- Email: [email protected]
Microprocessor-Based Equipment for Automatic Control in Power Generation and Distribution Systems
MICROPROCESSOR-BASED EQUIPMENT FOR AUTOMATIC CONTROL IN POWER GENERATION AND DISTRIBUTION SYSTEMS R. Parmella NE! Electronics Ltd., Reyrolle Protection, UK
Abstract. This paper describes a microprocessor-based system designed to provide control, telecontrol, telemetry and data logging functions in power generation, transmission and distribution systems. The Modular approach adopted for the hardware and software is examined and details are provided of the measures taken to ensure secure reliable operation in the electrically hostile environment associated with power systems. Examples of the application of the equipment are Given. The paper concludes that correct operation of microprocessor-based systems in such hostile environments can be ensured only by correctly identifying the hazards and incorporating into the equipment hardware and software means to overcome such hazards.
Keywords. Power system control; telecontrol; data acquisition; system integrity; power transmission; power generation; power distribution; microprocessors.
intelligent systems for decentralised The use of high-spee6 di~ital techniques in the electrically hostile environment associated with electrical power systems requires that special precautions be taken in the design of both the hardware and software sections of such equipment.
INTRODUCTION
ap~lication.
The worldwide development of power systems has produced a need for more extensive data acquisition, greater flexibility in remote control and increased complexity in switching and control iunctions following system fault conditions. As load patterns and growth alter, power system configurations are changed and there is an attendant requirement for associated control systems and protection to be adapted, if possible without incurrinG costly and time consuming penalties due to extensive hardware changes.
The microprocessor based system control equipment described in this paper has been specifically designed to meet the exacting enviro~~er.ta! con~i~io~s v~i!e ~rovi~1ng
maximum flexibility of usage through the modular design nature of both hardware and software.
Until recently the majority of data acquisition, control and switching functions have been accommodated by hardwired dedicated el.ectro.,.mechanical or semiconductor-based systems which, althouch successful in application suffer from a number of disadvantages including lack of flexibility, duplication of specification effort, and exclusion of self-testing facilities due to complexity and cost. These disadvantages have resulted in a trend toward the use of programmable equipment in place of hardwired systems.
DESIGN CONSIDERATIONS The design considerations for a multifunction processor based system must include both the expected areas of application and the environment in which the equipment must operate along with the detailed technical requ iremen t s . Application Areas Considerations of application have a great impact on the topology of microprocessor based systems and it is probable that any multi-functional system will have conflictin~ requirements in terms of memory size, speed, interfacing capability, flexibility and cost. The main areas of application considered during the development of the system were:-
There has also been limited experience in the use of centralised computer techniques applied to substation control. This experience has indicated the need for decentralised intelligent systems which may or may not be linked into an overall controlling function. The advent of microprocessor technology has provided the means of economically producing programoable,
(a)
173
Power System Post-Fault Switching.
174
R. Parmella
(b)
Fower Station Auxiliary Plant Control.
(c)
Telecontrol.
(d)
Telemetry and data logging.
(e)
Alarm Annunciation.
The requirements in each of the application areas differ considerably and, if a standard approach is to be adopted, the design should cater for ex,andable interfacing with a modular ap~roach to both the hardware and software of the system. Environment Electrical power systems create an electrically hostile environment in which high s~eed digital equipment must operate with a high degree of integrity. Impulsive and continuous interference signals may be introduced into wiring due to insulation breakdown, protective gap flashover, differential rise of earth potential during system fault conditions as well as many other causes. Radiated interference of wide bandwidth may be generated by a variety of sources on the system, one of the most notable being the noise generated by isolator operation. In order to provide a consistent testing and acceptance procedure for equipment operating in this environment, standards relating to impulse and interference tests have been derived for relays and protection schemes in particular IEC 255-4, Appendix E speci~ies impulse and interference tests for static equipment. During the design of the present equip~ent it was considered essential that, as a minimum requirement, it should meet the require~ents of IEC 255-4 and that further tests relating to fast transients and radiated field interference should be examined. To comply with these test requirements it was considered essential that safeguards should be incorporated into the hardware and software early in the development stage and be integrated into the total system operation. Commissioning and Maintenance The introduction of microprocessor based systems can require a considerable change in the role and attitude of staff employed in the commissioning and maintenance of equipment, particularly if the staff are familiar with hard wired schemes. Training of staff in the disciplines associated with microcom,uters does much to overcome this difficulty, but in order to achieve the necessary level of understanding it is necessary to provide a man-machine interface which allows for comprehensive yet comprehendable tests to be performed. It was therefore necessary to incorporate hardware and software in the system to allow man-machine dialogue to occur with ease.
