ANALYSIS OF AUTOMATED DISTRIBUTION SYSTEMS SCHEMES

Transcription

1 ANALYSIS OF AUTOMATED DISTRIBUTION SYSTEMS SCHEMES Mauricio Sperandio 1 Edison A.C. Aranha Neto 1 Jorge Coelho 1 Rodrigo Ramos 2 1 LabPlan \ UFSC, Brazil 2 CELESC ABSTRACT The objective of this paper is to present the main schemes of distribution automation used worldwide, and to describe the communication systems that can be employed. The benefits and economical results obtained with the utilization of automated systems from some utilities of different countries are showed, indicating their performance. Finally the strategic aspects of the implementation of these systems are discussed. 1. INTRODUCTION This paper makes a prospection of the experiences with automation of distribution networks, showing that the accomplishment of this type of project perform an essential role for the improvement of the electrical power supply quality, emphasizing the benefits and economical results obtained, mainly in the case of countries as Japan and USA, that heavily invest in the automation of their distribution systems. In the Brazilian case the automation is in an initial stage due to growing regulation of the electric sector, that is compelling the utilities to offer more quality in the services provided, always looking for cost reductions, increasing the efficiency and competitiveness. The automation of the electric power distribution systems, together with the operational evolution, speeds up the process of systems restoration under fault, because they reduce the need of in loco maintenance that can be prejudiced by traffic jams, long distances and unfavorable accesses. Consequently, this improve the continuity indicators and the customer's satisfaction by the following factors: - Reduction of the duration index (time) of supply interruption under fault, or momentary by maneuvers; - Monitoring voltage and loading levels; - Better operational reliability for system maneuvers; - Reduction of the operational costs; - Increase of the sold energy; - Essential loads priority. Some automation schemes that can be put into practice are presented, as the use of the protection in the overhead distribution network, switches and automatic reclosers, remote or automated switching between feeders, and monitoring of electrical measures. The decision about the configuration plan (types, amount and location of equipments) is discussed, to determine the gains that can be obtained with the application of each item mentioned above, and consequently the costs involved in that process. In Brazil, the distribution automation still comprehends mainly the substation. Though, there is a lot to develop in terms of automation of the networks and of the Operation Center (OC). There are already projects being implemented in some utilities, inclusive with development of own technology, but it is necessary to discuss the performance of different configurations, and especially their cost/benefit relations. Because of that, a strategic vision of the implantation of such systems is presented, emphasizing the improvement of global performance of the utility. 2. AUTOMATION LEVELS In the literature about distribution automation are presented several protection schemes and designs to isolate faults and to restore the system, however exist some types that stand out, being more commonly used by different companies. We will show a compilation of the main types defined here in 4 automation levels, summarizing their action form and main components. A justification is outlined for the most suitable configuration for each network type: rural, urban of small load and big load. It worth remind that the existence of equipments provided with reclose ability requires the coordination with other protection equipments, according to a sequence of preset operations. The objective of this coordination is to avoid that transitory faults cause the operation of protection devices that don t have reclose ability, like fuses. The objective of selectivity is to operate the more appropriate protection equipment immediately, such in case of permanent defects, only the smallest possible part of the network is turned off. The coordination and selectivity study is done by superposition of the characteristic time vs. current curves of the several equipments, usually in a bi-log graphic, adapting the action of each equipment Substation Automation: Given a reliable generation and transmission system, the first step to continue the improvement of the supply quality is the automation of the distribution substation. Beginning with the automatic bus arrangement control, in order to allow fast load transfers and reliability in the maneuvers, till the installation of relays in the exit of the feeders to monitor the voltage and current and to operate the protection of the circuit. The most common configuration is a relay and a circuit breaker with recloser function in the feeder, coordinated with the fuses along the main line (Figure 1). This avoids that a great part of fuses blows, and its consequent change, reducing interruption time (SAIDI),

2 since 80% of the faults are non-permanent types. That can result in an index value reduction from 40 to 50%, and as the reclosing time is of just few cycles, the fault may not even be computed (SAIFI). However, the lost of selectivity by the fuse saving transmits the fault till the substation and the relay trip affects the whole feeder [1]. channel can control them from the OC. Many of those switches already supervise the voltage and currents data, which transmitted to OC, allow the surveillance in real time of the loading along the feeder. Figure 1: S/s recloser relay coordinating with fuses With the use of digital relays, the configuration of logical selectivity and the fast adaptation of the adjustments grant flexibility to the system. It is necessary to considerate the protected spur load, in order to avoid an unnecessary opening or even the impossibility of the fuse to blow. This kind of coordination may reduce the useful life of the links, because those fuses are exposed to currents above their nominal specification for some cycles. This is a low cost scheme that serves as a base to decrease the interruptions duration considerably due to non-permanent faults. It is not recommended for feeders where there are critical loads sensitive to momentary voltage drops, if those do not have appropriate protection. It is more suitable for feeders that supply rural or small load urban areas. It can also serve as backup protection when the use of reclosers along the feeder, what is shown next Recloser Automation: The use of advanced protection for radial networks, where reclosers are allocated in distant points of the substation, allows the clearing of temporary faults without affecting the whole feeder, and in case of permanent faults it isolates a smaller portion of it (Figure 2). Considering a fault rate and loads equally distributed, the allocation of a recloser in the middle of the feeder reduces the Energy Not Supplied (ENS) in 25% in relation to the scheme shown in the item 2.1.