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Inter-relation between experiment and analogue simulation, mathematical models and computation technique in calculations of electric power systems
Inter-relation Between Experiment and Analogue Simulation, Mathematical Models and Computation Technique in Calculations of Electric Power Systems V. A. VENIKOV Introduction
The automation of electric power systems is entering a new phase of development. Automatic control equipments which even in the recent past were auxiliary devices are becoming of fundamental importance and have a decisive influence on all the conditions of the system and of transient processes. Hitherto devices used to control the conditions of electric power systems tended to operate to a set programme. A number of such devices, for example, the excitation field regulators first developed in the Soviet Union and now widely used, introduced simple computer elements such as differentiating controls acting according to the differential coefficient of a magnitude and thus, to some extent, capable of 'predicting' changes of conditions. It is to be expected that the immediate future will see much more rapid and fuller use of both digital and analogue computer techniques in the processes of power system control. Automatic control employing such techniques will be used both to establish optimum steady conditions and to control the system during transients. Use of Computers
Three main trends may be noted in considering the use of mathematical electronic machines to solve power engineering problems. The first trend, which should be developed in the immediate future, is the application of mathematical electronic machines for economic analysis of power system operation. The machines will be applied for economic calculations of projects for the design and automatic distribution of loads between stations in a power system. In the first stage the staff will use computer data as auxiliary information; subsequently the computed data will be appropriately transformed and used directly to control the equipment that changes the system conditions. Of course, the introduction of such devices into systems will require preliminary experiments, on physical models to start with. Further development of computer technique will open the second field of application of computers for the control of transient processes. Use will be made of high-speed analysis of the processes combined with rapid effect on transient conditions. Computer control of these rapid processes will become feasible when their operating speed will be fast enough to take less time than the transient process. Even more than in the control of steady conditions tests prior to introducing such equipment into power systems will be important. The third trend of computer application consists in calculating new designs, of control equipment as well as design and
operating calculations of power system conditions. In this case, rapid performance of experiments, usually between calculations, is even more necessary. New possibilities of calculating conditions and control of automated power systems will lead to the appearance of new quantitative relationships and qualitative properties. In all formulations of new investigations it is necessary to pay special attention to experiment to make the initial theoretical statement more accurate, to confirm the numerical values of parameters and to avoid errors due to incorrect evaluation of physical effects. Such errors can be particularly serious when using large computers. It should be emphasized that new designs of machines and apparatus in future power systems, developed as a result of the application of automatic control, will require accurate mathematical description of the processes that occur both in the new elements and in the systems as a whole. An example of such new elements in power systems are generators of super-high rating, the characteristics of which, even at present, cannot be considered in isolation from the action of automatic control. The design of these generators has to allow for the possibility of automatic control. Further examples are the new asynchronized synchronous machine (All-Union Scientific Research Institute of the Electrical Industry, Scientific Research Institute of Electro-Mechanics); the high-voltage generator (Moscow Power Institute) ; the high-power controlled reactor with pre-magnetization (Power Institute of the Academy of Science); electronic and ionic apparatus connected directly to the power part of power systems and participating in the formation of system conditions (Moscow Power Institute). A further typical feature is the appearance in power systems of new techniques associated with the utilization of atomic power, which have necessitated the introduction of quite new electrical constructions and installations, such as the electromagnetic pumps used in atomic power stations. Combination of Experiment, Simulation and Analysis
A purely analytical approach to the study of processes occurring in individual pieces of new equipment or in their interaction with other equipment and the power system as a whole is impossible. Special attention needs to be paid to the combination of full-scale experiment, analogue simulation and mathematical analysis which makes use of the experimental data, clarifies the concept of the processes and thus ensures optimum control. Experiment is in no way in contradiction to analytical investigations, including those made on high-speed computers; on the contrary, it is an inseparable part of the cycle of engineering investigations based both on analysis and
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on direct experimental verification of theoretical concepts. The complication of power systems, the increase in the capacity of systems as a whole and of their individual parts makes direct experimentation in systems more and more difficult, however, so that laboratory investigations on analogues of various types and sizes assume greater importance. The task of such experimental investigations includes verification of the correctness of the mathematical formulation of the problems, correction of the coefficients that enter into the mathematical characteristics of the effects studied, general characteristics of system behaviour when various elements are present in it, and lastly testing components the rapid adjustment of which is difficult or often impossible in a large power system. Such experimental adjustment investigations become more important with increasing use of automatic equipment and increasing complication of control methods. Preliminary testing and adjustment of individual controller parts. and verification of their inter-action, should be carried out on physical models before the new elements are introduced dir'!ctly into a large power system. The problem of the part to be played by man assumes special importance in view of the increasing use of computer-control technique in power systems. Installations based on methods of physical modelling offer boundless possibilities for the solution of problems of this kind and for overcoming the difficulties that are bound to arise before equipment of this kind is used in large power systems. Staff training for work in the new conditions of automated systems can also be greatly simplified by the use of physical models. For all these reasons it is necessary to consider methods of mathematical investigation and physical modelling as a single whole, in particular for controlling integrated automated electric power systems. It is typical of these systems that some parameters can only be stated arbitrarily, so that the mathematical description of the processes becomes less definite. Hence it is essential for the engineers to attain accurate physical concepts. In investigating conditions of integrated systems computer technique and automatics should more and more supplement one another, without impairing the use of analogues and physical models.
