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Some Applications and Trends in Automatic Control of Thermal Power Plants
THERMAL POWER PLANT CONTROLS
SOME APPLICATIONS AND TRENDS IN AUTOMATIC CONTROL OF THERMAL POWER PLANTS
J. Debelle*, R. Baeyens**, J. Cl. Lemoine** and F. Van de Meulebroeke** *Generation Department, EBES, Brussels, Belgium **Electronic f!I Automatic Section, Laborelec, Rhodes St . -Genese, Belgium Abstract. The report attempts to give a synthesis of special realizati ons and limited experiences in the field of automatic control of therma l power plants. The functions ensured by the automatic con tro l are examined under the aspect of performances and behaviour expected of the generating units in the overall power system operation, including normal operation, shut-down, start-up and their behaviour during incidents. Technological aspects of the equipments are considered. The role played by process computers as far as DDC and monitoring are concerned is examined, including the man/machine interfaces and the design of control rooms. Benef it, reliability and standardization of co ntrol systems are also investigat ed . Current trends in all these fields are presented. Keywords. Power plants, automatic control, computer applications, direct digital control, boilers control, turbines control. carried out to improve our knowledge of processes and control techniques. As a result, models were devel oped very representative of the inves tigated phenomena, and, control systems, often quite sophisticated, we re put into application. The same is also reflected at tech nological level : various manufacturers have commercialized equipments tha t wer e specifically dir ec ted towards applicat ion in the electrical power system field. Reading through the Survey Papers and publications presented at IFAC's Cong resses over a large number of years, it is striking that a substantial part of the pr oblems have been known for a long time and that a lot of remarks that have been put forward are still up-to-date. Could this mean that only slight progress has been made in control techniques f o r this field ? Certainly not, because th e increase of our knowledge and the dev e lopment of our means has now turned many ideas earlier reported into proven rather than experimental achievements.
INTRODUCTION. Electric power generation, transmission and distribution constitute and industrial process having its own very specific characteristics. It is not our intention to review details which have often been developed here but it is nevertheless important to mention some of the major aspects which strongly influence the conception of power generation processes and hence the related automatisms (56,57). The notion of continuity governs the design of generating units, transmission and distribution networks, equipment redundancy, as well as permanent and instantaneous management of the overall system : power and frequency control, voltage level control, load modulation of generating units including shut-downs and start-ups, and of course also appropriate actions during disturbances such as outage of a generating unit, power-line trip-outs, short-circuits in transmission network, frequency drop. A feature of the system is f avourable however in that various elements of th e process are often quite similar. Generating units for instance can be arranged in a few main groups such as hydraulic units, conventional thermal units, PWR or BWR nuclear units, gas turbine sets, ... Not being faced with a multitude of of ten widely differing processes, the electricity sector has been in a position to focus its research and development efforts on a limited number of problems. This particular aspect is also apparent in the field of automatic control of power stations and systems : various studies were
PART TAKEN BY THE GENERATING UNITS. The part generating units take must be seen in the electric power system as a whole "Generation - Transmission - Distribution". This truism unfortunately does not always mat eria lize at generating unit operating lev e l (nor does it in network operation). We feel this remark is particularly pertinent when cons idering the "power units networks" system behaviour under major dis57
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turbances. The turbine-alternator interconnecting shaft section constitutes a physical border-line enabling either side to carry over part of its problems to the other. Solutions obtained in this way thus often favour either the boiler, turbine or network selfishness. The actual studies however bring forward the non-negligeable effects of the network on boiler and turbine behaviour and vice versa. Numerous studies have been made on linear behaviour of boilers, turbines and networks, and the results have led to development of automatic control systems which are effective in the steady state. But as soon as generating units are considered under major disturbances or failures affecting both power units and networks, then linearization assumptions are no longer valid and the approach has to be shifted towards all aspects of non-linear behaviour of the process. This important step has been made possible through the elaboration of increasingly detailed mathematical models, and particularly through resorting to high-performance computer simulation aids. This stage is of fundamental significance, because the investigated phenomena (consequences of short-circuits, frequency drops, ... ) fortunatly hardly ever occur and experiments cannot be carried out in practice. As their consequences may be critical, it ~s necessary to set up the automatic control systems which will enable making the involved equipment respond appropriately. AUTOMATIC CONTROL OF FOSSIL-FIRED BOILERS. Automatic control of fossil-fired boilers is a problem which is considered as having been solved for a number of years now when operated at rated capacity and when the perturbations affecting the process are of a small amplitude, i.e. not exceeding la % of the maximum perturbance amplitude (6). For some years, other preoccupations have came to light : automatic control under large load modulations (boiler operation between 100 % and 40 % of the load, or even less) ; automatic control during unit startup or during internal incidents such as, for instance, feedpump cut-out or fan stalling (9, 12, 26, 31, 39, 53, 75). This extension of tasks expected from automatic control systems has had the following consequences : - introduction into automatic control systems of non-linear actions (limiters, signal selectors, automatic adjustment of control coefficients, compensation of nonlinear static characteristics). - a more through analysis of interactions between the various physical variables and the use of non-interacting or coordinated
automatic control systems. Without being intended to replace safety systems, automatic control nevertheless plays an important part in operational safety and availability. For instance, in case of a feedwater pump cut-out event in a boiler fed with two such pumps, steam generation will automatically be reduced by 50 % while maintaining pressures, temperatures, 02 excess in flue gas within acceptable limits throughout the transient period and subsequently enabling proper operation at this new load. In such a case, automatic control of the boiler-turbine set is switched over from "boiler following turbine" mode to the "turbine following boiler" mode. By such action, the impact of the failure on the power generation has been reduced from 100 % (outage of the unit for some tens of minutes) to 50 %.
It must be admitted that to date, the conventional PID controller remains practically all over applied, with its well know properties and its fundamental advantage of being easy to implement. Many theoretical and simulation studies have been carried out concerning application of optimal control theories to boilers (70). The performances reached by the latter kinds of control often remains comparable to that obtained with more traditional control systems. Criteria used for optimization do not always conform to the actual boilers operation criteria, and it is noted regularly that the elements of weighted matrices have been chosen "a posteriori" to obtain a satisfactory compromise for the transient behaviour of the process (27). Furthermore, implementing these control techniques is quite complex and the relation between the physics of the process and the control system cannot be easily assimilated by the user. Hence, "quasi optimal" or "sub-optimal" solutions may often prove more interesting. In chosing the control structures, modern theories are valuable for they lead to a better approach to the problems. Selecting a control scheme for small perturbat ions dynamic behaviour remaining similar to the PID control system, but which in the event of large disturbances would enable to take into consideration various non-linearities and interactions, is fundamental (9, 12). Functional block diagrams obtained in this way will always include the two following points : - estimation of the variables representing the system, obtained from accessible signals. - elaboration of control variables, through a set of feedback, and feedforward paths coming from the process itself and from its estimator. Acceptable simplifying assumptions will provide "quasi-optimal" solutions, easier to implement (49, 59, 60).
Automatic Control of Thermal Power Plants
Some examples
The electric servomotors have a non-linear behaviour as soon as the range of small perturbations is left : the motor performs as an integrator between 0 and 100 % of the range, the deviation signal driving the motor via a non-linear control including a deadband, a small proportional zone (thus linear) and then a saturation zone (Figure I) (45). Therefore, the servomotor for most of the time operates at the maximum speed. As the times required to effect 0 to 100 % of the range are between 10 to 60 seconds, effects of actuators speed limitation must not be neglected in relation to the time constants of the process. The following figures illustrate the overall behaviour of the controlled process with a control structure enabling to take into account the actuator 's non-linear behaviour (fig. 3 and 4). The control structure used here is represented in fig. 3 and is obtained from a control scheme based on a direct model of the process (estimation of the disturbances) and an inverse model (compensation of the process dynamic behaviour) (fig. 5-a, 5-b) (9,
14).
The aim of this control is to adjust the feedwater input flow to the load required from the boiler. Any unbalance between the heat flux provided at the boiler by the fuel to meet the load and the heat flux removed by the feedwater induces modifications in the behaviour of some parameters thereby providing a measurement of the unbalance. Formerly, the unbalance value was measured by the pilot-tube temperature which is related to the large thermal inertia of the boiler and therefore gives a filtered and hence delayed information. A deepgoing theorical study of transient phenomena in an evaporator has shown obviously the existence of variables presenting a very fast response to heat flux variations , which led to the idea of using one of these variables in control as a measurement of disturbances. This study is based upon solving of infinitesimal equations describing thermal and mass exchanges and the fluid flow in a section of the evaporator. It gives the system transfer functions corresponding to feedwater flow and heat flux variations. It is also noticeable that the effects due to specific volume variations of the water/ steam mixture are much faster than effects of enthalpy fluctuations in the pilot tubes. Hence the proposal to measure induced disturbances due to heat flux variations by means
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of pressure drop variation measurements through a venturi fitted into the evaporator outlet. The simple control structure achieved in this way responds much faster to heat flux fluctuations than the previous structure. Effects of boiler behaviour in the transient
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Various simplified models of boilers have been developed (7, 13, 36). They enable to analyse the impact of network disturbances on boiler behaviour : flow, pressure and temperature fluctuations. Reciprocally it appears more and more necessary to take into account boiler response in studies concerning network transient stability. As the considered phenomena have a duration of from one to some tens of seconds, it has been generally accepted that they would only slightly disturb the boilers. But, coordination of boiler/turbine controls leads to an as fast as possible response by the boiler to electric load, turbine valves position and steam flow variations. The severest constraints for the boiler occur during closure (almost instantaneous) and reopening (slower) of the turbine valves during certain network disturbances. For the boiler, the reopening speed should not be less than 20 % per second.
