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884 PROCEEDINGS OF THE IEEE, VOL. 62, NO. 7, PY 1974

Real-Time Computer Control of Power Systems

TOMAS E. DY LIACCO, SENIOR MEMBER, IEEE

Invited Pa@

Abstract-A dramatic trmsfommtion in system monitoring and control is taking place in the electric utility industry. Hew control centers are being equipp& with multiprocessor real-time computers syming and controlling the generation and 1 * *nsystemvia hqh-qwd data-acquisition subsystems and interacting with the human operator via dynamic, color, graphic displays. Within the computer aa many .as a.hrmdred or more programs are available to m n in.a multiprogrammirrg environment in response to changing poner-system conditions and to operators demands.

The main objective of thie new development m on-line control is the enhancement of the security of the power system in order to matrtrin a high reliability of electric power service.

The concept of security control is dimumd and the noteworthy featurea of its present slate of development in recently designed and inscalled control centers are described.

T INTRODUCTION

H E EVOLUTION of automation in electric power systems has been marked by outstanding developments in the application of state-of-the-art technology in

both instrumentation and control. Many of these applications are required for local control to carry out such functions as regulation, -switching, protective relaying, generator loading, etc., using measurements obtained locally. In certain cases, such as in protective relaying, the 1-1 control logic also requires information from one or more remote jocations, such information being transmitted via telephone lines, power- line carrier, or microwave facilities.

Local control has evolved to the point where minicomputers are being used for specific purposes such as boiler- turbine controls, or to consolidate at a substation hitherto separate instrumentation and control functions. There have been some remarkable developments in the use of digital computers for local control. However, in the discussion of real-time computer control of power systems, the main em- phasis of this paper will be on the systemwide control from a central location. A more comprehensive summary of the developments in both local and central controls may be found in [4].

In an overall centralized control system, all local controls become functionally resident a t the lowest level of a multilevel control hierarchy [l], the highest level being at the central control. In becoming part of a hierarchy, certain local controls require information links with higher level controls.

The implementation of central control in power systems originated from two independent control requirements and developed into two separate centralized control systems. One central control system is for the supervisory control and indication of transmission and/or distribution equipment. In addition to equipment status indication and simple alarming functions the supervisory control system provided the dis- patcher at the center with the means to remotely actuate

Manuscript received December 31,1973. The author is with the Cleveland Electric I l l d ~ t i n g Company,

Cleveland, Ohio44101.

various station equipment, primarily circuit breakers. The centralization of supervisory control could be carried out at one location or in several district offices, dictated more or less by the geography of the territory served, and by the organi- zational structure of the electrical operation division of the utility company.

The other central control system is for the automatic control of the outputs of generating units in order to meet the continuous changes in load demands. The central control of generation which started out as a direct regulatory type of control to make system generation match the minute-to- minute changes in load was improved in the 1950s with the addition of an optimizing-control level. The optimizing control automatically allocated the generation requirement in such a way that the total operating cost was a minimum. This two-level control system for generation soon became a universal standard. The direct-control. level, known in the industry as automatic generation control (or by the more common designation of load-frequency control), became mandatory for power systems which are interconnected; the optimizing-control level, known as economic-dispatch control, became a necessity for systems with a significant amount of thermal generating units.

The supervisory-control and the generation-control functions each had its data-gathering system and its own central and remote hardware. Generation control evolved from an analog system to a digitally directed analog and finally, in the 1960s, to a direct digital-control systeni. Similarly, supervisory-control systems evolved from one hardwired master per remote to one hardwired master for several remotes and finally to a digital-computer master. Thus by the end of the 1960s there were in service two types of digital-computer control installations-the dispatch computer and the supervisory-control computer-using small computers and requiring no more data than what was essential either for generation dispatch or for supervisory control. Digital telemetry was also coming into use to replace analog telemetry. Up to this time the man-machine interface consisted of stripchart recordings, loggers, indicating lights, annunciator windows, console switches or pushbutton panels, thumbwheels, and4 other special-pur- hardware. More recently, black and white and color CRTs are being used for either the dispatch computer or the supervisory-control computer.