many other kinds of equipment are being developed with the aid of preliminary investigations en complete phYSical models. The steady increase in the power of models and the combination of experiment with analytical calculations by high-speed electronic computers will rapidly lead to the best results. It is important to emphasize that investigations on models should not be considered as replacing full-scale experimentation nor analytical investigation. On the contrary, as a means of making the mathematical description of effects more accurate, they lead to rapid evolution of adequate mathematical methods of investigation. Mathematical formulations should be in line with the physics of the problem and should give quick solutions of adequate, but not excessive, accuracy. Therefore, besides introducing modern continuous and discrete computer techniques for the control and design of power systems, the special computer installations hitherto used will have to be maintained, apparently for quite a long time. Thus d.c. calculating panels will doubtlessly continue to be used for their simplicity and, after improvement, can be relied on for approximate solutions of immediate problems such as power flow in complex systems, short-circuit currents, etc. In solving such problems the computer cannot and indeed should not compete with the simple calculating panel. A.c. calculating boards will be used more and more to determine the distribution of active and reactive loads in complicated systems with a large number of stations. At present they are applied in combination with discrete machines for determining power distribution coefficients between the elements of a system. These coefficients are first calculated on the calculating board and are then introduced into the discrete machine; attempts to calculate these coefficients directly on the computer have so far been unsuccessful. Continuously operating mathematical models will probably be confined to applications in which they are used directly as parts of the automatic equipment (for example, elements for differentiation, summation and integration in various kinds of special controllers), and for calculations on processes for which there exists a partial but insufficiently accurate mathematical description, so that the use of a discrete machine is not justified. The use of a discrete machine is particularly to be recommended where high accuracy of calculation is essential. This includes problems of the distribution of reactive and active loads, determination of optimum conditions and economic operation of projected systems, i.e. highly accurate loss calculations. To summarize it may be repeated that both in analysing the operation of automated electrical systems, in direct adjustment of automatic equipment and in the design of parts of automatic systems, full-scale and model experiments and computer techniques, discrete and continuous, facilitate the solution of new problems; it is impermissible to consider one type of technique as opposed to the other. Experience of developing new automatic devices such as intense field regulators now operating in the Volga Hydroelectric Station, and new protective devices for the transmission line from this station to Moscow show that the automatic equipment can be completed most rapidly by adopting the following sequence: analogues are set up on the basis of general theoretical and physical concepts and are studied experimentally in a model of the system under conditions physically similar to those under which they will operate in practice. The model is used not only to work out the designs but also to elucidate their characteristics and to correct the
Simulation on Physical Models
Physical simulation is developing in two directions: (I) In studying the transient processes in both equipment and power systems as a function of time. This may be called 'incomplete' simulation because the electric, magnetic and other fields that occur in practice are not reproduced in the model. Establishing conditions similar to the processes occurring in full-scale equipment make it possible to reproduce these on smaller scale and to obtain results relevant to the system. The dynamic models of power systems widely used in the U.S.S.R., France, Austria, Germany, England and Australia are of this type. (2) In the direction of simulating processes in time and space, which may be called 'complete' simulation. This is particularly important in the design of new elements of automated systems, for which these physical models provide preliminary verification of operation and adjustment, since the mathematical characteristics of new equipment which has not been developed and tested on a full scale are necessarily unknown. Such methods of complete simulation are now extensively developed. Atomic reactors, electromagnetic pumps, high-voltage generators for voltages of 100-200 kV, new types of an asynchronized synchronous alternator and 4-A.R.C.4
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tion the collection and transmission of a considerable quantity of processed information is the group of problems associated with the economic distribution of active and reactive power between stations and individual parts of the system. This is now successfully solved by means of digital computers. In the present stage of development, however, the machine plays the part of adviser but does not yet directly control power systems. Machines for direct control should be developed in the very near future. Work in this direction is being carried out for both digital and analogue computer techniques. The influence of analogue and digital computer techniques can also be clearly seen in the development of automatic field control of alternators in power systems. The first stage of development of automatic field control was characterized by the application of electromechanical regulators which had a zone of instability and did not operate on small alterations or in the early stages of transients. Their role was limited to correcting the voltage, i.e. to restoring it some time after a change in conditions had occurred. Speed and frequency controllers were similarly constructed. All these controllers usually were characterized by the circumstance that each magnitude was controlled by a change in itself, there being no interaction of control from a number of changes. The second stage of control development, which may be termed the stage of intensified control, is typified by the more effective use of electronic controllers and compounding devices with no zone of insensitivity. This made it possible to evolve so-called artificial stability, with a limiting power for synchronous machines under steady-state conditions, characterized by transient e.mJ. and transient synchronous impedance. This, as it were, somewhat corrected the characteristics and parameters of the synchronous machine which, without control or with early methods of control, was characterized by synchronous impedance and no-load e.m.f. The application of field forcing operated by automatic control equipment made it possible to operate in the initial stages of the transient process and partially to improve the dynamic characteristics of the synchronous machine. At this stage of development alternator speed control was subject to no change and, separated as before from secondary control of frequency, was not directly influenced by changes in the electrical conditions, although there were some attempts to develop turbine emergency governors of this kind on an industrial scale. Frequency control devices, however, began to fulfil not only functions of secondary control, maintaining the mean frequency of the system, but also of economic load distribution between stations. Automatic control, as now developed, is called intense control; this may be considered as a third stage from which it should be possible to make rapid transition to the fourth stage of integrated control of power systems and cybernetic control. Intense field control is characterized by high intensity of action: high excitation ceilings and high amplification factors of control equipment become possible by using feedback and controlling not only the change in the parameters of the system but also their rate and acceleration of change. Intense control will make it possible partially or completely to exclude the influence of generator and transformer reactance on the limiting power that can be transmitted, so that under static conditions it will be possible to have a transmission limit set by the line. Here the function of intense field control is extended and the problem of maintaining the voltage becomes one of the partial problems of field control. The dynamic characteristics of systems fitted with intense
mathematical description of the processes. By subsequent analytical investigation the theory of the devices under development is improved, whereupon new analogues are constructed which are again tested and developed, so that the mathematical description of the process can be further corrected. The prototype models of controllers and protective devices made at the factories are again tested in the model after which an experimental series is manufactured. Before being installed in an operating system the automatic and protective devices are first adjusted on the model, after which a limited number of tests are made directly in the system. It was just such interaction between full-scale and model tests with analytical investigations that ensured rapid adjustment of the system for the Volga Hydro-electric Station, which was the first in the world to be supplied with intense field control and highly perfected protective equipment. Revision of Theoretical Concepts It should be noted that successful application of high-speed electronic digital computers for solving concrete technical problems and for control of systems will soon necessitate some reconsideration of the theory of operation of electrical power systems and their parts. This theory was often evolved owing to the engineer's need of overcoming mathematical difficulties in solving the equations of the system, yet the adequacy of the mathematical description of the physics of the process was frequently jeopardized in doing this. For example, parts of the Park-Gorev two-reaction theory which arbitrarily distinguishes two axes in a synchronous machine, originated in the difficulty of solving equations with periodic coefficients. Other calculation methods for electric systems also involve assumptions remote from the physical effects, and sometimes also from the essence of the matter-the outcome of attempts to find simplified engineering solutions. . Equations with periodic coefficients can now be solved on digital computers, and so there will probably be no need to write the equations of synchronous machines with reference to arbitrary direct and quadratic axes. Hence, the general tworeaction theory of the operation of electrical systems may prove unnecessary. Both the theory and its mathematics can be expressed in matrix formulation and appropriate analysis, relieving the engineer of difficulties associated with numerical solutions. New equations will be needed, however, which correspond to both the physics of the effect and the capabilities of computers used for analysis and direct control of these conditions. Such changes in the theory of systems, their operation and components must necessarily be accompanied by experimental investigations, lest perspective and adequate representation of the physical effects be lost. Combination of full-scale experiment, simulation and computer techniques is therefore necessary.