DESCRIPTION OF EQUIPMENT The system control equipment is intelligent stand-alone equipment composed of a range of standard modules housed in one or more 19 inch racks. Rack Housing A typical rack layout is shown in Fig.l. The rack is sub-divided into two areas a screened high-security electrically qu~et area with no direct access to the outside world, and an interface area which provides communication with the electrically hostile outside world. The microcomputer modules are located in the screened area and the interface modules are located in the outside access area. Data transfer between the two areas is provided only after the interface modules have suppressed harmful interference effects present on the external wiring which enter via terminals on the rear of the rack. The sub-division of the system allows the microcomputer section and interfacing section to expand and contract independently to meet specific application requirements while ensuring that the effects of radiated and lead-borne interference are removed. Interfacing System The arrangement of the interfacing system and the level of isolation provided is shown in Fi~.2. The power supply and each input and output interface to plant provide a 5kV impulse barrier between the microcomputer and the external connections and between the input circuits and earth. In addition each plant-connected terminal has 5kV impulse isolation to all other terminals. Connections from the modem section of the microcomputer to external communication circuits e.g. voice frequency telegraph circuits, ar~ made via isolating transformers which provide a 15kV rms barrier between the communication circuit and the microcomputer equipment. The use of high power surge suppression and isolation techniques effectively prevents damage to, and maloperation of, the equipment due to lead-borne interference signals. Furthermore the techniques employed mean that as far as external connections are concerned the microcomputer based system can be treated in the same way as a standard piece of electromechanical or semiconductor based equipment. No special precautions are required for wiring, insulation or continuity testing. The individual modules which constitute the interfacing system are as follows:(a) Plant Control Interface Module; This module consists of double pole reed relays fed via buffer amplifiers from the microcomputer. Each module can control up to eight times of plant and each plant items is double-pole switched to preserve security in the case of earth fault on the inter-
Microprocessor-based equipment for automatic control connection wiring. (b) Status Input Interface: This module enables either normally open or normally closed plant contacts or A.C. signals to be monitored by the equipment. The electrically noisy in~ut signals are surge suppressed and fed into opto-couplers which provide isolation. Each module caters for up to 16 plant contacts and will accept single or double wire inputs. (c) Analogue Interface Module: The analogue interface module is designed to accept up to 16 analogue inputs which can consist either of a current source in the range 0 to ±lOmA or a voltage source in the range 0 to ±lOV. Analogue inputs are assessed on a polled or interrupt basis under software control and are multiplexed after being processed by scaling amplifiers on the module. If simultaneous sampling of analogue inputs is required the scaling amplifiers may be replaced by sample and hold circuits. (d) Power Supply Uodule: The dc/dc converter power supply provides a range of input voltages, which is generally available to all modules with final regulation for the logic circuits, to the required voltage level provided on each module. Protection circuits are incorporated into the power supply to protect against damage due to overload. Over-voltage and under-voltage cut-off circuits are also included to prevent maloperation of the equipment under faulty conditions. A master relay, controlled by the CPU, provides a switched supply to all plant control modules. Microcomputer System Hardware: The microcomputer system interconnection is arranged on a standard motherboard technique which facilitates maximum permutations of modules. Modules may be plugged into a motherboard in any order and the motherboard can be expanded to facilitate 8, 16 or 20 modules. Each module in the microcomputer system has a self-contained power supply which incorporates current limit and fold back characteristics. The individual modules which constitute the microcomputer system are as follows:(a) CPU Module: This module can be considered to be a self-contained system since it not only contains the processing unit but also the system synchronisation circuits, bus driver capability, a standard 20mA loop or RS232 interface and electrically re-programmable read only memory (EPROM) and random access memory (RAM) in which the system executive software resides. The CPU module will support 64K bytes of memory and in excess of lK bytes input/output capability without necessity for additional decoding.
175
Eight levels of vectored interrupt, a real time 24 hour clock with lOmS resolution and a system manual reset and indication are also provided. (b) RAM/EPROM Module: This module provides the memory facility for the microcomputer system. Each module can contain a maximum of 24K bytes of memory which can be either electrically programmable read only memory (EPROM or random access memory (RAM) or combinations of each in lK word blocks. Each memory module in a system is assigned a unique address by manual operation of a selection circuit mounted on the module. (c) I/O Module: This module provides the interface between the interfacing system located in the electrically noisy section of the rack and the microcomputer system. All I/O modules are identical and accept data bus, address bus and control lines from motherboard bus system and each I/O module in the system can be uniquely addressed by suitable coding provided by manual operation of a selection circuit mounted on the module. Each module contains 9 ports (8 data lines each) which are programmable as input or output and can be configured in either interrupt or polled mode. (d) Modem Module: This module enables data to be transferred to and from remote points over standard communication circuits. The modem provides parallel to serial data conversion and is under software control for the selection of data rate and transmit/ receive mode format. The serial data is passed over the communication link