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Distribution automation using microcomputer technology
Electric Power Systems Research, 1 ( 1 9 7 7 1 7 8 ) 131 - 137 © Elsevier Sequoia S.A., L a u s a n n e - - P r i n t e d in t h e N e t h e r l a n d s
131
Distribution A u t o m a t i o n Using Microcomputer Technology
B. D O N R U S S E L L
Department of Electrical Engineering, Texas A &M University, College Station, Texas 77843 (U.S.A.) (Received D e c e m b e r 1, 1 9 7 7 )
SUMMARY
The development of microcomputers has resulted in numerous applications in the areas of power system protection, control, and data acquisition. Micros are currently being used for communications processors in remote terminal units and in sequence of events recorders. They provide a degree of "intelligence" and flexibility previously unknown. Future applications include their use as dedicated control modules in substation automation schemes and as replacements for traditional protective relays. The combined advantages of low cost and high performance will result in widespread use in power system applications.
INTRODUCTION
As early as 1921, Commonwealth Edison, using telephone lines as the communication medium, placed various distribution substations under limited control. Soon many utilities were controlling limited substation functions over dedicated lines using one master station for each remote terminal unit. From these beginnings we have progressed to advanced supervisory control systems using digital computers and high,speed digital communication links. Supervisory control and data acquisition (SCADA) has become a way of life and a real part o f daffy system operations for many utilities. We have the ability today to replace relay based and hardwired solidstate remote terminal units (RTU) in substations with "intelligent" remotes capable of making local decisions. We have the capability of communicating over distribution lines to control remote equipment, read meters, control load, etc. H o w have utilities reacted to these changes ? H o w well has the market place reflected advances in technology? Are we
headed toward a totally automated substation ? It is these questions, as well as others, that we wish to discuss.
A U T O M A T I O N - - W H A T IS I T ?
The traditional approach to substation operations can best be classified as nominal control with electromechanical protection. As new functional needs developed, a piece of equipment was designed with its own sensors, communication links, data storage, etc. The result of this "need breeds design" style of growth has resulted in the orderly addition of functions to substations. However, it has also produced numerous independent pieces of hardware superimposed on the substation. This has led to functional inefficiency and unnecessary redundancy in transducers, data channels, storage capability, etc., as represented in Fig. 1. Substation automation can be loosely defined as the a t t e m p t to consolidate as many control, protection, and operational functions as possible into an integrated computer-based control system. This system would interface to the outside world for data transmission and
POWerLines Events Recorder
Relaying
DataStorage
Breaker
Logger J
Etc.
Fig. 1. T r a d i t i o n a l s u b s t a t i o n f u n c t i o n a l implementa-
tion.
132 Power Lines
comm0,i¢o,io~I
(~at puts
Fig. 2. Computer-based functional implementation. control inputs. It would acquire and analyze data from all key points in the substation to support its various functional responsibilities. As Fig. 2 shows, this would greatly reduce the number of transducers and data channels required to perform these functions. We have actually reduced hardware complexity and replaced it by intelligence capability (computer software). Since logical and computational ability now rests in the substation, additional actions can be performed including local closed-loop control functions. As we progress out of the substation, automation can be described as the a t t e m p t to remotely control equipment out in the system, control load at the utilization level, read meters automatically, etc. All of these functions will probably be controlled from the substation but they represent a very distinct technology based on a reliable communications system. The purpose of these systems is to extend our control all the way down to load levels -- a degree of control unknown in the past. There are m a n y reasons for pursuing distribution automation. The increased size and complexity of substations including the need for greater security, improved service continuity and reliability, and heavier loads with reduced transformer reserve margins all demand a more sophisticated approach to system operations. The need for load management both at the substation level and at point of utilization calls for increased automation including improved system communications. Many utilities point to operational improvements they expect to achieve. Others have found apparent economic savings to justify automation. Whatever their reasons, utilities are moving ever closer to an automated distribution system from the substation down to the load.