Automatic Devices for Power Systems
The development of the control of power systems, in line with the application of principles of cybernetics, passes more and more from solving stabilization problems and planned change of. controlled variables to the solution of problems of logic combined with simpler control problems. The introduction of logical action into automatic control systems is based on the application of digital, continuously operating computers. In the development of modern automatic control equipment it is typical to employ the condition of invariancy by avoiding steady deviations and transient components of error. Typical of cybernetic problems which require for their solu50
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EXPERIMENT AND ANALOGUE SIMULATION, MATHEMATICAL MODELS AND COMPUTATION IN CALCULATIONS control will also change insofar as field control corresponding to the first and second differential coefficients of operating parameters will, by means of field forcing and weakening, improve the dynamic characteristics of the system directly after the disturbance, combined with optimum damping of the disturbance. The static limit of transmission during intense field control will then become the limit set by the possibilities of the transmission line, and the dynamic limit will be the same as the static. Intensive operation on the transient process will actually exceed that of field control. Investigations carried out on computers and with physical models have been used to develop and perfect equipment for the electrical retardation of generators that accelerated under fault conditions. This electrical retardation device is controlled by devices that react to sudden unloading with the resultant sudden change in turbine torque. These devices can raise dynamic stability to the level of static stability, rendering the system stable under any short-circuit conditions. A further improvement in controlling load impedances is control not only in proportion to the power thrown off but also according to the differential coefficient of speed and acceleration of the particular alternators. With this type of control the parameters controlled may be made practically invariant, however severe the disturbance. The use of controlled retardation devices combined with intense field control also ensures stability when steam turbines are used, apart from main transmission lines, on intersystem links where till now it had been necessary to maintain a high stability reserve, so that their transmission capacity was not fully utilized.
In effecting intense field control of turbines it is rational to use combined methods of control (according to changes in control magnitude and its differential coefficient). Intense control of field, torque and speed of water-driven and steam-driven electric power systems ensures not only improvement of steady-state operating conditions with increased system stability but also facilitates auto-restoration of synchronous operation of any station that may have dropped out of synchronism. Thus there appears the concept of resultant stability of a system. In a system having resultant stability some parts may run asynchronously for a short time, synchronous operation being restored automatically as soon as consumer operating conditions permit. The presence of such a group of controlled devices means transition to integrated control reacting to changes in several parameters of the power system and acting simultaneously on several characteristics of the machine. A further stage should be transition to automatic control, which combines the integrated controllers mentioned above in a single unit that will determine the best operating conditions of the system and then make the necessary adjustments to secure the best effect. Subsequent development should be in the direction of cybernetics, evaluating the development of the process according to information received in transient and steady-state conditions and accordingly adjusting the control system to optimum conditions. The application of methods of intense and integrated control will permit a complete solution of the problem of ensuring the quality of electric power in systems with sudden changes of load.
DISCUSSION H. F. R . TAYLOR (U.K.) What are the parameters of the largest power system that can be simulated on the model; what is the number of power stations, alternators, their ratings, the number of transmission lines, their voltage and their lengths? V. A. VENIKOV, in reply. The possibility of simulating electric power systems with physical or mathematical models is not limited by the power of the system, its length or the number of power stations and machines involved to be studied. However, on physical models the main problems studied are usually those which reveal the physical nature of phenomena, the verification of the effects of new machines, apparatus, controllers, etc. Therefore, it is inadvisable to simulate on such models an excessive number of power stations and individual machines within these power stations. It is possible to limit oneself to representing the basic units and to substitute a large power station, for instance, the Volga Hydroelectric Station imeni V. I. Lenin, by two machines. This is fully adequate for studying the influence of this station on the remaining parts of a system. Dynamic (physical) models contain 5 to 30 machines, which simulate large stations and sub-stations with synchronous compensators. This number of machine models is sufficient for studying the processes which occur in the interconnected power systems of Siberia, Ural and the European part of the Soviet Union. The major part of the model is built to operate at normal industrial frequency . The models which simulate large alternators of 50, lOO and even 300 MW have ratings of 5 to 30-40 kW. As mentioned above, these machines simulate, in some cases, large power stations as, for instance, a 2,000 MW hydraulic power station. Transmission lines with voitages up to 700 kV (experimental ones) are simulated by equivalent circuits. Models of 800 kV d.c. transmission lines are also in existence; the length of the
transmission lines simulated by the models corresponds to the transmission lines which are being built or projected for the Soviet power system. Thus, for instance, models of 1,000 km transmission lines of 400-500 kV exist; and models of d.c. transmission lines 600 km long and of 800 kV are also in existence, as also are models of transmission lines 2,0002,500 km long, which simulate the future transmission lines to be built in Siberia. L. K. KIRCHMAYER (U.S.A.) In the U.S.A. much work has been carried out with the use of digital computers for analysis of systems which were investigated previously on d.c. and a.c. analogues; these computer programmes, which will be automated in the near future, open up extensive possibilities for studying power systems. Do you follow the same trend in your work? V. A. VENIKOV, in reply. In our research work and also in the design and operation of power systems, mathematical simulation and digital computers are used very extensively in addition to physical models. Much work has been carried out also in relation to automation of a.c. computing models (calculation desks). However, irrespective of the development and perfection of such equipment, we also build, develop and perfect physical (dynamic) models of power systems on which the principal problems of the operation of electrical systems are being studied; the accuracy of differential equations which define the transient processes and also the mathematical definition of new apparatus, controllers and new powertransmission circuits are being improved. Application of digital computers is justified, in the first instance, in cases in which a high mathematical computing accuracy is required. These machines are particularly useful in economic calculations, calculations of the load distribution between stations, etc. Therefore, it can be stated that all the means of studying power systems have their own fields of
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v. A. VENIKOV application and do not exclude each other, although the advantage of application of one or the other method of analysis may change with the progress of time.
