by executing the two-phase neutral current reduction algorithm. Fig. 10 shows the phase currents and neutral currents of the test feeder before and after phase I rephasing, which have been solved by three-phase load flow analysis based on the feeder loading in Fig. 2. By rephasing of lateral L1 from (A,B,C) to (C,A,B), the neutral current has been reduced from 95 (A) to 67 (A), which implies that the problem of feeder tripping by LCO protective relay has been solved successfully. By comparing to the phase currents in Fig. 2 before phase I rephasing, the hourly magnitudes of phase current have been reduced while the magnitudes of phase currents, and , have been increased, which illustrates the effectiveness of neutral current reduction by rephasing of lateral L1. To achieve the power loss reduction by further enhancing the three-phase balance of test feeder in Fig. 8, Phase II algorithm for neutral current reduction has been applied to identify the laterals and distribution transformers for rephasing. It is found that lateral L2 should be selected for rephasing from (A,B,C) to (C,A,B) and the OYD transformer at node N87 should be rephasing from (A,*,C) to (*,C,A). By executing the load flow analysis for the test feeder with new configuration after rephasing, the hourly three-phase currents and neutral current at feeder outlet have been solved as shown in Fig. 11. It is found that the three-phase unbalance has been improved and the peak value of neutral current at 2 PM has been reduced from 67 A to 36 A. After solving the rephasing strategy for the test feeder in Table V, Taipower engineers have completed the field works for rephasing of laterals L1, L2 and the OYD transformer at location of N87. To verify the effectiveness of the rephasing strategies derived by the proposed expert system to enhance three-phase balance of distribution systems, the actual hourly neutral currents of the test feeder have been collected by the SCADA system. Fig. 12 shows the hourly neutral currents of test feeder before and after executing the proposed rephasing strategy. It is found that the peak value of neutral current has been reduced from 95 A to 38 A and the average value of daily neutral current has also been improved from 72 A to 28 A. Therefore the proposed rephasing strategy solved by the expert system can therefore reduce the neutral current to enhance the three-phase balance of distribution feeders.
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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 proposed rephasing strategy, the three-phase load flow analysis has been executed for the test feeder before and after rephasing of laterals and distribution transformers. Fig. 13 shows the loss percentage which is defined as the ratio of feeder power loss with respect to feeder power loading. By executing the rephasing strategy for the test feeder, the peak power loss has been reduced from 5.3% to 4.2% at 8 PM and the daily power loss has been improved from 4% to 2.8% after the enhancement of three-phase balance by rephasing two laterals and one OYD transformers. To illustrate the cost benefit of rephasing strategy, the total power loss cost reduction, the customer service interruption cost and
Fig. 10. Three-phase currents and neutral current of Feeder BD31 after Phase I rephasing.
TABLE V PROPOSED REPHASING STRATEGIES FOR FEEDER BD31
the labor cost to perform the rephasing work for the test feeder have been solved as shown in Table VI. The total power loss cost
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TABLE VII NEUTRAL CURRENTS OF FEEDER BD31 AND FEEDER BD32 BEFORE AND AFTER LOAD TRANSFER FOR SERVICE RESTORATION
Fig. 11. Three-phase currents and neutral current of Feeder BD31 after Phase II rephasing.
Fig. 12. Neutral current of test feeder before and after rephasing.
B. Enhancement of Load Transfer Capability by Rephasing Scheme Besides the reduction of neutral current, the rephasing strategy can also enhance the capability of load transfer for service restoration of distribution systems after fault contingency [5], [6], [8]. For the distribution system configuration of test feeder BD31 in Fig. 8, there is an open-tie switch at node N202 for load transfer between Feeder BD31 and Feeder BD32. When a fault occurs at the location of feeder outlet, the faulted zone is isolated first by opening the circuit breaker of Feeder BD31. The unfaulted but out of service sections are then restored by closing the open tie switch at N202 to complete the load transfer from Feeder BD31 to Feeder BD32. Table VII shows the neutral currents of both feeders before and after load transfer. Without performing the rephasing scheme for Feeder BD31, the neutral current of Feeder BD32 after load transfer will be increased from 57 A to 149 A, which will activate the LCO relay to cause service interruption of Feeder BD32. When the rephasing scheme for both feeders are applied to enhance three-phase balance in advance, the neutral currents of Feeder BD31 and BD32 are reduced to be 36 A and 23 A, respectively. To perform the load transfer for the same fault contingency, the neutral current of Feeder BD32 after load transfer for service restoration has been reduced to 55 A and the over neutral current problem can be prevented effectively. VI. CONCLUSIONS
Fig. 13. Power loss percentage of test feeder before and after rephasing.
TABLE VI REDUCTION 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 customer service interruption cost and the labor cost are $6407 and $469, respectively, for the executions of the proposed rephasing strategy.
