1.1 Distribution Automation (DA) and Flexible Control
As a consequence of deregulation and restructuring, the focus of electric utilities is shifting from a conservative "reliability at all costs'' emphasis to a more efficient and economic emphasis. Electric Utilities are turning to distribution automation to increase the options for real-time computation, communication, and control. The evolution of distribution system automation has been dictated by the level of sophistication of existing monitoring and control technologies. Just as the Supervisory Control And Data Acquisition (SCADA) system monitors the generation and transmission system, the distribution automation system monitors the distribution system.
Although distribution systems are a significant part of power systems, advances in distribution control technology have lagged considerably behind advances in generation and transmission control. However, recent progress in computer and communication technology has made distribution automation realizable . The progress of distribution automation has been relatively slow due to reluctance of utilities in spending money on automation. Many utilities have found it difficult to justify automation based purely on cost-benefit numbers. However, distribution automation provides many intangible benefits, which should be given consideration while deciding to implement distribution automation. Unbundling of electric services in the future is likely to make distribution automation more attractive because distribution companies will be operating as independent entities.
Automation allows utilities to implement flexible control of distribution systems, which can be used to enhance efficiency, reliability, and quality of electric service. Flexible control also results in more effective utilization and life-extension of the existing distribution system infrastructure. The efforts of various utilities, research organizations, and individuals towards automating the distribution systems has led to the following definition of distribution automation systems as given in an IEEE publication :
''A system that enables an electric utility to remotely monitor, coordinate and operate distribution components in a real-time mode from remote locations."
In general, those functions that can be automated in distribution systems can be classified into two categories, namely, monitoring functions and control functions. Monitoring functions are those needed to record (1) meter readings at different locations in the system, (2) the system status at different locations in the system, and (3) events of abnormal conditions. The monitored data are not only useful for day to day operation but also for system planning. Supervisory Control and Data Acquisition (SCADA) systems perform some of these monitoring functions. The control functions are related to switching operations, such as switching a capacitor, or reconfiguring feeders. In addition to these functions, system protection can also be a part of overall distribution automation schemes. Some customer related functions, such as remote load control, remote meter reading, and remote connect/disconnect may also be considered as distribution automation functions. A complete list of distribution automation functions as defined by EPRI is available in various articles and reports [3,4,5].However, we will focus our attention mainly on control functions.
The functions mentioned above are performed in a relatively slow time frame (minutes to hours). These devices are not designed to endure frequent switching. Recently, several new devices have been developed which allow rapid control. Application of distribution-level Flexible AC Transmission System (FACTS) devices such as the Static Condenser (STATCON) for distribution system control has been demonstrated . These devices are continuously controlled and respond in real-time to system changes. Coordination of a STATCON with Load-Tap-Changer (LTC) and mechanically-switched capacitors reduces fluctuations in system voltage, improving the quality of service.
Electric power quality has become an increasingly problematic area in power system distribution systems. Power quality may be defined as "the measurement, analysis, and improvement of bus voltage, usually a load bus voltage, to maintain that voltage to be a sinusoid at rated voltage and frequency ". A direct correlation exists between the lack of electric power quality delivered to the customer and the number of complaints received from the customer. As a result, EPRI has directed substantial research efforts into the development of advanced technologies to improve the performance of utility distribution systems. The technology, called custom power, seeks to integrate modern power electronics-based controllers such as the solid-state breaker (SSB), the STATCON, and the Dynamic Voltage Restorer (DVR) with distribution automation and integrated utility communications to deliver a high grade of electric power quality to the end user.
The main advantage of SSB over its electromechanical counterpart is switching speed. Not only does the SSB react in one cycle of the power frequency or less, but it also experiences less degradation after repeated applications. In the event of a momentary outage on one feeder, a SSB could switch over to another feeder so that the customer load would be unaffected by the feeder switching. The SSB usually is operated in conjunction with the STATCON which is a shunt-connected device capable of generating or absorbing real and reactive power at its ac output terminal when fed from an energy storage source at its dc input terminal. The distribution level STATCON is essentially a voltage-source inverter which can also inject currents of regulated amplitude, phase, and harmonic content into the utility system via a step-up transformer. This device can perform the following functions: regulate the terminal voltage to minimize voltage sag effects, filter harmonic currents  and counteract power line disturbances. Another effective device is the DVR which is connected in series with the distribution line between the source side of a feeder and the load terminals. By injecting voltages of regulated amplitude, phase, and harmonic content into the line via an insertion transformer, the DVR can improve the quality of the terminal voltage during line disturbances when the quality of source-side voltage is poor. This device is capable of providing continuously-variable series line compensation. The DVR can also limit fault currents by injecting a lagging voltage in quadrature with the fault current resulting in an increased effective line impedance. Thus, the DVR can "block" load-side harmonic currents from propagating into the distribution system.