Studies have demonstrated that should reopening of the turbine be too slow, coming back to the initial mechanical power of the turbine would be difficult and too slow: opening of valves (by-pass, safety) boiler side, combustion difficulties through lack of air ; functioning of the turbine steampressure limiters (which reduce the turbine power output). In certain cases such difficulties may result in unit outage even though the electric disturbance is already over. STEAM TURBINE CONTROL. In the field of steam turbine control a similar evolution has taken place as for boiler control, in that the role of automatic control systems are extended to ob tain a more adequate behaviour and a better transient response as units under severe variations as those resulting from network disturbances. An extension of the traditional speed governor functions enable it to take an important part in the field of transient stability. This extension has an effect on equipment development as well as on the computation means.
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~~~~~~_~~_~bi~_~~~_~~i~~~~~i~~_~~_~~~ig~_~~~
studies.
Traditional functions. The traditional functions of speed controllers for turbines are the following : - during start-up, run up, programmed or not, and, if required, including monitoring of the turbine's thermal constraints, limiting the maximum speed gradient either automatically or manually - synchronizing the unit via the synchronizer - loading during start-up and effecting programmed load changes (with or without thermal stresses monitoring) - taking part in the load-frequency control by : . primary control, load modulation in function of the controller statism . secondary control : load changes (automatic or manual) upon the load set-point - keeping the speed - below the overspeed tripout threshold in the event of sudden load rejection by tripping of the circuit breaker. Certain types of electro-hydraulic controlers carry out all these functions, others perform part of them or with certain alternative functions. The last point however is the essential one for all turbine speed controllers and any additional functions are in fact logical extensions of this last point.
Turbine units can also be subject to sudden load variations caused by events other than those mentioned above. The main among these are : - network disturbances (short-circuits). These cause total or partial unloading (depending upon the electric distance) followed by sudden re-loading when the fault has been cleared. Under these circumstances the following is required from the speed controller: I) Fast closing of the control and/or intercept valves, with a degree of closure adapted to the gravity of the fault. Such action will enable extending the stability limit for the first swing ~tability criterium. 2) That this action be not too prolonged, after elimination of the fault, by such on/ off closure actions at the moment of the fault and too slow re-opening of control valves. Ideal opening speeds it seems are between 20 and 50 7. per second. An exemple of such a behaviour is given on fig. 6. - changeovers to isolated network. These cause sudden load variations in the synchronous network to the new load value imposed by the isolated system (lower or higher than the initial load). The controller which imposed the load generation in the synchronous network has to take up again its function as a speed controller to limit frequency excurcions (either way) in the isolated system.
With a view to assign to the speed controller a substantial task in extending the area of the first swing stability, various studies were undertaken concerning fast valving. As a rule these studies investigated how the existing possibilities of fast closure of turbine control valves, necessary to limit overspeed in case of load rejection, could be made useful in case of network disturbances. Fast re-opening appears to be more and more necessary (I, 20, 52, 68). The need for appropriate reaction to network disturbances leads to conditioning the action of threshold accelerometers and speed 1imiters, and to setting a maximum time for reopening. Deepgoing studies are being conducted on these various points (32) . A more thourough study of electro-hydraulic speed controllers and their behaviour under abnormal conditions (disturbances, change over to isolated network) has become possible due to the realization of specialized equipment such as the turbine simulator used in Belgium. It enables to loop the actual controller and steam inlet actuators within a simulated model of the turbine and rotating masses (72). Investigations of the actual incidence of the mechanical torque on the transient stability have called for more realistic representations of all the elements defining this torque. Generally these models are mainly aimed at non-linearities appearing in the various components, rather than at the high order of the transfer functions used. IEEE has proposed non-linear speed controller models, taking into account the inlet valves non-linearities (travel limitations, unlike limitation of closing and opening speeds). This proposal however does not include the representation of a threshold accelerometer nor the MP-LP body preceded by the intercept valves (40). Other studies represent these elements in more detailed models (11, 33, 71). In the same context aimed at realistic representation, particular attention was paid to the turbine model in the scope of investigations into torsional stresses of the turbine-generator shaft during large transients (3, 5, 14, 16, 34, 44, 51). The effects of speed control on stability of synchronous network and particularly of isolated systems and the behaviour of the latter under power deficiency were fully described in several papers (2, 8, 10, IS, 18, 73). In the field of turbine speed control an increasing trend has come about for developing the contribution towards transient stability, by adapting existing equipments or designing new equipments. Emphasis having been put on the good behaviour after fault clearing, the controller is required to handle the actions
Automatic Control of Thermal Power Plants
of control valves with a maximum of efficiency. Computer programs enabling analysis of speed controller performances have followed this evolution by development of models stressing the importance of non-linearities in the control loop. AUTOMATIC CONTROL OF NUCLEAR BOILERS. Automatic control of nuclear boilers and particularly that of reactors is a very specific field, resulting of the process itself and the operating conditions of nuclear reactors (19). The feeling is that reactor control is essentially designed for reactor operation but that the requirements of the overall process, including turbine generator and network, are often neglected. For some time now, studies attempt to better describe the performance potential of nuclear units and the part they have to play in the overall power generation system. Currently, the user cannot easily intervene at the design level of reactor control loops, but the dialogue seems to be starting. In the more conventional parts of nuclear units, the same principles apply as for fossil fuel fired power units. The following example illustrates the striving to obtain an as complete as possible automatic control system working from 0 to lOO % of the load range and during start-up periods.
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structure (water level and steam pressure in the first stage of the turbine, which is an "image" of the steam flow). These problems were solved and the control structure, including a preset adaptif control of the gain, in function of the feedwater temperature, enable to maintain the water level at its set point value for any load conditions. As a result, start-up times are reduced and operation is made easier. CANDU reactor control (48, 65). A remarkable application of computerized control is certainly that of the CANDU reactors (Pickering, Gentilly, etc ... ) Control is carried out by a dual central computer, with automatic transfer from the master computer to the other which permanently works in parallel. Ea ch computer checks its inputs and its control performances as well as its own operation. This has called for extensive development of auto-tests and automatic transfer from one computer to the other. The computer provides automatic control for the reactor power, drum pressure, temperature gradients, turbine run-up, fuel loader. All the classical monitoring functions are also included. This dual-computer system seems very reliable and only very slightly affects the overall unit availability (at Pickering, unavailability due to the computer system is less than 0.5 %). ROLE OF DIGITAL PROCESS COMPUTERS.
The water level in steam generators must be maintained between narrow limits. A too high level could result in water in the steam entering the turbine HP section with danger of priming the turbine blades. A too low level means that water reserves would be insufficient should emergency cooling of the reactor become necessary. During large transients, this constraint may result in reactor scram or turbine trip-out entailing substantial reduction of power availability. To improve the water level control, a threeelement control structure was developed based upon structures proven in conventional thermal power plants (water level, feedwater flow, steam flow). Problems appeared, among other things, in connection with the substantial delay in level response to feedwater flow variation (because the feedwater temperature differs from the saturation temperature) calling for special precautions. Another problem is that of the temporary inverse time response of the level to steam flow control variation (shrink/swell phenomena). At low load (feedwater and steam flows less than about la %), the flow signals are no longer valid, and the level control system must be changed over to a two-element
Both in conventional and in nuclear power plants operation digital process computers are playing an increasing part (4, 29, 30, 47, 58, 69, 76). It must be admitted that although a number of DDC experiments have been made with satisfactory results, the systematic use of computers for in-line control functions is far from systematic yet (35, 54, 74). The great majority of tasks entrusted to computers consist in on-line type functions, monitoring and assistance to the operators, without automatic feedback to the process. The integration of such equipment has strongly modified control room design (61, 62, 63).
We will only mention here the really industrial DDC systems, and particularly some of the applications in the U.S. and Belgium (17, 22, 25, 46, 64, 67). In these cases, after a first implementation, a policy was decided to equip in this way all new (fossil fired) power units. The following considerations mainly concern DDC aoplications at Martins Creek (units 3 and 4) and at Langerlo (units I and 2). The concern direct digital control systems
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for boiler control. Turbine controls are not included as they are often supplied by the turbine manufa c ture~ (conventional analog/ logic equipment). There are however other applications which do include direct digital control of the turbine. In the examples examined, there is a single computer and back-up in case of computer failure is provided by simplified analog controllers (In Langerlo however, the DDC remains operational, even if the disk memory fails). The use of process computers enabled to highly improve boiler control, not so much by improving dynamic responses of control loops, but mainly by permitting the use of more complex control structures and hence to better control the power unit in case of large disturbances (see the § on Automatic control of fossil-fired boilers). This considerable amelioration in comparison with conventional (analog/logic) control systems however is felt less at the Langerlo plant, because the boilers of previous units are yet equipped with sophisticated analogue control equipment which already include such functions. Improvements mentioned have been made possible by the developme.nt of digital control programs based upon a large choice of calculation modules : - PID controller (but with possibility of max. or min. limits or incremental limit on the output value) - numerous dynamic functions (including dead time) limiters - signal selectors - non-linear functions - logic-type functions - incremental computation - validation of input and output signals - automatic modification of control loops and thus control functions possibility to automatically modify the control coefficients in function of any other variable (preset adaptif control). The importance of these functions becomes obvious in the following example : for the boiler control, at the Langerlo power plant, 32 different type algorithms are used nearly 1000 times in the following proportions (25): - logic algorithms 15 % - arithmetic algorithms 36 % - non-linear algorithms 30 % - I, PI, PID algorithms 2 % - dynamic algorithms (lead-rag, ... ) 2 % - interface algorithms ( i n p u t s / I S % outputs) This shows the very low proportion of dynamic algorithms used (4 %). PID type control remains the most widely used. The first advantage of DDC is related to the large number of possibilities it offers to develop sophisticated control structures. Another point is that increasing the control loop complexity will in no way reduce its reliability and availability. The second advantage is its high degree of intervention flexibility into DDC programs, which enables modifications and adaptations
to be carried out in only a few hours. The disadvantage of a DDC system remains the necessity to ha ve a simplified back-up system. The need for providing coherent change over between computer control and back-up control furthermore sometimes limits control functions improvements. 2. ~~~i~~Ei~g_~~~_~~~i~~~~~~_~~_~~~_~E~E~~~E~ functions.