Over the years the improvements in generation control and in supervisory control had been in the hardware used and in the efficiency of the control techniques. The basic ob- jectives had remained the same, Le., to control generation and to control remotely located devices or equipment. Near the end of the 19609, however, power-system engineers began analyzing the entire system operation problem from a systems viewpoint, motivated by the evident need for a more comprehensive and more effective operating control than had been conventionally available to the power-system dis-

DY LIACCO: COMPUTER CONTROL OF POWER SYSTEMS 885

patcher. We are now witnessing in the decade of the 1970s the beginnings of a new wave of power-system control systems, much broader in scope of system monitoring and control due to the integration of operating functions, and the addition of a new dimension-system security.

The addition of system-security considerations to the strictly generation-dispatch and supervisory-control requirements has caused a quantum jump in the evolution of real- time central control of power systems by digital computer. This transformation is growing in numbers throughout the world. In many cases utility companies are in the process of replacing or augmenting existing digital-dispatch or supervisory-control computers less than 10 years old with the new security-control systems of the 1970s. This paper discusses the concept of security control and the current state of its implementation in power systems.

SECURITY CONTROL Provision for system security has always been an inherent

part of sound system design. However, due to economic and other considerations, only so much security can be built into a system. I t has generally been assumed that as long as a system is built according to prevailing design standards, any abnormality in operation requiring control beyond that provided for by conventional, automatic devices would be taken care of by the human operator. Historically, this assumption had been blithely made without providing the opqrator with adequate tools to aid him in the complex decision-making processes concerning security. In some companies where the security problem had become quite acute, procedures had been instituted to .aid the operator by way of security programs run off-line on general-purpose computers, in certain cases using remote terminals or, in a few rare instances, a dedicated network analyzer or analog computer. These applications represent a transitional step from conventional control to security control.

Security control or a security control system may be defined as a system of integrated automatic and manual controls for the maintenance of electric power service under all conditions of operation [2]. Note from this definition that security control is a significant departure from the traditional dispatch control or supervisory-control systems. Firstly, the proper integration of all the necessary automatic- and manual- control functions requires a total systems approach with the human operator being an integral part of the control-system design. Secondly, the mission of security control is all-en- compassing, recognizing that control decisions by the man- computer system must be made not ju s t when the power system is operating normally but also when i t is operating under abnormal conditions. As power systems have grown in size and have become more tightly coupled, the problem of making the right operating decisions under varying conditions has become extremely difficult.

The Three Operating States In [ l ] and [2] the operating conditions of a power system

are characterized in terms of three operating states-normal, emergency, and restorative. Let us review this concept as i t is helpful in the discussion of security control and the current state of its development.

The power system may be assumed as being operated under conditions expressible in the form of two sets of constraints [2], [SI :

G(x , u) = O load constraints H ( x , u)ZO operating constraints

where G and H are function vectors and

x vector of dependent variables; u vector of independent or control variables.

Sollberger [SI, in extending the concept of security control to industrial systems in general, calls one set of constraints the load constraints and the other set the operating constraints, and generalizes both into inequalities. In our discussion the equalities are the load constraints while the inequalities are the operating constraints. Fundamentally, the load constraints impose the physical equations which satisfy the requirements that the load demands will be met by the system; while the operating constraints impose maximum or minimum operating limits (e.g., loading limits, voltage limits, etc.) on variables associated with the component parts of the system.

If both the load and operating constraints are satisfied, the system is said to be in the normal operating state. In response to the relatively small minute-to-minute changes in load, a power system may be considered as going from one normal state to another and each normal state may be assumed to be a quasi-steady-state condition.

On the occurrence of a severe disturbance (e.g., a large load change, a loss of generation, a short circuit) a system may settle down to a new normal state or may go to either an emergency or restorative operating state. In the emergency state the operating constraints are not satisfied. In the restorative state the operating constraints are satisfied but not the load constraints.

Two types of emergency may be noted. The first type occurs when, after a disturbance, the power system remains stable and continues operating but with the operating constraints not fully met, i.e., with some equipment loading limits exceeded or with abnormal voltage levels at certain locations. This type of emergency condition which we shall refer to as steady-state emergency may be tolerated for a reasonable period of time, generally allowing corrective action to be taken. Such corrective action should be effective enough to prevent or limit damage to the overloaded equipment. The second type of emergency occurs when, because of a disturbance, the power system becomes unstable, during which time both the operating and the load constraints are not being met. This type of emergency condition which we shall refer to as dynamic instability takes place in a very short period of time and, unless a proper fast corrective action is taken, ends up with the power system in the restorative state, i.e., in a partial or a total shutdown.

