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Automatic generation control
Automatic generation control H Glavitsch and J Stoffel Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 ZOrich, Switzerland
Automatic generation control, one o f the functions o f a centralized system control centre, is discussed from the point o f view o f the steady-state loadfiow between areas, the control algorithm, noninteractive control, optimum control and its impact on security enhancement, as well as the various interfaces to upper and lower level control functions. The importance o f AGC in sparsely and highly interconnected systems is outlined, and present day problems are resolved.
I. Introduction
Within power system control, automatic generation control (AGC), known formerly as load-frequency control, plays a role quite distinct from that of other control functions. The main reason for its uniqueness is that it has a number of features not attributable to other functions in the same configuration. The main points are that AGC is ~rtrue centralized function, it operates in real time as well as closed loop, and it has a strong interface with other functions oriented towards the economy and security of the power system. Pictured within the framework of system control functions, AGC is an executive function which realizes the results of higher level algorithms (see Figure 1). It has to be emphasized, however, that AGC algorithms are of particular importance despite this executive feature. With the realization of interconnections in the 1930s, the problem of system control has already been recognized and the basic ideas of tie-line bias control have been formulated. 1,2 They were developed further in the English literature and were well documented by Cohn. 3 The principles were convincing and could be implemented by analogue means. In the UCPTE system, load-frequency control was established in the early 1950S. 4 There is long experience with systems of this kind including also digital versions, and the performance achieved has been excellent.
the net tie-line power is at the same time the power flow over the interconnecting lines. For highly interconnected areas, this is no longer the case. The net interchange may be completely different from the power flow crossing an area, and area control loses its effect on this power flow. The losses caused by the power flow have to be covered by the area, and a partner not participating in the power exchange has to pay for an interchange between external companies. The cost of regulating power is another point. Maintaining frequency and time in the system means operating the regulating units with frequent load changes. When new generating units are added, there is a tendency not to let them participate in the regulation. This leads to increased duty of the regulating units. Irrespective of the choice of regulating units, i.e. hydro or thermal, such an operation is quite uneconomical, and the question arises as to the choice of the participation. Further, if it is necessary to realize a tight control programme, how much regulating power is essential and what should be the response of these units'? A larger participation of units in the control task has not been implemented because of economic considerations given to new large units. Being aware of these problems, one has to look differently at the missing realizations of optimal control and other advanced features of system control. Missing regulation power per se excludes any advance in the improvement of control. However, efforts have been made to develop AGC and design algorithms as well as models which take into account the constraints and requirements of the real system.S, 6, 7 These approaches all require that more information is made available to the control system and some
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When areas are interconnected by just a few lines and the interconnection is concentrated on one side of the area,
Vol 2 No i January 1980
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In recent years, however, questions have been raised as to the appropriateness of AGC principles for modern system control, particularly when systems are becoming increasingly interconnected. The problem seems to lie in the uncontrollable inter-area power flow and the cost of regulating power.
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21
basic strategy is changed in order to arrive at the desired inrprovement. The objective of this paper is to develop these features.
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II. Tie-line power-frequency control principle In order to appreciate power-frequency control, two features o f the control system must be considered, namely the way in which output quantities of an area are measured and combined to a controlled variable, and the actual control algorithm itself.
Area ~ N
Figure 2 Connection of single area over small number of tie-lines
The first feature determines the so-called area control error (ACE) and allows each area to operate as an equal partner in the system with no superseding centralized control. Assuming that each area aims at keeping its ACE lo zero and that there is one common system frequency leads to a well defined steady-state interchange, This will become apparent from the following system of equations representing the various areas. ACE1 =Pox
P1
10Bl(f
Jol)
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=P1 +P2 + . . . + P n
where Pol is the net interchange schedule, B1 is the frequency bias setting, jol is the frequency schedule, ACE 1 is the area control error, and n is the number o f areas. Written in matrix form, this gives:
] 1
10 B
-~,-
-Pol + 1 0 B l f o l
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0
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Interconnected power system, many areas, paths
of power flow
0
The matrix is regular (for B 1 ~> 0, at least one of tile BlS > 0) and flrerefore a solution for any setting of the Pols and Jols exists. Tire scheduled interchange values and tire scheduled frequency, together with the frequency bias settings, are the coefficients in this system of equations. The actual frequency and the net interchanges are the variables. These equations will always have a solution irrespective of the size o f the coefficients, i.e. even if the scheduled quantities are not consistent. This also makes the principle of tie-line power-frequency control quite robust, but it leads to errors. Any incorrect setting o f a partner of the system affects the overall interchange schedule. Assuming that the schedule is correct and that tile interchange is realized accordingly, there are two cases to consider. One is a system configuration in which an area is interconnected by a small number o f lines, as illustrated in Figure 2. In this case, the average loading o f these lines corresponds to the net interchange o f the area and hence
the loading of the lines can be closely controlled by the interchange schedule. This is the kind of application where AGC has proved to be most successful. The second case is one in which the area is highly interconnected, e.g. as an area surrounded by other areas. In a simplified way, it can be represented by the schematic in Figure 3. The areas are rectangles and there are interconnecting lines at all sides. The net interchange per area is the difference between the input and output power of the area. A power transfer across an area does not appear in the net interchange. The control o f power flow, being one of the original objectives o f AGC, is thus no longer given since the other areas will determine the power flow through the area. What happens if an area I transmits power to another area K within an interconnected system is indicated in Figure 3 by arrows. The power flow is determined by the loadtlow equations in which the net interchanges enter as equality constraints. Depending on the line impedances, the power flow can take on quite unexpected patterns even if the sending and receiving areas are adjacent to each other. The net interchange turns out to be a weak constraint and cannot prevent overloading of lines. The individual tie-line flow is beyond the control of tire local utility, and losses
Additional signals(stateinformation)
incurred by the power transfer have to be covered by local generation. These features are highly undesirable from the point of view of the area and make AGC unattractive. Hence local utilities are becoming increasingly interested in schemes that allow individual control of lines and of which the socalled optimal power flow may be one form of realization. The problem is not easily resolved since an optimal power flow in the true sense can be implemented only in a centralized system whereas the present system consists of a number of independent areas. So, in order to achieve the above objective, a closer coordination between areas will be necessary. One approach worth mentioning is found in a paper dealing with the assessment of inter-area capabilities, a It should be mentioned that the reactive power flow causes similar problems, and due consideration should be given to that portion of the power flow as well. III. Control algorithm The steady-state equation above indicates the requirements necessary for achieving the desired control objective, namely to keep the ACE at zero or at least at a constant small value. The control algorithm, the second important feature o f AGC, is designed to fulfil this objective. Assuming controllability of the interconnected power system, a high-gain amplifier or an integrating regulator will be able to achieve a small error in ACE or even a diminishing value. This function is implemented by a regulator (hardware, continuous, sampled) or by an EDC-program on a computer. The detailed algorithms vary depending on the school of thought: in Europe, a proportional integral {PI) algorithm is used ; in the USA, a proportional scheme is employed. The PI algorithm is represented by the following control law:
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Y = Cp. ACE + - - .