The functions of monitoring and assistance to the operators in power plants are always handled by process computers and can therefore be considered as classical equipment together with the information means to the operator, such as printers and CRT displays (37, 41, 55). The tasks managed are well known and are enencount e red in any industry equipped with process computers. They consist mainly in monitoring of alarms and changes of state, display on CRT or printing of analog values, graphical representation on CRT's of process circuits with the state of actuators and values of physical parameters, printing of pre-and post-incident perturbographic reports, counting programs (for statistical purposes), operator's guides (display on CRT of operating instructions or procedures). The main advantage of these functions is to provide the operator and operating staff with a clearer picture of the process state, its evolution at any moment and to assist considerably during and after disturbances. Although this advantage is hard to evaluate in terms of money, users consider that the improv ement in monitoring and display of data has made operation more safe and more reliable. Printing of all in/out alarms and changes of state in their chronological order (discrimination time between 10/20 msec and 300/ 500 msec) is of paramount importance to analyse the cause of an incident and its consequences.
Introduction of process computers and new functions of assistance to the operators trends to considerably modify the design of control rooms (23, 42, 43, 50). These are more and more designed for power unit operation based upon the use of visual displays, permitting to strongly reduce the number of indicators and alarm panels on the control consoles (21). Power units operation remains however possible in the event of failure of the computer system. Nevertheless, in the case of the CANDU reactors, operation cannot be continued longer than a set time limit should the computer system break down completely. Indeed, various reactor automatic control functions are handled by these computers.
Automatic Control of Thermal Power Plants
An important point to comply with is the following ; the man/machine interfaces must be designed in the operator's natural language and not that of the computer. To obtain good readability on CRT's a too large number of different colours must be avoided as well as overloading the screen (a density of 25 to 30 % of the screen with data seems to be an optimum) (21). TECHNOLOGICAL ASPECTS OF CONTROL SYSTEMS. At the present time, most of the control systems mentioned above are carried out by conventional analog electronic devices. The functions available are not numerous (PI, PID controllers, simple lag filter, lead-lag filter, a few non-linear functions) and thus sophisticated control structures are not easy to design. During the last ten years a much more complete list of modules has been made available by certain manufacturers . This progress has made it possible to realize much more sophisticated control schemes; non-interactive control, introduction of nonlinearities , preset au t omatic adaptation of control coefficents, automatic modification of control structures, ... However , the systems thus obtained are complex and call upon the highest performances of the modu lar electronic technology . The main constraints met with this technology are ; complexity of wiring, non homogeneous mixture of analog and logic type functions, problems of intergrating the user ' s control philosophy within the manufacturers design philosophy , lack of flexibility for future modifications. The use of digital process computers has made it possible to solve a large number of these problems . The success of the various applications encountered is however tied to the fact that applicatio n software has been specially developed or adapted for the users, even by the users. The use of process computers has led to two new problems ; one, the need to add a simplified standby analog control system , the other, the complexity of using real time monitors . These two aspects are tied with the centralization of the control functions. Recent solutions are now available on the market in the form of decentralized digital controllers designed on the basis of microcomputers which can combine most of the advantages of the various technologies . It should allow the decentralization of control functions (regrouping all the trol functions of one control loop or manipulating one actuator) to be allied with the intrinsic flexibility of digital technology (possibility to program certain special or sophisticated functions) . A great effort has also been made in improving the intrinsic reliability of modern
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control equiprnents, both analog and digital (66) .
An automatic monitoring of these equipments can detected most failures and then force the concerned control loops to a control mode (manual control, for instance) which prevents any effect of the failure on the quality of the generation. The application of these "operational safety" principles enables improving the already very high level of reliability of the electronic equipments ; the effect of a failure of a control device (including the instrumentation and the actuators) on the generation availability has become practically negligible . It should also be noted that te rapid evolution of the technology results in the automatic control systems becoming obsolescent earlier than the generating units . The only solution to this problem appears to us to be the standardization of the equipments, of the signals transmitting the information, and, above all , of the interfaces. This standardization would enable that, in the future , an old equipment could be replaced by a more modern one as soon as, for example, all input - output connections would be in the range 4- 20 mA. Another example of standardization is that of the HP- IP interconnection system standardized by IEEE and IEC. The work carried out by IEC in this matter is thus of paramount importance to the users. BENEFITS FROM AUTOMATIC CONTROL SYSTEMS . The benefits arising from the use of automatic control systems are often strongly debated. When referring to the benefit aspect it is essential to clearly define the criteria used in the calculations. As an example we shall mention here just one very simple case where there is no doubt of the benefit brought about by an automatic control system. It concerns the example described before, regarding the water level control in the steam generators of a PWR type nuclear unit . The problem was well known , some years ago, for the water level control of drum type boilers. Manual control of this level is very difficult at very low load (below 25 % of CMR) and during start - ups leads to trip-outs (1 per 3 start-up) . A full range automatic control eliminates these trips and consequently a loss in time of around 3 hours. Given that P is the CMR, T is the time lost, C is the cost of the replacement energy, then the gain (G), or the avoided loss, obtained by adequate automatic control amounts to ; G = P.T . C. For nuclear units, where P ~ 1000 MW, this gain can easily amount to hundreds of thousands of dollars per year.
J. Debelle, e t al .
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The improvement in the bahaviour of generating units during network incidents can also be calculated but this is much more complicated. SOME CONSIDERATIONS ON THE GAP BETWEEN THEORY AND PRACTICE. A survey of the various applications of automatic control in power plants lets become apparent the gap that exists between the modern theory of automatic control resulting in experimental applications, limited in number and duration, and the practice of automatic control as it is actually performed in almost all power stations. We believe that this gap is due to several r easons of which some have been clearly shown in a recent report (28). We will only mention the following aspects : - the user has rarely much time to devote to theoretical studies and often has to solve problems very quickly. Therefore, he will always be ready to accept solutions that are simple and easy to put into operation, even if they are sub-optimal. - certain studies, confining themselves to well defined parts of a process, do not always take correctly into account the reciprocal influences of other sub-systems. Besides, the knowledge of these influences is often a priori ill-defined. - normally, the power plant staff does not included high level specialists in the various fields and particularly in that of automatic control. These specialists are found in the departments concerned with research and development or in operation assis tance and one of their roles is precisely that of decanting the mass of theoretical works in order to extract new solutions that would meet the industrial requirements. - the gap between theory and practice is also caused by the gap ex isting between the control specialist and the process specialist, the final user. A permanent dialogue and an effort from each to better understand the other is most essential. and finally, - the optimum curve 1S often flat and consequently the practical sub-optimal solutions do not result in disasters.
wider context. The improvement in the performances of a control loop, in case of small perturbations, is of lesser interest to the user. However, efforts are made to develop sophisticated control enabling operation in an automatic mode over an extensive range (almost 0 to 100 % CMR) and also appropriate reaction of the automatic control system to the various incidents affecting the process. Digital process computers are playing an ever important role in the operation of power stations. DDC industrial applications are still limited in number but their success ensures a promising future. This is mainly due to the fact that more complex control structures can be easily realized and modified by means of the programmable digital technology. The use of process computer for monitoring, supervision and aid to the operator is becoming more and more generalized, this is resulting in control rooms being designed taking into account the use of CRT displays. The contribution that theory can bring to industrial applications is a long process and to make theoretical results available to the user, and thereby rendering the expected service, requires numerous and repeated efforts. These efforts must be made and followed up, for, to the user nothing is more practical than sound theory. BIBLIOGRAPHY . I.
2.
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4.
CONCLUSIONS. 5. The paper is far from being complete and does not deal with all aspects nor with all realizations in the automatic control of power stations. The views expressed and the examples given have mainly the aim to show up the problems facing users and how automatic control techniques can help them to meet these problems. The generating units are an integral part of the all power system and the performances of their control are examined in a wider and
6.
7.
