The Evolution of Real Time Control Applications to Power Systems

SESSION - PLENARY PAPERS SESSION [I-PLENARY

Copyright ~ CC) IFAC IFAC Rul Real Ti"", Time Digital Conuol Control Applications Applications Copyrighc Cuadalajara ico 198~ Guadalajara.. M.~ Mexico 198~

THE EVOLUTION OF REAL TIME CONTROL APPLICATIONS TO POWER SYSTEMS N. Cohn N.Cohn 14'7 1457 Noble Road,}enkintown, Road,Jenkintown, PA 19046, USA

Abstract. Paralleling the extensive growth and expansion of interconnected and Csnada Canada during the past electric power systems in the United States snd sixty years years,, has been the related need to regulate generation in the constituent areas, and the power flow between them, to achieve equitable, rreliable e liable and economic system and area operation. Many individuals and groups have made IIl8de contributions to these objectives objectives.. These constitute the evolution of the syatem system and area real time control art from modest, tentative beginnings to the comprehensive, broadly scoped and highly cspable capable present day on-line digital control systems systems.. This paper presents one indi individual's view, based largely on peraonal personal experience and observation, of significant steps in this evolutionary process. The paper deals primarily with the analog phases of these developments, many of the philosophies and basic to current digital executions. techniques of which remain bssic Keywords. Computer control, power system control, power station control, dispatching,, distributed control systems, large scale systems. load dispatching INTRODUCTION

The history of a discipline is probably best written, certainly most objectively written, by a non-participant in the events being recounted and evaluated. When reviewed by a participant, as in this instance, it is quite likely to have an autobiographical castt and perhaps even a bias. In any event, cas I hope that this review of earlier techniques, which I have been asked to undertake, and which recalls some of the steps and experiences that got us to where we are, may be helpful in providing a better basis for comprehending and appreciating the advanced technologies that thst are practiced in today's toda y 's computer-directed world. This paper will be based largely on the observations and experiences of my fifty-five years in this field, forty-five of these with Leeds & & Northrup (Philadelphia, 1927-29 and 1955-72; San Francisco, 1929-36; Chicago, 1937-55) and ten years (1972-present) as a consultant. It will discuss activities in the United States and Canada, Canads, without at all diminishing diminishing the importance of work done elsewhere. It will describe developments that occurred primarily in the analog snalog domain, many of the latter-day philosophies and techniques of which remain basic in current digital executions. Present day digital technology has of course moved far beyond the limitations of the analog domain, and introduced greatly expanded and valuable valuable real resl time monitoring and control techniques for integrated integrated power systems. A session including including a presentation on the evolution of digital control for energy control centers, Carpentier Carpentier (1983), is scheduled later in this this symposium. symposiulII.

Over the years, many individuala individuals and engineering and operations groups have contributed to the definition of system operating objectives and to the formulation, appraisal, revision and implementation of techtechniques for achieving them them.. To try to name all of thelll them would be impossible. To name reasonable . Despite none would scarcely be reasonable. its risks, I will identify some, primarily in this paper's bibliographic references. Others may see origins or events differently. In any event, with this paper I salute all individuals, whether specifically identified in it or not, who over the years have contributed to the development and application of real time power systems control. Useful bibliographic tabulations are Premin(1960 ) and IEEE (1977, 1981). Earlier ger (1960) Horehistorical reviews are Brandt (1953), Morehouse (1965) and McDaniel MCDaniel (1974). ( 1974). Throughout the history of the power industry, dependable, real time automatic control to insure inaure safe, reliable, responsive operaoperanecessary element of power tion has been a necessary true a system installations. That was true hundred years ago at Thomas Edison's first central generating station at 255-257 Pearl York, placed into operation on Street in New York, Sept. 4, 1882, and generally regarded as marking the founding of the electric power IEEE (1982). Each Each of the staindustry, IEEE tion'a six 100 100 kw generators was waa equipped tion's for control with speed governors, lineal for descendsnts of James James Watt's pioneering descendants to feedback feedback control. contribution to

N. Cohn

2

engineers are permitted a sympathetControl engineer. uckle when reading re ading of ic and undentandlng understanding ch chuckle Edison's early problems (later of course Ediaon'. resolved) with the Pearl St. governors, governors , encounte r ed when first trying to run two encountered generators in parallel. generator.

now speak of, like Caul, I now Gaul, divided into three separate parts: psrts: a boiler room, a turbine-generator room, and an electrical aswitchboard wit chboard room, with heavy walls separati ng them, lest, l e8t , one can a88ume, t here be ting assume, there communication between them.