engineering calculations was to save calculation time which justified the fact that, to some extent, their physical nature was artificial. I consider that Mr. Bills' question expresses a desire concerning the trends along which we should work and in this respect I am in full agreement with him.
J. H. WESTCOTT (U.K.) How does the inadequacy of the theory of two reactions manifest itself in the physical processes in which automatic control of the electric machinery is applied? Can you give additional information on the method of measuring the parameters of machines for the purpose of using them for analysis by means of digital computers? V. A. VENIKOV, in reply. If it is necessary to carry out more accurate calculations of processes which are in operation in electrical machinery in power systems it is necessary to take into consideration a large number of factors, the accuracy of which was not so important earlier. For instance, in the analysis of forced regulation, plotting of the areas of stability and of the optimum setting of the regulators cannot be effected without taking into consideration the variation of the reactance of the machines with varying conditions of operation. This variation is so strong that for certain conditions the value of the vertical resistances is less than the value of the horizontal resistances. A change in the parameters along one axis influences the processes along the other axis. Assumption of independence of the processes along the longitudinal and the transverse axes does also not appear to be justified in a number of cases. It is possible that for physical simulation and for calculation, systems of equations may be more suitable which interrelate directly one electrical circuit with another which is shifted relative to it by means of an electric circuit simulating the electrical machine. It is possible that, due to the necessity of solving equations with periodic coefficients, the mathematical difficulties which arise in this case may be eliminated by the use of modern computers. The methods of solving this problem are still disputable, but it is necessary to work on these solutions. Additional parameters and factors, which have to be determined in more accurate investigations of regimes of complicated systems which are fitted with highly effective controllers, differ greatly. These include: influence of saturation oil the reactance of the machines; variation of the frequency of the current, etc. It is, for instance, necessary to determine the so-called frequency characteristics which are becoming increasingly important in the above-mentioned calculations. All these improvements in the accuracy of the parameters and definition of the characteristics are being achieved by experiments on analogues and by analytical calculations and also by full-scale experiments on large hydro-alternators and turbo-alternators. Work proceeding in this field in the Soviet Union has been reported on in a special paper presented at the CIGRE meeting in Paris.
KHARLAMOV (U.S.S.R.) To what extent do you propose to use telemechanical equipment for comprehensive control by means of computers? V. A. VENIKOV, in reply. It is obvious that telemechanical and teletransmission apparatus should be used in realizing comprehensive control of a system and control of a system by means of computers; but, they should only be used when justified from the point of view of the tasks involved; solutions without teletransmission and telemechanic apparatus are preferable. Comprehensive control, which I discussed, assumes greater use of parameters such as frequency and voltage which exist throughout the system, instead of individual parameters, such as angle, which have to be teletransmitted. However, means for obtaining information for teletransmission and telemechanization of this information should be introduced gradually into power systems and the latter should prove justified from the technical and economic point of view.
N. A. KARTVELlSHVILI (US.S.R.) The development of power systems, their automation and application of computer techniques will basically change the requirements regarding the control processes, calculation of power systems, power distribution, etc. What are Mr. Venikov's views on this problem? V. A. VENIKOV, in reply. A detailed reply to this question would require a complete paper. Briefly, an important additional requirement in the new theory will be the investigation of complex systems which contain a large number of elements, study of systems with distributed parameters, and also the effect of non-linearities. Development in the field of computers will make it possible to change those mathematical formulations of problems, which are directly linked with the difficulties of solving equations, to approximate the physical phenomena more closely. Therefore, it is necessary to combine experimental investigations with improvements in the analytical theory. J. LATOUR (Poland) I believe that the most valuable part of the paper of Venikov is its philosophical content. It is fully justified to assume that individual methods of getting better knowledge will allow concepts to be developed which agree more closely with the processes they represent and will enable their character to be predicted from definite assumptions. These methods have their advantages and disadvantages. Important conclusions are derived from the arguments of the paper. The complexity of the studied processes do indeed require a combination of various methods which are applied in a certain sequence worked out in advance. By this method a sufficient approximation of the solution which is required for practical purposes will be achieved. The correctness of the basic assumptions made in the paper, illustrated by concrete cases which relate to the selection of a method of control of powerful generators, is convincing. Speeding up the solution of complicated processes by the methods described in the paper is of great importance. Therefrom the practical conclusion is reached that combination of the experimental and analytical investigations is a well developed method with a great future.