The overall objective of security control may be restated as follows: to keep the power system operating in the normal state, i.e., to prevent or to minimize the departures from the normal state into either the emergency or the restorative state. To realize an effective strategy for carrying out this objective let us look more closely into the concept of system security.

The Concept of System Security System security may be considered as the ability of a

power system in normal operation to undergo a disturbance without getting into an emergency condition. The system is then said to be secure. On the other hand, a normal operat-

886 PROCEEDINGS OF THE IEEE, JULY 1974

ing system would be insecure if there is a disturbance which could bring about an emergency operating condition. If one considers all possible disturbances i t would be impossible to find a secure power system. In practice system security is determined with reference to an arbitrary subset of the complete disturbance set. This subset is called the next-contingency set. The choice of the composition of the next- contingency set is dictated by the probability of occurrence of the contingency within the next short period of time (in the order of minutes) and the consequences to the system should the contingency occur. In most power systems the next-contingency set includes, as a minimum, the following types of disturbances:

1) any circuit out; 2) any generating unit out; 3) any phase-to-phase or 3-phase short circuit.

Other types of disturbances may be added, the more disturbances included in the next-contingency set the more stringent the system-security requirements become.

For a given next-contingency set, the set of all normal operating states may be partitioned into two disjoint subsets-secure and insecure. That is, a normal operating system is either secure or insecure. We see then that for security control to accomplish its objective of preventing or minimizing departures from the normal state it would be highly desirable to be able to manipulate the system so that it stays as long as possible in the secure region of the normal state. This is a remarkably different control philosophy from that of the traditional dispatch computer. Its implications in terms of data requirements, information processing, control design and algorithms, man-machine interfaces, and computer requirements can be readily appreciated.

Strategy for Security Control The effectiveness of security control in keeping the power

system secure for as long as possible depends heavily on the control done during the normal operating state. We will refer to this control as preventive control since its preventive character is what distinguishes i t from conventional control. Preventive control should not only take care of all of the traditional functions involved in controlling generation to meet the load but should also determine the actual operating conditions of the system, assess system secudty for bgth types of emergency (i.e., steady-state emergency and dynamic instability), and determine the corrective action to be taken in case the system were insecure. Consideration of these tasks leads us to the discussion of three important ideas: security monitoring, security analysis, and security-constrained optimization.

Security Monitoring: Security monitoring is the on-line identification and the dynamic display (to the human operator) of the actual operating conditions of the power system. Security monitoring requires a systemwide instrumentation on a greater scale and variety than that required by a control system without security monitoring. To gather all the system data every few seconds and bring it to a control location requires a high-speed digital data-acquisition system interfaced with a central computer system. The central computer system operates in real-time and supervises the data acquisition, processes the data received, and supports the display sub- system. Finally, the man-machine requirements entail the the use of devices such as color CRTs for display and as interface for operator inputs and operator-initiated controls.

Security monitoring involves the processing, of the measured data to determine the system operating conditions and the violations of the operating constraints H(w, u)BO. Also part of security monitoring is the on-line determination of the network topology as required for display and for models used by other on-line functions.

The large amount of telemetry required, with its attend- an t problems of errors in measurement and in data transmission, has justified considerations of exploiting redundan- cies in measurement so as to obtain best estimates of the system variables using Kalman filtering and other stochastic approximation methods. This function, referred t o h the industry as state estimation [lo]-[14], [26], is becoming accepted as a necessary part of security monitoring.

We see then that just the addition of security monitoring to conventional functions creates a new type of system control using state-of-the-art hardware systems and sophisticated data-processing methods.

Security Analysis: Security analysis is the determination of the security of the system based on a next-contingency set. While a security analysis may be made for both steady-state emergency and dynamic instability the trend has been to have a separate analysis for each of the two types of emergency. One reason for this is the extreme difficulty of implementing a dynamic security analysis with present methods of stability evaluation. On the other hand, for steady-state security analysis several approaches are possible and are in use. Basically, these approaches start with a knowledge of the present state of the system as obtained from the security monitoring function. The system is then tested for various next contingencies by, in effect, solving for the changes in the system conditions for a given contingency and checking the new values against the operating constraints.