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Distributionfactors-EDCprogram
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Figure4 Overall control scheme of AGC
is given which processes frequency and tie-line power according to the above control law. The output is then multiplied by participation factors before it is sent to the individual regulating units. The participation is determined by economic considerations supplemented by knowledge about the type of unit (speed of response). At the regulating unit, the control signal acts as a setpoint in the speed governor. In this schematic, control actions interface at various points, one being at the centralized level between the load-frequency controller and a possible economic dispatch algorithm, and another being at the local level between the governor and the centralized control. There, interfaces are quite determinant for control performance and will be treated in more detail below.
CE.dt
IV. Noninteractive control where Cp is the proportional gain, T N is the time constant, ACE is the area control error, and Y is the output of controller, with typical values Cp = 0 . 1 - 0 . 3 and T N = 30 100 s. Since the integral term has a relatively small gain, it assures the decrease of ACE to zero but has little effect on the dynamics of the controller. This is given mainly by the proportional term. Filtering and delays have additional effects as will be discussed below. This control law is also the point at which two extensions of the basic algorithm can be introduced. One is noninteractive control and the other is optimal control, both of which will be elaborated below. However, it can be stated here that extensions are oriented mainly towards dynamic effects, the speed of response and the participation of areas and regulating units. The basic objective of keeping ACE at zero or at a small value remains the same. One importan't addition to the basic algorithm is the weighting of the outgoing control signal by participation factors which can be best understood by means of Figure 4, which shows the overall control scheme of AGC. The block
For a system without AGC, a load change within an area will be balanced by the prime mover and governor action alone. When steady state is reached, each generating unit will contribute according to its droop (speed regulation). When AGC is superimposed, the net contribution of an area can be reduced or even compensated by an appropriate adjustment of the frequency bias setting. Then the regulating units will be activated only if the load change occurs within the own area. A plot o f some of the relevant quantities, as shown in Figure 5, illustrates this mode of operation quite clearly. In this case, a load change was introduced in a neighbouring area, the frequency dropped and the area assisted the neighbour by supplying power in a transient mode. The control signal and the prime mover output, however, hardly changed. In steady state, all deviations went back to zero. This principle of noninteraction has been further developed by Quazza 9 with the object of suppressing the remaining tie-line power flow in the transient mode and avoiding interaction between frequency and tie-line power.
0
This means that a change in tie-line power should not affect frequency control and a variation in frequency should not affect the control of tie-line power.
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Control actions are confined to the area in which the load change takes place in order that the area controller can be designed without having to consider the dynamics o f the neighbouring area. A profound understanding o f large systems control must be attributed to Quazza in developing this concept. 9
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These objectives are achievable under assumptions o f linearized systems and so-called stiff areas as well as of the realizabitity o f the required transfer functions in the control system.
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In today's interconnected systems, the emphasis has changed somewhat. In areas where large generating units are installed, the necessary reserve can hardly he maintained within the area. So when a unit is lost, the area relies on its neighbours.
From the point of view o f modern control theory, this was not considered the best possible result. With the availability of state estimation and optimum control theory, models and algorithms were developed which were aimed at an improvement in the response o f frequency and tie-line power. The best known result is that o f Elgerd and Fosha s,6 which is based on the existing control structure. The feedback coefficients are determined such that the minimum of a performance index is achieved.
The data acquisition system is a sampled data system and the regulating units have constraints. Hence the required transfer functions cannot be realized and the neighbouring areas supply power to the troubled area. This can be seen in Figure 5. There is also a conflict between noninteraction and support to the neighbour in terms of the amount and the duration of the support, as a recent IEEE committee report l° indicates. The reserves in the neighbouring areas have an economic value and have to be justified. It seems, however, that a reserve in all areas in proportion to their spinning power is becoming accepted.
An extension of this procedure, observer estimation detection (OED), was suggested by Glavitsch and Galiana u and comprises an estimator, a detector and a linear quadratic regulator. The concept is based on the idea that, in order to achieve an improved control, the dynamic model of the area has to be extended and states or output variables of this model have to be included in the objective function. Since some o f the variables are not accessible, an observer estimator has to be used. The ACE could not be dispensed with and had to be included as a feedback variable since it determined the final values.
V. Optimum control The general practice in the design o f the area controller is to use a Pl algorithm (see section Ill). This gives adequate response considering the stability requirement and the performance of regulating units.
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The weighting o f the various quantities within the objective function allowed different strategies of control to be realized, including a minimum interaction with the neighbouring area, support of the neighbour and good frequency control. Two examples of optimum responses are shown in Figures 6 and 7.
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The first example is the result o f a case in which tile area has to cover a load change inside its boundaries. Although the response is optimal, a certain power flow from the outside cannot be suppressed. The second is to demonstrate that control o f frequency o f the system can be improved by supporting the troubled neighbour by a small transient increase of the tie-line power. One o f the basic difficulties in achieving true optimum control lies in the fact that the final values of variables are not known at the initiation of the disturbance. A linear quadratic regulator would require the difference between actual values and final steady-state values as an input. Even in a simulated result, these inputs can only be estimated.
direction will have to be justified by a more profound evaluation of the worth o f the gains in the response.
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VI. Optimal power flow and security enhancement
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Figure7 Optimum control with OED (disturbance in neighbouring area, A load = 0.032 p.u.) (a) Afl, (b) APti e, (c) (i) AU1 (ii) APml In practice, this deficiency is covered by other deteriorating effects, such as measurement errors, nonlinearities, model errors, constraints, etc. In fact, these items determine and actually limit the achievable improvements of control. What is of greater importance, however, is the response of the system to different types of disturbances. It seems appropriate that the area controller reacts depending on the size and the type. A proposal of how this should be done is given in reference 11 and the distinction is made by the amplitude of the load or tie-line power change. Four classifications are chosen: (i) (ii) (iii) (iv)
smaller than 2% smaller than 5% larger than 5% own area larger than 5% neighbouring area.