O.J. AANSTAD, J.E. LOKAY "Fast Valve Control can Improve Turbine Generator Response to Transient Disturbances" Westinghouse Eng. July 1970. M. ABE et al. "A study of Fast valving as an Aid to power transient stability" IFAC Symposium Melbourne 2/77 A. ABOLINS, D. LAMBRECHT, J.S. JOYCE, L.T. ROSEHBERG "Effect of clearing short circuits and automatic reclosing on torsional stress and life expenditure of turbine generator shafts" IEEE - P.A.S. - 95 - n° I - Jan.-Feb. 1976 J. ADAMS, RP BROWNGARDT, RLG VERA "Power plant computer control fot the 1970' s" R.T.H. ALDEN and P.J. NOLAN "Evaluating alternative models for power system dynamic stability studies" IEEE - P.A.S. - 95 - n° 2 - March/ April 1976 G. ALLARD, F. LAUBLI, D. LE FEBVE "Regimes transitoires dans les chaudieresCalcul des fonctions de transfert - Utilisation des calculateurs par le constructeur". Journees AIM, Mai 1970. PM ANDERSON, S NANAKORN "An analysis and comparison of certain
Automatic Control of Thermal Power Plants
low-order boiler models". ISA 74. 8. P.H. ASHMOLE, D.R. BATTLEBURY, R.K. BOWDLER. "Power-system model for lage frequency disturbances" Proc. lEE - Vol. 121 - N° 7 - July 1974 9. R. BAEYENS et J. DE BELLE "La conduite automatique des centrales thermiques. Methodes d'analyse - Regulations fonctionnelles". Conference SRBE ~ IBRA 75. 10. M.S. BALDWIN, H.S. SCHENKEL "Determination of frequency decay rates during periods of generation deficiency". IEEE - P.A.S. - Vol. 95 - nO I - Jan./ Feb. 1976. 11. C. BARBIER "Les modiHes des machines electriques et leurs organes de commande dans les etudes de comportement dynamique des reseaux". Revue E - Vol. VIII - N° 8 - 1976. 12. V. BERLEMONT ET J. DEBELLE "Some aspects of the automatic control of the start up of a drum boiler". IFAC Dusseldorf 10/68 13. WG. BLOETHE, PM ANDERSON "Application of a low-order boiler model to transient stability studies". IEEE 14. P. B~LDERL, T. KULIG, D. LAMBRECHT "Die Torsionsmomente in Turbinen- und Generatorweller bei Kurzschlussen, Fehlsynchronisierung und Kurzschluss abschaltung". "Die Beurteilung der Torsions beanspruchung in dem Wellen von Turbosatzen bei wiederholt auftretenden Storungen im Laufe der Betriebszeit". ETZ-A - 96 Jahrgang - Heft 4 - April 1975 15. A. BONNELIE et M. LECRIQUE "Le reglage de vitesse des turbo-alternateurs a resurchauffe alimentant les reseaux d'energie electrique". REG - Octobre 1973, t.82 - nO 10 16. M. CANAY "Beanspruchung von Turbosatzen bei wiederkehrender Netzspannung" ETZ-A - 96 Jahrgang - Heft 4 - April 1975 17. NM CHAMPA, JW KING, AW HOWARD. "Conesville Unit 4 - DDC application to a coal fired supercritical generating unit" ISA 74 18. C. CONCOP.DIA "Performance of interconnected Systems following disturbances". IEEE Spectrum - June 1965. 19. KF COOPER, BA MUTAFELIJA "Control System optimization for a large pressurized water reactor steam supply system". 5th IFAC/IFIP Conference - The Hague 6/77 20 E.W. CUSHING, G.E. DREHSLER, W.P. KILLGOAR, H.G. MARS HALL , H.R. STEWART. "Fast valving as an aid to power system transient stability". IEEE - P.A.S. nO 4 - 9172
65
21. MM. DANCHAK "CRT Display for Power Plants" Instrumentation Technology 10/76 22. JM. DANIELS and FT SANDT "DDC comes of age at Martins Creek" ISA - IPI - 74 23. DE BEAR "Plant Operator's Computer Interface" Instrumentation Technology 10/75 24. J. DEBELLE & al. "Regulation des generateurs de vapeur Exper;pnce acquise : Doel I" Common report : ACEC - EBES - LABORELEC TRACTIONEL / July 75. 25. J. DEBELLE et al. "First belgian application of a digital computer for the control of a 280 MW boiler of the thermal power station at GENK-LANGERLO" 5th IFAC/IFIP Conference - The Hague 6/77 26. J. DEBELLE et R. VASQUEZ "Experience acquise en conduite automatique des chaudieres a circulation naturelle". Congres AIM - Liege 74. 27. R. DETTINGER, E. WELFONDER "Application of Optimal Multivariable Control with Process computer in a Thermal Power Plant". IFAC Symposium - Melbourne 77 28. G. DUYFJES, P.J. de JONG and HB VERBRUGGEN "Questionnaire on Applications of modern theory to computer control in the process industry - Results and comments". 5th IFAC/IFIP Conference - The Hague 6/77 29. T. DY LIACCO "Digital computer applications in the control of electric power systems". 5th IFAC/IFIP Conference - The Hague 6/77. 30. D. ERNST "Digital Control in power systems" 4th IFAC/IFIP Conference - Zurich 3/74 31. F.H. FENTON, F. LAUBLI "The flexibility of the supercritical boiler as a partner in power system design and operation" IEEE Winter Power Meeting, New York, Jan. 31 to Febr. 5, 1971. 32. J.C. GOUGEUIL et M. LECRIQUE. "Quelques questions touchant la regulation de vitesse et son incidence sur le fonctionnement des reseaux". SEE - 12e section - Journee du 18 mars 1975. 33. J.C. GOUGEUIL et F. MAURY "Analyse des influences de differents facteurs sur la stabilite des reseaux". RGE - Mars 1974. 34. P.A.L. HAM "Turbine and Governor Modelling" Symposium on Power System Dynamic - Univ. Manches ter 17-21 Sept. 1973. 35. R. HARDT, W. LATZEL "DDC in Power Plant - Application and operational results" 4th IFAC/IFIP Conference - Zurich 3/74.
66
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36. CJ HERGET, CU PARK "Parameter identification and verifi cation of low-order boiler models" . IEEE PES Winter Meeting, NY 1/76. 37 . D. HILTON and D. WELBOURNE "Hartlepool and Heysham' on-line data processing systems". Nuclear Engineering International 1/73 38. R. HOCEPIED "Automatic Control of the Level of steam generators in PWR Nuclear Power Stations" ACEC Review nOs 3-4/75 39 . IEEE WG on Power response to load change. "MW Response of Fossil fueled steam Units" 1972 . 40. IEEE Committee report. "Dynamic models for steam- and hydro turbines in power system studies" IEEE - P . E. S . Winter Meeting - Jan . 28 Feb. 2, 1973. 4 I. MW JERVIS "On-line computers in power stations". Proceedings lEE 8/72. 42. RE JOHNSON "One man control desk for nuclear power plant" . Energy International 6/74 43. G. KAPLAN "Power plant control : displays, computers and man" . IEEE Spectrum 11/74. 44 . B. KULICKE, A. WEBS "Elektromechanisches Verhalten von Turbosatzen bei kurzschlussen in Kraftwerksnahe". ETS-A - 96 Jarhgang - Heft 4 - April 1975. 45 . JC. LEMOINE "Structure de regulation d'un processus pilote par un organe de commande presentant des non- linearites et un caractere dynamique plus lent que celui du processus" . Rapport LABORELEC 2.401.54/G2/JCL/nv 9/76. 46 . WM. LONN, GH MOORE ,. BM SPECKMAN "Operating experience with dual DDC Computer systems". Pittsburg Power Plant unit 7. 47. JE . LUNDE "Advanced control and automation ln nuclear power plants". Nuclear Engineering International 1/73. 48. TB MAHOOD "Computer control at Pickering" Nuclear Engineering International 1/73. 49. MENENDEZ, LEBOURGEOIS "Un nouvel algorithme de commande dynamique optimale" . Automatisme 4/73 50. JE MEYERS, TG SHULTZ "The design of a computer based power plant control center" . IEEE Nuclear Power Syst . Symposium 12/74. 51. P . J. NOLAN, N. K. SINHA "Eigenvalue sensitivities of power systems including network and shaft Dynamics" . IEEE - P . A. S . - 95 - n° 4 - July/Aug. 1976
52. H. PARK "Fast Turbine Valving". IEEE - P . A. S. - May/June 73. 53. J.V . PESNAK and YK NGO "Control of Pulverised Fuel fired boilers for large instantaneous load changes". IFAC Symposium - Melbourne 2/77 . 54 . A. POSSENTI "A direct digital control system for a steam power plant". 4th IFAC/IFIP Conference - Zurich 3/74. 55. REJ PUTMAN "Design for Computer Control". Instrumentation Technology 10/74 . 56. G. QUAZZA "The state of the art in automatic control for electric power systems" . 4th IFAC Congress - Warsaw 6/69. 57 . G. QUAZZA, E. FERRARI "Role of Power Station Control in overal system operation". Symposium on Real-Time control of electric power systems - BBC - Baden 7 1. 58 . JB REID , JG SELMECZY "Computers in nuclear plants beyond 1985" Westinghouse . 59 . RICHALET & al. "IDCOM Conduite algorithmique des processus industriels de fabrication". ADERSA/GERBIOS 6/76. 60 . RICHALET & al. "Model algorithmics control of indus trial processes" 5th IFAC/IFIP Conference - The Hague 6/77. 61 . JE RIJNSDORP and WB ROUSE "Design of man- machine interfaces in process control". 5th IFAC/IFIP Conference - The Hague 6/77. 62 . SE RIPPON "Taking the complexity out of the control complex" . Nuclear Engineering International 1/73. 63 . WB ROUSE "Design of Man-Computer Interfaces for On-Line Interactive System" . Proceeding IEEE 6/75 64. FT SANDT "Martins Creek SES - Unit 3 & 4. Computer System (including DDC) - Implementation and status" EEl - Atlanta 10/75 . 65. TE SMITH "Experience with process control computers in the Canadian nuclear power program". 6th IFAC Congress - Boston 75. 66 . M. SPlRA "Construction de la surete de fonctionnement operationnelle des systemes d' automatisme de conduite" . Automatisme 11/76. 67 . M. STANGIL "Computer and controls - Martins Creek SES - Units 3 & 4". Pensylvania Electric Ass. - 10/75 .
Automatic Control of Thermal Power Plants
68. A.C. SULLIVAN, H. YEE "Fast governing of turbogenerators during transients". Proc. lEE, Vol. 120, n° 3, March 1973. 69. R. URAM, W. GlRAS "Computer Control of a Power Plant". Control Engineering 12/72. 70. P. URONEN, L. YLINIEMI. "Experimental comparison and application of different DDC-algorithmes". 5th IFAC/IFIP Conference - The Hague 6/77 71. F. VAN DE MEULEBROEKE. "Invloed van de voorstelling van de snelheidsregelaar van stoomturbines bij de studie van transiente verschijnselen in electrische netten" Kontaktgroep "STERKSTROOM" N.F.W.O. R.U.G. - 12 december 1974.