An Edison biographer, Conat ((1979), writes, Ediaoo bi ographer, Const 19 79) , WTitea, "Wi th one eengine ng i ne running, ru nnin g , everything everythi ng was waa "With fine. f ine. 'Then we started scarted another engine', Ediaoo reported, r epor ted , ''and and threw them 1n Edison in parallel llel.. Of all the circusea circuses aince since Adam was born , we had the worst. One engine would born, stop,. and the other would run up to 1000 atop tbey see-sawed. lee- aa wed. The revolutions. Then they trouble was w•• with the governors governors..',ItR

Automatic Control

NI atl11 As 1n in the very beginning, there are still

speed governor governors. . . Such governors throughout cothe system, syatem, together with the frequency coefficient of connected customer load, l oad, and system stored energy the variation of aystem ener gy as ss a function of frequency, serve as the basic self-regulat ing forces of the system. self-regulating Jollyman (1927) describes the utilization utilizstion of t hese effects for regulation of an isolated these system.. These governing effects aare, system r e, howsingly- dimensioned and lack l ack geographiever, singly-dimensioned cal discrimination in reaponding responding to load changes on an interconnected i nterconnected system, sys tem, Cohn (1971b).. They therefore require (1971b) req uire supplesupple~entary area contro l s to reallocate generamentary controls generaisfy individual individua l t i on changes iinn order to sst tion satisfy interconnected area responsibilities and int erconnected srea objectives, which include programmed bulk areas,, and economic power transfers to other areas and secure operation within the area. s rea . Recounting the development of such supplemensupple~n­ tary controls is the prime pri me objective object i ve of this paper. POWER IINDUSTRY NDUSTRY -- 1927

To provide a background reference for the evolution and syssod growth of on-line power aystems controls controls,, let's teu let ' s see what the domestic domest i c power industry was like at the time I embarked on my car eer. In the forty-five forty- f i ve career. years ssince Street ince Pearl St reet it had had what then would have been called great growth.. great growth generating capability i ng capabi lity was about The U.S. generat Nikola 25,000 MW. MW . Thomas Edison and Nikol a Tesla, the gisnt giant geniuses of the electric elect ric power field, were still aliVe, alive, and worki working. ng . The dc-ac dc- ac battle between their respective technologies had been resolved in favor of the t he t hough there were still many metrolatter, though politan ateas areas that were we re distributing dc power. No one cou could ld have visualized then t hen that dc would one day be back, aa as the preferred medium for llong ong distance, extra high voltage transmission tranlmilaion lines, and for asynchronous interconnections. nOUI inter connect i o ns . In contust pr esent capability of contrast to the present 600,000 seems amal smalll 600 ,000 MW, KW , the 1927 19 27 capability aeems indeed. i ndeed. Also at that time, transmission distances voltages were lower, transmission di a t ances smaller. shorter, and generating units uni ts lmall er. And in the context of present da dayy plant and sys8Y8tem coordination, we can note that t hst most fossil fueled power plants planta were at the t he time

IInn 1927, suppl ementary automat i c control was supplementary automatic indeed in its infancy. Voltage control was, it is i s true, true , regularly used. Boiler Boile r feed water control control was cus customary, watet tomary, and boiler boi l er combustion control was relatively System bus tion control relstively new. new . Syatem control cont r ol depended primarily on generator speed governors, supplemented apeed 8upplemented by manual control. Preminge r (1960), only three con trol. In Preminger papera 1922-1 928. papers are listed for the period 1922-1928. There was waa aatt that time, tllDe, and snd for fo r some time thereafte r, relatively little contro thereafter, controll sa practiced in i n recent theory. Simulation as control experiyears was not no t available for cont r ol expe rimentation. mentation . It was not, no t , however, especially missed. For the following two decades exmisaed. s, perimentation on the best of all simulator simulators, themselve s, was feasible, feasible , and power ayatems systems themselves, was practiced. Teleme t ering was quite limited. limited . A good watt Telemetering transducer was wa s not available. Such power telemetering as actually occurred was execcuted uted with wi th a fairly complex and expensive expenaive transmitting t r ansmitting unit, generally gene rally over dc telephone lines. li nes . Some telemetering telemete ri ng was wa s done mete r s , ffrom t om impulse generators on watthour meters, frequently overr carrier, which tended frequent l y ove t ended to t o be limited computat i on, lill:llted and noisy. Analog computation, where whe t e executed, executed , was generally gene rally done with servo driven slidewires. We had not yet entered the electronic el ect r onic age. Analog computers, as we later knew them, t hem, were we re yet to to come,, and digital computera computers were still far fa r come off in the distant diatant future. future . Interconnections Intetconnec t ions