A review of the commercial market offers great insight into the development and acceptance of distribution automation. Many vendors offering extensive product lines and services have entered the market place in the last five years. For example, approximately 20 manufacturers of various types of supervisory control and data acquisition systems are currently vying for contracts in this area. Approximately 25 manufacturers are developing automated distribution communication systems anticipating a strong market. While it is generally accepted that utilities cannot support this number of manufacturers in either of these areas, the active efforts of so many vendors shows great promise for the speedy application of new automation technologies. Many manufacturers will go out of business, but the intense competition should push forward the state of the art at a rapid pace. While most utilities are interested in adopting some phase of distribution automation, the overall sales volume for systems is not large. It is projected that an average $100 million per year in supervisory control systems sales can be expected over the next few years. Gross sales are increasing rapidly and discussions with utilities indicate that very few will be operating w i t h o u t the benefit of supervisory systems by 1985. However, the total projected sales are too small to support the large number of manufacturers. As for automation beyond the substation level, no industry wide market has developed, although m a n y manufacturers have high hopes. A few trial systems using power line carrier have been installed and we are all aware of the available ripple control systems and those using radio and telephone communications. It is generally recognized that this market will develop in parallel with and tied to the increasing need for load management.
FUNCTIONAL DESCRIPTION From a technical viewpoint, what functions are required to automate various levels of the distribution system ? The Substation Computer Working Group of the PES Relaying Committee of IEEE has suggested the following list of substation automation functions: Primary fault detection Fault classification
133
Breaker tripping Fault location Back-up fault detection Bus bar protection Transformer protection Electrical Gas pressure Gas analysis Moisture content, etc. Adaptive relay settings Adaptive breaker control Breaker failure Reclosing control Voltage and reactive control Switching control in discrete steps Load shedding and rejection sectionalizing Load survey Load projection Automatic load management Automatic meter reading coordination Oscillography Pre-post-fault data and analysis Substation security Communications control Encoding, coding, error control Substation carrier supervisory Data logging Analog metering, demand metering MW MVAR Power factor Voltage Voltage angle Current Frequency Line loss and MWh calculations Transformer information Breaker and disconnect status Relay status Tap settings All other contact status Sequence of events Environment checks Weather information Control room temperature Temperature of other elements Computer equipment checks Power supply monitor Control of operator console Real time clock The Mitre Corporation, in a recent project for the Electric Power Research Institute, sug-
gests the following categories of distribution (feeder) automation functions: (1) Load management. Discretionary loadswitching, variable-rate billing, time-of
EXAMPLES
Numerous commercial and research organizations are currently attempting to apply this advanced technology. These advances in distribution substation automation can be understood by viewing the technological progress made with remote terminal units (RTUs). Relay-based and electronic manually operated remotes represent more than two-thirds of the supervisory remotes currently in use. However, when we look at replacement units and new purchases we find that 75% are advanced SCADA system RTUs which are typically computer controlled. Even these advanced systems are no less than five years behind current technology. Intelligent remotes are commercially available and are functionally more advanced. In due course, substation computers will be available to perform RTU functions as well as local control. Progress toward automated substations has been slow but steady over the past five years. We all recall Tri-State's Wray substation project. By using two GRI 99 minicomputers, a limited number of automation functions was implemented, including capacitor bank control, transformer loading analysis, a n d data
134 validation. A more recent a t t e m p t at advanced substation automation is the General Electric's PROBE project. The PROBE system is centered around a "data base" formed by frequently sampling and storing data at all critical points in the substation. These data categories are current, voltage, time, status, and temperature. By using this data base an a t t e m p t is being made to implement the following functions: Air system monitor Alarm annunciation Audible noise monitor Automatic reclosing Automatic bus sectionalizing Breaker failure protection Capacitor bank control {substation and feeder) Fault location Fault recording Feeder communications interface Feeder d e p l o y m e n t and switching Instrumentation and metering Load shedding Local load and contingency evaluation Message buffering Relating relay setting to load R e m o t e control and data reporting Sequence o f events recording Synchronism check Transformer load capability monitoring Transformer load feeder matching Transformer LTC control Voltage regulator control Appropriate protection functions The PROBE system is intended to replace such typical substation hardware as a SCADA RTU, fault recorders, sequence of events recorders, metering devices, and certain relays. Such defrayed equipment should go a long way toward cost justifying such a system. M o d e m automation systems have been made possible by recent advances in computer and communications technology. Specifically, the advent of the microcomputer has opened up new possibilities of economically providing computational ability at the substation level. Since no full-scale automation systems are commercially available, we must look to specific applications to see what impact microcomputers have had on the industry.