For class (i), there is no real need for a fast response: the disturbances are mostly random and the system should he aimed at reducing the variance of the frequency and power variations. For class (ii), an efficient reaction within the area is needed without too great an effect on the neighbours. Classes (iii) and (iv) represent emergencies and the overall system should respond in this case. This brief discussion indicates that the application of optimal control must be geared towards specific incidents in the systems when there is a real need. Since for most of the time no serious incidents occur, the system should behave quietly without any large excursion of the regulating units which are also a part of optimal control. A number of papers and proposals tend towards the same objective.V, 12,13, 14 Control logic, analogue filtering and nonlinear optimal control 12 are the methods employed. Again, it must be emphasized that improvements can be realized only if more information about the system state is available. This requires estimation and observation at a greater frequency than is envisaged today. A step in this
In the introduction, it was pointed out that, although AGC is taking care of the power balance in the system, the algorithm with its output function acts as an executive for economic dispatch and security control. As the functions are set up today (see Figure 1) dispatching and security are located on a higher level and respond at a considerably slower rate. They are based on a steady-state model of the power system, and it would be unreasonable to interfere at a faster rate. In the area of optimal power flow, the algorithms have been perfected and the theory is well developed, is The algorithms are able to handle the full AC power flow with constraints of both dependent and independent variables.16, 17, 18 For online applications, rescheduling for security enhancenlent is becoming more and more important, and efficient handling of constraints and sensitivity relationships are the main features. A stage has been reached at which implementation is quite feasible due to considerable improvement in the convergence and storage space utilization. Availability of these tools allows power flow corrections by the AGC algorithm, i.e. by setting the participation factors of individual units accordingly. There are, however, two major obstacles which confine this type of control to a limited number of cases. One is the fact that very often the number of regulating units is small and their location is such that the controllability of the power flow is rather reduced. The second point is related to the location of the area within ttle interconnected power system. As pointed out in section I1, it will be difficult to control the power flow when the major power transfer through an area is dictated from the outside. Again, this amounts to a lack of controllability. Keeping the net interchange fixed amounts to similar limitations. In order to overcome these shortcomings, it would be necessary to introduce a floating net interchange which would also allow the tie-line power to be included in economic dispatch. In addition, AGC would have to be complemented by switching operations to augment the possibility for rescheduling. The consequence is that several areas have to coordinate their actions quite closely, and this leads to pool operation, which is already done in certain countries, as the existing pools in the USA indicate.
VII. Interfacing with economic dispatch and primary control In generation control, several control levels exist, namely economic dispatch (EDC), frequency tie-line power control (LFC) and unit control. Each of these levels has its time scale and each has an output according to its objective.
In order to serve tile overall objective o f security and economy, the way in which these levels interface must be well defined. For clariiy, a schematic of these three levels is given in Figure 8, where tire points o f interaction are also indicated. The structure o f these three levels is not unique, but corresponds to the present implementation o f AGC. It could well be that upon the availability o f appropriate economic models, EDC is located below LFC. This would mean that participation factors are worked out dynamically. The interlacing problem, however, remains.
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Tire unit control is considered first. The output of AGC is realized in the unit via setpoint control whereby the telemetered quantity can be tile setpoint itself or an increment. In any case, tile setpoint will be stored at the input of the unit, so that if the telemetry system fails, the unit will operate according to the last setpoint transmitted. All monitoring of tire unit, for high and low limits, maximum permissible increase of output, response, etc., is done at this level. Tire monitoring function is implemented ira tile AGC computer in an output unit which is connected to tile generating unit by telecontrol. Any modifications or improvements at this level affect the unit response to a small degree only. Tire merit lies mainly in tire closer observation of plant limits and a better knowledge of unit performance at the AGC level. The interlhce at tire AGC level is more complex and permits realization o f various strategies. At the moment, the general philosophy o f control is such that frequency/ tie-line control has first priority. Economic dispatch with its setpoints follows the former or acts independently. This can best be discussed by considering one of the power allocation algorithms. 19
i where Psi is the setpoint of the unit, Phi is the base loading o f i t h unit, Pi is the participation factor, Y is the area demand in MW, m is tile number of units under AGC, and ~ p i = 1. Y is the output o f tire frequency/tie-line power controller and it is assumed, for the moment, that it equals tire sum o f tire base loads. Tire system is balanced and it operates
Economic dispatch
EO
30
40
50
Time (t),s
Figure9 Two different speeds of response {a) response of fast unit, (b) response of slow unit
in a state according to economic dispatch which determined tire base load points. Since two different running modes for LFC (faster) and EDC exist, a load change affects Y first and Phi at a later stage. Depending on the size of tire participation factor Pi, a unit can be made to respond quickly to an increase in Y and is eventually restored to a smaller value after EDC has determined the load point. If the participation is small, the unit will essentially follow tire EDC program. This double action within tile AGC level at this interface can give rise to considerable discussion o f which the attain issue is tire worth of tire fast response of the regulating units (large participation) (see Figure 9). Tire increase of regulating power and subsequent decrease is not desirable from the point of view o f the unit (cost, wear, maintenance). Tire power system, however, depends upon fast response. It could be implemented in a larger number of units, which usually is not tile case. If tire participation factors of fast regulating units are not set to a high value, the other units are driven harder, they reach their limits, and Y increases, which again drives tile regulation units up. So for large excursions of Y, tile problem cannot be resolved satisfactorily at this point. Tire real problem is tile definition of the objective o f control for a given class o f disturbances (see section V). If control action is needed in case o f an emergency, tile regulating units should be allowed to participale substantially irrespective of subsequent backswings. If frequency and tie-line power deviations are not critical, participation can be small and tire Pi factor can be set according to economic considerations. This is an area where optimum control on a sound economy security basis can bring about real improvements.
\ Load-frequency control
Interface
Unit control
Interface
Speed overnor
Figure 8
Control levels and interfaces in AGC system
So far the Y and P,,is have been considered to act rodependently upon the setpoint. The setpoint can also be made to respond to logic combinations of their deviations, e.g. a raise in the EDC requirement is realized only if the LFC requirement asks for a raise at tile same time. It is lhen permitted to go to the unit. I lence this mode is called permissive conlroJ. When the t'.[)C is realized in any case, tile mode is called mandatory control. Mandatory control is required for tile realization of an EDC schedule in the steady state of the power system.
In the permissive control mode, a certain smoothing effect is achieved.
Table 1 Filtering, logic decision and thresholds on various levels
Level VIII.
Filtering, logic decision and thresholds
In the early applications of load-frequency control, analogue systems were used and the measurements were taken continuously and simultaneously. In modem telemetry systems and the associated data acquisition systems, quantities such as tie-line power are sampled. The sampling instants cannot be the same at the various tic-lines; hence, the tie-line powers are taken nonsimuhaneously and their sum contains a noise term (aliasing). It can be kept small by increasing the acquisition rate. In addition, filtering will decrease the error and can take place either at the transducer level or within the AGC controller. Usually, both methods are applied. Filtering at the AGC controller level, however, introduces quite a delay since the sampling interval is of the order of 2 5 seconds and filtering means considering at least 2 - 5 sampling periods, tlence, a step change in load can be detected only after roughly 10 seconds. The methods of filtering, as applied today, are quite simple and there is room for improvement, e.g. by using the dynamic model of the area (estimation).