nterco nBy 1927, the potential benefit of iinterconnections nect i ons between adjacent areas, sharing generation genera tion and reserves, r eaerves , and in i n some cases construction, plant cons truct i o n, had been recognized and the practice started. atarted. Comments by the t he late Samuel Insull (1921), whatever his faults faul t s may later have been, reflect considerable underatanding understanding of the value and the probable future extension of interco nnections. Humphery (1927) provides interconnections. a comprehensive summary of interconnections as they had then been developed In in the northeast, In in the mid-Atlantic mid- Atlantic states, and outlines in the Chicago region. His paper ou tlines the potential benefits of interconnection. More particularly, howeve however, r, it emphasizes prevailing problems,, by no means prevai l ing operating problems res olved, such as control cont r ol of frethen yet resolved, quency, control control of power pover flow and proper dispatching. di 8pstching . It emphasizes that at that t hst stage of the game, existence atage gsme, the exi stence of interconnections didn't connection. didn 't mean their continued capability. Clearly, challenging control cont r ol problems lie ahead. shead.

Evolution Time Control Applications Evo lution of Real Tim~ Applicati ons Other interconnections at interconnecti ons already in service st that time include early ea rly ties of the Southern Company Pool and ties in i n California and the Pae:ifi c No rt hwest . The Pennsylvania-New Pacific Northwest. Je r sey Pool, later to t o be the PennsylvaniaJersey New Je Jersey-Maryland r sey-Maryland Pool, was within a few f ew months ~nths of being established.

The ccurrent urrent full fu ll extent of interconnections, related sub-stations s ub-ststions and plants planta in the Un ited Sta tes and Canada, 230 kV and above, United States is NERC ((1981). I s shown in NERC 1981). A map of North American interconnected inte r connected control areas appears (1981).. ap pears in U. S. Dept. Dept . of Energy (1981) 1I think it is clear clea r that the extensive growth correspongr owth of interconnections, i nte r connecti ons , the t he corre spondding ing increase in the number of generating gene rating stations, s tations , and the asignificant i gn ificant differences in theirr sizes and incremental efficiencies, thei efficienciea, introduced I nt r oducen substantial hierarchical hierarchica l multivariable varia ble multi-level control problems. problems . We will sho shortly rtly see how these have been approached. pr oached. Measurement ~!easurement

Developments Develo pments

i s a self-evident se lf-evident maxim that what you It is cannot measure, directly or inferentially, you cannot e:ontrol, control, or at least leaat you ought nott tr tryy to control. It will be clear clear from no frOlll the discuss discussion ion thus far, that two of the majo r parameters involved in power systems major control megawatt con trol are system frequency f requency and megawstt load, ithe r to generaInad , the latter applying eeither tors or transmission tie lines, or both. Apparatus Ap paratus for fo r making such sue:h measurements prior to t o 1924 19 24 was of limited limit ed flexibility flexibili ty or precipree:ision, sion , or of inadequate applicability appli e:ability to control systems, costly. systems , or far too t oo e:ostly. Three development developments, s , one initially inlt1 ally unrelated to power systems activi activities ti es and two that tha t occurred occu rred virtually ssimultaneously illlult aneousl y but totally ta lly independently of each other, othe r, filled the measurement voids for power powe r systems applications and were major factors in stimulating early st imulating the ea rly work in power systems real real time control. These developments were: 1. The self balancing bslanci ng potentiometer hightorque to rque servo recorder, ree:order , invented by Leeds (1912).

adaptation self-balan22.. The adapt a tion of the Leeds self-balanccing i ng recorder to t o a self-balancing ac Wien brige frequency f requency recorder reco rder by Wunsch (1925). 3. 3 . The Lincoln thermal therlll81 converter, introduced in 1924 by Lincoln Linco ln Meter Company of described Canada, as de scribed in Lincoln (1929). These have played so important a role in the development of power systems control that I should like to t o say a few words ~ords about each of them. Leeds recorder. recorder . This Thi s instrument ina trument was originally developed for the automatic measurement of small de such as dc potentials auch aa those encountered with thermocouples t her~ocouples or resistance thermometer circuits. the r ~o~ete r ci r cuit s. It was a revolution-