67
72. F. VAN DE MEULEBROEKE "Simulateur analogique de turbine a vapeur a resurchauffe". Note d'information L/E n° 25 (mars 1975). 73. F. VAN DE MEULEBROEKE "Comportement en frequ ence en res ea u isole". Rapport L/E - 2/308/I/VDM. 74. VON D. ZELGERT "Erfahrung mit einer DDC-Regelung an einem Kraftwerksblo c k der Chemischen Werke Huls AG". VGB Kraftwerkste chnik 2/75. 75. JP. WAHA "Multivariabl e technical control sys tems. Survey of appli ca ti ons in power plants and power distribution systems". 2nd IFAC Symposium - Dusseldorf 10/71. 76. BR. WELCH. "Computer systems in GEGB nucl ea r power stations". Nuclear Engineering International 1/73.
J. Debell e , et al .
68
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69
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J. Debelle, et al
70
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71
Automatic Control of Thermal Power Pl ants
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J. Debell e , et al
72
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SOME APPLICATIONS AND TRENDS IN AUTOMATIC CONTROL OF THERMAL POWER PLANTS
J. Debelle*, R. Baeyens**, J. Cl. Lemoine** and F. Van de Meulebroeke** *Generation Department, EBES, Brussels, Belgium **Electronic f!I Automatic Section, Laborelec, Rhodes St . -Genese, Belgium Abstract. The report attempts to give a synthesis of special realizati ons and limited experiences in the field of automatic control of therma l power plants. The functions ensured by the automatic con tro l are examined under the aspect of performances and behaviour expected of the generating units in the overall power system operation, including normal operation, shut-down, start-up and their behaviour during incidents. Technological aspects of the equipments are considered. The role played by process computers as far as DDC and monitoring are concerned is examined, including the man/machine interfaces and the design of control rooms. Benef it, reliability and standardization of co ntrol systems are also investigat ed . Current trends in all these fields are presented. Keywords. Power plants, automatic control, computer applications, direct digital control, boilers control, turbines control. carried out to improve our knowledge of processes and control techniques. As a result, models were devel oped very representative of the inves tigated phenomena, and, control systems, often quite sophisticated, we re put into application. The same is also reflected at tech nological level : various manufacturers have commercialized equipments tha t wer e specifically dir ec ted towards applicat ion in the electrical power system field. Reading through the Survey Papers and publications presented at IFAC's Cong resses over a large number of years, it is striking that a substantial part of the pr oblems have been known for a long time and that a lot of remarks that have been put forward are still up-to-date. Could this mean that only slight progress has been made in control techniques f o r this field ? Certainly not, because th e increase of our knowledge and the dev e lopment of our means has now turned many ideas earlier reported into proven rather than experimental achievements.
INTRODUCTION. Electric power generation, transmission and distribution constitute and industrial process having its own very specific characteristics. It is not our intention to review details which have often been developed here but it is nevertheless important to mention some of the major aspects which strongly influence the conception of power generation processes and hence the related automatisms (56,57). The notion of continuity governs the design of generating units, transmission and distribution networks, equipment redundancy, as well as permanent and instantaneous management of the overall system : power and frequency control, voltage level control, load modulation of generating units including shut-downs and start-ups, and of course also appropriate actions during disturbances such as outage of a generating unit, power-line trip-outs, short-circuits in transmission network, frequency drop. A feature of the system is f avourable however in that various elements of th e process are often quite similar. Generating units for instance can be arranged in a few main groups such as hydraulic units, conventional thermal units, PWR or BWR nuclear units, gas turbine sets, ... Not being faced with a multitude of of ten widely differing processes, the electricity sector has been in a position to focus its research and development efforts on a limited number of problems. This particular aspect is also apparent in the field of automatic control of power stations and systems : various studies were
PART TAKEN BY THE GENERATING UNITS. The part generating units take must be seen in the electric power system as a whole "Generation - Transmission - Distribution". This truism unfortunately does not always mat eria lize at generating unit operating lev e l (nor does it in network operation). We feel this remark is particularly pertinent when cons idering the "power units networks" system behaviour under major dis57
58
J. Debelle, et a l .
turbances. The turbine-alternator interconnecting shaft section constitutes a physical border-line enabling either side to carry over part of its problems to the other. Solutions obtained in this way thus often favour either the boiler, turbine or network selfishness. The actual studies however bring forward the non-negligeable effects of the network on boiler and turbine behaviour and vice versa. Numerous studies have been made on linear behaviour of boilers, turbines and networks, and the results have led to development of automatic control systems which are effective in the steady state. But as soon as generating units are considered under major disturbances or failures affecting both power units and networks, then linearization assumptions are no longer valid and the approach has to be shifted towards all aspects of non-linear behaviour of the process. This important step has been made possible through the elaboration of increasingly detailed mathematical models, and particularly through resorting to high-performance computer simulation aids. This stage is of fundamental significance, because the investigated phenomena (consequences of short-circuits, frequency drops, ... ) fortunatly hardly ever occur and experiments cannot be carried out in practice. As their consequences may be critical, it ~s necessary to set up the automatic control systems which will enable making the involved equipment respond appropriately. AUTOMATIC CONTROL OF FOSSIL-FIRED BOILERS. Automatic control of fossil-fired boilers is a problem which is considered as having been solved for a number of years now when operated at rated capacity and when the perturbations affecting the process are of a small amplitude, i.e. not exceeding la % of the maximum perturbance amplitude (6). For some years, other preoccupations have came to light : automatic control under large load modulations (boiler operation between 100 % and 40 % of the load, or even less) ; automatic control during unit startup or during internal incidents such as, for instance, feedpump cut-out or fan stalling (9, 12, 26, 31, 39, 53, 75). This extension of tasks expected from automatic control systems has had the following consequences : - introduction into automatic control systems of non-linear actions (limiters, signal selectors, automatic adjustment of control coefficients, compensation of nonlinear static characteristics). - a more through analysis of interactions between the various physical variables and the use of non-interacting or coordinated
automatic control systems. Without being intended to replace safety systems, automatic control nevertheless plays an important part in operational safety and availability. For instance, in case of a feedwater pump cut-out event in a boiler fed with two such pumps, steam generation will automatically be reduced by 50 % while maintaining pressures, temperatures, 02 excess in flue gas within acceptable limits throughout the transient period and subsequently enabling proper operation at this new load. In such a case, automatic control of the boiler-turbine set is switched over from "boiler following turbine" mode to the "turbine following boiler" mode. By such action, the impact of the failure on the power generation has been reduced from 100 % (outage of the unit for some tens of minutes) to 50 %.
It must be admitted that to date, the conventional PID controller remains practically all over applied, with its well know properties and its fundamental advantage of being easy to implement. Many theoretical and simulation studies have been carried out concerning application of optimal control theories to boilers (70). The performances reached by the latter kinds of control often remains comparable to that obtained with more traditional control systems. Criteria used for optimization do not always conform to the actual boilers operation criteria, and it is noted regularly that the elements of weighted matrices have been chosen "a posteriori" to obtain a satisfactory compromise for the transient behaviour of the process (27). Furthermore, implementing these control techniques is quite complex and the relation between the physics of the process and the control system cannot be easily assimilated by the user. Hence, "quasi optimal" or "sub-optimal" solutions may often prove more interesting. In chosing the control structures, modern theories are valuable for they lead to a better approach to the problems. Selecting a control scheme for small perturbat ions dynamic behaviour remaining similar to the PID control system, but which in the event of large disturbances would enable to take into consideration various non-linearities and interactions, is fundamental (9, 12). Functional block diagrams obtained in this way will always include the two following points : - estimation of the variables representing the system, obtained from accessible signals. - elaboration of control variables, through a set of feedback, and feedforward paths coming from the process itself and from its estimator. Acceptable simplifying assumptions will provide "quasi-optimal" solutions, easier to implement (49, 59, 60).
Automatic Control of Thermal Power Plants
Some examples
The electric servomotors have a non-linear behaviour as soon as the range of small perturbations is left : the motor performs as an integrator between 0 and 100 % of the range, the deviation signal driving the motor via a non-linear control including a deadband, a small proportional zone (thus linear) and then a saturation zone (Figure I) (45). Therefore, the servomotor for most of the time operates at the maximum speed. As the times required to effect 0 to 100 % of the range are between 10 to 60 seconds, effects of actuators speed limitation must not be neglected in relation to the time constants of the process. The following figures illustrate the overall behaviour of the controlled process with a control structure enabling to take into account the actuator 's non-linear behaviour (fig. 3 and 4). The control structure used here is represented in fig. 3 and is obtained from a control scheme based on a direct model of the process (estimation of the disturbances) and an inverse model (compensation of the process dynamic behaviour) (fig. 5-a, 5-b) (9,
14).