EDC
Filtering,logic decisions Estimation of steady-state operating point,
forecasting of near future operating point LFC
Filtering of tie-line power, estimation of ramps, logic operations on economic participation, regulation and assist power, economic limits, suppression of noise, suppression of ACE when below certain thresholds, mandatory and permissive action
Unit
Plant limits, ramps, raise/lower pulses of
control
variable duration, timing, mandatory and permissive action
Generally, a smoothing of the control output is achieved and efficient control action is maintained when an emergency occurs. These algorithms are easily implemented on digital controllers with the additional benefit of producing alarms when some adverse situation appears (excess area control error, loss of tie-line measurement, etc.).
IX. Existing problem areas Filtering is just one of the processes applied to the signals involved in AGC. In recent years, the use of logic decisions and thresholds ll" 13,14 has been added which is probably responsible for many of the recent improvements in AGC. The idea behind all this is the adaptation of the mode of control to the type of dynamic process or disturbance. This has already been recognized by Ross. 7 The power system process is not unique for which one single control algorithm is good enough. Unfortunately, there are no appropriate models for the description of the various processes, l lence heuristic methods and approximations are used. The basic idea is to suppress certain quantities if they are not significant or to produce an output only if a number of quantities form a certain logic pattern. One example was given in section V, when four classes of disturbance were discussed. II This included also the so-called random noise rejection of reference 14. When security is at stake, a security overriding logic 14 can be included which has some similarity with class (iii) (class (c) of reference I 1). Logic functions may not be confined to the AGC level but can also be applied to the unit control level. Examples, such as unit blocking and a scheme which has permissive control features, are given in reference 14. The various processes can again be illustrated by a schematic such as that in Table 1, which is organized according to the levels (unit control, LFC, etc.). For each level, a number of examples for filtering processes and logic decisions are given. The main motivation for all these schemes is the reduction o f wear, unnecessary control actions and even counterproductive excursions of controllers.
Interconnected operation of a power system is affected by the following factors, which also determine problem areas and topics for further research. In the developed countries, interconnections still grow and the single area is becoming embedded in a very dense system. The new generating units in excess of 1000 MW are base loaded and there is a tendency not to let them participate in the regulating function. Other units, e.g. coal-fired ones geared for regulation, are not as responsive as gas- or oil-fired units. In developing countries, the interconnections grow as well, for economic and security reasons. The problems associated with these types of system are characterized to a greater extent by difficulties in maintaining stable operation and uninterrupted interconnections. The problem areas evolving from these factors are related to the following points which have been emphasized in the paper. In highly interconnected systems, the tie-line bias principle is becoming increasingly ineffective. The need arises to control tie-lines individually. Power transfers across areas being neither the generating nor the sending end of its power interchange require the allocation of the associated losses. To compensate for the loss of large generating units, AGC should produce an adequate response under the support of the neighbouring areas.
Tight control of frequency, tie-line power and inadvertent interchange are to be re-examined. "[he worth of a fast response in regulating power has to be evaluated against the slower units, but also against deviations in tie-line power and frequency.
X. Acknowledgement Presented at the IFAC Symposium on Computer Applications in Large-Scale Power Systems, New Delhi (1979), this state-of-the-art lecture was dedicated to the late Professor G Quazza, who contributed substantially to research in this area.
References 1 Graner, H 'Regel- und Steuerverfahren fur den Elek-
XI.
trizit~itsverbundbetrieb' ETZ Vol 60 (1939) pp 12691276
Osanna, J, Graner, H and Hofmann, F 'Frequenz- und Leistungssteuerung (Netzkennliniensteuerung) von Netzverb~inden' German Patent No. 634025 (February 1931) Cohn, N Control o f generation and power How on interconnected systems Wiley, USA (1966) Bloch, H, Frey, W and Luder, H 'lnterconnection between Switzerland and the West European power system' CIGRE 1962 Report 318, 318 bis
E!gerd, O I and Fosha, C E 'Optimum megawattfrequency control of multi-area electric energy systems' IEEE Trans Vol PAS-89 (1970) pp 556-563 Fosha, C E and Elgerd, O I 'The megawatt-frequency control problem: a new approach via optimal control theory' IEEE Trans Vol PAS-89 (1970) pp 563-577 Ross, C W 'Error adaptive control computer for interconnected power systems' IEEE Trans Vol PAS-85 (1966) pp 742-749
Najaf-Zadeh, K and Andersen, S W 'Sensitivity analysis in optimal simultaneolJs power interchange' IEEE Trans Vol PAS-97 No 6 (1978) pp 2405-2415
Quazza, G 'Non-interacting control of interconnected electric power systems' IEEE Trans Vol PAS-85 (1966) pp 727-741 10 'IEEE committee report: current operating problems associated with automatic generation control' IEEE Trans Vol PAS-98 (1979) pp 88-96 11 Glavitseh, H and Galiana, F D 'Load-frequency control with particular emphasis on thermal power stations' Real-time control of electric power systems Elsevier (1972) Handschin, E (Ed.) 12 Galiana, F D and Glavitsch, H 'State adaptation in power systems control' IEEE Trans Vol PAS-92 (1973) pp 1670-1678 13 de Mello, F P, Mills, R J and B'Rells, W F 'Automatic generation control Part I - Process modelling' IEEE Trans Vol PAS-92 (1973) pp 710-715 14 de Mello, F P, Mills, R J and B'Rells, W F 'Automatic generation control Part II - Digital control techniques' IEEE Trans Vol PAS-92 (1973) pp 716-724 15 Carpentier, J 'Optimal power flows' Int. J. Electr. Power& Energy Syst. Vol 1, No 1 (April 1979) pp 3 - 1 5 16 Glavitsch, H 'Economic load dispatching and corrective rescheduling using online information of the system state' Proc. PICA (1973) pp 412-420 17 Nielsen, J E Economic load dispatching under security related constraints Thesis Dept EE, Technical University of Denmark, Lyngby (1976) 18 Wagner, G Lastflusssteuerung bei unzu/#ssigen Betriebszust#nden in Hochspannungsnetzen PhD Thesis, Techn. Hochschule Aachen (1978) 19 Weiss, J 'Digital load frequency control in a multi-area power system' Proc. 4th IFAC/IFIP Int. conf. on digital computer applications to process control ZSrich, Switzerland (March 19-22, 1974) Vol 94, p 394 20 'IEEE standard definition of terms for automatic generation control on electric power systems' IEEE Trans Vol PAS-89 (1970) pp 1358-1364
Automatic generation control, one o f the functions o f a centralized system control centre, is discussed from the point o f view o f the steady-state loadfiow between areas, the control algorithm, noninteractive control, optimum control and its impact on security enhancement, as well as the various interfaces to upper and lower level control functions. The importance o f AGC in sparsely and highly interconnected systems is outlined, and present day problems are resolved.