G. BILLS (U.S.A.) In his paper Mr. Venikov deals with problems relating to the two-reaction theory. Does he consider to be justified a similar system which uses the symmetrical components in the analysis of power systems? In the U.S.A. digital computers are available which can rapidly process a large quantity of data. The required accuracy can be achieved without using symmetrical components, therefore it is no longer necessary to change over from physical quantities to symmetrical components and vice versa. V. A. VENIKOV, in reply. There is no doubt that modern computers will make the use of symmetrical components unnecessary since basically their introduction into electrical
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The automation of electric power systems is entering a new phase of development. Automatic control equipments which even in the recent past were auxiliary devices are becoming of fundamental importance and have a decisive influence on all the conditions of the system and of transient processes. Hitherto devices used to control the conditions of electric power systems tended to operate to a set programme. A number of such devices, for example, the excitation field regulators first developed in the Soviet Union and now widely used, introduced simple computer elements such as differentiating controls acting according to the differential coefficient of a magnitude and thus, to some extent, capable of 'predicting' changes of conditions. It is to be expected that the immediate future will see much more rapid and fuller use of both digital and analogue computer techniques in the processes of power system control. Automatic control employing such techniques will be used both to establish optimum steady conditions and to control the system during transients. Use of Computers
Three main trends may be noted in considering the use of mathematical electronic machines to solve power engineering problems. The first trend, which should be developed in the immediate future, is the application of mathematical electronic machines for economic analysis of power system operation. The machines will be applied for economic calculations of projects for the design and automatic distribution of loads between stations in a power system. In the first stage the staff will use computer data as auxiliary information; subsequently the computed data will be appropriately transformed and used directly to control the equipment that changes the system conditions. Of course, the introduction of such devices into systems will require preliminary experiments, on physical models to start with. Further development of computer technique will open the second field of application of computers for the control of transient processes. Use will be made of high-speed analysis of the processes combined with rapid effect on transient conditions. Computer control of these rapid processes will become feasible when their operating speed will be fast enough to take less time than the transient process. Even more than in the control of steady conditions tests prior to introducing such equipment into power systems will be important. The third trend of computer application consists in calculating new designs, of control equipment as well as design and
operating calculations of power system conditions. In this case, rapid performance of experiments, usually between calculations, is even more necessary. New possibilities of calculating conditions and control of automated power systems will lead to the appearance of new quantitative relationships and qualitative properties. In all formulations of new investigations it is necessary to pay special attention to experiment to make the initial theoretical statement more accurate, to confirm the numerical values of parameters and to avoid errors due to incorrect evaluation of physical effects. Such errors can be particularly serious when using large computers. It should be emphasized that new designs of machines and apparatus in future power systems, developed as a result of the application of automatic control, will require accurate mathematical description of the processes that occur both in the new elements and in the systems as a whole. An example of such new elements in power systems are generators of super-high rating, the characteristics of which, even at present, cannot be considered in isolation from the action of automatic control. The design of these generators has to allow for the possibility of automatic control. Further examples are the new asynchronized synchronous machine (All-Union Scientific Research Institute of the Electrical Industry, Scientific Research Institute of Electro-Mechanics); the high-voltage generator (Moscow Power Institute) ; the high-power controlled reactor with pre-magnetization (Power Institute of the Academy of Science); electronic and ionic apparatus connected directly to the power part of power systems and participating in the formation of system conditions (Moscow Power Institute). A further typical feature is the appearance in power systems of new techniques associated with the utilization of atomic power, which have necessitated the introduction of quite new electrical constructions and installations, such as the electromagnetic pumps used in atomic power stations. Combination of Experiment, Simulation and Analysis
A purely analytical approach to the study of processes occurring in individual pieces of new equipment or in their interaction with other equipment and the power system as a whole is impossible. Special attention needs to be paid to the combination of full-scale experiment, analogue simulation and mathematical analysis which makes use of the experimental data, clarifies the concept of the processes and thus ensures optimum control. Experiment is in no way in contradiction to analytical investigations, including those made on high-speed computers; on the contrary, it is an inseparable part of the cycle of engineering investigations based both on analysis and
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on direct experimental verification of theoretical concepts. The complication of power systems, the increase in the capacity of systems as a whole and of their individual parts makes direct experimentation in systems more and more difficult, however, so that laboratory investigations on analogues of various types and sizes assume greater importance. The task of such experimental investigations includes verification of the correctness of the mathematical formulation of the problems, correction of the coefficients that enter into the mathematical characteristics of the effects studied, general characteristics of system behaviour when various elements are present in it, and lastly testing components the rapid adjustment of which is difficult or often impossible in a large power system. Such experimental adjustment investigations become more important with increasing use of automatic equipment and increasing complication of control methods. Preliminary testing and adjustment of individual controller parts. and verification of their inter-action, should be carried out on physical models before the new elements are introduced dir'!ctly into a large power system. The problem of the part to be played by man assumes special importance in view of the increasing use of computer-control technique in power systems. Installations based on methods of physical modelling offer boundless possibilities for the solution of problems of this kind and for overcoming the difficulties that are bound to arise before equipment of this kind is used in large power systems. Staff training for work in the new conditions of automated systems can also be greatly simplified by the use of physical models. For all these reasons it is necessary to consider methods of mathematical investigation and physical modelling as a single whole, in particular for controlling integrated automated electric power systems. It is typical of these systems that some parameters can only be stated arbitrarily, so that the mathematical description of the processes becomes less definite. Hence it is essential for the engineers to attain accurate physical concepts. In investigating conditions of integrated systems computer technique and automatics should more and more supplement one another, without impairing the use of analogues and physical models.