PRESENT APPLICATIONS The present applications of microcomputers to power substation operations center around the data acquisition and remote supervisory functions. These systems have proved ideal for use as intelligent remotes, communications interfaces, and data loggers. One such system is the substation sequential events recorder {SER) offered by Rochester Instruments. The primary function of SERs is to record time of operation of various pieces of equipment such as circuit breakers, protective relays, and reclosing devices. Such information is extremely important in reconstructing an exact time picture of the events associated with a system fault or other operating emergency. While events recorders have been used for years, they have taken on a new dimension by incorporating microprocessor technology to provide intelligence to the systems. The programming flexibility of the microprocessor as opposed to hardwired logic allows for system modifications to fit individual needs. Additional m e m o r y can be added to the microprocessor to provide "interpretive" ability or "events analysis" as indicated in Fig. 3. This means that the events can not only be stored, tagged, and logged b u t they can be analyzed and various reports issued providing specific information to meet customer needs. Another interesting feature is the interfacing of several SERs in remote substations to a microprocessor at a central location. This central unit can sort the data from several SERs and provide a system sequential log for use by operating personnel. Microprocessor technology has been used in a new generation of graphic display systems which are being adopted b y power system operators. Display generators utilizing hardwired control logic have been used for many years b u t cost v e r s u s performance trade-offs have limited their use by power system operators. Microprocessor-based systems offer high flexibility and performance for moderate costs and have opened up new areas of application. Systems such as the Interactive Graphic Color Display CRT offered by Aydin Controls are widely used by utilities. The advantages of these systems are numerous. Great flexibility of language and display characteristics allows for modifications to fit individual needs. The amount of data transfer between the display
135 Monitor Points
Events Recorder
~
_~
Log
Events
Anolysis
]
1
Recommended Action
Fig. 3. Intelligent events recorder.
generator and host CPU can be greatly reduced. The intelligence of the system allows for the storage of cursor moves and characters for later display in multiple character form. Additionally, the reprogramming capability allows for the system to be adapted to fit existing operating systems and CPU/software configuration. Microprocessors have, to date, had some impact on system operations, mostly in the area of remote terminal units for substations. It is the primary function of these units to provide data concerning the system and to serve as a "control" interface to the master supervisory control system. As the complexity of power systems has grown so has the desire to have information concerning the system. This has placed a burden on many existing data acquisition systems. With numerous substations to monitor and each system having several hundred points, older hardwired systems have difficulty performing their functions within a reasonable time period. For example, status changes which are determined by the central computer require that information about all points be brought to a central location. This burdens communications links and places an unnecessary burden on the central CPU. The logical alternative is the reporting of "exceptions" or only those points which have changed status during the last time period. This necessarily requires intelligence at the substation and it is for such purposes that a microcomputer-based RTU is best suited. With local substation intelligence numerous other functions can be performed such as data analysis, trend detections, load management, equipment diagnostics, etc. The flexibility of the microprocessor allows these to be readily accomplished. Several commercial systems exist which provide the basic functions required of any RTU. Control Systems Industries offers a
microprocessor-based RTU, as does TRW Controls. In the CSI system an intelligent programmable RTU is designed around a microcomputer constructed o f T T L MSI chips. The TRW 2000 microprocessor is a 16-bit bipolar MSI/ LSI processor using low-power Schottky technology. TRW uses the TRW 2000 for control of communications, control of the remote terminal unit, and for man/machine interface. This philosophy of using common hardware throughout the overall supervisory control and data acquisition system (SCADA) provides distinct advantages in terms of maintenance training, limited spare parts, and operator familiatiry with the system. Another manufacturer has utilized a microprocessor in a SCADA remote to solve a particular problem. Seldom do utilities purchase and install complete SCADA systems at one time from one vendor. These systems tyl~ically grow as the system grows and as finances permit. The result is equipment of different designs from different manufacturers which must be integrated into a functional system. Since message formats and protocol vary extensively this can be a major obstacle. Leeds and Northrup Company has used a microprocessor to help interface their SCADA remote to other systems of older design or from different manufacturers. This unit can be made to look like other SCADA units allowing utilities to purchase a Leeds and Northrup remote for use with other systems. Again, the flexibility of the microcomputer makes alterations and system adaptations possible which were obviously impossible with previous RTUs. As can be seen, microcomputer technology has made great inroads into the SCADA syw terns used by utilities. However, the future holds even broader uses which may have more significant impact.