in the form of a frequency shift keyed voice frequency signal via 15kV rms isolating transformers. The protocol format employed in the transmission and receipt of data is controlled by either a dedicated software module resident in the memory field allocated to user orientated software or, in the case of standard protocol formats, by dedicated hardware. The modem module also provides an RS232 or 20mA loop output for communication with local teletype equipment. (e) ADC Module: This module provides analogue to digital conversion. The ADC can be arranged for 12, 10 or 8 bit resolution with optional codes for unipolar and bipolar operation and is accessed by the CPU under software control. Decoding for the time division multiplexing of analogue input signals enables up to 128 analogue inputs to be accommodated in blocks of 16 which are accessed on a polled basis. (f) System Module: This module permits access to the microcomputer system by the test equipment for the purpose of diagnosing faults and exercising the system under manual or automatic conditions. It also permits the microcomputer to be extended into a multiple rack system. The module contains 64 words of manually selectable read only memory which allows user set point parameters to be
176
R. Parrnella
allocated enabling the equipment to be adapted to chanGing requirements without introducing programme alterations. (g) Bulk ~emory Storage: A bulk memory storaee facility is available in modular form using a cassette recorder. This can contain a maximum of lOOK bytes of memory. It can be interfaced with a standard I/O module for data logging and oscilloperturbograph applications. Software To maintain uniformity in structure and to allow application of the modular concept the software for the equipment is categorised into three main grou[-s as shown in Fig.3. Executive Software: The executive program consists of an algorithm known as the executive skeleton which calls up operational elements of the algorithm called executive functions. The executive functions are concerned primarily with the verification of the functional capacity of the microcomputer, diaGnostic routines, and the initialisation of the system. Supervisor Software: The supervisor software is mainly responsible for the scheduling of tasks to be carried out by the microcomputer and is also subdivided into three main areas:(a) Schedule - a ~roGram which controls the overall running of the system in terms of time sharing, and the management and book-kee~ing aspects of interrupt driven and polled systems. (b) Reduced Capacity Handling - a program which is user dependent and which defines those functions that the microcomputer is allowed to perform when certain error conditions exist or when certain resources are not available. (c) User Program - a program which controls the overall sequencinG of the system for a specific application. The program calls the User Functions and Scheduler proerams to execute the required activities. User Functions: The User Functions are a set of programs which prOVide the functional operational elements of the system. Each proGram in the set is a self-contained module which can be called by the User Program to perform a particular function, e.g. convert analogue data to digital values, pass parameters from one routine to another, load pr0t':rams to specific areas of RA!.! from eiven locations. This arrangement allows software to be written in a modular form as each software module is written for a specific function with general parameters which can be assigned particular values when the specific application is defined.
Security The security of both the hardware and software sections of the equipment is based upon a range of diagnostic routines contained in the executive software which is exercised as part of the initialisation procedure and are subsequently repeated during normal service. The diagnostic routines may be looked on as a background program. to be executed whenever user programs are suspended awaitin~ a resource or may be executed at a rate set initially by the user. The self-diagnostics rely on certain intelligence being present in the system at all times therefore the executive programs, of which the diagnostic routines constitute a substantial part, are located in memory which is physically situated on the CPU module. If this assumed intelligence should become faulty, watchdog circuits on the CPU module operate to prevent maloperation of peripheral equipment. The watchdog circuit is a simple, reliable hardware device which requires a reset signal to be generated by the CPU at regular intervals. If the reset siGnal is not generated, or if the reset line becomes fixed in any single state, the equipment is disabled in a controlled manner and alarms are prOVided. The self-diagnostic routines contain a number of programs that will teat and verify executive, supervisory and user software, test the availability of other modules in the system and record failures in error files situated on the CPU module. When the equipment is started an initialisation progra~_starts with the CPU performing extremely simple functions, gradually increasing the complexity of the task until the CPU module has been fully exercised and then proceeds to check other modules in the system. To ensure that each system can operate only with the program dedicated to it each EPROt{ memory chip has three bytes programmed to provide information unique to the particular system. The information consists of the serial nunber of the equipment, the chip reference position in the memory field and a code which defines the contents of the chip for use by an error checking propram. The initialisation program examines this data to determine the personality of the memory contents and if incorrect prevent further proeress. Faulty or wrongly inserted memory chips are also located using this program. Each interface module is coded to prOVide the identify of the type of interface, these codes being checked by initialisation program to ~etermine whether the correct type of interface is connected to each section of the I/O. This procedure ensures that the multi-way cables connecting the interface section to the microcomputer section are correct. The executive, supervisory and user programs are cycles by an error checking routines which uses the data obtained from the memory during the initialisation
Microprocessor-based equipment for automatic control procra~ to determine the validity of blocks of software prior to execution.