I. Introduction
Within power system control, automatic generation control (AGC), known formerly as load-frequency control, plays a role quite distinct from that of other control functions. The main reason for its uniqueness is that it has a number of features not attributable to other functions in the same configuration. The main points are that AGC is ~rtrue centralized function, it operates in real time as well as closed loop, and it has a strong interface with other functions oriented towards the economy and security of the power system. Pictured within the framework of system control functions, AGC is an executive function which realizes the results of higher level algorithms (see Figure 1). It has to be emphasized, however, that AGC algorithms are of particular importance despite this executive feature. With the realization of interconnections in the 1930s, the problem of system control has already been recognized and the basic ideas of tie-line bias control have been formulated. 1,2 They were developed further in the English literature and were well documented by Cohn. 3 The principles were convincing and could be implemented by analogue means. In the UCPTE system, load-frequency control was established in the early 1950S. 4 There is long experience with systems of this kind including also digital versions, and the performance achieved has been excellent.
the net tie-line power is at the same time the power flow over the interconnecting lines. For highly interconnected areas, this is no longer the case. The net interchange may be completely different from the power flow crossing an area, and area control loses its effect on this power flow. The losses caused by the power flow have to be covered by the area, and a partner not participating in the power exchange has to pay for an interchange between external companies. The cost of regulating power is another point. Maintaining frequency and time in the system means operating the regulating units with frequent load changes. When new generating units are added, there is a tendency not to let them participate in the regulation. This leads to increased duty of the regulating units. Irrespective of the choice of regulating units, i.e. hydro or thermal, such an operation is quite uneconomical, and the question arises as to the choice of the participation. Further, if it is necessary to realize a tight control programme, how much regulating power is essential and what should be the response of these units'? A larger participation of units in the control task has not been implemented because of economic considerations given to new large units. Being aware of these problems, one has to look differently at the missing realizations of optimal control and other advanced features of system control. Missing regulation power per se excludes any advance in the improvement of control. However, efforts have been made to develop AGC and design algorithms as well as models which take into account the constraints and requirements of the real system.S, 6, 7 These approaches all require that more information is made available to the control system and some
I Long4ermoptimizalion I .........
I
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When areas are interconnected by just a few lines and the interconnection is concentrated on one side of the area,
Vol 2 No i January 1980
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In recent years, however, questions have been raised as to the appropriateness of AGC principles for modern system control, particularly when systems are becoming increasingly interconnected. The problem seems to lie in the uncontrollable inter-area power flow and the cost of regulating power.
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21
basic strategy is changed in order to arrive at the desired inrprovement. The objective of this paper is to develop these features.
Area ~
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-----
II. Tie-line power-frequency control principle In order to appreciate power-frequency control, two features o f the control system must be considered, namely the way in which output quantities of an area are measured and combined to a controlled variable, and the actual control algorithm itself.
Area ~ N
Figure 2 Connection of single area over small number of tie-lines
The first feature determines the so-called area control error (ACE) and allows each area to operate as an equal partner in the system with no superseding centralized control. Assuming that each area aims at keeping its ACE lo zero and that there is one common system frequency leads to a well defined steady-state interchange, This will become apparent from the following system of equations representing the various areas. ACE1 =Pox
P1
10Bl(f
Jol)
ACE2 =P02
P2
10B2(f
./o2)
ACE,,=Pon
Pn
lOBn(J"
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0
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II II
i
~
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j
=P1 +P2 + . . . + P n
where Pol is the net interchange schedule, B1 is the frequency bias setting, jol is the frequency schedule, ACE 1 is the area control error, and n is the number o f areas. Written in matrix form, this gives:
] 1
10 B
-~,-
-Pol + 1 0 B l f o l
ACE1-
l0
B
P2
P02 + 10B2-/02
ACE2
Bn
Pn
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ACE.
1 10 1
1
1
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0
f
Interconnected power system, many areas, paths
of power flow
0
The matrix is regular (for B 1 ~> 0, at least one of tile BlS > 0) and flrerefore a solution for any setting of the Pols and Jols exists. Tire scheduled interchange values and tire scheduled frequency, together with the frequency bias settings, are the coefficients in this system of equations. The actual frequency and the net interchanges are the variables. These equations will always have a solution irrespective of the size o f the coefficients, i.e. even if the scheduled quantities are not consistent. This also makes the principle of tie-line power-frequency control quite robust, but it leads to errors. Any incorrect setting o f a partner of the system affects the overall interchange schedule. Assuming that the schedule is correct and that tile interchange is realized accordingly, there are two cases to consider. One is a system configuration in which an area is interconnected by a small number o f lines, as illustrated in Figure 2. In this case, the average loading o f these lines corresponds to the net interchange o f the area and hence
the loading of the lines can be closely controlled by the interchange schedule. This is the kind of application where AGC has proved to be most successful. The second case is one in which the area is highly interconnected, e.g. as an area surrounded by other areas. In a simplified way, it can be represented by the schematic in Figure 3. The areas are rectangles and there are interconnecting lines at all sides. The net interchange per area is the difference between the input and output power of the area. A power transfer across an area does not appear in the net interchange. The control o f power flow, being one of the original objectives o f AGC, is thus no longer given since the other areas will determine the power flow through the area. What happens if an area I transmits power to another area K within an interconnected system is indicated in Figure 3 by arrows. The power flow is determined by the loadtlow equations in which the net interchanges enter as equality constraints. Depending on the line impedances, the power flow can take on quite unexpected patterns even if the sending and receiving areas are adjacent to each other. The net interchange turns out to be a weak constraint and cannot prevent overloading of lines. The individual tie-line flow is beyond the control of tire local utility, and losses
Additional signals(stateinformation)
incurred by the power transfer have to be covered by local generation. These features are highly undesirable from the point of view of the area and make AGC unattractive. Hence local utilities are becoming increasingly interested in schemes that allow individual control of lines and of which the socalled optimal power flow may be one form of realization. The problem is not easily resolved since an optimal power flow in the true sense can be implemented only in a centralized system whereas the present system consists of a number of independent areas. So, in order to achieve the above objective, a closer coordination between areas will be necessary. One approach worth mentioning is found in a paper dealing with the assessment of inter-area capabilities, a It should be mentioned that the reactive power flow causes similar problems, and due consideration should be given to that portion of the power flow as well. III. Control algorithm The steady-state equation above indicates the requirements necessary for achieving the desired control objective, namely to keep the ACE at zero or at least at a constant small value. The control algorithm, the second important feature o f AGC, is designed to fulfil this objective. Assuming controllability of the interconnected power system, a high-gain amplifier or an integrating regulator will be able to achieve a small error in ACE or even a diminishing value. This function is implemented by a regulator (hardware, continuous, sampled) or by an EDC-program on a computer. The detailed algorithms vary depending on the school of thought: in Europe, a proportional integral {PI) algorithm is used ; in the USA, a proportional scheme is employed. The PI algorithm is represented by the following control law:
1;a
Y = Cp. ACE + - - .