F U T U R E APPLICATIONS
One of the most significant areas of microcomputer application in the future will center around system protection. Power systems have traditionally been protected from overload or fault conditions by dedicated electromechanical hardware. Recently dedicated solid-state electronic systems have made some inroads, but protection philosophy has not changed. Replacing these systems with microcomputer-
136 based hardware will offer significant advantages. Needless to say, the new systems must perform as well as the existing protection hardware. Yet, even if their performance only equals the existing systems, other advantages will force adoption of the new systems. Computer-based systems can perform self
Z2 [ ...........
Ulit I I
Eectrcol
i
~J " I Ifluid gas
I ';;i."~:g~'l I (c . . . . . t ond Potential) contaminant I 1 -] I Transducers. I I I°''~°'` I
Fig. 4. Microcomputer-based transformer protection. advantage of "seeing" the whole substation or the "big picture" and, having this broader base, control decisions could be made locally or passed through to a central control computer for action. The use of microcomputers for various functions in a feeder automation system can easily be projected. Microprocessor-based metering devices will be used at the residential level to store meter data and transmit it when called upon. Micros will be used as area data gathering centers to assist in transferring meter data from many points to a central billing computer. They may also be called upon to manage area load b y equitably distributing load curtailments as directed b y a central energy management computer. Where decisions must be made and data must be stored or transferred we can expect to see microcomputer-based systems.
PRO AND CON
While we may be impressed b y the capabilities of microcomputer automation, we must always investigate the economics of replacing traditional systems with new technology. An overall economic conclusion cannot be reached since the facts of each application must be carefully considered. We can say that the continuing decrease in the cost of automation equipment coupled with steady increase in substation and feeder functional complexity point toward a favorable economic evaluation for many automation projects. For example, a discrete c o m p o n e n t RTU which cost over $15 000 ten years ago can now be replaced by a functionally superior device costing one fourth as much. With microcomputer-based remotes and control devices the cost per function will be even less. As indicated in Fig. 5,
137
TRADITIONAL) TOTAL COST
dimension to this development and should allow for the economic application of new automation technologies.
UTOMATION)
BIBLIOGRAPHY
FUNCTIONAL COMPLEXITY
Fig. 5. Substation control costs.
automation techniques are cost-effective when compared with traditional substation control for functionally complex substations. We should also note that other factors besides cost will play a significant role in the adoption of microcomputer-based automation. Some of these are Hardware susceptibility to electromagnetic interference Effects of non-electrical environmental conditions Overall system reliability Complexity o f equipment maintenance To be determined Each of these areas must be thoroughly researched and their problems satisfactorily solved if full micro-automation is to be accepted b y utilities.