Pattern recognition has been tried with some promising results [SI, [IS], [I61 as an alternative approach to security analysis but the method has not been put to use on-line. Es- sentially, a small set of on-line measurements are used as security indicators. A security function of these indicators will then classify the state of the system as being secure or insecure. The security function or pattern classifier is derived by a learning algorithm based on a training set of off-line studies. The appeal of the pattern-recognition approach is the trans- fer of the need for full-scale simulation studies (especially dynamic stability studies) from the on-line computer with limited capabilities to an off-line computer with more and better modeling and computation resources. However, further work remains to be done to develop a satisfactory pattern-recognition method for on-line implementation.

Security-Constrained Optimization: If the system were found to be insecure by the security-analysis function the next problem is to determine whether the system can be made secure. This becomes a security-constrained optimization problem where we have to find the best operating condition which satisfies not only the load constraints and the operating constraints but also the security constraints. That is, minimize F ( x , u) subject to

G(r , u)=O load constraints H(w, u)hO operating constraints S(x, u) 2 0 security constraints

where F is the operating cost function. The security-function vector S may consist of all the load

and operating constraints for each of the next contingencies whose occurrences would cause emergencies [17], [18]. Or


S(x, u)LO may be the single security function used in the pattern-recognition method of security analysis [SI. The security-constrained optimization would determine the best corrective action for making a system secure. The extra cost involved in implementing this corrective action would be presented to the operator who would then decide whether to carry it out or not.

Theoretically, the optimizing level of preventive control may be formulated from the very outset using the complete set of constraints, as indicated in the preceding paragraph. In practice this is not necessary nor is i t desirable. The optimizing-control algorithm would not only be too complex to implement on-line but would result in an unnecessary in- crease in operating expense and in computer overhead.

The combined strategy of security monitoring, security analysis, and lastly security-constrained optimization is a correct and practical approach. For the present it is the only viable approach to on-line preventive control. I t is a good illustration of the power of decomposition and multilevel organization of a control system to accomplish a complex control objective [6]. To summarize, the strategy works as follows: The optimizing control is formulated as simply as possible using only the load constraints and a very small subset of the operating constraints (e.g., only the generator limits but not the equipment loading limits). In most cases, the optimizing control is decomposed into the real power optimization (or economic dispatch) and the reactive power optimization (or var dispatch). Since the changes in var dispatch are for many systems less frequent than the changes in economic dispatch, var dispatch is usually treated as an open-loop adaptive control. Further simplification of the economic dispatch is obtained by using for load constraints a single function equating the algebraic sum of the power injections and the system losses to zero. The security-monitoring function working independently of the economic dispatch detects on- line when certain operating constraints become semi-binding or binding. When this happens the economic dispatch should be modified so that the semi-binding and binding constraints are included in the set of operating constraints. That is, the size of the operating constraint set considered is adjusted with the number of likely and actual violations. The security- analysis function, either working independently or triggered by the security-monitoring function, checks the security of the system. If the system were insecure, a security-constrained optimizing program would then be run to find the corrective action.

I t is readily evident that the development of a good preventive control enhances tremendously the ability of a system to stay secure and therefore minimizes the departures into the abnormal conditions of the emergency and restorative states. Both emergency control and restorative control are still needed for a complete security-control system. However, these controls are difficult to develop and implement for a variety of reasons. Let it suffice for this paper to remark that preventive control is relatively easier to implement being well within the capabilities of present-day technology and analysis. Furthermore, the fact that preventive control offers a workable control strategy gives impetus to its higher priority of development. Thus most of the applications of the security-control concept that are being developed in the power industry are in the area of preventive control. The development of emergency control and restorative control is currently of a lower priority and no significant innovations have as yet been implemented.

MODERN CONTROL-SYSTEM INSTALLATION The implementation of security control systems which

started with the coming of the 1970s is gaining momentum throughout the power systems in the world. By the end of 1973 there were approximately 30 systems in operation, under construction, or on order, which fall in the category of the new generation of modern, state-of-the-art control systems.

Based on what is being developed in the power industry a composite modern control system for a bulk power system would have the following features and on-line functions.

