AbstractâA multiagent-based Distribution Automation. System (DAS) is developed for service restoration of distribution systems after a fault contingency.
Fault Detection, Isolation and Restoration using a Multiagent-based Distribution Automation System (1)
Chia-Hung Lin1, Hui-Jen Chuang2, Chao-Shun Chen3, Chung-Sheng Li3, and Chin-Ying Ho2
(2) Department of Electrical Engineering Department of Electrical Engineering National Kaohsiung University of Applied Sciences Kao Yuan University Kaohsiung 807, Taiwan Lu Chu 821, Taiwan
Department of Electrical Engineering National Sun Yat-Sen University Kaohsiung 804, Taiwan
the FTUs along the feeder. The fault flags are then reported to the MS from the RTUs of substations to determine the faulted line section according to the combination of fault flags and network topology .
Abstract—A multiagent-based Distribution Automation System (DAS) is developed for service restoration of distribution systems after a fault contingency. In this system, Remote Terminal Unit (RTU) agents, Main Transformer (MTR) agents, Feeder Circuit Breaker (FCB) agents, and Feeder Terminal Unit (FTU) agents of the Multiagent System (MAS) are used to derive the proper restoration plan after the faulted location is identified and isolated. To assure the restoration plan complies with operation regulation, heuristic rules based on standard operation procedures of Taipower’s distribution system are included in the best first search of the MAS. For fault contingency during summer peak season when the capacity reserves of supporting feeders and main transformer are not enough to cover the fault restoration, load shedding scheme is derived for the MAS to restore service to as many key customers and loads as possible. A Taipower distribution system with 43 feeders is selected for computer simulation to demonstrate the effectiveness of the proposed methodology. The results show that by applying the proposed multiagent-based DAS, service of distribution systems can be efficiently restored.
Fig. 1. The distribution automation system of Taipower.
Index Terms—Multiagent system, Distribution automation system, Fault Detection, Isolation and Restoration (FDIR)
When a fault is detected, the FDIR immediately opens the nearest boundary line switches to isolate the faulted feeder section from both directions. The upstream out-of-service sections are then restored by closing the circuit breaker of the distribution feeder. To restore the electricity service for the downstream unfaulted sections, a restoration strategy is derived by maximizing the area of service restoration with the minimum number of switching operations. When the faulted feeder section is repaired, FDIR can be activated to provide the reverse switching sequence to return the distribution system back to the pre-fault configuration. FDIR is designed to be able to handle multiple faults occurring simultaneously or within a time window. The number of switching operations can be reduced by preventing further reconfiguration within several hours after fault occurrence by considering the load estimation of all line sections for the future hours. Due to the complexity and expansion of distribution systems, conventional centralized and regulated control systems tend to be inadequate due to their deficiencies in robustness, openness, and flexibility. Furthermore, the centralized control systems are highly sensitive to system failures because these systems rely on single decision-making software components or human operators to handle a very large amount of data processing on a powerful central computing facility with high communication capabilities. Multiagent systems are used in a wide range of
I. INTRODUCTION Power quality has, in recent years, become more and more of a critical concern for utility customers. Power companies have to improve service in order to retain customers in the deregulated market. For this reason, distribution automation systems have been implemented at Taipower as an intelligent way to enhance the reliability and operation efficiency of distribution systems. Among all functions achieved by DAS, Fault Detection, Isolation and Restoration (FDIR) is considered to be most important. The objective of FDIR is to reduce service restoration time from an average of 58 minutes to less than 5 minutes for a permanent fault contingency of distribution feeders. To achieve this, a fully integrated DAS in Fig. 1 is designed to include a Master Station (MS) with application software, Remote Terminal Units (RTUs) in the substations, Feeder Terminal Units (FTUs), and automatic line switches along primary feeders . With the progress of monitoring and control functions of the DAS, real-time fault identification and isolation becomes possible. When a permanent fault occurs in the distribution system, FDIR is triggered by an automatic feeder breaker trip in real-time operation, and starts to detect the fault location based on the fault flags generated by the over-current relays of