As part of the self-diagnostic routines the protection circuits associated with the power supplies situated on each module and the main power sup~ly module are examined. If a module power supply indicates a faulty situation then that particular module is closed down and the system is either closed down, kept running with reduced capacity or re-started. The priority of such decisions is detailed by the reduced capacity handlinG ~rogram and is dependent upon the user identifyine the priority of functions and resources. The nain power sup~ly unit is designed to provide a power reservoir sufficient to maintain the system lone enough for the CPU to close down the system safely in the event of a main power supply failure. To complement the range of diagnostic routines the watchdog circuits which detect both hardware and software faults, each module in the microcomputer system has integral surge su~pression to prevent damage to, and maloperation of, a module in the unlikely event of interference bypassine the interface circuits. Although the security arrangements detailed involve a penality in terms of speed of execution and cost they are considered to be essential to ensure consistent and correct operation in such a hostile environment. Commissioninc and Uaintenance Both commissioning and maintenance aspects of the equipment require that a comprehensive man/microcomputer interface be provided to allow, in the former case, complete verification of the system and, in the latter case, a means of assimilating the conveying failure information. The test equipment associated with the system control equipment is therefore designed on the basis of intelligent diagnostic and plant simulation equipment for co~issioning purposes and a local access point for fault investigation. The self-diagnostic proGrams record failure information in error files located on the CPU module and by interrogating these error files the status of the eqUipment may be verified. This is done by means of a standard ZOmA loo? or RS232 interface which is provided as part of the standard hardware on the CPU module and is accessible via a socket located on the front of the module. Where local indication of events and conditions is required a standard front panel can be housed on the front of the rack. The information display area consists of a 16-element matrix with each element comprising four light emitting diodes and an 8-digit alpha numeric display. A set of functional keys allows data to be interroGated and displayed locally. These facilities provide a powerful man-
177
machine interface to provide maximum information retrieval and display with maximum understanding by the operator.
APPLICATIONS The eqUipment described has been designed to cover the application areas stated in section 3. These are specific areas of application but the inherent extensible and flexible nature of the equipment makes it suitable for use in most types of industry where a high electrical noise environment would be encountered. Multi-Station Application A typical example of the application of the equipment in a multi-station role is the provision of a computer-based telecontrol system for the control, monitoring and acquisition of data for an electrical distribution network. In this application it is intended that a central computer would communicate with computers at a number of district control centres, with each district control centre communicating with system control equipment at each of a number of substations. Status, alarm, analogue and control information is collected by the substation system control equipme~t and is communicated to the district control via a multi-drop communication circuit which also services all other substations. The capacity of such an installation is dependent upon the types of modules plugged into the motherboard bus system. For an eight module motherboard, however, the followinc capacities would be typical: 48K bytes of memory (2 RAI.I/EPnOl,1 Modules) 144 lines of input/output 32 analogue signals 1.I0dules)
(2 Analogue Interface
80 plant status signals Modules) 24 plant controls -
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Power System Post-Fault Switching Application A typical example of this type of application is the substation arrangement shown in simplified form in Fig.4. The arrangement consists of six 132kV lines feeding duplicate busbars connected to two 132/33kV transformers. Eight 33kV feeders are connected to the busbars on the 33kV side of the two transformers. The control scheme is intended to provide autoreclosing and isolation facilities for faults on any part of the system. Although the microcomputer equipment was capable of providing all of the required facilities in one integrated scheme employing one CPU and three racks of interfacing modules it was decided for
178
R. Parmella
reasons of security to split the scheme into three independent sections. This ~rangement employed almost the same hardware as the single integrated scheme in terms of cost but greatly increased the overall security of the scheme. The advantages of separating the scheme in this manner were as follows:(a) The software for each section was simplified due to the lack of interaction between sections. This also meant the software was more easily checked. (b) Failure in anyone section of the equipment did not influence the other sections. (c) The system could be commissioned in stages which greatly simplified the commissiong requirements. (d) The introduction of modifications due to system developQent or operational problems would be simplified. Hardware Requirements: The total hardware associated with the scheme consists of:(i)
3 - 19 inch racks.
(ii)
3 - CPU modules.
(iii)
3 - System modules (192 words of manually selectable memory).
(iv)
9 - I/O modules (684 data lines).
(v)
15 - Status Interface modules (240 status monitoring points).
(iv)
16 - Plant Control modules control signals).
(vii)
3 - Power Supply Uodules.
(1~8
The plant control modules are connected to ~rovide additional security in that each module has one element connected as a check contact. In the event of a contact controlling a function on a given feeder becoming faulty the module can be isolated via the check contact and all other circuits can proceed normally.
Software Requirements: A simplified flow diagram of the software for Section 1 of the scheme is shown in Fig.5. This program is written for one circuit breaker and in this example is capable of being executed by six circuit breakers simultaneously. The software is written to cater for a variety of system conditions which determine the sequence of operation for a given section of the 132kV circuits, e.g. busbar and line voltage, circuit breaker position, autoreclose in/out, dead line charge, dead bar charge etc. Fig.6 shows a simplified flow diagram for the autoreclose requirements of the 33kV feeders. The program is written for one feeder and in this example may be executed by eight feeders simultaneously.
CONCLUSIONS A flexible and extensible microprocessorbased system for use in the high electrical noise environments associated with electrical power generation and distribution systems has been described. From the experience gained during the development of the equipment it is concluded that secure operation of microprocessor based systems in these enviromaents can be obtained only by correct recognition of the hazards associated with such applications and the incorporation into the equipment of means to overcome such hazards. The means provided to protect the equipment should be an inherent part of the design and operation of both the hardware and software sections of the equipment.