many other kinds of equipment are being developed with the aid of preliminary investigations en complete phYSical models. The steady increase in the power of models and the combination of experiment with analytical calculations by high-speed electronic computers will rapidly lead to the best results. It is important to emphasize that investigations on models should not be considered as replacing full-scale experimentation nor analytical investigation. On the contrary, as a means of making the mathematical description of effects more accurate, they lead to rapid evolution of adequate mathematical methods of investigation. Mathematical formulations should be in line with the physics of the problem and should give quick solutions of adequate, but not excessive, accuracy. Therefore, besides introducing modern continuous and discrete computer techniques for the control and design of power systems, the special computer installations hitherto used will have to be maintained, apparently for quite a long time. Thus d.c. calculating panels will doubtlessly continue to be used for their simplicity and, after improvement, can be relied on for approximate solutions of immediate problems such as power flow in complex systems, short-circuit currents, etc. In solving such problems the computer cannot and indeed should not compete with the simple calculating panel. A.c. calculating boards will be used more and more to determine the distribution of active and reactive loads in complicated systems with a large number of stations. At present they are applied in combination with discrete machines for determining power distribution coefficients between the elements of a system. These coefficients are first calculated on the calculating board and are then introduced into the discrete machine; attempts to calculate these coefficients directly on the computer have so far been unsuccessful. Continuously operating mathematical models will probably be confined to applications in which they are used directly as parts of the automatic equipment (for example, elements for differentiation, summation and integration in various kinds of special controllers), and for calculations on processes for which there exists a partial but insufficiently accurate mathematical description, so that the use of a discrete machine is not justified. The use of a discrete machine is particularly to be recommended where high accuracy of calculation is essential. This includes problems of the distribution of reactive and active loads, determination of optimum conditions and economic operation of projected systems, i.e. highly accurate loss calculations. To summarize it may be repeated that both in analysing the operation of automated electrical systems, in direct adjustment of automatic equipment and in the design of parts of automatic systems, full-scale and model experiments and computer techniques, discrete and continuous, facilitate the solution of new problems; it is impermissible to consider one type of technique as opposed to the other. Experience of developing new automatic devices such as intense field regulators now operating in the Volga Hydroelectric Station, and new protective devices for the transmission line from this station to Moscow show that the automatic equipment can be completed most rapidly by adopting the following sequence: analogues are set up on the basis of general theoretical and physical concepts and are studied experimentally in a model of the system under conditions physically similar to those under which they will operate in practice. The model is used not only to work out the designs but also to elucidate their characteristics and to correct the
Simulation on Physical Models
Physical simulation is developing in two directions: (I) In studying the transient processes in both equipment and power systems as a function of time. This may be called 'incomplete' simulation because the electric, magnetic and other fields that occur in practice are not reproduced in the model. Establishing conditions similar to the processes occurring in full-scale equipment make it possible to reproduce these on smaller scale and to obtain results relevant to the system. The dynamic models of power systems widely used in the U.S.S.R., France, Austria, Germany, England and Australia are of this type. (2) In the direction of simulating processes in time and space, which may be called 'complete' simulation. This is particularly important in the design of new elements of automated systems, for which these physical models provide preliminary verification of operation and adjustment, since the mathematical characteristics of new equipment which has not been developed and tested on a full scale are necessarily unknown. Such methods of complete simulation are now extensively developed. Atomic reactors, electromagnetic pumps, high-voltage generators for voltages of 100-200 kV, new types of an asynchronized synchronous alternator and 4-A.R.C.4
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tion the collection and transmission of a considerable quantity of processed information is the group of problems associated with the economic distribution of active and reactive power between stations and individual parts of the system. This is now successfully solved by means of digital computers. In the present stage of development, however, the machine plays the part of adviser but does not yet directly control power systems. Machines for direct control should be developed in the very near future. Work in this direction is being carried out for both digital and analogue computer techniques. The influence of analogue and digital computer techniques can also be clearly seen in the development of automatic field control of alternators in power systems. The first stage of development of automatic field control was characterized by the application of electromechanical regulators which had a zone of instability and did not operate on small alterations or in the early stages of transients. Their role was limited to correcting the voltage, i.e. to restoring it some time after a change in conditions had occurred. Speed and frequency controllers were similarly constructed. All these controllers usually were characterized by the circumstance that each magnitude was controlled by a change in itself, there being no interaction of control from a number of changes. The second stage of control development, which may be termed the stage of intensified control, is typified by the more effective use of electronic controllers and compounding devices with no zone of insensitivity. This made it possible to evolve so-called artificial stability, with a limiting power for synchronous machines under steady-state conditions, characterized by transient e.mJ. and transient synchronous impedance. This, as it were, somewhat corrected the characteristics and parameters of the synchronous machine which, without control or with early methods of control, was characterized by synchronous impedance and no-load e.m.f. The application of field forcing operated by automatic control equipment made it possible to operate in the initial stages of the transient process and partially to improve the dynamic characteristics of the synchronous machine. At this stage of development alternator speed control was subject to no change and, separated as before from secondary control of frequency, was not directly influenced by changes in the electrical conditions, although there were some attempts to develop turbine emergency governors of this kind on an industrial scale. Frequency control devices, however, began to fulfil not only functions of secondary control, maintaining the mean frequency of the system, but also of economic load distribution between stations. Automatic control, as now developed, is called intense control; this may be considered as a third stage from which it should be possible to make rapid transition to the fourth stage of integrated control of power systems and cybernetic control. Intense field control is characterized by high intensity of action: high excitation ceilings and high amplification factors of control equipment become possible by using feedback and controlling not only the change in the parameters of the system but also their rate and acceleration of change. Intense control will make it possible partially or completely to exclude the influence of generator and transformer reactance on the limiting power that can be transmitted, so that under static conditions it will be possible to have a transmission limit set by the line. Here the function of intense field control is extended and the problem of maintaining the voltage becomes one of the partial problems of field control. The dynamic characteristics of systems fitted with intense
mathematical description of the processes. By subsequent analytical investigation the theory of the devices under development is improved, whereupon new analogues are constructed which are again tested and developed, so that the mathematical description of the process can be further corrected. The prototype models of controllers and protective devices made at the factories are again tested in the model after which an experimental series is manufactured. Before being installed in an operating system the automatic and protective devices are first adjusted on the model, after which a limited number of tests are made directly in the system. It was just such interaction between full-scale and model tests with analytical investigations that ensured rapid adjustment of the system for the Volga Hydro-electric Station, which was the first in the world to be supplied with intense field control and highly perfected protective equipment. Revision of Theoretical Concepts It should be noted that successful application of high-speed electronic digital computers for solving concrete technical problems and for control of systems will soon necessitate some reconsideration of the theory of operation of electrical power systems and their parts. This theory was often evolved owing to the engineer's need of overcoming mathematical difficulties in solving the equations of the system, yet the adequacy of the mathematical description of the physics of the process was frequently jeopardized in doing this. For example, parts of the Park-Gorev two-reaction theory which arbitrarily distinguishes two axes in a synchronous machine, originated in the difficulty of solving equations with periodic coefficients. Other calculation methods for electric systems also involve assumptions remote from the physical effects, and sometimes also from the essence of the matter-the outcome of attempts to find simplified engineering solutions. . Equations with periodic coefficients can now be solved on digital computers, and so there will probably be no need to write the equations of synchronous machines with reference to arbitrary direct and quadratic axes. Hence, the general tworeaction theory of the operation of electrical systems may prove unnecessary. Both the theory and its mathematics can be expressed in matrix formulation and appropriate analysis, relieving the engineer of difficulties associated with numerical solutions. New equations will be needed, however, which correspond to both the physics of the effect and the capabilities of computers used for analysis and direct control of these conditions. Such changes in the theory of systems, their operation and components must necessarily be accompanied by experimental investigations, lest perspective and adequate representation of the physical effects be lost. Combination of full-scale experiment, simulation and computer techniques is therefore necessary.