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applications requiring flexibility and adaptability with a rapidly changing environment due to their distributed nature, modularity, and ease of implementation. Agent-based computation has been studied for several years in the field of distributed artificial intelligence – and applied to power systems such as a strategic power infrastructure defense system , protection systems , , energy-management systems , power system restoration , , power markets , a transformer condition monitoring system , and distribution system restoration , . Nagata et al.  presented a two-level hierarchical MAS architecture for distribution system service restoration. Reference  presented a MAS with three types of agents using Java Agent Development Framework (JADE) as a multiagent framework to exchange information and determine a feasible restoration strategy. This paper describes an application of MAS technologies to implement a FDIR function based on a DAS structure to derive the restoration plan for a typical radial distribution system in Taipower after a fault occurs. The communication among agents is implemented in JADE, which is compliant with the Foundation for Intelligent, Physical Agents (FIPA) . The MAS architecture supports direct or indirect interactions among decision making components of the FDIR. After a permanent fault occurs, knowledge of restoring the outof-service area is obtained from the interaction of the operation rules used by experienced dispatchers and the system database. With this knowledge base, the MAS uses its distributed characteristics to simultaneously and efficiently find various proper switching operations to restore distribution system service. To provide better service quality to critical loads in the system, the proposed MAS is also designed to give priorities to the service restoration of hospitals, police stations, etc., in case of system capacity reserve shortage. To estimate the loadings of service zones along each feeder and the reserve capacity of the supporting main transformers more accurately, typical load patterns of residential, commercial, and industry customers are included in the knowledge base. The customers within each service zone are identified by the Outage Management System (OMS) and the power consumption of the customers is retrieved from the Customer Information System (CIS) in Taipower. By integrating the typical load patterns and power consumption of customers, the hourly power demand of each service zone is determined. The following sections will describe restoration problem formulation, multiagent architecture for FDIR, as well as to demonstrate the effectiveness of the proposed methodology with a Taipower distribution system with 43 feeders. The results show that by applying the proposed multiagent-based DAS, service of distribution systems can be restored more efficiently as compared to the conventional centralized DAS system.
1) To restore service to as many customers as possible within unfaulted but out-of-service area. 2) To first restore service to key customers with higher service priorities. 3) To restore service as quickly as possible by minimizing the number of switching operations. 4) To maintain the open loop system configuration after service restoration for better protective coordination of distribution systems. 5) To not overload any components in the distribution system. In this study, Taipower’s operation rules are used to search for the proper restoration plan, after the fault location is identified and isolated by opening the boundary line switches. In order to determine the service restoration plan, the proposed switching operation strategy of this paper will give preference to remotely operated automatic line switching devices. The operation rules are derived according to the rule of thumb of the experienced operators to comply with the operation regulation for the service restoration of fault contingences.