CONCLUSION
Distribution automation is here to stay! From the substation down to a pole top remote unit or residential meter reading device we see a rapidly expanding technology. The advent of microcomputers has added a new
1 D. G. Berkowitz, S. A. Jordan and D. L. Nickel, The distribution line carrier system -- versatile and economical, 1977 Control of Power Systems Conf. Record, No. 77CHl168, 4 REG. 5. 2 J. R. Goodman and N. C. Raleigh, Automated distribution, 1976 Control of Power Systems Conf. Record, No. 76CH1057-9, REG. 5. 3 G. P. Gurr, PROBE -- A feasibility demonstration of substation and distribution automation, American Power Conf., 39th Annual Meeting, Chicago, Illinois, April 20, 1977. 4 N. Jagoda, R. D'Auteuil, H. Baker, R. Abbott and E. Jones, Network control, load management and meter reading techniques using power line communications, 1976 Control of Power Systems Conf. Record, No. 76CH1057-9, REG. 5. 5 J. L. Jones, The application of microprocessors to power system data acquisition and control, 1977 Control of Power Systems Conf. Record, No. 77CHl168, 4 REG. 5. 6 L. C. Markel and P. B. Layfield, Economic feasibility of distribution automation, 1977 Control of Power Systems Conf. Record, No. 77CHl168, 4 REG. 5. 7 R. E. Milligan and S. Nilsso~, Digital fault data acquisition and recording in substations, 1977 Control of Power Systems Conf. Record, No. 77CHl168, 4 REG. 5. 8 Mitre Corporation, The automated distribution system: an assessment of communications alternatives, Electrical Power Res:~Inst., EL-157, Vol. 1, September 1976. 9 B. Don Russell, Present and future applications of microcomputers in power system control, Micro77 Computer Conf. Record, IEEE. 10 B. Don Russell, A microcomputer based substation control system, 1976 Control of Power Systems Conf. Record, No. 76CH1057-9, REG. 5. 11 G. H. Seaverns, Seven automatic meter reading and control installations, 1977 Control of Power Systems Record, No. 77CHl168, 4 REG. 5.
131
Distribution A u t o m a t i o n Using Microcomputer Technology
B. D O N R U S S E L L
Department of Electrical Engineering, Texas A &M University, College Station, Texas 77843 (U.S.A.) (Received D e c e m b e r 1, 1 9 7 7 )
SUMMARY
The development of microcomputers has resulted in numerous applications in the areas of power system protection, control, and data acquisition. Micros are currently being used for communications processors in remote terminal units and in sequence of events recorders. They provide a degree of "intelligence" and flexibility previously unknown. Future applications include their use as dedicated control modules in substation automation schemes and as replacements for traditional protective relays. The combined advantages of low cost and high performance will result in widespread use in power system applications.
INTRODUCTION
As early as 1921, Commonwealth Edison, using telephone lines as the communication medium, placed various distribution substations under limited control. Soon many utilities were controlling limited substation functions over dedicated lines using one master station for each remote terminal unit. From these beginnings we have progressed to advanced supervisory control systems using digital computers and high,speed digital communication links. Supervisory control and data acquisition (SCADA) has become a way of life and a real part o f daffy system operations for many utilities. We have the ability today to replace relay based and hardwired solidstate remote terminal units (RTU) in substations with "intelligent" remotes capable of making local decisions. We have the capability of communicating over distribution lines to control remote equipment, read meters, control load, etc. H o w have utilities reacted to these changes ? H o w well has the market place reflected advances in technology? Are we
headed toward a totally automated substation ? It is these questions, as well as others, that we wish to discuss.
A U T O M A T I O N - - W H A T IS I T ?
The traditional approach to substation operations can best be classified as nominal control with electromechanical protection. As new functional needs developed, a piece of equipment was designed with its own sensors, communication links, data storage, etc. The result of this "need breeds design" style of growth has resulted in the orderly addition of functions to substations. However, it has also produced numerous independent pieces of hardware superimposed on the substation. This has led to functional inefficiency and unnecessary redundancy in transducers, data channels, storage capability, etc., as represented in Fig. 1. Substation automation can be loosely defined as the a t t e m p t to consolidate as many control, protection, and operational functions as possible into an integrated computer-based control system. This system would interface to the outside world for data transmission and
POWerLines Events Recorder
Relaying
DataStorage
Breaker
Logger J
Etc.
Fig. 1. T r a d i t i o n a l s u b s t a t i o n f u n c t i o n a l implementa-
tion.
132 Power Lines
comm0,i¢o,io~I
(~at puts
Fig. 2. Computer-based functional implementation. control inputs. It would acquire and analyze data from all key points in the substation to support its various functional responsibilities. As Fig. 2 shows, this would greatly reduce the number of transducers and data channels required to perform these functions. We have actually reduced hardware complexity and replaced it by intelligence capability (computer software). Since logical and computational ability now rests in the substation, additional actions can be performed including local closed-loop control functions. As we progress out of the substation, automation can be described as the a t t e m p t to remotely control equipment out in the system, control load at the utilization level, read meters automatically, etc. All of these functions will probably be controlled from the substation but they represent a very distinct technology based on a reliable communications system. The purpose of these systems is to extend our control all the way down to load levels -- a degree of control unknown in the past. There are m a n y reasons for pursuing distribution automation. The increased size and complexity of substations including the need for greater security, improved service continuity and reliability, and heavier loads with reduced transformer reserve margins all demand a more sophisticated approach to system operations. The need for load management both at the substation level and at point of utilization calls for increased automation including improved system communications. Many utilities point to operational improvements they expect to achieve. Others have found apparent economic savings to justify automation. Whatever their reasons, utilities are moving ever closer to an automated distribution system from the substation down to the load.