The demonstration of the feasibility of distribution automation through various pilot projects has increased the interest of the technical community in this field. More conferences and symposia relating to distribution automation are being held in the recent years as compared to a few years ago. The number of manufacturers offering distribution automation equipment has increased substantially. Some years ago reliability of equipment was a major concern. The equipment available now is more reliable and robust compared to the older generation. However, there are still several issues, which are an obstacle to wide spread implementation of distribution automation. These issues include cost of the equipment, absence of hardware and software standards, and availability of application software. EPRI has been active in promoting open systems and in forming standards for hardware and software relating to distribution automation. Standardization will allow the users of distribution automation systems to mix and match components manufactured by different manufacturers, and also to port software from one platform to other.
Implementation of flexible control in distribution control requires careful thinking and planning. As discussed in a presentation , the utilities can either adopt the "top-down" approach or the "bottom-up" approach. The top-down approach is the revolutionary approach in which a large-scale fully-integrated system performs most or all of the functions performed by various individual devices in the distribution system. The bottom-up approach is evolutionary in the sense that a manual device for performing a particular function is replaced by a new device or system to perform the same function automatically. In addition, devices and systems can also be installed to perform new functions not available in the older traditional distribution systems.
The top-down approach is expensive and requires major modifications in the utility operation, and thus, it is not suitable for many utilities. The bottom-up strategy is more suitable for them. This approach allows utilities to adjust to changes at a more measured pace and to install automated systems for the most immediate needs. However, the most difficult task for a utility contemplating flexible control of distribution system is to identify the functions to be automated [2,11]. The needs of every utility are dependent on geographic location, operating philosophy, and financial situation. Therefore, a careful screening of all the possible control functions is imperative before implementing any of them.
1.2 Relationship of DA to SCADA and AM/FM
1.2.1 SCADA Systems
The SCADA systems has been in use in the transmission and subtransmission systems for many years now. Hence, the technology associated with them has become quite mature. The application of the SCADA systems in distribution systems in very recent. An increased interest in distribution automation has led to an increase in use of the SCADA systems in distribution systems. In fact, many functions performed by the SCADA systems, particularly data acquisition, are an integral part of distribution automation. System data is very essential for distribution automation because without data control decisions can not be made. However, the SCADA systems are very different from distribution automation systems. In the SCADA systems the control is supervisory, where an operator looks at the data and makes decision to take control action. In distribution automation systems, most decision are made by the computer and corresponding control actions are performed with very little intervention by the operator.
Except for the control part, the SCADA systems are very similar to distribution automation systems. Thus it is natural to think that development of distribution automation should be on a SCADA platform. However, that has not been the case. Distribution automation grew independent of SCADA, mainly because the communication needs of initial distribution automation systems were different from the SCADA systems existing at that time. Load control and remote meter reading have always been parts of distribution automation systems, therefore, the communication systems needed for distribution automation required that communication be available between individual customers and the control station. Moreover, SCADA technology was itself not very mature at that time. Thus, developers of distribution automation systems used different software platforms and different languages for their system. This meant that those utilities who were interested in distribution automation had to learn a new operating environment. The distribution automation system manufacturers have realized this problem and have formed alliance with the SCADA manufacturers to integrate the two systems. Moreover, many SCADA manufacturers have entered the distribution automation market, and therefore, there is more integration between the two systems is noticeable in the recent years.
1.2.2 AM/FM Systems
Almost parallel to this development, a development has taken place in the automated mapping and facilities management (AM/FM) arena. Advent of high powered graphics computer has accelerated progress in this field. Most of the development in the AM/FM area has been in the land management area. The pipeline industry has also been making use of this technology. More recently, the power companies have started using the AM/FM technology. In an AM/FM system the electrical service maps are superimposed on the geographical maps. With the help of these maps and the database associated with those maps, the utilities can manage their distribution facilities more efficiently. Some of the common functions performed by the AM/FM systems are distribution system design, facility mapping, right of way/permit tracking, facilities inventory, and system and equipment maintenance [ ]. Most of these functions do not have the real-time feature which is an important ingredient of distribution automation. However, some functions for which real-time feature is important can be performed using AM/FM systems. These functions include outage analysis and system restoration. In the event of an outage, the calls from customers are displayed on the system maps [ ]. Then, from the outage pattern possible causes of outage are determined. The maps are then used to direct crew to perform switching operations or switches can be operated remotely.