1) Hierarchical structure consisting of several levels of computer systems. Examples: a) National center, regions, divisions. b) Power pool, member companies, divisions. c) System center, divisions or substations.

2) Hierarchical structure of control functions, i.e., preventive, emergency, restorative, and each divided into 3 layers d i r e c t , optimizing, adaptive.

3) Dual processors or multiprocessors plus redundant peripherals.

4) High-speed digital data telemetry and state-of-the-art data-acquisition equipment.

5) Color CRTs with graphics for interactive display. 6) Dynamic, color, wallboard group display. 7) Automatic generation control. 8) Economic dispatch control. 9) Automatic voltage (var) control. 10) Automatic circuit restoration [19]. (Re-energizes

circuits which have been dropped out of service by a disturbance.)

11) Supervisory control (breakers, capacitors, transformer taps, generating unit startup, and shutdown).

12) Scheduling of generation resources, Le., fossil, hydro, nuclear (long term, medium term, and short term).

13) Security monitoring. 14) State estimation. 15) Steady-state security analysis. 16) Dynamic security analysis. 17) Automatic system trouble analysis [19]. (Processes

breaker and protective relaying operations to analyze system disturbances.)

18) On-line load flow. (An interactive load flow available to the system operator to determine power flows in the network for a given set of conditions.)

19) Optimum power flow. (An optimal load-flow solution where the load-flow equations are solved with an optimization routine to minimize the objective function, usually, but not necessarily, the total operating cost.)

20) On-line short-circuit calculation. 21) Short-term load forecast. (Predicts the next 24-h

load curve.) 22) Bus-load forecasting. (Predicts the load a t each indi-

vidual load point.) 23) Various support programs for generation dispatching,

such as: schedule for startup and shutdown of units, power interchange transactions with neighboring systems, calculation of generation reserves, etc.

24) Logging and historical data management. There is no control system that has all of the functions

just enumerated. In general, except in the case of a few power systems, the control system designs that have been developed possess only rather small subsets of this list. In the Appendix, Table I lists some basic data on control systems which are in service, under construction, or on order, as of November 1973. This table is intended to convey a rough picture of the

888 PROCEEDINGS OF THE IEEE, JULY 1974

.*w w . 1 sW,.avc


TABLE I (Continued)

la In 1973 I

I

I I s A ./ ' '

X

Implemented by analog controller. Legend for on-line functions:

ACR Automatic Circuit Restoration. OLF On-Line Load Flow.

ASTA Automatic System Trouble Analysis. OSC On-Line Short Circuit. AGC Automatic Generation Control. OPF Optimum Power Flow.

AVC Automatic Voltage/Var Control. SA Steadyatate Security Analysis. EC Emergency Control. SBC Supervisory Breaker Control.

EDC Economic Dispatch Control. NOX Minimum NO. Emission Dispatch. SM Security Monitoring.

SE State Estimation.

SVC Supervisory Voltage Control.

890 PROCEEDINGS OF THE IEEE, JLXY 1974

types of systems that have been or are being developed in the power industry in approximately the first half of this decade.

In Table I several hierarchical computer systems are in evidence, most of them being 2-level hierarchies. System 24 is a 3-level hierarchy, while the combination of Systems 2 and 15 forms a 4-level hierarchy.

Of the systems already in service, System 7 is of particular interest. Controlling a small power system, consisting of 9 busses and 3 major transmission lines [25], the Tokke control center of the Norwegian Water Resources and Electricity Board has the distinction of having the first on-line application in the world of state estimation for security monitoring of a power system. I t also has an emergency control function which automatically reduces generation at the appropriate plant in case of problems resulting from a transmission outage.

For a large, interconnected system the first successful experimental results of state estimation were obtained in September 1972 in a portion of the American Electric Power system [26].

Another interesting development is the use by System 11 of the optimum power flow for calculating cost coefficients which are factored in the economic-dispatch-control calcula- tions. The vector X of cost coefficients is given by

X = ( - F)-* grad, Pr where J T is the transpose of the Jacobian matrix of the power- flow equation G(x, u) =0, and grad, P, is the gradient of the power at the reference bus used in the power-flow equation. This is the first on-line application to economic dispatch of a concept rigorously treated for both real and reactive power dispatching with equality and inequality constraints by Car- pentier [24] in 1962.