A review of the commercial market offers great insight into the development and acceptance of distribution automation. Many vendors offering extensive product lines and services have entered the market place in the last five years. For example, approximately 20 manufacturers of various types of supervisory control and data acquisition systems are currently vying for contracts in this area. Approximately 25 manufacturers are developing automated distribution communication systems anticipating a strong market. While it is generally accepted that utilities cannot support this number of manufacturers in either of these areas, the active efforts of so many vendors shows great promise for the speedy application of new automation technologies. Many manufacturers will go out of business, but the intense competition should push forward the state of the art at a rapid pace. While most utilities are interested in adopting some phase of distribution automation, the overall sales volume for systems is not large. It is projected that an average $100 million per year in supervisory control systems sales can be expected over the next few years. Gross sales are increasing rapidly and discussions with utilities indicate that very few will be operating w i t h o u t the benefit of supervisory systems by 1985. However, the total projected sales are too small to support the large number of manufacturers. As for automation beyond the substation level, no industry wide market has developed, although m a n y manufacturers have high hopes. A few trial systems using power line carrier have been installed and we are all aware of the available ripple control systems and those using radio and telephone communications. It is generally recognized that this market will develop in parallel with and tied to the increasing need for load management.
FUNCTIONAL DESCRIPTION From a technical viewpoint, what functions are required to automate various levels of the distribution system ? The Substation Computer Working Group of the PES Relaying Committee of IEEE has suggested the following list of substation automation functions: Primary fault detection Fault classification
133
Breaker tripping Fault location Back-up fault detection Bus bar protection Transformer protection Electrical Gas pressure Gas analysis Moisture content, etc. Adaptive relay settings Adaptive breaker control Breaker failure Reclosing control Voltage and reactive control Switching control in discrete steps Load shedding and rejection sectionalizing Load survey Load projection Automatic load management Automatic meter reading coordination Oscillography Pre-post-fault data and analysis Substation security Communications control Encoding, coding, error control Substation carrier supervisory Data logging Analog metering, demand metering MW MVAR Power factor Voltage Voltage angle Current Frequency Line loss and MWh calculations Transformer information Breaker and disconnect status Relay status Tap settings All other contact status Sequence of events Environment checks Weather information Control room temperature Temperature of other elements Computer equipment checks Power supply monitor Control of operator console Real time clock The Mitre Corporation, in a recent project for the Electric Power Research Institute, sug-
gests the following categories of distribution (feeder) automation functions: (1) Load management. Discretionary loadswitching, variable-rate billing, time-of
EXAMPLES
Numerous commercial and research organizations are currently attempting to apply this advanced technology. These advances in distribution substation automation can be understood by viewing the technological progress made with remote terminal units (RTUs). Relay-based and electronic manually operated remotes represent more than two-thirds of the supervisory remotes currently in use. However, when we look at replacement units and new purchases we find that 75% are advanced SCADA system RTUs which are typically computer controlled. Even these advanced systems are no less than five years behind current technology. Intelligent remotes are commercially available and are functionally more advanced. In due course, substation computers will be available to perform RTU functions as well as local control. Progress toward automated substations has been slow but steady over the past five years. We all recall Tri-State's Wray substation project. By using two GRI 99 minicomputers, a limited number of automation functions was implemented, including capacitor bank control, transformer loading analysis, a n d data
134 validation. A more recent a t t e m p t at advanced substation automation is the General Electric's PROBE project. The PROBE system is centered around a "data base" formed by frequently sampling and storing data at all critical points in the substation. These data categories are current, voltage, time, status, and temperature. By using this data base an a t t e m p t is being made to implement the following functions: Air system monitor Alarm annunciation Audible noise monitor Automatic reclosing Automatic bus sectionalizing Breaker failure protection Capacitor bank control {substation and feeder) Fault location Fault recording Feeder communications interface Feeder d e p l o y m e n t and switching Instrumentation and metering Load shedding Local load and contingency evaluation Message buffering Relating relay setting to load R e m o t e control and data reporting Sequence o f events recording Synchronism check Transformer load capability monitoring Transformer load feeder matching Transformer LTC control Voltage regulator control Appropriate protection functions The PROBE system is intended to replace such typical substation hardware as a SCADA RTU, fault recorders, sequence of events recorders, metering devices, and certain relays. Such defrayed equipment should go a long way toward cost justifying such a system. M o d e m automation systems have been made possible by recent advances in computer and communications technology. Specifically, the advent of the microcomputer has opened up new possibilities of economically providing computational ability at the substation level. Since no full-scale automation systems are commercially available, we must look to specific applications to see what impact microcomputers have had on the industry.