Tx
l
~ie - Tie-linepower
Load-frequency control
~f - Frequency i of area
Distributionfactors-EDCprogram
L1
u,
r
[
]
Level!
U2 ~
Level 2
Powersystem - Area Z"
Figure4 Overall control scheme of AGC
is given which processes frequency and tie-line power according to the above control law. The output is then multiplied by participation factors before it is sent to the individual regulating units. The participation is determined by economic considerations supplemented by knowledge about the type of unit (speed of response). At the regulating unit, the control signal acts as a setpoint in the speed governor. In this schematic, control actions interface at various points, one being at the centralized level between the load-frequency controller and a possible economic dispatch algorithm, and another being at the local level between the governor and the centralized control. There, interfaces are quite determinant for control performance and will be treated in more detail below.
CE.dt
IV. Noninteractive control where Cp is the proportional gain, T N is the time constant, ACE is the area control error, and Y is the output of controller, with typical values Cp = 0 . 1 - 0 . 3 and T N = 30 100 s. Since the integral term has a relatively small gain, it assures the decrease of ACE to zero but has little effect on the dynamics of the controller. This is given mainly by the proportional term. Filtering and delays have additional effects as will be discussed below. This control law is also the point at which two extensions of the basic algorithm can be introduced. One is noninteractive control and the other is optimal control, both of which will be elaborated below. However, it can be stated here that extensions are oriented mainly towards dynamic effects, the speed of response and the participation of areas and regulating units. The basic objective of keeping ACE at zero or at a small value remains the same. One importan't addition to the basic algorithm is the weighting of the outgoing control signal by participation factors which can be best understood by means of Figure 4, which shows the overall control scheme of AGC. The block
For a system without AGC, a load change within an area will be balanced by the prime mover and governor action alone. When steady state is reached, each generating unit will contribute according to its droop (speed regulation). When AGC is superimposed, the net contribution of an area can be reduced or even compensated by an appropriate adjustment of the frequency bias setting. Then the regulating units will be activated only if the load change occurs within the own area. A plot o f some of the relevant quantities, as shown in Figure 5, illustrates this mode of operation quite clearly. In this case, a load change was introduced in a neighbouring area, the frequency dropped and the area assisted the neighbour by supplying power in a transient mode. The control signal and the prime mover output, however, hardly changed. In steady state, all deviations went back to zero. This principle of noninteraction has been further developed by Quazza 9 with the object of suppressing the remaining tie-line power flow in the transient mode and avoiding interaction between frequency and tie-line power.
0
This means that a change in tie-line power should not affect frequency control and a variation in frequency should not affect the control of tie-line power.
-
2
0
_ 40 -60
Control actions are confined to the area in which the load change takes place in order that the area controller can be designed without having to consider the dynamics o f the neighbouring area. A profound understanding o f large systems control must be attributed to Quazza in developing this concept. 9
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These objectives are achievable under assumptions o f linearized systems and so-called stiff areas as well as of the realizabitity o f the required transfer functions in the control system.
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In today's interconnected systems, the emphasis has changed somewhat. In areas where large generating units are installed, the necessary reserve can hardly he maintained within the area. So when a unit is lost, the area relies on its neighbours.
From the point of view o f modern control theory, this was not considered the best possible result. With the availability of state estimation and optimum control theory, models and algorithms were developed which were aimed at an improvement in the response o f frequency and tie-line power. The best known result is that o f Elgerd and Fosha s,6 which is based on the existing control structure. The feedback coefficients are determined such that the minimum of a performance index is achieved.
The data acquisition system is a sampled data system and the regulating units have constraints. Hence the required transfer functions cannot be realized and the neighbouring areas supply power to the troubled area. This can be seen in Figure 5. There is also a conflict between noninteraction and support to the neighbour in terms of the amount and the duration of the support, as a recent IEEE committee report l° indicates. The reserves in the neighbouring areas have an economic value and have to be justified. It seems, however, that a reserve in all areas in proportion to their spinning power is becoming accepted.
An extension of this procedure, observer estimation detection (OED), was suggested by Glavitsch and Galiana u and comprises an estimator, a detector and a linear quadratic regulator. The concept is based on the idea that, in order to achieve an improved control, the dynamic model of the area has to be extended and states or output variables of this model have to be included in the objective function. Since some o f the variables are not accessible, an observer estimator has to be used. The ACE could not be dispensed with and had to be included as a feedback variable since it determined the final values.
V. Optimum control The general practice in the design o f the area controller is to use a Pl algorithm (see section Ill). This gives adequate response considering the stability requirement and the performance of regulating units.
o
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The weighting o f the various quantities within the objective function allowed different strategies of control to be realized, including a minimum interaction with the neighbouring area, support of the neighbour and good frequency control. Two examples of optimum responses are shown in Figures 6 and 7.
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The first example is the result o f a case in which tile area has to cover a load change inside its boundaries. Although the response is optimal, a certain power flow from the outside cannot be suppressed. The second is to demonstrate that control o f frequency o f the system can be improved by supporting the troubled neighbour by a small transient increase of the tie-line power. One o f the basic difficulties in achieving true optimum control lies in the fact that the final values of variables are not known at the initiation of the disturbance. A linear quadratic regulator would require the difference between actual values and final steady-state values as an input. Even in a simulated result, these inputs can only be estimated.
direction will have to be justified by a more profound evaluation of the worth o f the gains in the response.
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Figure7 Optimum control with OED (disturbance in neighbouring area, A load = 0.032 p.u.) (a) Afl, (b) APti e, (c) (i) AU1 (ii) APml In practice, this deficiency is covered by other deteriorating effects, such as measurement errors, nonlinearities, model errors, constraints, etc. In fact, these items determine and actually limit the achievable improvements of control. What is of greater importance, however, is the response of the system to different types of disturbances. It seems appropriate that the area controller reacts depending on the size and the type. A proposal of how this should be done is given in reference 11 and the distinction is made by the amplitude of the load or tie-line power change. Four classifications are chosen: (i) (ii) (iii) (iv)
smaller than 2% smaller than 5% larger than 5% own area larger than 5% neighbouring area.