[2] Sub Chave Fusível R Automatic Recloser Figure 2: Reclosers on the radial network There a re low cost reclosers that work by blowing up up to three fuse links (Figure 3.a), helping to reduce the duration of non-permanent faults, but still requiring maintenance to change the fuses. The automatic switch (Figure 3.b) can open and close the circuit by action of their local relays, or the RTU and a communication R (a) (b) Figure 3: Recloser types: (a) With 3 fuses (b) Automatic with RTU; This is the best scheme for long radial feeders, allowing a better index decrease than just the use of coordination with the substation, because its selectivity in the operation. Even so, the gains are not proportional to the number of reclosers. Depending on the feeder configuration, more than two or three reclosers no longer bring benefits to justify their costs (Figure 4). Figure 4: Interruption time reduction vs. number of reclosers 2.3. Circuit Automation in Open Ring: This is the most used scheme in the companies that adopt automation in their networks. The circuit in ring, or loop, minimizes the fault affected area as larger the number of switches or smaller the section among them. The scheme is based on a circuit with two sources and a normally open point, as shown in Figure 5. The feeders can be part of the same substation, however with a small loss of reliability. The switches have fault detection relays and can work normally closed () or normally open (NO). They depend on a communication channel between themselves and the DAS (Distribution Automation System), to coordinate the most appropriate switches status for each situation, to make the insulation and restoration of sections under fault. F 1 F 2 DAS Communication Channel Figure 5: Automatic open ring scheme NO

3 In the Figure 5 example exist six independent protection zones, where is necessary that the conductors are dimensioned to assist five of them, for maximum efficiency. In case of a permanent fault, just the section between two switches is immediately isolated to await repair. Automatic reclose functions may also be used. It is a very reliable scheme that guarantees minimum outage levels. Ideal for protection of important customers as industrial areas or hospitals. An example is Lakeland Electric & Water (LEW), Florida USA, that automated an industrial district of more than m 2, very affected by lightings and storms. The fact of the damages be usually severe and take from 45 minutes to several hours to be repaired could no longer be tolerated by the consumers. Then LEW made the decision of installing 6 automated switches in the system, dividing the distribution system in 4 areas. The system automatically detects and isolates the section under fault, reestablishing adjacent sections in less than 1 minute and allowing the repair team to act directly in the defect cause Full Automation: Nowadays, the most complex distribution automation scheme is the large-scale use of "intelligent" switches, with local measurement and inter-communication, acting as the open ring model, but with more switching possibilities (Figure 6). It accomplishes the isolation of sections under fault and the restoration of not affected areas in a fast and safe way. Another great advantage is allowing the fast network reconfiguration seeking to minimize losses and/or the load of the feeders. In this scheme, the distribution network becomes observable and controllable. With a great number of samples of the current and voltage conditions, a complete system profile can be obtained in real time, allowing a global performance optimization study. The main advantages of the implementation of those systems are: the gains with the decrease of ENS; a more effective feeders operation, postponing big investments as new substations or reconductoring of feeders; decrease and reallocation of service teams; maneuvers reliability and demand management for load shedding. F 1 F 2 Closed Open Figure 6: Meshed distribution network full automated Japan is a pioneer country in the distribution automation and have been expanding and improving their systems for more than 20 years, leaving it with the smallest F 3 F 4 world interruption time index, very ahead of other countries that already use similar systems (Figure 7). Interruption time / feeder / year (min) Without Automation USA UK France Japan Figure 7: World interruption time w/wo automation. The great load density and the quality demanded by the consumers took the Chubu Electric Power Company (CEPCO) to implement one of the most modern automated distribution systems already in One of the biggest cities under its concession, with more than 8 millions consumers, had automated 13% of all the maneuver switches in The outage time decreased drastically for faults in the automated system. The time for beginning of the service decreased in 25 minutes, the time until the end of the restoration operation decreased in 53 minutes, and the time of restoration of the section under fault decreased in 30 minutes, because of the "meshed" system and the operations management that execute optimized reconfiguration algorithms. One of the main advantages of an extensive system as this is the reliability and agility of the scheduled maneuvers, that in this case, are in average almost one a day. [3] In 1994 the utility responsible for the energy distribution in Hong Kong began a great automation project, implanting a total of RTUs, measurement points and remote control of more objects, along 6 years. Such project involved the participation of more than 50 specialists of different areas for more than 65 thousand hours, with a commissioning rhythm of 3 to 4 new RTUs per day of work. 3. COMMUNICATION SYSTEMS Fundamental piece in the process of automation and telecontrol of distribution networks, the communication systems should be critically evaluated, because its bad operation is even able to worsening the interruptions time [4]. The most used protocols are the DNP 3.0 in the communication of the reclosers or switches with the concentrator and IEC in the communication of the concentrators with the control center Radio The communication via radio is nowadays the most used in the automation of distribution networks, mainly due its low cost and the availability of its technology. It allows the data transmission through different channels and frequency bands.