The aim of this control is to adjust the feedwater input flow to the load required from the boiler. Any unbalance between the heat flux provided at the boiler by the fuel to meet the load and the heat flux removed by the feedwater induces modifications in the behaviour of some parameters thereby providing a measurement of the unbalance. Formerly, the unbalance value was measured by the pilot-tube temperature which is related to the large thermal inertia of the boiler and therefore gives a filtered and hence delayed information. A deepgoing theorical study of transient phenomena in an evaporator has shown obviously the existence of variables presenting a very fast response to heat flux variations , which led to the idea of using one of these variables in control as a measurement of disturbances. This study is based upon solving of infinitesimal equations describing thermal and mass exchanges and the fluid flow in a section of the evaporator. It gives the system transfer functions corresponding to feedwater flow and heat flux variations. It is also noticeable that the effects due to specific volume variations of the water/ steam mixture are much faster than effects of enthalpy fluctuations in the pilot tubes. Hence the proposal to measure induced disturbances due to heat flux variations by means
59
of pressure drop variation measurements through a venturi fitted into the evaporator outlet. The simple control structure achieved in this way responds much faster to heat flux fluctuations than the previous structure. Effects of boiler behaviour in the transient
i!~~III!Y=fI~I~~----------------------------
Various simplified models of boilers have been developed (7, 13, 36). They enable to analyse the impact of network disturbances on boiler behaviour : flow, pressure and temperature fluctuations. Reciprocally it appears more and more necessary to take into account boiler response in studies concerning network transient stability. As the considered phenomena have a duration of from one to some tens of seconds, it has been generally accepted that they would only slightly disturb the boilers. But, coordination of boiler/turbine controls leads to an as fast as possible response by the boiler to electric load, turbine valves position and steam flow variations. The severest constraints for the boiler occur during closure (almost instantaneous) and reopening (slower) of the turbine valves during certain network disturbances. For the boiler, the reopening speed should not be less than 20 % per second.
Studies have demonstrated that should reopening of the turbine be too slow, coming back to the initial mechanical power of the turbine would be difficult and too slow: opening of valves (by-pass, safety) boiler side, combustion difficulties through lack of air ; functioning of the turbine steampressure limiters (which reduce the turbine power output). In certain cases such difficulties may result in unit outage even though the electric disturbance is already over. STEAM TURBINE CONTROL. In the field of steam turbine control a similar evolution has taken place as for boiler control, in that the role of automatic control systems are extended to ob tain a more adequate behaviour and a better transient response as units under severe variations as those resulting from network disturbances. An extension of the traditional speed governor functions enable it to take an important part in the field of transient stability. This extension has an effect on equipment development as well as on the computation means.
60
J. Debelle, e t al.
~~~~~~_~~_~bi~_~~~_~~i~~~~~i~~_~~_~~~ig~_~~~
studies.
Traditional functions. The traditional functions of speed controllers for turbines are the following : - during start-up, run up, programmed or not, and, if required, including monitoring of the turbine's thermal constraints, limiting the maximum speed gradient either automatically or manually - synchronizing the unit via the synchronizer - loading during start-up and effecting programmed load changes (with or without thermal stresses monitoring) - taking part in the load-frequency control by : . primary control, load modulation in function of the controller statism . secondary control : load changes (automatic or manual) upon the load set-point - keeping the speed - below the overspeed tripout threshold in the event of sudden load rejection by tripping of the circuit breaker. Certain types of electro-hydraulic controlers carry out all these functions, others perform part of them or with certain alternative functions. The last point however is the essential one for all turbine speed controllers and any additional functions are in fact logical extensions of this last point.
Turbine units can also be subject to sudden load variations caused by events other than those mentioned above. The main among these are : - network disturbances (short-circuits). These cause total or partial unloading (depending upon the electric distance) followed by sudden re-loading when the fault has been cleared. Under these circumstances the following is required from the speed controller: I) Fast closing of the control and/or intercept valves, with a degree of closure adapted to the gravity of the fault. Such action will enable extending the stability limit for the first swing ~tability criterium. 2) That this action be not too prolonged, after elimination of the fault, by such on/ off closure actions at the moment of the fault and too slow re-opening of control valves. Ideal opening speeds it seems are between 20 and 50 7. per second. An exemple of such a behaviour is given on fig. 6. - changeovers to isolated network. These cause sudden load variations in the synchronous network to the new load value imposed by the isolated system (lower or higher than the initial load). The controller which imposed the load generation in the synchronous network has to take up again its function as a speed controller to limit frequency excurcions (either way) in the isolated system.
With a view to assign to the speed controller a substantial task in extending the area of the first swing stability, various studies were undertaken concerning fast valving. As a rule these studies investigated how the existing possibilities of fast closure of turbine control valves, necessary to limit overspeed in case of load rejection, could be made useful in case of network disturbances. Fast re-opening appears to be more and more necessary (I, 20, 52, 68). The need for appropriate reaction to network disturbances leads to conditioning the action of threshold accelerometers and speed 1imiters, and to setting a maximum time for reopening. Deepgoing studies are being conducted on these various points (32) . A more thourough study of electro-hydraulic speed controllers and their behaviour under abnormal conditions (disturbances, change over to isolated network) has become possible due to the realization of specialized equipment such as the turbine simulator used in Belgium. It enables to loop the actual controller and steam inlet actuators within a simulated model of the turbine and rotating masses (72). Investigations of the actual incidence of the mechanical torque on the transient stability have called for more realistic representations of all the elements defining this torque. Generally these models are mainly aimed at non-linearities appearing in the various components, rather than at the high order of the transfer functions used. IEEE has proposed non-linear speed controller models, taking into account the inlet valves non-linearities (travel limitations, unlike limitation of closing and opening speeds). This proposal however does not include the representation of a threshold accelerometer nor the MP-LP body preceded by the intercept valves (40). Other studies represent these elements in more detailed models (11, 33, 71). In the same context aimed at realistic representation, particular attention was paid to the turbine model in the scope of investigations into torsional stresses of the turbine-generator shaft during large transients (3, 5, 14, 16, 34, 44, 51). The effects of speed control on stability of synchronous network and particularly of isolated systems and the behaviour of the latter under power deficiency were fully described in several papers (2, 8, 10, IS, 18, 73). In the field of turbine speed control an increasing trend has come about for developing the contribution towards transient stability, by adapting existing equipments or designing new equipments. Emphasis having been put on the good behaviour after fault clearing, the controller is required to handle the actions
Automatic Control of Thermal Power Plants
of control valves with a maximum of efficiency. Computer programs enabling analysis of speed controller performances have followed this evolution by development of models stressing the importance of non-linearities in the control loop. AUTOMATIC CONTROL OF NUCLEAR BOILERS. Automatic control of nuclear boilers and particularly that of reactors is a very specific field, resulting of the process itself and the operating conditions of nuclear reactors (19). The feeling is that reactor control is essentially designed for reactor operation but that the requirements of the overall process, including turbine generator and network, are often neglected. For some time now, studies attempt to better describe the performance potential of nuclear units and the part they have to play in the overall power generation system. Currently, the user cannot easily intervene at the design level of reactor control loops, but the dialogue seems to be starting. In the more conventional parts of nuclear units, the same principles apply as for fossil fuel fired power units. The following example illustrates the striving to obtain an as complete as possible automatic control system working from 0 to lOO % of the load range and during start-up periods.
61
structure (water level and steam pressure in the first stage of the turbine, which is an "image" of the steam flow). These problems were solved and the control structure, including a preset adaptif control of the gain, in function of the feedwater temperature, enable to maintain the water level at its set point value for any load conditions. As a result, start-up times are reduced and operation is made easier. CANDU reactor control (48, 65). A remarkable application of computerized control is certainly that of the CANDU reactors (Pickering, Gentilly, etc ... ) Control is carried out by a dual central computer, with automatic transfer from the master computer to the other which permanently works in parallel. Ea ch computer checks its inputs and its control performances as well as its own operation. This has called for extensive development of auto-tests and automatic transfer from one computer to the other. The computer provides automatic control for the reactor power, drum pressure, temperature gradients, turbine run-up, fuel loader. All the classical monitoring functions are also included. This dual-computer system seems very reliable and only very slightly affects the overall unit availability (at Pickering, unavailability due to the computer system is less than 0.5 %). ROLE OF DIGITAL PROCESS COMPUTERS.
The water level in steam generators must be maintained between narrow limits. A too high level could result in water in the steam entering the turbine HP section with danger of priming the turbine blades. A too low level means that water reserves would be insufficient should emergency cooling of the reactor become necessary. During large transients, this constraint may result in reactor scram or turbine trip-out entailing substantial reduction of power availability. To improve the water level control, a threeelement control structure was developed based upon structures proven in conventional thermal power plants (water level, feedwater flow, steam flow). Problems appeared, among other things, in connection with the substantial delay in level response to feedwater flow variation (because the feedwater temperature differs from the saturation temperature) calling for special precautions. Another problem is that of the temporary inverse time response of the level to steam flow control variation (shrink/swell phenomena). At low load (feedwater and steam flows less than about la %), the flow signals are no longer valid, and the level control system must be changed over to a two-element
Both in conventional and in nuclear power plants operation digital process computers are playing an increasing part (4, 29, 30, 47, 58, 69, 76). It must be admitted that although a number of DDC experiments have been made with satisfactory results, the systematic use of computers for in-line control functions is far from systematic yet (35, 54, 74). The great majority of tasks entrusted to computers consist in on-line type functions, monitoring and assistance to the operators, without automatic feedback to the process. The integration of such equipment has strongly modified control room design (61, 62, 63).