Automatic Devices for Power Systems
The development of the control of power systems, in line with the application of principles of cybernetics, passes more and more from solving stabilization problems and planned change of. controlled variables to the solution of problems of logic combined with simpler control problems. The introduction of logical action into automatic control systems is based on the application of digital, continuously operating computers. In the development of modern automatic control equipment it is typical to employ the condition of invariancy by avoiding steady deviations and transient components of error. Typical of cybernetic problems which require for their solu50
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EXPERIMENT AND ANALOGUE SIMULATION, MATHEMATICAL MODELS AND COMPUTATION IN CALCULATIONS control will also change insofar as field control corresponding to the first and second differential coefficients of operating parameters will, by means of field forcing and weakening, improve the dynamic characteristics of the system directly after the disturbance, combined with optimum damping of the disturbance. The static limit of transmission during intense field control will then become the limit set by the possibilities of the transmission line, and the dynamic limit will be the same as the static. Intensive operation on the transient process will actually exceed that of field control. Investigations carried out on computers and with physical models have been used to develop and perfect equipment for the electrical retardation of generators that accelerated under fault conditions. This electrical retardation device is controlled by devices that react to sudden unloading with the resultant sudden change in turbine torque. These devices can raise dynamic stability to the level of static stability, rendering the system stable under any short-circuit conditions. A further improvement in controlling load impedances is control not only in proportion to the power thrown off but also according to the differential coefficient of speed and acceleration of the particular alternators. With this type of control the parameters controlled may be made practically invariant, however severe the disturbance. The use of controlled retardation devices combined with intense field control also ensures stability when steam turbines are used, apart from main transmission lines, on intersystem links where till now it had been necessary to maintain a high stability reserve, so that their transmission capacity was not fully utilized.
In effecting intense field control of turbines it is rational to use combined methods of control (according to changes in control magnitude and its differential coefficient). Intense control of field, torque and speed of water-driven and steam-driven electric power systems ensures not only improvement of steady-state operating conditions with increased system stability but also facilitates auto-restoration of synchronous operation of any station that may have dropped out of synchronism. Thus there appears the concept of resultant stability of a system. In a system having resultant stability some parts may run asynchronously for a short time, synchronous operation being restored automatically as soon as consumer operating conditions permit. The presence of such a group of controlled devices means transition to integrated control reacting to changes in several parameters of the power system and acting simultaneously on several characteristics of the machine. A further stage should be transition to automatic control, which combines the integrated controllers mentioned above in a single unit that will determine the best operating conditions of the system and then make the necessary adjustments to secure the best effect. Subsequent development should be in the direction of cybernetics, evaluating the development of the process according to information received in transient and steady-state conditions and accordingly adjusting the control system to optimum conditions. The application of methods of intense and integrated control will permit a complete solution of the problem of ensuring the quality of electric power in systems with sudden changes of load.
DISCUSSION H. F. R . TAYLOR (U.K.) What are the parameters of the largest power system that can be simulated on the model; what is the number of power stations, alternators, their ratings, the number of transmission lines, their voltage and their lengths? V. A. VENIKOV, in reply. The possibility of simulating electric power systems with physical or mathematical models is not limited by the power of the system, its length or the number of power stations and machines involved to be studied. However, on physical models the main problems studied are usually those which reveal the physical nature of phenomena, the verification of the effects of new machines, apparatus, controllers, etc. Therefore, it is inadvisable to simulate on such models an excessive number of power stations and individual machines within these power stations. It is possible to limit oneself to representing the basic units and to substitute a large power station, for instance, the Volga Hydroelectric Station imeni V. I. Lenin, by two machines. This is fully adequate for studying the influence of this station on the remaining parts of a system. Dynamic (physical) models contain 5 to 30 machines, which simulate large stations and sub-stations with synchronous compensators. This number of machine models is sufficient for studying the processes which occur in the interconnected power systems of Siberia, Ural and the European part of the Soviet Union. The major part of the model is built to operate at normal industrial frequency . The models which simulate large alternators of 50, lOO and even 300 MW have ratings of 5 to 30-40 kW. As mentioned above, these machines simulate, in some cases, large power stations as, for instance, a 2,000 MW hydraulic power station. Transmission lines with voitages up to 700 kV (experimental ones) are simulated by equivalent circuits. Models of 800 kV d.c. transmission lines are also in existence; the length of the
transmission lines simulated by the models corresponds to the transmission lines which are being built or projected for the Soviet power system. Thus, for instance, models of 1,000 km transmission lines of 400-500 kV exist; and models of d.c. transmission lines 600 km long and of 800 kV are also in existence, as also are models of transmission lines 2,0002,500 km long, which simulate the future transmission lines to be built in Siberia. L. K. KIRCHMAYER (U.S.A.) In the U.S.A. much work has been carried out with the use of digital computers for analysis of systems which were investigated previously on d.c. and a.c. analogues; these computer programmes, which will be automated in the near future, open up extensive possibilities for studying power systems. Do you follow the same trend in your work? V. A. VENIKOV, in reply. In our research work and also in the design and operation of power systems, mathematical simulation and digital computers are used very extensively in addition to physical models. Much work has been carried out also in relation to automation of a.c. computing models (calculation desks). However, irrespective of the development and perfection of such equipment, we also build, develop and perfect physical (dynamic) models of power systems on which the principal problems of the operation of electrical systems are being studied; the accuracy of differential equations which define the transient processes and also the mathematical definition of new apparatus, controllers and new powertransmission circuits are being improved. Application of digital computers is justified, in the first instance, in cases in which a high mathematical computing accuracy is required. These machines are particularly useful in economic calculations, calculations of the load distribution between stations, etc. Therefore, it can be stated that all the means of studying power systems have their own fields of
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v. A. VENIKOV application and do not exclude each other, although the advantage of application of one or the other method of analysis may change with the progress of time.