4 Its main advantages are the possibility of monitoring system variables in real time and the wireless communication. Its main disadvantage is the connection difficulty in height unevenness, urban centers and long distances. It is also more sensitive to electromagnetic interferences and weather conditions, turning it the system with the smallest reliability Telephony Phone-line It was the first resource employed in telecontrol of remote reclosers, by modems in common commuted phone-lines or dedicated lines. It is sensitive to atmospheric discharges, and possesses a small data band and transmission rate. Nowadays is still in use as backup system, or it was replaced by the fiber optic. No report of the use or test of ADSL technology for this purpose was found. Cellular The cellular network can be a viable mean of communication for distribution automation. It presents a reduced installation cost and quick implantation, mainly where there is communication difficulty through other means, because its wireless characteristic. However, it presents a higher answer time in relation to usual means of communication, as radio or fiber optic. With the development of new resources as the GPRS (technology of fast access to internet), it will allow to supervise the distribution equipments without the need of accomplish calls, reducing the costs and increasing the speed of the system. Satellite A last option for switches control in isolated places without the possibility of other communication types. It can be feasible using a compact protocol that makes little communication with the operation center Fiber Optic Fiber optic is the only option for "Total Automation" scheme type, shown in the item 2.4, due to its large data band, permanent communication in high transmission rates and very high reliability. It is totally immune to electromagnetic field noises and it can be used in a scheme of multiplexed ring that guarantees the operation even in case of a segment loss. It is the system that demands the largest installation investment, but don t have operation costs or service tax PLC - Power Line Communications Communication system that uses high frequency modulation through the own medium and/or low voltage distribution lines. It is a recent technology, still little disseminated, but promises to revolutionize the measurement and control of distribution systems. 4. ECONOMICAL ANALYSIS To begin the evaluation of the implementation of an distribution system automation first is necessary to acquire a series of variables, such as the length of the main feeders and some secondary extensions, transformer numbers per feeder, number of consumers per transformer, average demand per consumer, percentile of consumption classes and the interruption cost per class. Then, the economical analysis should raise the investment costs for each one of the automation levels, as the number of switches, relays, measurement, auxiliary sources, communication systems, project and commissioning. With that is possible to do reliability studies to obtain the continuity indicators and the values of the losses. In general, increasing the automation level reduces the interruption costs, but with a very superior increase of the investment costs. In some cases, the interruption costs decreased in up to 80% with the substitution of only 4 manual switches by remotely operated reclosers. The experience of some utilities shows that the automation of rural areas is profitable, overcoming a return tax of 10% a year, except for the most advanced schemes as the ring one. In small urban areas (residential predominance), usually the interruption costs do not justify the investment in automation, but the continuity goals established by the regulatory policy can be a decisive factor. The automation of large urban networks is very advantageous, because it improves the reliability and the flexibility of the distribution system, reducing the interruption times, the Energy Not Supplied and it increases the efficiency of the feeders. In case of huge consumers groups, the gains with the decrease of ENS can arrive to more than US$ 1.35 million a year. Defined the base configuration, among the presented schemes, a sensitivity analysis can demonstrate the influence of certain aspects in the automation gains. For example, how much some quality indicator or reliability increases with the addition of a new recloser. The experience of Yancheng Power Bureau's (YPB), chinese utility that is implementing automation in its distribution system with 5 substations and 42 feeders since 1996, shows that in 4 years its Distribution Automation System (DAS) acted in more than 200 occasions reducing the interruption time from an average of 35 minutes for less than 3 minutes. The DAS added 130MWh of consumed energy a year, and is estimated that more than US$ 18 millions of social production value. It is expected that the DAS implementation will reach the financial return within five years. Northern States Power (NSP) - USA, used with great success the automation of the distribution to indefinitely postpone a substation expansion that would cost US$ 2 millions. NSP would need to expand the largest of the two substations of the city adding a third source of 43.4 MVA or, alternatively, with the automation of five feeders among the substations, to allow fast load transfers during emergency situations. When the financial justifications were developed for each potential solution, the decision was clear: the smallest