For class (i), there is no real need for a fast response: the disturbances are mostly random and the system should he aimed at reducing the variance of the frequency and power variations. For class (ii), an efficient reaction within the area is needed without too great an effect on the neighbours. Classes (iii) and (iv) represent emergencies and the overall system should respond in this case. This brief discussion indicates that the application of optimal control must be geared towards specific incidents in the systems when there is a real need. Since for most of the time no serious incidents occur, the system should behave quietly without any large excursion of the regulating units which are also a part of optimal control. A number of papers and proposals tend towards the same objective.V, 12,13, 14 Control logic, analogue filtering and nonlinear optimal control 12 are the methods employed. Again, it must be emphasized that improvements can be realized only if more information about the system state is available. This requires estimation and observation at a greater frequency than is envisaged today. A step in this
In the introduction, it was pointed out that, although AGC is taking care of the power balance in the system, the algorithm with its output function acts as an executive for economic dispatch and security control. As the functions are set up today (see Figure 1) dispatching and security are located on a higher level and respond at a considerably slower rate. They are based on a steady-state model of the power system, and it would be unreasonable to interfere at a faster rate. In the area of optimal power flow, the algorithms have been perfected and the theory is well developed, is The algorithms are able to handle the full AC power flow with constraints of both dependent and independent variables.16, 17, 18 For online applications, rescheduling for security enhancenlent is becoming more and more important, and efficient handling of constraints and sensitivity relationships are the main features. A stage has been reached at which implementation is quite feasible due to considerable improvement in the convergence and storage space utilization. Availability of these tools allows power flow corrections by the AGC algorithm, i.e. by setting the participation factors of individual units accordingly. There are, however, two major obstacles which confine this type of control to a limited number of cases. One is the fact that very often the number of regulating units is small and their location is such that the controllability of the power flow is rather reduced. The second point is related to the location of the area within ttle interconnected power system. As pointed out in section I1, it will be difficult to control the power flow when the major power transfer through an area is dictated from the outside. Again, this amounts to a lack of controllability. Keeping the net interchange fixed amounts to similar limitations. In order to overcome these shortcomings, it would be necessary to introduce a floating net interchange which would also allow the tie-line power to be included in economic dispatch. In addition, AGC would have to be complemented by switching operations to augment the possibility for rescheduling. The consequence is that several areas have to coordinate their actions quite closely, and this leads to pool operation, which is already done in certain countries, as the existing pools in the USA indicate.
VII. Interfacing with economic dispatch and primary control In generation control, several control levels exist, namely economic dispatch (EDC), frequency tie-line power control (LFC) and unit control. Each of these levels has its time scale and each has an output according to its objective.
In order to serve tile overall objective o f security and economy, the way in which these levels interface must be well defined. For clariiy, a schematic of these three levels is given in Figure 8, where tire points o f interaction are also indicated. The structure o f these three levels is not unique, but corresponds to the present implementation o f AGC. It could well be that upon the availability o f appropriate economic models, EDC is located below LFC. This would mean that participation factors are worked out dynamically. The interlacing problem, however, remains.
q.
,
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Tire unit control is considered first. The output of AGC is realized in the unit via setpoint control whereby the telemetered quantity can be tile setpoint itself or an increment. In any case, tile setpoint will be stored at the input of the unit, so that if the telemetry system fails, the unit will operate according to the last setpoint transmitted. All monitoring of tire unit, for high and low limits, maximum permissible increase of output, response, etc., is done at this level. Tire monitoring function is implemented ira tile AGC computer in an output unit which is connected to tile generating unit by telecontrol. Any modifications or improvements at this level affect the unit response to a small degree only. Tire merit lies mainly in tire closer observation of plant limits and a better knowledge of unit performance at the AGC level. The interlhce at tire AGC level is more complex and permits realization o f various strategies. At the moment, the general philosophy o f control is such that frequency/ tie-line control has first priority. Economic dispatch with its setpoints follows the former or acts independently. This can best be discussed by considering one of the power allocation algorithms. 19
i where Psi is the setpoint of the unit, Phi is the base loading o f i t h unit, Pi is the participation factor, Y is the area demand in MW, m is tile number of units under AGC, and ~ p i = 1. Y is the output o f tire frequency/tie-line power controller and it is assumed, for the moment, that it equals tire sum o f tire base loads. Tire system is balanced and it operates
Economic dispatch
EO
30
40
50
Time (t),s
Figure9 Two different speeds of response {a) response of fast unit, (b) response of slow unit
in a state according to economic dispatch which determined tire base load points. Since two different running modes for LFC (faster) and EDC exist, a load change affects Y first and Phi at a later stage. Depending on the size of tire participation factor Pi, a unit can be made to respond quickly to an increase in Y and is eventually restored to a smaller value after EDC has determined the load point. If the participation is small, the unit will essentially follow tire EDC program. This double action within tile AGC level at this interface can give rise to considerable discussion o f which the attain issue is tire worth of tire fast response of the regulating units (large participation) (see Figure 9). Tire increase of regulating power and subsequent decrease is not desirable from the point of view o f the unit (cost, wear, maintenance). Tire power system, however, depends upon fast response. It could be implemented in a larger number of units, which usually is not tile case. If tire participation factors of fast regulating units are not set to a high value, the other units are driven harder, they reach their limits, and Y increases, which again drives tile regulation units up. So for large excursions of Y, tile problem cannot be resolved satisfactorily at this point. Tire real problem is tile definition of the objective o f control for a given class o f disturbances (see section V). If control action is needed in case o f an emergency, tile regulating units should be allowed to participale substantially irrespective of subsequent backswings. If frequency and tie-line power deviations are not critical, participation can be small and tire Pi factor can be set according to economic considerations. This is an area where optimum control on a sound economy security basis can bring about real improvements.
\ Load-frequency control
Interface
Unit control
Interface
Speed overnor
Figure 8
Control levels and interfaces in AGC system
So far the Y and P,,is have been considered to act rodependently upon the setpoint. The setpoint can also be made to respond to logic combinations of their deviations, e.g. a raise in the EDC requirement is realized only if the LFC requirement asks for a raise at tile same time. It is lhen permitted to go to the unit. I lence this mode is called permissive conlroJ. When the t'.[)C is realized in any case, tile mode is called mandatory control. Mandatory control is required for tile realization of an EDC schedule in the steady state of the power system.
In the permissive control mode, a certain smoothing effect is achieved.
Table 1 Filtering, logic decision and thresholds on various levels
Level VIII.
Filtering, logic decision and thresholds
In the early applications of load-frequency control, analogue systems were used and the measurements were taken continuously and simultaneously. In modem telemetry systems and the associated data acquisition systems, quantities such as tie-line power are sampled. The sampling instants cannot be the same at the various tic-lines; hence, the tie-line powers are taken nonsimuhaneously and their sum contains a noise term (aliasing). It can be kept small by increasing the acquisition rate. In addition, filtering will decrease the error and can take place either at the transducer level or within the AGC controller. Usually, both methods are applied. Filtering at the AGC controller level, however, introduces quite a delay since the sampling interval is of the order of 2 5 seconds and filtering means considering at least 2 - 5 sampling periods, tlence, a step change in load can be detected only after roughly 10 seconds. The methods of filtering, as applied today, are quite simple and there is room for improvement, e.g. by using the dynamic model of the area (estimation).