We will only mention here the really industrial DDC systems, and particularly some of the applications in the U.S. and Belgium (17, 22, 25, 46, 64, 67). In these cases, after a first implementation, a policy was decided to equip in this way all new (fossil fired) power units. The following considerations mainly concern DDC aoplications at Martins Creek (units 3 and 4) and at Langerlo (units I and 2). The concern direct digital control systems
62
J. Debelle, et al .
for boiler control. Turbine controls are not included as they are often supplied by the turbine manufa c ture~ (conventional analog/ logic equipment). There are however other applications which do include direct digital control of the turbine. In the examples examined, there is a single computer and back-up in case of computer failure is provided by simplified analog controllers (In Langerlo however, the DDC remains operational, even if the disk memory fails). The use of process computers enabled to highly improve boiler control, not so much by improving dynamic responses of control loops, but mainly by permitting the use of more complex control structures and hence to better control the power unit in case of large disturbances (see the § on Automatic control of fossil-fired boilers). This considerable amelioration in comparison with conventional (analog/logic) control systems however is felt less at the Langerlo plant, because the boilers of previous units are yet equipped with sophisticated analogue control equipment which already include such functions. Improvements mentioned have been made possible by the developme.nt of digital control programs based upon a large choice of calculation modules : - PID controller (but with possibility of max. or min. limits or incremental limit on the output value) - numerous dynamic functions (including dead time) limiters - signal selectors - non-linear functions - logic-type functions - incremental computation - validation of input and output signals - automatic modification of control loops and thus control functions possibility to automatically modify the control coefficients in function of any other variable (preset adaptif control). The importance of these functions becomes obvious in the following example : for the boiler control, at the Langerlo power plant, 32 different type algorithms are used nearly 1000 times in the following proportions (25): - logic algorithms 15 % - arithmetic algorithms 36 % - non-linear algorithms 30 % - I, PI, PID algorithms 2 % - dynamic algorithms (lead-rag, ... ) 2 % - interface algorithms ( i n p u t s / I S % outputs) This shows the very low proportion of dynamic algorithms used (4 %). PID type control remains the most widely used. The first advantage of DDC is related to the large number of possibilities it offers to develop sophisticated control structures. Another point is that increasing the control loop complexity will in no way reduce its reliability and availability. The second advantage is its high degree of intervention flexibility into DDC programs, which enables modifications and adaptations
to be carried out in only a few hours. The disadvantage of a DDC system remains the necessity to ha ve a simplified back-up system. The need for providing coherent change over between computer control and back-up control furthermore sometimes limits control functions improvements. 2. ~~~i~~Ei~g_~~~_~~~i~~~~~~_~~_~~~_~E~E~~~E~ functions.
The functions of monitoring and assistance to the operators in power plants are always handled by process computers and can therefore be considered as classical equipment together with the information means to the operator, such as printers and CRT displays (37, 41, 55). The tasks managed are well known and are enencount e red in any industry equipped with process computers. They consist mainly in monitoring of alarms and changes of state, display on CRT or printing of analog values, graphical representation on CRT's of process circuits with the state of actuators and values of physical parameters, printing of pre-and post-incident perturbographic reports, counting programs (for statistical purposes), operator's guides (display on CRT of operating instructions or procedures). The main advantage of these functions is to provide the operator and operating staff with a clearer picture of the process state, its evolution at any moment and to assist considerably during and after disturbances. Although this advantage is hard to evaluate in terms of money, users consider that the improv ement in monitoring and display of data has made operation more safe and more reliable. Printing of all in/out alarms and changes of state in their chronological order (discrimination time between 10/20 msec and 300/ 500 msec) is of paramount importance to analyse the cause of an incident and its consequences.
Introduction of process computers and new functions of assistance to the operators trends to considerably modify the design of control rooms (23, 42, 43, 50). These are more and more designed for power unit operation based upon the use of visual displays, permitting to strongly reduce the number of indicators and alarm panels on the control consoles (21). Power units operation remains however possible in the event of failure of the computer system. Nevertheless, in the case of the CANDU reactors, operation cannot be continued longer than a set time limit should the computer system break down completely. Indeed, various reactor automatic control functions are handled by these computers.
Automatic Control of Thermal Power Plants
An important point to comply with is the following ; the man/machine interfaces must be designed in the operator's natural language and not that of the computer. To obtain good readability on CRT's a too large number of different colours must be avoided as well as overloading the screen (a density of 25 to 30 % of the screen with data seems to be an optimum) (21). TECHNOLOGICAL ASPECTS OF CONTROL SYSTEMS. At the present time, most of the control systems mentioned above are carried out by conventional analog electronic devices. The functions available are not numerous (PI, PID controllers, simple lag filter, lead-lag filter, a few non-linear functions) and thus sophisticated control structures are not easy to design. During the last ten years a much more complete list of modules has been made available by certain manufacturers . This progress has made it possible to realize much more sophisticated control schemes; non-interactive control, introduction of nonlinearities , preset au t omatic adaptation of control coefficents, automatic modification of control structures, ... However , the systems thus obtained are complex and call upon the highest performances of the modu lar electronic technology . The main constraints met with this technology are ; complexity of wiring, non homogeneous mixture of analog and logic type functions, problems of intergrating the user ' s control philosophy within the manufacturers design philosophy , lack of flexibility for future modifications. The use of digital process computers has made it possible to solve a large number of these problems . The success of the various applications encountered is however tied to the fact that applicatio n software has been specially developed or adapted for the users, even by the users. The use of process computers has led to two new problems ; one, the need to add a simplified standby analog control system , the other, the complexity of using real time monitors . These two aspects are tied with the centralization of the control functions. Recent solutions are now available on the market in the form of decentralized digital controllers designed on the basis of microcomputers which can combine most of the advantages of the various technologies . It should allow the decentralization of control functions (regrouping all the trol functions of one control loop or manipulating one actuator) to be allied with the intrinsic flexibility of digital technology (possibility to program certain special or sophisticated functions) . A great effort has also been made in improving the intrinsic reliability of modern
63
control equiprnents, both analog and digital (66) .
An automatic monitoring of these equipments can detected most failures and then force the concerned control loops to a control mode (manual control, for instance) which prevents any effect of the failure on the quality of the generation. The application of these "operational safety" principles enables improving the already very high level of reliability of the electronic equipments ; the effect of a failure of a control device (including the instrumentation and the actuators) on the generation availability has become practically negligible . It should also be noted that te rapid evolution of the technology results in the automatic control systems becoming obsolescent earlier than the generating units . The only solution to this problem appears to us to be the standardization of the equipments, of the signals transmitting the information, and, above all , of the interfaces. This standardization would enable that, in the future , an old equipment could be replaced by a more modern one as soon as, for example, all input - output connections would be in the range 4- 20 mA. Another example of standardization is that of the HP- IP interconnection system standardized by IEEE and IEC. The work carried out by IEC in this matter is thus of paramount importance to the users. BENEFITS FROM AUTOMATIC CONTROL SYSTEMS . The benefits arising from the use of automatic control systems are often strongly debated. When referring to the benefit aspect it is essential to clearly define the criteria used in the calculations. As an example we shall mention here just one very simple case where there is no doubt of the benefit brought about by an automatic control system. It concerns the example described before, regarding the water level control in the steam generators of a PWR type nuclear unit . The problem was well known , some years ago, for the water level control of drum type boilers. Manual control of this level is very difficult at very low load (below 25 % of CMR) and during start - ups leads to trip-outs (1 per 3 start-up) . A full range automatic control eliminates these trips and consequently a loss in time of around 3 hours. Given that P is the CMR, T is the time lost, C is the cost of the replacement energy, then the gain (G), or the avoided loss, obtained by adequate automatic control amounts to ; G = P.T . C. For nuclear units, where P ~ 1000 MW, this gain can easily amount to hundreds of thousands of dollars per year.
J. Debelle, e t al .
64
The improvement in the bahaviour of generating units during network incidents can also be calculated but this is much more complicated. SOME CONSIDERATIONS ON THE GAP BETWEEN THEORY AND PRACTICE. A survey of the various applications of automatic control in power plants lets become apparent the gap that exists between the modern theory of automatic control resulting in experimental applications, limited in number and duration, and the practice of automatic control as it is actually performed in almost all power stations. We believe that this gap is due to several r easons of which some have been clearly shown in a recent report (28). We will only mention the following aspects : - the user has rarely much time to devote to theoretical studies and often has to solve problems very quickly. Therefore, he will always be ready to accept solutions that are simple and easy to put into operation, even if they are sub-optimal. - certain studies, confining themselves to well defined parts of a process, do not always take correctly into account the reciprocal influences of other sub-systems. Besides, the knowledge of these influences is often a priori ill-defined. - normally, the power plant staff does not included high level specialists in the various fields and particularly in that of automatic control. These specialists are found in the departments concerned with research and development or in operation assis tance and one of their roles is precisely that of decanting the mass of theoretical works in order to extract new solutions that would meet the industrial requirements. - the gap between theory and practice is also caused by the gap ex isting between the control specialist and the process specialist, the final user. A permanent dialogue and an effort from each to better understand the other is most essential. and finally, - the optimum curve 1S often flat and consequently the practical sub-optimal solutions do not result in disasters.
wider context. The improvement in the performances of a control loop, in case of small perturbations, is of lesser interest to the user. However, efforts are made to develop sophisticated control enabling operation in an automatic mode over an extensive range (almost 0 to 100 % CMR) and also appropriate reaction of the automatic control system to the various incidents affecting the process. Digital process computers are playing an ever important role in the operation of power stations. DDC industrial applications are still limited in number but their success ensures a promising future. This is mainly due to the fact that more complex control structures can be easily realized and modified by means of the programmable digital technology. The use of process computer for monitoring, supervision and aid to the operator is becoming more and more generalized, this is resulting in control rooms being designed taking into account the use of CRT displays. The contribution that theory can bring to industrial applications is a long process and to make theoretical results available to the user, and thereby rendering the expected service, requires numerous and repeated efforts. These efforts must be made and followed up, for, to the user nothing is more practical than sound theory. BIBLIOGRAPHY . I.
2.
3.
4.
CONCLUSIONS. 5. The paper is far from being complete and does not deal with all aspects nor with all realizations in the automatic control of power stations. The views expressed and the examples given have mainly the aim to show up the problems facing users and how automatic control techniques can help them to meet these problems. The generating units are an integral part of the all power system and the performances of their control are examined in a wider and
6.
7.