5 cost solution was the distribution automation, allowing NSP to postpone the largest expense for at least six years, opting for an investment of US$ in 16 maneuver systems and three pedestal panels units with remote supervision, each one with an radio RTU, plus 16 capacitors banks mounted in pedestals. 5. DISCUSSION The strategic matter of the automation is in the effectiveness of the companies in applying the resources that they dispose, or exploiting favorable conditions, as the land relief and the demand to be supplied, searching to reach certain objectives as improvement of the quality indicators and reduction of losses. Many utilities adopt turn-key solutions, hiring an automation service company to do the whole project and commissioning of the system. That initiative is the fastest form of implementing such resources, however, leaves the utility dependent and bring a limited knowledge to their technical body. The most appropriate mean seems to be involving specialists of each area of the utility (equipments, protection, telecommunications, financial, etc) in a pilot project, establishing a step-by-step system through the different automation levels, evaluating the characteristics of the consumers group that will be supplied [5]. It is a slow process, but in a long term will bring more benefits to the utility. The importance of project development inside the own company and the use of non-proprietary technologies and protocols is highlighted in [6], where the beginning of a large-scale automation scheme is presented. The open protocols allow the communication between different manufacturer s equipments, and the possibility of software and firmware revisions postpone its obsolescence. Another point to be considered in the automation adoption is the expansion planning. The use or the intent to implement an automated system changes this paradigm, because new feeders and substations should also attend to the requirements of the automation, seeking for increase its range and flexibility. Brazil is still disadvantaged in relation to the most developed countries, which have been investing in automation for more than twenty years. Because of that, the brazilian utilities should not think about comparing themselves to those systems, but must engage strategies that bring more benefits than just an immediate indicators reduction, sometimes at exorbitant or even unnecessary costs. A well-structured project should consider multiple criteria, as the minimization of the number of necessary reclosers, reduction of fault indices, essential loads priority and the monitoring of network measures. That, together with the appropriate communication system planning, if necessary, should reach the desired objectives at a minimum cost, allowing the capital return by the gains with the automation in less time. 6. COLUSION The main aspects, advantages and disadvantages of the distribution systems automation were presented, as also the different project schemes, the communication systems, and the experience of some utilities that already use this kind of resources in their networks, summarizing the gains and analysis reported in the literature. Automation is a consequence of the technological evolution necessary to attend the consumer s demand for supply quality. Brazil is still a lot behind the main countries that already adopt the automation of their distribution systems, mainly concerning the advanced protection of overhead distribution lines, but several utilities demonstrate interest or already develop pilot projects in that area. 7. REFEREES [1] Warren, C.M.: The Effect of Reducing Momentary Outages on Distribution Reliability Indices, IEEE Transactions on Power Delivery. Vol. 7. No. 3. July [2] Sandez, G.; Tarchini, J.: Mejora en la Calidad del Servicio Mediante la Implementación de un Sistema Integrado de Telecontrol de la Red de Distribución, AR-02, IV CIERTEC, São Paulo, September [3] Oono, H., Kawaharasaki, M.: A New Large Scale DAS in CEPCO, Transactions on Power Systems, Vol.7, No.2, May [4] Ying He, Söder, L., Allan, R.N.:0 Distribution Automation: Impact of Communication System on Reliability of Automatic Control, IEEE Porto Power Tech Proceedings, Vol. 3, pag:6, [5] Radmilović, B.: Pilot Project for Remote Monitoring and Control System in Medium Voltage Electrical Distribution Network, Regional Conference and Exhibition on Electricity Distribution, Serbia and Montenegro, [6] Gacek, D., Geynisman, O., Proudfoot, D., Minnick, K.: Migrating from SCADA to Automation, IEEE/PES Transmission and Distribution Conference and Exposition, vol.1, pag: , ADDRESS OF AUTHOR Mauricio Sperandio LabPlan / EEL / CTC / UFSC - Campus Universitário Trindade, CEP , Florianópolis SC / Brazil