The AM/FM systems are generally very data intensive. In addition to the distribution system data, they also need geographical mapping data. Some of the system data is also used for the distribution automation system. Thus, to make efficient use of the system databases, they can be shared by different systems. To make such data sharing feasible, the AM/FM system and distribution automation system can be connected via a computer network, such as Ethernet. Yet another approach is to fully integrate the AM/FM and distribution automation systems with one main operating computer and several workstations. The main drawback of this approach is that it will radically change the operation of the company. It will cut across operations, planning, billing, and facilities management departments. All these departments will require coordination in operating this system and will have to learn to use the same operating system for their different tasks.
1.2.3 Integration of DA, SCADA and AM/FM
The integration of the SCADA and the distribution automation systems appears to be inevitable. However, full integration of the AM/FM and the distribution automation/SCADA will take some time or it may not happen. Presently, one can find examples of utilities using AM/FM systems or distribution systems. Those using AM/FM systems have very little distribution automation. Similarly, those using distribution automation systems have none or limited mapping facilities. The choice of one or the other system is based on the importance the utility places on different functions.
1.3 Communications Infrastructure
An integrated Distribution Management System will require a communications infrastructure to communicate with individual customer locations and control points in the distribution system on one side, and with the Energy Management System on the other side. Generally, such a communications infrastructure is a hybrid system utilizing different communication medium for different parts. Some of the earlier distribution automation systems used telephone for communication between the control center and the substation, and communication from substation to the customers and control points was based on power-line carrier or radio. Power-line carrier was an obvious choice because a link to all the points of communications was available; it was only a question on installing the right equipment to accomplish communication. Power-line carrier based communications suffered from heavy attenuation in certain parts of the distribution system. Thus, gradually popularity of power-line carrier has decreased over a period of time. However, a technology, which uses power lines and is based on shifting zero-crossing of the current waveform, is still being used successfully. Earlier radio systems also had a problem because of limited range and their inability to send signal across obstacles, such as tall buildings. Recently, with the development of packet radio technology and the availability of 900 MHz spectrum to electric utilities has made radio a very popular communication medium. Currently available radio systems can communicate with points in a large area very reliably.
Developments in the fiber-optic technology has made it a viable communication medium for certain applications. However, the cost is high to make it suitable for widespread implementation for communication in distribution systems. Cellular telephone and satellite are communications mediums which have been experimented by some utilities. Efforts are also underway by concerned parties to implement Integrated Services Digital Network (ISDN). Such network will be available for different services including data communication for electric utilities. Prevalence of such networks will provide many exciting opportunities to electric utilities for implementation of distribution automation functions.
- A. Pahwa and J.K. Shultis, Assessment of the Present Status of Distribution, Report No. 238, Engineering Experiment Station, Kansas State University, Manhattan, KS, March 1992.
- D. Bassett, K. Clinard, J. Grainger, S. Purucker, and D. Ward, Tutorial Course: Distribution Automation, IEEE Publication 88EH0280-8-PWR.
- T. Moore, "Automating the Distribution Network," EPRI Journal, September 1984, pp. 22-28.
- J.B. Bunch, Guidelines for Evaluating Distribution Automation, EPRI Report EL-3728, November 1984.
- T. Kendrew, "Automated Distribution," EPRI Journal, January/February 1990, pp.46-48.
- J.S. Paserba, N.W. Miller, S.T. Naumann, M.G. Lauby, and F.P. Sener, "Coordination of a Distribution Level Continuously Controlled Compensation Device with Existing Substation Equipment for Long Term Var Management," Paper No. 93 SM 437-4 PWRD, IEEE PES Summer Meeting, Vancouver, Canada, July 1993.
- G.T. Heydt, Electric power Quality, Stars in a Circle Publications, West Lafayette, IN, 1991.
- J. Douglas, "Power Quality Solutions,", IEEE Power Engineering Review, v. 14, no. 3, March 1994.
- J.C. Balda, K.J. Olejniczak, C. Wang, B. Barbré, and M. Samotyj, "Comments on the derating of Distribution transformers Serving Nonlinear Loads," Proceedings of the Second Intl. Conf. on Power Quality: End-Use Applications and Perspectives, Atlanta, GA, Sept. 28-30, 1992.
- E.A. Undren and J.R. Benckenstein, "Protective Relaying in Integrated Distribution Substation Control Systems," Presentation for Panel Session on Integration of Demand-Side Management and Distribution Automation, IEEE Power Engineering Society Winter Meeting, Atlanta, Georgia, Feb 90.
- E.H. Davis, S.T. Grusky, and F.P. Sioshansi, "Automating the Distribution System: An Intermediary View for Electric Utilities," Public Utilities Fortnightly , Jan 19, 1989, pp. 22-27.