To solve the three-phase unbalance problem for distribution feeders, the optimal rephasing strategy of laterals and distribution transformers has been proposed by applying the expert system in this paper. The hourly loading of each distribution transformer and each high voltage customer has been solved according to the typical load patterns of customer classes and the energy consumption of customers served. The attributes of distribution components are retrieved from the database of outage management system in Taipower to determine the feeder network topology and to prepare the input data file for computer simulation. By executing the three-phase load flow analysis, the phase currents and neutral currents of all service zones, laterals and primary trunk sections have been derived. To derive the optimal rephasing strategy to enhance the threephase balance of distribution feeders, a two-phase neutral current reduction algorithm has been developed by including the reductions of neutral current, the feeder power loss cost, the customer interruption cost, and the labor cost to perform the
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rephasing of laterals and distribution transformers. To demonstrate the effectiveness of the expert system to enhance threephase balance of distribution systems, a distribution feeder of Taipower has been selected for computer simulation to derive the rephasing strategy. By Phase I algorithm, the over neutral current problem has been solved successfully after rephasing of one lateral proposed. By Phase II algorithm, the neutral current has been further reduced by rephasing of another lateral and one distribution transformer to achieve the system power loss reduction. After executing the rephasing strategy by Taipower engineers, the phase currents and neutral currents of the test feeder have been collected by the SCADA system. By comparing to the phase currents and neutral current before rephasing, the threephase balance has been improved significantly by the proposed rephasing strategy. Besides solving the problem of over neutral current, the customer service interruption cost and the labor cost to perform the rephasing work can also be justified by the reduction of system power loss. Moreover, the capability of load transfer between distribution feeders for service restoration after fault contingency has also been enhanced by applying the rephasing scheme to improve three-phase balance of distribution systems. REFERENCES [1] C. S. Chen, C. Y. Chang, and S. Y. Jan, “Effect of open-wye open-delta transformers on the operation of distribution systems,” Elect. Power Syst. Res., vol. 10, no. 3, pp. 167–174, 1986. [2] J. Zhu, M. Y. Chow, and F. Zhang, “Phase balancing using mixedinteger programming,” IEEE Trans. Power Syst., vol. 13, no. 4, pp. 1487–1492, Nov. 1998. [3] J. Zhu, G. Bilbro, and M. Y. Chow, “Phase balancing using simulated annealing,” IEEE Trans. Power Syst., vol. 14, no. 4, pp. 1508–1513, Nov. 1999. [4] T. H. Chen and J. T. Cherng, “Optimal phase arrangement of distribution transformers connected a primary feeder for system unbalance improvement and loss reduction using a genetic algorithm,” IEEE Trans. Power Syst., vol. 15, no. 3, pp. 994–1000, Aug. 2000. [5] C. C. Liu, S. J. Lee, and S. S. Venkata, “An expert system operation aid for restoration and loss reduction of distribution system,” IEEE Trans. Power Syst., vol. 3, no. 2, pp. 619–626, May 1988. [6] J. S. Wu, K. L. Tomsovic, and C. S. Chen, “A heuristic search approach to feeder switching operations for overload, faults, unbalanced flow and maintenance,” IEEE Trans. Power Del., vol. 6, no. 4, pp. 1579–1586, Oct. 1991. [7] C. S. Chen, C. H. Lin, H. J. Chuang, C. S. Li, M. Y. Huang, and C. W. Huang, “Optimal placement of line switches for distribution automation systems using immune algorithm,” IEEE Trans. Power Syst., vol. 21, no. 3, pp. 1209–1217, Aug. 2006. [8] G. L. Ockwell, “Implementation of network reconfiguration for Taiwan power company,” in Proc. 2003 IEEE Power Eng. Soc. General Meeting, Toronto, ON, Canada, Jul. 2003. [9] J. C. Hwang, “Assessment of air condition load management by load survey in Taipower,” IEEE Trans. Power Syst., vol. 16, no. 4, pp. 910–915, Nov. 2001.
[10] C. S. Chen, J. C. Hwang, and C. W. Huang, “Application of load survey systems to proper tariff design,” IEEE Trans. Power Syst., vol. 12, no. 4, pp. 1746–1751, Nov. 1997. [11] Load Research Manual, Association of Edison Illumination Companies, 1990. [12] G. Toefson, R. Billinton, G. Wacker, E. Chan, and J. Aweya, “A Canadian customer survey to assess power system reliability worth,” IEEE Trans. Power Syst., vol. 9, no. 1, pp. 443–450, Feb. 1994. Chia-Hung Lin (S’95-M’98) received the B.S. degree from National Taiwan Institute of Technology, Taipei, Taiwan, R.O.C., in 1991, the M.S. degree from University of Pittsburgh, Pittsburgh, PA, in 1993, and the Ph.D. degree in electrical engineering from University of Texas at Arlington in 1997. He is presently a full Professor at National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan. His area of interest is distribution automation and computer applications to power systems.
Chao-Shun Chen (S’81-M’84) received the B.S. degree from National Taiwan University, Taipei, Taiwan, R.O.C., in 1976 and the M.S. and Ph.D. degrees in electrical engineering from the University of Texas at Arlington in 1981 and 1984, respectively. From 1984 to 1994, he was a Professor in the Electrical Engineering Department at National Sun Yat-Sen University, Kaohsiung, Taiwan. From 1989 to 1990, he was on sabbatical at Empros Systems International. Since October 1994, he has been working as the Deputy Director General of Department of Kaohsiung Mass Rapid Transit. From February 1997 to July 1998, he was with the National Taiwan University of Science and Technology as a Professor. From August 1998 to January 2008, he was with the National Sun Yat-Sen University 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 computer control of power systems, electrical, and mechanical system integration of mass rapid transit systems.
Hui-Jen Chuang (S’98-M’02) received the B.S. and M.S. degrees in electrical engineering from National Taiwan University of Science and Technology, Taipei, Taiwan, R.O.C., in 1990 and 1992, respectively, and the Ph.D. degree in 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 system analysis of mass rapid system.
Ming-Yang Huang received the M.S. degree in electrical engineering from National Cheng Kung University, Tainan, Taiwan, R.O.C., in 1993. He is currently pursuing the Ph.D. degree in electrical engineering of National Sun Yat-Sen University, Kaohsiung, Taiwan.
Chia-Wen Huang received the B.S. degree in electronic engineering from National 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 load survey and development of master plans for demand-side manager and integrated resource planning.