ACKNOWLEDGEMENTS The development of the e~uipment described in this paper was undertaken by N.E.I. Electronics Ltd., Reyrolle Protection. The author wishes to thank the Company for permission to pUblish the paper and to acknowledge the helpful and enthusiastic support of colleagues in its preparation.
Microprocessor-based equipment for automatic control
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Micro_cor.pu~~_C.?ntrol EqUipment
R. Parmella
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182
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Fig.6 - Simplified Application Software Flow Diagram for Autoreclose of 33kV Circuit Breakers ~~.------
Abstract. This paper describes a microprocessor-based system designed to provide control, telecontrol, telemetry and data logging functions in power generation, transmission and distribution systems. The Modular approach adopted for the hardware and software is examined and details are provided of the measures taken to ensure secure reliable operation in the electrically hostile environment associated with power systems. Examples of the application of the equipment are Given. The paper concludes that correct operation of microprocessor-based systems in such hostile environments can be ensured only by correctly identifying the hazards and incorporating into the equipment hardware and software means to overcome such hazards.
Keywords. Power system control; telecontrol; data acquisition; system integrity; power transmission; power generation; power distribution; microprocessors.
intelligent systems for decentralised The use of high-spee6 di~ital techniques in the electrically hostile environment associated with electrical power systems requires that special precautions be taken in the design of both the hardware and software sections of such equipment.
INTRODUCTION
ap~lication.
The worldwide development of power systems has produced a need for more extensive data acquisition, greater flexibility in remote control and increased complexity in switching and control iunctions following system fault conditions. As load patterns and growth alter, power system configurations are changed and there is an attendant requirement for associated control systems and protection to be adapted, if possible without incurrinG costly and time consuming penalties due to extensive hardware changes.
The microprocessor based system control equipment described in this paper has been specifically designed to meet the exacting enviro~~er.ta! con~i~io~s v~i!e ~rovi~1ng
maximum flexibility of usage through the modular design nature of both hardware and software.
Until recently the majority of data acquisition, control and switching functions have been accommodated by hardwired dedicated el.ectro.,.mechanical or semiconductor-based systems which, althouch successful in application suffer from a number of disadvantages including lack of flexibility, duplication of specification effort, and exclusion of self-testing facilities due to complexity and cost. These disadvantages have resulted in a trend toward the use of programmable equipment in place of hardwired systems.
DESIGN CONSIDERATIONS The design considerations for a multifunction processor based system must include both the expected areas of application and the environment in which the equipment must operate along with the detailed technical requ iremen t s . Application Areas Considerations of application have a great impact on the topology of microprocessor based systems and it is probable that any multi-functional system will have conflictin~ requirements in terms of memory size, speed, interfacing capability, flexibility and cost. The main areas of application considered during the development of the system were:-
There has also been limited experience in the use of centralised computer techniques applied to substation control. This experience has indicated the need for decentralised intelligent systems which may or may not be linked into an overall controlling function. The advent of microprocessor technology has provided the means of economically producing programoable,
(a)
173
Power System Post-Fault Switching.
174
R. Parmella
(b)
Fower Station Auxiliary Plant Control.
(c)
Telecontrol.
(d)
Telemetry and data logging.
(e)
Alarm Annunciation.
The requirements in each of the application areas differ considerably and, if a standard approach is to be adopted, the design should cater for ex,andable interfacing with a modular ap~roach to both the hardware and software of the system. Environment Electrical power systems create an electrically hostile environment in which high s~eed digital equipment must operate with a high degree of integrity. Impulsive and continuous interference signals may be introduced into wiring due to insulation breakdown, protective gap flashover, differential rise of earth potential during system fault conditions as well as many other causes. Radiated interference of wide bandwidth may be generated by a variety of sources on the system, one of the most notable being the noise generated by isolator operation. In order to provide a consistent testing and acceptance procedure for equipment operating in this environment, standards relating to impulse and interference tests have been derived for relays and protection schemes in particular IEC 255-4, Appendix E speci~ies impulse and interference tests for static equipment. During the design of the present equip~ent it was considered essential that, as a minimum requirement, it should meet the require~ents of IEC 255-4 and that further tests relating to fast transients and radiated field interference should be examined. To comply with these test requirements it was considered essential that safeguards should be incorporated into the hardware and software early in the development stage and be integrated into the total system operation. Commissioning and Maintenance The introduction of microprocessor based systems can require a considerable change in the role and attitude of staff employed in the commissioning and maintenance of equipment, particularly if the staff are familiar with hard wired schemes. Training of staff in the disciplines associated with microcom,uters does much to overcome this difficulty, but in order to achieve the necessary level of understanding it is necessary to provide a man-machine interface which allows for comprehensive yet comprehendable tests to be performed. It was therefore necessary to incorporate hardware and software in the system to allow man-machine dialogue to occur with ease.