II. RESTORATION PROBLEM FORMULATION
Rule3: The load of the entire interrupted feeder is transferred to the supporting feeder with the largest capacity reserve. Rule4: If the supporting feeder does not have enough capacity reserve to supply the demand of all out-of-service areas, load is transferred to additional supporting
A. Operation Rules for Feeder Fault Contingencies For a short circuit fault on the feeder, all boundary line switches are opened to isolate the faulted zones. The feeder circuit breaker is then closed to restore service to upstream customers if the fault is not on the feeder outlet. For downstream restoration, the operation rules to perform switching operation for service restoration have to comply to the following rules: (1) Restoration for feeder fault: Rule1: An out-of-service area can only be restored by another supporting feeder by closing the normally open tie switch between the two feeders. The supporting feeder and main transformer should not be overloaded after taking over the load demand of the zone with interrupted service. Rule2: If the main transformer which serves the supporting feeders does not have enough capacity reserve for the zones with interrupted service, load has to be transferred from the supporting main transformer to neighboring transformers in order to increase the spare capacity of the supporting main transformer. If overload problem still occurs after load transfer, load is then shedded according to the service priority of customers before service restoration. (2) Fault on the feeder outlet:
The purpose of the DAS system described in this paper is to develop a service restoration plan for fault contingencies to assist distribution system dispatchers with restoring power service of outaged customers. The objectives of service restoration in Taipower are:
feeders. Rule5: If the main transformer which supplies the supporting feeder does not have enough capacity reserve for the zones of interrupted service, the same procedures as Rule 2 are considered for service restoration. For service restoration of downstream unfaulted but out of sections, the available capacity reserve of the potential supporting feeders and main transformers are evaluated before the operation rules are applied to find the restoration plan. In doing so, load is transferred based on current flows of line switches and feeder loading, which are collected by FTUs of the SCADA system. With the execution of the restoration plan, the open-tie switch that connects the unfaulted but out-of-service zone and the energized zone is regarded as a switch candidate. If there are several switch candidates, the one with the largest capacity reserve and within the same substation service territory will be selected. To simplify the load transfer and minimize the affected area during reconfiguration, the supporting feeder served by the same substation is selected for service restoration.
will be identified by message 1-7. The attributes of each agent for Feeder 1 are identified and illustrated in Table I.
Fig. 2. Multiagent system for fault detection, isolation and restoration.
III. MULTIAGENT ARCHITECTURE FOR FDIR In this paper, we created an agent model that has the same FDIR functionality as the existing Taipower DAS. This new agent-based system would serve as the basis for enhancements in the FDIR simulation and decision support system. The multiagent architecture that is designed to find the proper switching operation for FDIR is introduced. In order to test the proposed FDIR model, communication simulation is carried out based on the JADE platform. Using the JADE platform ensures compliance to the FIPA standards while providing agent communication and a graphical user interface for monitoring agents. As shown in Fig. 2, the entire system consists of two layers-One layer is the MAS with the agents, and the other is the DAS. There are four types of agents in the proposed multiagent architecture: FTU Agents (FTUAs), FCB Agents (FCBAs), MTR Agents (MTRAs), and RTU Agents (RTUAs). The total number of agents corresponds to the total number of FTUs, FCBs, MTRs, and RTUs of the DAS. Each MAS agent can exchange information with one major electrical component such as the FTU, FCB, MTR, or RTU. Corresponding agents defined as neighboring agents include: the two electrical components in the DAS layer which are connected with each other, and FCBs and open-tie switch in a feeder-pair. These two types of neighboring agents are identified by topology processing. In each type, one agent interacts with its neighboring agents; each agent makes control decisions based on the information received from its neighboring agents and data from the electrical component in the DAS. After the agent makes a control decision, it sends the execution command to the electrical component and present status to its neighboring agents. Figure 3 shows the information track among agent communication for the sample feeders as shown in Fig. 3. The topology information FCB1→FTU1→FTU2→FTU3→FCB2
Fig. 3. Topology information track among agents for the sample feeders. TABLE I ATTRIBUTES OF EACH AGENT FOR SAMPLE FEEDER BY TOPOLOGY INFORMATION TRACK
A. Service restoration on feeder fault Assume that a permanent fault F1 between FTU1 and FTU2 is detected by the over-current relay of FTU1, as shown in Fig. 2. Fig. 4 shows the sequence diagram for the case of a permanent fault occurring on feeder. The steps for FDIR, along with agent behavior at each step, are as follows: Step 1: FTU1 sends a message to the upstream FCBA to trip FCB1. The fault is then isolated by sending an isolation message to FTU As and opening the boundary line switches of FTU1 and FTU2. Step 2: After isolating the F1 fault, upstream unfaulted services are restored by closing FCB1. Step 3: FCBA queries the loading messages from the FCBA and MTRA of the supporting feeder, and the main transformer which supplies the supporting feeder. If more than one supporting feeder can supply the
downstream unfaulted service zones, FCBA will select the supporting feeder which has the largest spare capacity for restoration. Step 4: FCBA sends restoration message to the downstream FTUA which corresponds to the open-tie switch, and closes the open-tie switch to restore service to the downstream zone. Step 5: If the supporting feeder does not have enough spare capacity for the interrupted service zone, FCBA will select an adapted downstream FTUA to open the switch and to shed partial loads of the release feeder.