engineering calculations was to save calculation time which justified the fact that, to some extent, their physical nature was artificial. I consider that Mr. Bills' question expresses a desire concerning the trends along which we should work and in this respect I am in full agreement with him.
J. H. WESTCOTT (U.K.) How does the inadequacy of the theory of two reactions manifest itself in the physical processes in which automatic control of the electric machinery is applied? Can you give additional information on the method of measuring the parameters of machines for the purpose of using them for analysis by means of digital computers? V. A. VENIKOV, in reply. If it is necessary to carry out more accurate calculations of processes which are in operation in electrical machinery in power systems it is necessary to take into consideration a large number of factors, the accuracy of which was not so important earlier. For instance, in the analysis of forced regulation, plotting of the areas of stability and of the optimum setting of the regulators cannot be effected without taking into consideration the variation of the reactance of the machines with varying conditions of operation. This variation is so strong that for certain conditions the value of the vertical resistances is less than the value of the horizontal resistances. A change in the parameters along one axis influences the processes along the other axis. Assumption of independence of the processes along the longitudinal and the transverse axes does also not appear to be justified in a number of cases. It is possible that for physical simulation and for calculation, systems of equations may be more suitable which interrelate directly one electrical circuit with another which is shifted relative to it by means of an electric circuit simulating the electrical machine. It is possible that, due to the necessity of solving equations with periodic coefficients, the mathematical difficulties which arise in this case may be eliminated by the use of modern computers. The methods of solving this problem are still disputable, but it is necessary to work on these solutions. Additional parameters and factors, which have to be determined in more accurate investigations of regimes of complicated systems which are fitted with highly effective controllers, differ greatly. These include: influence of saturation oil the reactance of the machines; variation of the frequency of the current, etc. It is, for instance, necessary to determine the so-called frequency characteristics which are becoming increasingly important in the above-mentioned calculations. All these improvements in the accuracy of the parameters and definition of the characteristics are being achieved by experiments on analogues and by analytical calculations and also by full-scale experiments on large hydro-alternators and turbo-alternators. Work proceeding in this field in the Soviet Union has been reported on in a special paper presented at the CIGRE meeting in Paris.
KHARLAMOV (U.S.S.R.) To what extent do you propose to use telemechanical equipment for comprehensive control by means of computers? V. A. VENIKOV, in reply. It is obvious that telemechanical and teletransmission apparatus should be used in realizing comprehensive control of a system and control of a system by means of computers; but, they should only be used when justified from the point of view of the tasks involved; solutions without teletransmission and telemechanic apparatus are preferable. Comprehensive control, which I discussed, assumes greater use of parameters such as frequency and voltage which exist throughout the system, instead of individual parameters, such as angle, which have to be teletransmitted. However, means for obtaining information for teletransmission and telemechanization of this information should be introduced gradually into power systems and the latter should prove justified from the technical and economic point of view.
N. A. KARTVELlSHVILI (US.S.R.) The development of power systems, their automation and application of computer techniques will basically change the requirements regarding the control processes, calculation of power systems, power distribution, etc. What are Mr. Venikov's views on this problem? V. A. VENIKOV, in reply. A detailed reply to this question would require a complete paper. Briefly, an important additional requirement in the new theory will be the investigation of complex systems which contain a large number of elements, study of systems with distributed parameters, and also the effect of non-linearities. Development in the field of computers will make it possible to change those mathematical formulations of problems, which are directly linked with the difficulties of solving equations, to approximate the physical phenomena more closely. Therefore, it is necessary to combine experimental investigations with improvements in the analytical theory. J. LATOUR (Poland) I believe that the most valuable part of the paper of Venikov is its philosophical content. It is fully justified to assume that individual methods of getting better knowledge will allow concepts to be developed which agree more closely with the processes they represent and will enable their character to be predicted from definite assumptions. These methods have their advantages and disadvantages. Important conclusions are derived from the arguments of the paper. The complexity of the studied processes do indeed require a combination of various methods which are applied in a certain sequence worked out in advance. By this method a sufficient approximation of the solution which is required for practical purposes will be achieved. The correctness of the basic assumptions made in the paper, illustrated by concrete cases which relate to the selection of a method of control of powerful generators, is convincing. Speeding up the solution of complicated processes by the methods described in the paper is of great importance. Therefrom the practical conclusion is reached that combination of the experimental and analytical investigations is a well developed method with a great future.
G. BILLS (U.S.A.) In his paper Mr. Venikov deals with problems relating to the two-reaction theory. Does he consider to be justified a similar system which uses the symmetrical components in the analysis of power systems? In the U.S.A. digital computers are available which can rapidly process a large quantity of data. The required accuracy can be achieved without using symmetrical components, therefore it is no longer necessary to change over from physical quantities to symmetrical components and vice versa. V. A. VENIKOV, in reply. There is no doubt that modern computers will make the use of symmetrical components unnecessary since basically their introduction into electrical
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