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Automatic Control of Thermal Power Plants
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21. MM. DANCHAK "CRT Display for Power Plants" Instrumentation Technology 10/76 22. JM. DANIELS and FT SANDT "DDC comes of age at Martins Creek" ISA - IPI - 74 23. DE BEAR "Plant Operator's Computer Interface" Instrumentation Technology 10/75 24. J. DEBELLE & al. "Regulation des generateurs de vapeur Exper;pnce acquise : Doel I" Common report : ACEC - EBES - LABORELEC TRACTIONEL / July 75. 25. J. DEBELLE et al. "First belgian application of a digital computer for the control of a 280 MW boiler of the thermal power station at GENK-LANGERLO" 5th IFAC/IFIP Conference - The Hague 6/77 26. J. DEBELLE et R. VASQUEZ "Experience acquise en conduite automatique des chaudieres a circulation naturelle". Congres AIM - Liege 74. 27. R. DETTINGER, E. WELFONDER "Application of Optimal Multivariable Control with Process computer in a Thermal Power Plant". IFAC Symposium - Melbourne 77 28. G. DUYFJES, P.J. de JONG and HB VERBRUGGEN "Questionnaire on Applications of modern theory to computer control in the process industry - Results and comments". 5th IFAC/IFIP Conference - The Hague 6/77 29. T. DY LIACCO "Digital computer applications in the control of electric power systems". 5th IFAC/IFIP Conference - The Hague 6/77. 30. D. ERNST "Digital Control in power systems" 4th IFAC/IFIP Conference - Zurich 3/74 31. F.H. FENTON, F. LAUBLI "The flexibility of the supercritical boiler as a partner in power system design and operation" IEEE Winter Power Meeting, New York, Jan. 31 to Febr. 5, 1971. 32. J.C. GOUGEUIL et M. LECRIQUE. "Quelques questions touchant la regulation de vitesse et son incidence sur le fonctionnement des reseaux". SEE - 12e section - Journee du 18 mars 1975. 33. J.C. GOUGEUIL et F. MAURY "Analyse des influences de differents facteurs sur la stabilite des reseaux". RGE - Mars 1974. 34. P.A.L. HAM "Turbine and Governor Modelling" Symposium on Power System Dynamic - Univ. Manches ter 17-21 Sept. 1973. 35. R. HARDT, W. LATZEL "DDC in Power Plant - Application and operational results" 4th IFAC/IFIP Conference - Zurich 3/74.
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36. CJ HERGET, CU PARK "Parameter identification and verifi cation of low-order boiler models" . IEEE PES Winter Meeting, NY 1/76. 37 . D. HILTON and D. WELBOURNE "Hartlepool and Heysham' on-line data processing systems". Nuclear Engineering International 1/73 38. R. HOCEPIED "Automatic Control of the Level of steam generators in PWR Nuclear Power Stations" ACEC Review nOs 3-4/75 39 . IEEE WG on Power response to load change. "MW Response of Fossil fueled steam Units" 1972 . 40. IEEE Committee report. "Dynamic models for steam- and hydro turbines in power system studies" IEEE - P . E. S . Winter Meeting - Jan . 28 Feb. 2, 1973. 4 I. MW JERVIS "On-line computers in power stations". Proceedings lEE 8/72. 42. RE JOHNSON "One man control desk for nuclear power plant" . Energy International 6/74 43. G. KAPLAN "Power plant control : displays, computers and man" . IEEE Spectrum 11/74. 44 . B. KULICKE, A. WEBS "Elektromechanisches Verhalten von Turbosatzen bei kurzschlussen in Kraftwerksnahe". ETS-A - 96 Jarhgang - Heft 4 - April 1975. 45 . JC. LEMOINE "Structure de regulation d'un processus pilote par un organe de commande presentant des non- linearites et un caractere dynamique plus lent que celui du processus" . Rapport LABORELEC 2.401.54/G2/JCL/nv 9/76. 46 . WM. LONN, GH MOORE ,. BM SPECKMAN "Operating experience with dual DDC Computer systems". Pittsburg Power Plant unit 7. 47. JE . LUNDE "Advanced control and automation ln nuclear power plants". Nuclear Engineering International 1/73. 48. TB MAHOOD "Computer control at Pickering" Nuclear Engineering International 1/73. 49. MENENDEZ, LEBOURGEOIS "Un nouvel algorithme de commande dynamique optimale" . Automatisme 4/73 50. JE MEYERS, TG SHULTZ "The design of a computer based power plant control center" . IEEE Nuclear Power Syst . Symposium 12/74. 51. P . J. NOLAN, N. K. SINHA "Eigenvalue sensitivities of power systems including network and shaft Dynamics" . IEEE - P . A. S . - 95 - n° 4 - July/Aug. 1976
52. H. PARK "Fast Turbine Valving". IEEE - P . A. S. - May/June 73. 53. J.V . PESNAK and YK NGO "Control of Pulverised Fuel fired boilers for large instantaneous load changes". IFAC Symposium - Melbourne 2/77 . 54 . A. POSSENTI "A direct digital control system for a steam power plant". 4th IFAC/IFIP Conference - Zurich 3/74. 55. REJ PUTMAN "Design for Computer Control". Instrumentation Technology 10/74 . 56. G. QUAZZA "The state of the art in automatic control for electric power systems" . 4th IFAC Congress - Warsaw 6/69. 57 . G. QUAZZA, E. FERRARI "Role of Power Station Control in overal system operation". Symposium on Real-Time control of electric power systems - BBC - Baden 7 1. 58 . JB REID , JG SELMECZY "Computers in nuclear plants beyond 1985" Westinghouse . 59 . RICHALET & al. "IDCOM Conduite algorithmique des processus industriels de fabrication". ADERSA/GERBIOS 6/76. 60 . RICHALET & al. "Model algorithmics control of indus trial processes" 5th IFAC/IFIP Conference - The Hague 6/77. 61 . JE RIJNSDORP and WB ROUSE "Design of man- machine interfaces in process control". 5th IFAC/IFIP Conference - The Hague 6/77. 62 . SE RIPPON "Taking the complexity out of the control complex" . Nuclear Engineering International 1/73. 63 . WB ROUSE "Design of Man-Computer Interfaces for On-Line Interactive System" . Proceeding IEEE 6/75 64. FT SANDT "Martins Creek SES - Unit 3 & 4. Computer System (including DDC) - Implementation and status" EEl - Atlanta 10/75 . 65. TE SMITH "Experience with process control computers in the Canadian nuclear power program". 6th IFAC Congress - Boston 75. 66 . M. SPlRA "Construction de la surete de fonctionnement operationnelle des systemes d' automatisme de conduite" . Automatisme 11/76. 67 . M. STANGIL "Computer and controls - Martins Creek SES - Units 3 & 4". Pensylvania Electric Ass. - 10/75 .
Automatic Control of Thermal Power Plants
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J. Debell e , et al .
68
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,-1
'/
/
If y// Y
60
I
~
1
40
/ /
20
j
1--- _ _
2"
50%
1----
1)1/ V
1:&
1----
o
p
/:-;?
1"
10%
--~
2 4 6 8 10 t Aw=10% 1_closedloop l~ __ openloopAy l "_ _ closed loop Ay Amo .. 2"_ _ Aw=50% 2 _ .. "
Aw=100%3_
..
"
m (%1 Ac~ u ator
100
I
80
\
60
40
20
o Fig4
100°;'
/
\
50%
LV
I
~
I/~ 2
: Con~roller based
10%
4
12 t 6 8 '0 upon modeis, and taking into account
the nonJinear behaviour of the actuator
J. Debelle, et al
70
Process
w
..
F_l(p)
mo ...
0
-
m
SM
y
F(p)
Aduaror
... Fl (p I
I'
- '-
F 61( p) = inverse model of F(p) Fl( ):: djrect model of F(p)
Fig 5a: Conl"rol structure based upon models (linear)
w
. -
~ .-
,/
K2
mo+-o-----
SM
m
y
F(p)
~
'---
Foc lp )
Fl I p)
Foclp) isa first order model of Flp) FlIp) is a second
order model of F(p)
Fig IS b: Control structrure based upon models/and taking Into account the non_ linear bet-aviour of the actuator
k
71
Automatic Control of Thermal Power Pl ants
pU
Pel Pm
Pel = electrical power
" \
\
,
.......
_-..,
~-1
.....
Pm
z:
mechanical
.5
it time O~'-&------~~----~-______ ~ __ ~ sec t .24 1. 2. 3. electrical incidentlnear the power plant)
Deg . 120
Rotor angle
90
30
pU
YHP= Yt.lP=
1.
HP valve position MP valVE' position
,- ...
--
....
{Op.nin g time lim +5p"/'" va Ives
.S
closing time lim-5pu/sec
.I,,'co {op.nin g time Iim.5p"/,", hydraulic convert'or
0
1.
closing time lim -Spu/sec
2.
3.
Fig 6 : Behaviour of a turbine EH controller, incase ofa ne twork i nciden t
sec
J. Debell e , et al
72
Fig 7.a:Doel LN uclear PWR uniL Startup_D iagrams of steam generatorA
%
LA
Automatic se~ point modification
1 OO-h-r--'+'----,-r--r-.-r-r-,,----,--r--. ~~~~~-
~~~i§l~t~~~~inin!aIij~j
50t r-- -
~ -tttii=tttjj=tttt:t=!=tt++=!=tt++=1=~+=1 t-
water level of the steam generator
t-"-
o ~±tlH±~~tlE~~~~
16~
aut"omatic
m:f+ FfwA
· ·i~f1 ·
...
50 ..
FmsA
Feed water flow
.,-
.-+
H
1( mm
Fig 7JfDoe(LNuclear PWR uniLStart up..Diagrams
at
steam generator A