DESCRIPTION OF EQUIPMENT The system control equipment is intelligent stand-alone equipment composed of a range of standard modules housed in one or more 19 inch racks. Rack Housing A typical rack layout is shown in Fig.l. The rack is sub-divided into two areas a screened high-security electrically qu~et area with no direct access to the outside world, and an interface area which provides communication with the electrically hostile outside world. The microcomputer modules are located in the screened area and the interface modules are located in the outside access area. Data transfer between the two areas is provided only after the interface modules have suppressed harmful interference effects present on the external wiring which enter via terminals on the rear of the rack. The sub-division of the system allows the microcomputer section and interfacing section to expand and contract independently to meet specific application requirements while ensuring that the effects of radiated and lead-borne interference are removed. Interfacing System The arrangement of the interfacing system and the level of isolation provided is shown in Fi~.2. The power supply and each input and output interface to plant provide a 5kV impulse barrier between the microcomputer and the external connections and between the input circuits and earth. In addition each plant-connected terminal has 5kV impulse isolation to all other terminals. Connections from the modem section of the microcomputer to external communication circuits e.g. voice frequency telegraph circuits, ar~ made via isolating transformers which provide a 15kV rms barrier between the communication circuit and the microcomputer equipment. The use of high power surge suppression and isolation techniques effectively prevents damage to, and maloperation of, the equipment due to lead-borne interference signals. Furthermore the techniques employed mean that as far as external connections are concerned the microcomputer based system can be treated in the same way as a standard piece of electromechanical or semiconductor based equipment. No special precautions are required for wiring, insulation or continuity testing. The individual modules which constitute the interfacing system are as follows:(a) Plant Control Interface Module; This module consists of double pole reed relays fed via buffer amplifiers from the microcomputer. Each module can control up to eight times of plant and each plant items is double-pole switched to preserve security in the case of earth fault on the inter-
Microprocessor-based equipment for automatic control connection wiring. (b) Status Input Interface: This module enables either normally open or normally closed plant contacts or A.C. signals to be monitored by the equipment. The electrically noisy in~ut signals are surge suppressed and fed into opto-couplers which provide isolation. Each module caters for up to 16 plant contacts and will accept single or double wire inputs. (c) Analogue Interface Module: The analogue interface module is designed to accept up to 16 analogue inputs which can consist either of a current source in the range 0 to ±lOmA or a voltage source in the range 0 to ±lOV. Analogue inputs are assessed on a polled or interrupt basis under software control and are multiplexed after being processed by scaling amplifiers on the module. If simultaneous sampling of analogue inputs is required the scaling amplifiers may be replaced by sample and hold circuits. (d) Power Supply Uodule: The dc/dc converter power supply provides a range of input voltages, which is generally available to all modules with final regulation for the logic circuits, to the required voltage level provided on each module. Protection circuits are incorporated into the power supply to protect against damage due to overload. Over-voltage and under-voltage cut-off circuits are also included to prevent maloperation of the equipment under faulty conditions. A master relay, controlled by the CPU, provides a switched supply to all plant control modules. Microcomputer System Hardware: The microcomputer system interconnection is arranged on a standard motherboard technique which facilitates maximum permutations of modules. Modules may be plugged into a motherboard in any order and the motherboard can be expanded to facilitate 8, 16 or 20 modules. Each module in the microcomputer system has a self-contained power supply which incorporates current limit and fold back characteristics. The individual modules which constitute the microcomputer system are as follows:(a) CPU Module: This module can be considered to be a self-contained system since it not only contains the processing unit but also the system synchronisation circuits, bus driver capability, a standard 20mA loop or RS232 interface and electrically re-programmable read only memory (EPROM) and random access memory (RAM) in which the system executive software resides. The CPU module will support 64K bytes of memory and in excess of lK bytes input/output capability without necessity for additional decoding.