PRESENT APPLICATIONS The present applications of microcomputers to power substation operations center around the data acquisition and remote supervisory functions. These systems have proved ideal for use as intelligent remotes, communications interfaces, and data loggers. One such system is the substation sequential events recorder {SER) offered by Rochester Instruments. The primary function of SERs is to record time of operation of various pieces of equipment such as circuit breakers, protective relays, and reclosing devices. Such information is extremely important in reconstructing an exact time picture of the events associated with a system fault or other operating emergency. While events recorders have been used for years, they have taken on a new dimension by incorporating microprocessor technology to provide intelligence to the systems. The programming flexibility of the microprocessor as opposed to hardwired logic allows for system modifications to fit individual needs. Additional m e m o r y can be added to the microprocessor to provide "interpretive" ability or "events analysis" as indicated in Fig. 3. This means that the events can not only be stored, tagged, and logged b u t they can be analyzed and various reports issued providing specific information to meet customer needs. Another interesting feature is the interfacing of several SERs in remote substations to a microprocessor at a central location. This central unit can sort the data from several SERs and provide a system sequential log for use by operating personnel. Microprocessor technology has been used in a new generation of graphic display systems which are being adopted b y power system operators. Display generators utilizing hardwired control logic have been used for many years b u t cost v e r s u s performance trade-offs have limited their use by power system operators. Microprocessor-based systems offer high flexibility and performance for moderate costs and have opened up new areas of application. Systems such as the Interactive Graphic Color Display CRT offered by Aydin Controls are widely used by utilities. The advantages of these systems are numerous. Great flexibility of language and display characteristics allows for modifications to fit individual needs. The amount of data transfer between the display
135 Monitor Points
Events Recorder
~
_~
Log
Events
Anolysis
]
1
Recommended Action
Fig. 3. Intelligent events recorder.
generator and host CPU can be greatly reduced. The intelligence of the system allows for the storage of cursor moves and characters for later display in multiple character form. Additionally, the reprogramming capability allows for the system to be adapted to fit existing operating systems and CPU/software configuration. Microprocessors have, to date, had some impact on system operations, mostly in the area of remote terminal units for substations. It is the primary function of these units to provide data concerning the system and to serve as a "control" interface to the master supervisory control system. As the complexity of power systems has grown so has the desire to have information concerning the system. This has placed a burden on many existing data acquisition systems. With numerous substations to monitor and each system having several hundred points, older hardwired systems have difficulty performing their functions within a reasonable time period. For example, status changes which are determined by the central computer require that information about all points be brought to a central location. This burdens communications links and places an unnecessary burden on the central CPU. The logical alternative is the reporting of "exceptions" or only those points which have changed status during the last time period. This necessarily requires intelligence at the substation and it is for such purposes that a microcomputer-based RTU is best suited. With local substation intelligence numerous other functions can be performed such as data analysis, trend detections, load management, equipment diagnostics, etc. The flexibility of the microprocessor allows these to be readily accomplished. Several commercial systems exist which provide the basic functions required of any RTU. Control Systems Industries offers a