EDC
Filtering,logic decisions Estimation of steady-state operating point,
forecasting of near future operating point LFC
Filtering of tie-line power, estimation of ramps, logic operations on economic participation, regulation and assist power, economic limits, suppression of noise, suppression of ACE when below certain thresholds, mandatory and permissive action
Unit
Plant limits, ramps, raise/lower pulses of
control
variable duration, timing, mandatory and permissive action
Generally, a smoothing of the control output is achieved and efficient control action is maintained when an emergency occurs. These algorithms are easily implemented on digital controllers with the additional benefit of producing alarms when some adverse situation appears (excess area control error, loss of tie-line measurement, etc.).
IX. Existing problem areas Filtering is just one of the processes applied to the signals involved in AGC. In recent years, the use of logic decisions and thresholds ll" 13,14 has been added which is probably responsible for many of the recent improvements in AGC. The idea behind all this is the adaptation of the mode of control to the type of dynamic process or disturbance. This has already been recognized by Ross. 7 The power system process is not unique for which one single control algorithm is good enough. Unfortunately, there are no appropriate models for the description of the various processes, l lence heuristic methods and approximations are used. The basic idea is to suppress certain quantities if they are not significant or to produce an output only if a number of quantities form a certain logic pattern. One example was given in section V, when four classes of disturbance were discussed. II This included also the so-called random noise rejection of reference 14. When security is at stake, a security overriding logic 14 can be included which has some similarity with class (iii) (class (c) of reference I 1). Logic functions may not be confined to the AGC level but can also be applied to the unit control level. Examples, such as unit blocking and a scheme which has permissive control features, are given in reference 14. The various processes can again be illustrated by a schematic such as that in Table 1, which is organized according to the levels (unit control, LFC, etc.). For each level, a number of examples for filtering processes and logic decisions are given. The main motivation for all these schemes is the reduction o f wear, unnecessary control actions and even counterproductive excursions of controllers.
Interconnected operation of a power system is affected by the following factors, which also determine problem areas and topics for further research. In the developed countries, interconnections still grow and the single area is becoming embedded in a very dense system. The new generating units in excess of 1000 MW are base loaded and there is a tendency not to let them participate in the regulating function. Other units, e.g. coal-fired ones geared for regulation, are not as responsive as gas- or oil-fired units. In developing countries, the interconnections grow as well, for economic and security reasons. The problems associated with these types of system are characterized to a greater extent by difficulties in maintaining stable operation and uninterrupted interconnections. The problem areas evolving from these factors are related to the following points which have been emphasized in the paper. In highly interconnected systems, the tie-line bias principle is becoming increasingly ineffective. The need arises to control tie-lines individually. Power transfers across areas being neither the generating nor the sending end of its power interchange require the allocation of the associated losses. To compensate for the loss of large generating units, AGC should produce an adequate response under the support of the neighbouring areas.
Tight control of frequency, tie-line power and inadvertent interchange are to be re-examined. "[he worth of a fast response in regulating power has to be evaluated against the slower units, but also against deviations in tie-line power and frequency.
X. Acknowledgement Presented at the IFAC Symposium on Computer Applications in Large-Scale Power Systems, New Delhi (1979), this state-of-the-art lecture was dedicated to the late Professor G Quazza, who contributed substantially to research in this area.
References 1 Graner, H 'Regel- und Steuerverfahren fur den Elek-
XI.
trizit~itsverbundbetrieb' ETZ Vol 60 (1939) pp 12691276
Osanna, J, Graner, H and Hofmann, F 'Frequenz- und Leistungssteuerung (Netzkennliniensteuerung) von Netzverb~inden' German Patent No. 634025 (February 1931) Cohn, N Control o f generation and power How on interconnected systems Wiley, USA (1966) Bloch, H, Frey, W and Luder, H 'lnterconnection between Switzerland and the West European power system' CIGRE 1962 Report 318, 318 bis
E!gerd, O I and Fosha, C E 'Optimum megawattfrequency control of multi-area electric energy systems' IEEE Trans Vol PAS-89 (1970) pp 556-563 Fosha, C E and Elgerd, O I 'The megawatt-frequency control problem: a new approach via optimal control theory' IEEE Trans Vol PAS-89 (1970) pp 563-577 Ross, C W 'Error adaptive control computer for interconnected power systems' IEEE Trans Vol PAS-85 (1966) pp 742-749
Najaf-Zadeh, K and Andersen, S W 'Sensitivity analysis in optimal simultaneolJs power interchange' IEEE Trans Vol PAS-97 No 6 (1978) pp 2405-2415
Quazza, G 'Non-interacting control of interconnected electric power systems' IEEE Trans Vol PAS-85 (1966) pp 727-741 10 'IEEE committee report: current operating problems associated with automatic generation control' IEEE Trans Vol PAS-98 (1979) pp 88-96 11 Glavitseh, H and Galiana, F D 'Load-frequency control with particular emphasis on thermal power stations' Real-time control of electric power systems Elsevier (1972) Handschin, E (Ed.) 12 Galiana, F D and Glavitsch, H 'State adaptation in power systems control' IEEE Trans Vol PAS-92 (1973) pp 1670-1678 13 de Mello, F P, Mills, R J and B'Rells, W F 'Automatic generation control Part I - Process modelling' IEEE Trans Vol PAS-92 (1973) pp 710-715 14 de Mello, F P, Mills, R J and B'Rells, W F 'Automatic generation control Part II - Digital control techniques' IEEE Trans Vol PAS-92 (1973) pp 716-724 15 Carpentier, J 'Optimal power flows' Int. J. Electr. Power& Energy Syst. Vol 1, No 1 (April 1979) pp 3 - 1 5 16 Glavitsch, H 'Economic load dispatching and corrective rescheduling using online information of the system state' Proc. PICA (1973) pp 412-420 17 Nielsen, J E Economic load dispatching under security related constraints Thesis Dept EE, Technical University of Denmark, Lyngby (1976) 18 Wagner, G Lastflusssteuerung bei unzu/#ssigen Betriebszust#nden in Hochspannungsnetzen PhD Thesis, Techn. Hochschule Aachen (1978) 19 Weiss, J 'Digital load frequency control in a multi-area power system' Proc. 4th IFAC/IFIP Int. conf. on digital computer applications to process control ZSrich, Switzerland (March 19-22, 1974) Vol 94, p 394 20 'IEEE standard definition of terms for automatic generation control on electric power systems' IEEE Trans Vol PAS-89 (1970) pp 1358-1364