175
Eight levels of vectored interrupt, a real time 24 hour clock with lOmS resolution and a system manual reset and indication are also provided. (b) RAM/EPROM Module: This module provides the memory facility for the microcomputer system. Each module can contain a maximum of 24K bytes of memory which can be either electrically programmable read only memory (EPROM or random access memory (RAM) or combinations of each in lK word blocks. Each memory module in a system is assigned a unique address by manual operation of a selection circuit mounted on the module. (c) I/O Module: This module provides the interface between the interfacing system located in the electrically noisy section of the rack and the microcomputer system. All I/O modules are identical and accept data bus, address bus and control lines from motherboard bus system and each I/O module in the system can be uniquely addressed by suitable coding provided by manual operation of a selection circuit mounted on the module. Each module contains 9 ports (8 data lines each) which are programmable as input or output and can be configured in either interrupt or polled mode. (d) Modem Module: This module enables data to be transferred to and from remote points over standard communication circuits. The modem provides parallel to serial data conversion and is under software control for the selection of data rate and transmit/ receive mode format. The serial data is passed over the communication link in the form of a frequency shift keyed voice frequency signal via 15kV rms isolating transformers. The protocol format employed in the transmission and receipt of data is controlled by either a dedicated software module resident in the memory field allocated to user orientated software or, in the case of standard protocol formats, by dedicated hardware. The modem module also provides an RS232 or 20mA loop output for communication with local teletype equipment. (e) ADC Module: This module provides analogue to digital conversion. The ADC can be arranged for 12, 10 or 8 bit resolution with optional codes for unipolar and bipolar operation and is accessed by the CPU under software control. Decoding for the time division multiplexing of analogue input signals enables up to 128 analogue inputs to be accommodated in blocks of 16 which are accessed on a polled basis. (f) System Module: This module permits access to the microcomputer system by the test equipment for the purpose of diagnosing faults and exercising the system under manual or automatic conditions. It also permits the microcomputer to be extended into a multiple rack system. The module contains 64 words of manually selectable read only memory which allows user set point parameters to be
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R. Parrnella
allocated enabling the equipment to be adapted to chanGing requirements without introducing programme alterations. (g) Bulk ~emory Storage: A bulk memory storaee facility is available in modular form using a cassette recorder. This can contain a maximum of lOOK bytes of memory. It can be interfaced with a standard I/O module for data logging and oscilloperturbograph applications. Software To maintain uniformity in structure and to allow application of the modular concept the software for the equipment is categorised into three main grou[-s as shown in Fig.3. Executive Software: The executive program consists of an algorithm known as the executive skeleton which calls up operational elements of the algorithm called executive functions. The executive functions are concerned primarily with the verification of the functional capacity of the microcomputer, diaGnostic routines, and the initialisation of the system. Supervisor Software: The supervisor software is mainly responsible for the scheduling of tasks to be carried out by the microcomputer and is also subdivided into three main areas:(a) Schedule - a ~roGram which controls the overall running of the system in terms of time sharing, and the management and book-kee~ing aspects of interrupt driven and polled systems. (b) Reduced Capacity Handling - a program which is user dependent and which defines those functions that the microcomputer is allowed to perform when certain error conditions exist or when certain resources are not available. (c) User Program - a program which controls the overall sequencinG of the system for a specific application. The program calls the User Functions and Scheduler proerams to execute the required activities. User Functions: The User Functions are a set of programs which prOVide the functional operational elements of the system. Each proGram in the set is a self-contained module which can be called by the User Program to perform a particular function, e.g. convert analogue data to digital values, pass parameters from one routine to another, load pr0t':rams to specific areas of RA!.! from eiven locations. This arrangement allows software to be written in a modular form as each software module is written for a specific function with general parameters which can be assigned particular values when the specific application is defined.
Security The security of both the hardware and software sections of the equipment is based upon a range of diagnostic routines contained in the executive software which is exercised as part of the initialisation procedure and are subsequently repeated during normal service. The diagnostic routines may be looked on as a background program. to be executed whenever user programs are suspended awaitin~ a resource or may be executed at a rate set initially by the user. The self-diagnostics rely on certain intelligence being present in the system at all times therefore the executive programs, of which the diagnostic routines constitute a substantial part, are located in memory which is physically situated on the CPU module. If this assumed intelligence should become faulty, watchdog circuits on the CPU module operate to prevent maloperation of peripheral equipment. The watchdog circuit is a simple, reliable hardware device which requires a reset signal to be generated by the CPU at regular intervals. If the reset siGnal is not generated, or if the reset line becomes fixed in any single state, the equipment is disabled in a controlled manner and alarms are prOVided. The self-diagnostic routines contain a number of programs that will teat and verify executive, supervisory and user software, test the availability of other modules in the system and record failures in error files situated on the CPU module. When the equipment is started an initialisation progra~_starts with the CPU performing extremely simple functions, gradually increasing the complexity of the task until the CPU module has been fully exercised and then proceeds to check other modules in the system. To ensure that each system can operate only with the program dedicated to it each EPROt{ memory chip has three bytes programmed to provide information unique to the particular system. The information consists of the serial nunber of the equipment, the chip reference position in the memory field and a code which defines the contents of the chip for use by an error checking propram. The initialisation program examines this data to determine the personality of the memory contents and if incorrect prevent further proeress. Faulty or wrongly inserted memory chips are also located using this program. Each interface module is coded to prOVide the identify of the type of interface, these codes being checked by initialisation program to ~etermine whether the correct type of interface is connected to each section of the I/O. This procedure ensures that the multi-way cables connecting the interface section to the microcomputer section are correct. The executive, supervisory and user programs are cycles by an error checking routines which uses the data obtained from the memory during the initialisation
Microprocessor-based equipment for automatic control procra~ to determine the validity of blocks of software prior to execution.