Fig. 5. Sequence diagram for the FDIR procedures of feeder outlet fault. IV. PRACTICAL SYSTEM STUDY To demonstrate the effectiveness of the proposed multiagent methodology, a distribution system of Fengshan District of Taipower is selected for the computer simulation to solve the FDIR problem. Fig. 6 depicts the one line diagram of the system which consists of 3 substations, 9 main transformers, 43 feeders, and 115 four-way line switches. By the proposed MAS architecture, 3 RTUAs, 9 MTRAs, 43 FCBAs, and 115 FTUAs are created. The current ratings of feeders are 450A. The current ratings of main transformers are 604A for the 25MVA main transformers, and 1450A for the 60MVA main transformers. The communication information monitoring between agents for the execution of FDIR procedures is shown diagrammatically in Fig. 7. For this case study, it is assumed that a permanent fault occurs at the outlet of Feeder BW46 with the circuit breaker CB30 tripped and service zones L32L37 de-energized. After querying the over-current flags of all FTU along the feeder, the fault location is identified. Because the supporting Feeder BW39 does not have enough capacity reserve to take over the total loading of Feeder BW46 as shown in messages 4-9, BW39 has to partially transfer its load (117A) to BW32 by performing switching operations (S5,S4) first. The out-of-service zones from L32 to L37 of BW46 are then served by BW39 after closing switch S34. Table II shows the service restoration strategy solved by the multiagent system for this fault.
Fig.4 shows these FDIR steps in the case of a permanent fault on a feeder. B. Restoration on feeder outlet fault Assume that a permanent fault F2 at the feeder outlet is detected by the over-current relay of FCB, as shown in Fig. 2. After isolating the fault F2 by tripping the FCB, the FDIR steps along with agent behaviors are the same as Step 3 ~5 of restoration on feeder fault in Section II-A. Fig. 5 shows the sequence diagram for the case of a permanent fault occurring at the feeder outlet. If the main transformer that serves the supporting feeders does not have enough capacity reserve for the restoration of unfaulted but out of service zone, load from the supporting main transformer is transferred to its neighboring transformers in order to increase its spare capacity. The steps and agent behavior involved are the same as that of restoration on feeder outlet fault in Section II-A. If the overload problem can not be solved completely by this load transfer, load shedding procedure is then performed according to the service priority of customers before service is restored.
Fig. 6. The one-line diagram of Taipower distribution system. TABLE II RESTORATION PLAN FOR CASE 1
Fig. 7. Communication information monitoring between agents for Case 1.
In this paper, an MAS based on the DAS structure is proposed for the service restoration of a distribution system with fault contingency. The operation rules for service restoration in Taipower have been included in the MAS so that the switching operation plan derived complies with the operation regulations. After the integration of these operation rules and distributed processing into the MAS, the switching operation plan to isolate the fault and restore the unfaulted but out of service areas is obtained for distribution systems. To demonstrate the effectiveness of the proposed methodology, an actual Taipower distribution system with 3 substations and 43 feeders was selected for computer simulation. The communication information monitoring between various agents has been applied to illustrate the procedure of information query and action request for line switching operation to achieve the fault restoration in a parallel processing manner. The service restoration plan is derived for the fault contingencies on the distribution feeders and main transformers in substations. When the capacity reserve of supporting feeders and main transformers is not enough to cover whole service restoration, the load shedding service zones are determined according to the service priority of customers. Compared to the current centralized DAS system at Taipower, the proposed MAS system with its distributed processing capabilities, is able to restore service after a fault contingency more efficiently.
           
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