Unique among those systems in service or under development is the automatic system trouble analysis function implemented in System 11. This function requires the monitoring of protective relaying operations, both primary protection and backup protection, in addition to monitoring circuit- breaker operations. In other systems only circuit-breaker status is monitored to obtain information about a short-circuit type of disturbance.

The inclusion of a short-circuit calculation as an on-line function by System 4 is another unique feature not found in other systems.

The control-system features and on-line functions mentioned in this section have been described rather sketchily, the object being to present an overview of what is being done. More details of specific control systems and of various functions, some of which may not have been referred to in this paper, may be found in the literature [8], [9], [19]-[23].

As mentioned earlier most of the systems are implementing only a small number of on-line functions. However, even a minimum of security-control functions would entail a rather sophisticated assemblage of computers, display devices, data-acquisition systems, and a host of real-time programs.

The requirements for the new type of control system have also caused demands for real-time computer operating systems which initiaIly were either nonexistent or inadequate for the application. Development efforts have been expended in creating special operating systems where none were available or in augmenting the real-time capabilities of what the computer manufacturer had to offer. A standard real-time operat-

ing system usually has to be modified in order to support the requirements for the following.

1) A dual computer configuration with intercomputer communication.

2) Provisions for maintaining backup for critical control funLtions and for automatic failover in case of various types of failure.

3) A multiprogramming environment where various modes of program execution are required.

4) Interfacing with the data-acquisition system and non- standard peripherals.

5 ) System initialization for various modes of startup. 6) Provisions for testing and for training.

CONCLUSIONS The need for security control is not unique to the power

industry [SI. However, because of the uniqueness of the prod- uct that the power industry supplies plus the unceasing need to maintain that supply, the implementation of security control with the aid of real-time computers is being pursued perhaps more vigorously in utilities than in other industrial processes. The power industry, in developing this new type of control system, has taken advantage of advances in real-time computer design, state-of-the-art developments in. datar acquisition systems, dynamic and interactive displays, systems engineering concepts and algorithms in mathematical programming, stochastic approximation, and estimation thepry.

More work remains to be done in the other more difficult areas of security control such as dynamic security analysis, emergency control, and restorative control. Research and analysis efforts are continuing to resolve some of these re- maining problems. Doubtless the years beyond 1975 will bear witness to further advances in the real-time computer control of power systems [7].

APPENDIX Table I is presented to give an overall picture of the new

types of control systems that are being implemented in the power industry. These control systems are designed for the central control of the overall power system and are distin- guished from earlier types of automatic control by the provision for system security.

Table I is fairly complete as far as the following criteria are concerned :

1) central control for generation and transmission; 2) use of dual or multiprocessors; 3) use of color CRTs for interactive display; 4) inclusion of, at least, the security-monitoring function

for security control.

Some systems not fully meeting these criteria have been included because of certain security-control features.

Although the author tried his best to gather as much information as possible about what is going on in the industry, there are doubtless some state-of-the-art systems which have escaped his attention, The author apologizes for these omissions.

Many new, but strictly supervisory-control or generation- control systems, are not included in Table I except as they belong to a computer hierarchy and satisfy criteria 2) and 3). I t should be mentioned, however, that these strictly supervisory-control systems have become modernized to the extent

DY LIACCO: COMPUTER CONTROL OF POWER SYSTEMS 89 1

that dual processors, mostly minicomputers, and color CRTs are being used.

I t should also be noted that many of the companies in Table I which have new control systems under development already have existing generation- and supervisory-control systems using digital computers, some with CRTs, which had been placed in service only a few years ago.

The last column of Table I lists the on-line functions planned for each control system. Only the major on-line functions are enumerated. Standard support programs and off-line functions which form a large part of the software of these control systems are not included.

ACKNOWLEDGMENT The author wishes to thank his many industry colleagues

in the United States, Europe, and Japan for their cooperation in putting together as up-to-date an information summary as possible for the tabulation (Table I ) of modern, state-of-the- art, control systems. Permission was obtained from each of the utilities represented to use the data in this paper. The data for the Japanese system were obtained through the good offices of the Central Research Institute of Electric Power Industry in Japan.

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