microprocessor-based RTU, as does TRW Controls. In the CSI system an intelligent programmable RTU is designed around a microcomputer constructed o f T T L MSI chips. The TRW 2000 microprocessor is a 16-bit bipolar MSI/ LSI processor using low-power Schottky technology. TRW uses the TRW 2000 for control of communications, control of the remote terminal unit, and for man/machine interface. This philosophy of using common hardware throughout the overall supervisory control and data acquisition system (SCADA) provides distinct advantages in terms of maintenance training, limited spare parts, and operator familiatiry with the system. Another manufacturer has utilized a microprocessor in a SCADA remote to solve a particular problem. Seldom do utilities purchase and install complete SCADA systems at one time from one vendor. These systems tyl~ically grow as the system grows and as finances permit. The result is equipment of different designs from different manufacturers which must be integrated into a functional system. Since message formats and protocol vary extensively this can be a major obstacle. Leeds and Northrup Company has used a microprocessor to help interface their SCADA remote to other systems of older design or from different manufacturers. This unit can be made to look like other SCADA units allowing utilities to purchase a Leeds and Northrup remote for use with other systems. Again, the flexibility of the microcomputer makes alterations and system adaptations possible which were obviously impossible with previous RTUs. As can be seen, microcomputer technology has made great inroads into the SCADA syw terns used by utilities. However, the future holds even broader uses which may have more significant impact.
F U T U R E APPLICATIONS
One of the most significant areas of microcomputer application in the future will center around system protection. Power systems have traditionally been protected from overload or fault conditions by dedicated electromechanical hardware. Recently dedicated solid-state electronic systems have made some inroads, but protection philosophy has not changed. Replacing these systems with microcomputer-
136 based hardware will offer significant advantages. Needless to say, the new systems must perform as well as the existing protection hardware. Yet, even if their performance only equals the existing systems, other advantages will force adoption of the new systems. Computer-based systems can perform self
Z2 [ ...........
Ulit I I
Eectrcol
i
~J " I Ifluid gas
I ';;i."~:g~'l I (c . . . . . t ond Potential) contaminant I 1 -] I Transducers. I I I°''~°'` I
Fig. 4. Microcomputer-based transformer protection. advantage of "seeing" the whole substation or the "big picture" and, having this broader base, control decisions could be made locally or passed through to a central control computer for action. The use of microcomputers for various functions in a feeder automation system can easily be projected. Microprocessor-based metering devices will be used at the residential level to store meter data and transmit it when called upon. Micros will be used as area data gathering centers to assist in transferring meter data from many points to a central billing computer. They may also be called upon to manage area load b y equitably distributing load curtailments as directed b y a central energy management computer. Where decisions must be made and data must be stored or transferred we can expect to see microcomputer-based systems.
PRO AND CON
While we may be impressed b y the capabilities of microcomputer automation, we must always investigate the economics of replacing traditional systems with new technology. An overall economic conclusion cannot be reached since the facts of each application must be carefully considered. We can say that the continuing decrease in the cost of automation equipment coupled with steady increase in substation and feeder functional complexity point toward a favorable economic evaluation for many automation projects. For example, a discrete c o m p o n e n t RTU which cost over $15 000 ten years ago can now be replaced by a functionally superior device costing one fourth as much. With microcomputer-based remotes and control devices the cost per function will be even less. As indicated in Fig. 5,
137
TRADITIONAL) TOTAL COST
dimension to this development and should allow for the economic application of new automation technologies.
UTOMATION)
BIBLIOGRAPHY
FUNCTIONAL COMPLEXITY
Fig. 5. Substation control costs.
automation techniques are cost-effective when compared with traditional substation control for functionally complex substations. We should also note that other factors besides cost will play a significant role in the adoption of microcomputer-based automation. Some of these are Hardware susceptibility to electromagnetic interference Effects of non-electrical environmental conditions Overall system reliability Complexity o f equipment maintenance To be determined Each of these areas must be thoroughly researched and their problems satisfactorily solved if full micro-automation is to be accepted b y utilities.
CONCLUSION
Distribution automation is here to stay! From the substation down to a pole top remote unit or residential meter reading device we see a rapidly expanding technology. The advent of microcomputers has added a new
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