As part of the self-diagnostic routines the protection circuits associated with the power supplies situated on each module and the main power sup~ly module are examined. If a module power supply indicates a faulty situation then that particular module is closed down and the system is either closed down, kept running with reduced capacity or re-started. The priority of such decisions is detailed by the reduced capacity handlinG ~rogram and is dependent upon the user identifyine the priority of functions and resources. The nain power sup~ly unit is designed to provide a power reservoir sufficient to maintain the system lone enough for the CPU to close down the system safely in the event of a main power supply failure. To complement the range of diagnostic routines the watchdog circuits which detect both hardware and software faults, each module in the microcomputer system has integral surge su~pression to prevent damage to, and maloperation of, a module in the unlikely event of interference bypassine the interface circuits. Although the security arrangements detailed involve a penality in terms of speed of execution and cost they are considered to be essential to ensure consistent and correct operation in such a hostile environment. Commissioninc and Uaintenance Both commissioning and maintenance aspects of the equipment require that a comprehensive man/microcomputer interface be provided to allow, in the former case, complete verification of the system and, in the latter case, a means of assimilating the conveying failure information. The test equipment associated with the system control equipment is therefore designed on the basis of intelligent diagnostic and plant simulation equipment for co~issioning purposes and a local access point for fault investigation. The self-diagnostic proGrams record failure information in error files located on the CPU module and by interrogating these error files the status of the eqUipment may be verified. This is done by means of a standard ZOmA loo? or RS232 interface which is provided as part of the standard hardware on the CPU module and is accessible via a socket located on the front of the module. Where local indication of events and conditions is required a standard front panel can be housed on the front of the rack. The information display area consists of a 16-element matrix with each element comprising four light emitting diodes and an 8-digit alpha numeric display. A set of functional keys allows data to be interroGated and displayed locally. These facilities provide a powerful man-
177
machine interface to provide maximum information retrieval and display with maximum understanding by the operator.
APPLICATIONS The eqUipment described has been designed to cover the application areas stated in section 3. These are specific areas of application but the inherent extensible and flexible nature of the equipment makes it suitable for use in most types of industry where a high electrical noise environment would be encountered. Multi-Station Application A typical example of the application of the equipment in a multi-station role is the provision of a computer-based telecontrol system for the control, monitoring and acquisition of data for an electrical distribution network. In this application it is intended that a central computer would communicate with computers at a number of district control centres, with each district control centre communicating with system control equipment at each of a number of substations. Status, alarm, analogue and control information is collected by the substation system control equipme~t and is communicated to the district control via a multi-drop communication circuit which also services all other substations. The capacity of such an installation is dependent upon the types of modules plugged into the motherboard bus system. For an eight module motherboard, however, the followinc capacities would be typical: 48K bytes of memory (2 RAI.I/EPnOl,1 Modules) 144 lines of input/output 32 analogue signals 1.I0dules)
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Power System Post-Fault Switching Application A typical example of this type of application is the substation arrangement shown in simplified form in Fig.4. The arrangement consists of six 132kV lines feeding duplicate busbars connected to two 132/33kV transformers. Eight 33kV feeders are connected to the busbars on the 33kV side of the two transformers. The control scheme is intended to provide autoreclosing and isolation facilities for faults on any part of the system. Although the microcomputer equipment was capable of providing all of the required facilities in one integrated scheme employing one CPU and three racks of interfacing modules it was decided for
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reasons of security to split the scheme into three independent sections. This ~rangement employed almost the same hardware as the single integrated scheme in terms of cost but greatly increased the overall security of the scheme. The advantages of separating the scheme in this manner were as follows:(a) The software for each section was simplified due to the lack of interaction between sections. This also meant the software was more easily checked. (b) Failure in anyone section of the equipment did not influence the other sections. (c) The system could be commissioned in stages which greatly simplified the commissiong requirements. (d) The introduction of modifications due to system developQent or operational problems would be simplified. Hardware Requirements: The total hardware associated with the scheme consists of:(i)
3 - 19 inch racks.
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3 - System modules (192 words of manually selectable memory).
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The plant control modules are connected to ~rovide additional security in that each module has one element connected as a check contact. In the event of a contact controlling a function on a given feeder becoming faulty the module can be isolated via the check contact and all other circuits can proceed normally.
Software Requirements: A simplified flow diagram of the software for Section 1 of the scheme is shown in Fig.5. This program is written for one circuit breaker and in this example is capable of being executed by six circuit breakers simultaneously. The software is written to cater for a variety of system conditions which determine the sequence of operation for a given section of the 132kV circuits, e.g. busbar and line voltage, circuit breaker position, autoreclose in/out, dead line charge, dead bar charge etc. Fig.6 shows a simplified flow diagram for the autoreclose requirements of the 33kV feeders. The program is written for one feeder and in this example may be executed by eight feeders simultaneously.
CONCLUSIONS A flexible and extensible microprocessorbased system for use in the high electrical noise environments associated with electrical power generation and distribution systems has been described. From the experience gained during the development of the equipment it is concluded that secure operation of microprocessor based systems in these enviromaents can be obtained only by correct recognition of the hazards associated with such applications and the incorporation into the equipment of means to overcome such hazards. The means provided to protect the equipment should be an inherent part of the design and operation of both the hardware and software sections of the equipment.
ACKNOWLEDGEMENTS The development of the e~uipment described in this paper was undertaken by N.E.I. Electronics Ltd., Reyrolle Protection. The author wishes to thank the Company for permission to pUblish the paper and to acknowledge the helpful and enthusiastic support of colleagues in its preparation.
Microprocessor-based equipment for automatic control
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