3

ary development and was a great stimulus sti~ulus to scientific and industrial indus trial measurement in many IqIny applications throughout the world. Its maj or characteristic e:haracteriati c was that in measuring major ve ry small slll811 electrical ele ctrical voltages, vol tages , it i t did not Dot very draw power from or or alter the measured voltage. In addition it possessed, pOssesaed, from fro~ its it s own energy source, adequate power to drive a teD inch wide pen Without without restraint on a ten chart,, to operate control contacts and to chart operate a number nu=ber of retransmitting r et ransmitting slidewires in independent circuits in which were reproduced the measured ~easured voltage at high levels for analog computation and automatic control use. I dare say s ay that when the inastrument truQent was developed in 1912 no one could have anticipated that it would become the cornerstone of frequency frequen cy and load measurement and control, serving auch such functions widely through World War 11 Il and beyond. Wunsch frequency fre uenc recorder. Sometime Somet i~e in 1923 Nevin Funk (president president of AIEE in 1943-44) then Chief Engineer of Philadelphia Electric Elee:tri c Company, COQpany , who had been using Leeds recorders for the measurement of generator genera t or and transformer temperatures was anxious to have an equally scale recorder for a precise equal l y open acale measur ement of system syste~ frequency. fr eq uency. He asked measurement tAN if it would yould not be pos lible to t o build L&N possible t llk was WII given to Felix Feli x such aI unit. The task Wunach in the Company's Engineering DepartWunsch ment. In due course cours e he adapted adapt ed a Leeds recorder to serve as a self balancing ac Wien bridge suitable for the precise measurement meaaure~ent of system frequency, frequency , using a range of 58 to 62 cycles e:ycles over a ten inch chart. The recorder was installed at Philadelphia Electric in 1924. Many followed elsewhere, and brought a whole new understanding of the nature nlture of frequency variations on power syssys tems. Lincoln thermal converter. converter . This unit inveninvented, I believe, by Prof. Prof . Paul Lincoln of Cornell (AIEE president 1914-15) and developed for practical use jointly j ointly by Lincoln, Louis Paine of Lincoln Meter Company Co~p any of Canada and Perry Borden 80rden of Hydro Electric Power Commission Com~ssion of Ontario was introduced in 1924 just j ust about coincident with -- but totally unrelated to -- the development develop~ent by Wunsch of the frequency recorder with which it was later to have so close and extensive an association. asaoe:iation. The thermal converter unusual the rmal conve rter was ~as a most unuaual device. It had no moving parts, developing developi ng a temperature differe difference nce between two selfcontained heaters which was directly proportionate to ac ae: power, independent of phase angle or frequency. It possessed posses sed high precision e: ision and stability. Self-contained thermocouples IDOcouples measured the temperature te~perature difference and turned out a dc de: milli-voltage mllli-voltage proportionate to ac power, power , of sufficient magnitude to t o permit measurement on a selfbalancing basis basiS by the Leeds Leed. dc de recorder referred dc outputs of a r eferred to t o earlier. earlier . The de: number nu~ber of converters converter. could be connected in series for reliable totalizing t otal i zing purposes, a new dimension for power system ayste~ dispatching.

,

N. eohn Cohn

4

Thermal converters were 1n in use extensively in starting 1n Canada canada atart ing with the Hydro Electric Power Co"!a.lon Commdssion of Ontario in 1926, HEPC KEPC Sangamo Meter Company had a (1926). The. The Sang.ao relationship with Lincoln Meter Company and in 1931 an In 80 arrangement arrange.ent was waa made between L&N, Sangamo and Lincoln Meter Heter Company for L&N to serve as the distributor of the units. Berve .a unlts. Thereafter the converters were very intimately related 1n in power systems applications to L&N recording and control controlling ling assemblies. in such applications multiplied Their use uu 1n and they were not withdrawn from sal salee until 1978, s.fter after more useful IDOre than fifty years of uae!ul application. LOAD FREQUENCY FREOUENCY CONTROL EARLY TECHNIQUES Dispatching for Manual Hanual Control Conventional practice 1n in power .ystema systems operations had been to depend on generator govergovertlons nors to respond to system load changes and to utilize manual ~nual adjustment of governor settings on one or more mo re machines to achieve desired distribution of generation between alternative sources. An early, 1924, central dispatching installation to facilitate such operation was that of the Philadelphia Electric Elec tric Company. Recorders showing the generation at each of their four stations, the total system generation and the first Wunsch recorder showing the system frequency were provided at the dispatching center. Thermal Converter These were pre-Lincoln Ther~l Conver ter days. The telemetering for the station load readings utilized Westinghouse Type R Kelvin Balance totalizing recorders. IInn each of the stations there was waa attached to the recorder a transmitting potentiometer slidewire, the output of which, connected to telephone lines, was measured potentiometripotentiometrically with a Leeds recorder at the central office. Retransmitting alidewires slidewires on each of the receiving recorders provided a voltage summation of the individual station loads and was recorded as totsl total system load by a fifth recorder. A Warren master clock provided the reference for periodic manual adjustment of the system speed to maintain time within wi thin limits considered appropriate. These were not close limits since it was not felt that close synchronous time was a service cOlUlitllM!nt commitment to customers. custolrlers.

The information continually available at the central diapatching dispatching center minimized minilrlized the need for communication cOlrlmunication with the individual plants, and the extent of manual adjustment adjustlrlent required to fulfill operating ope rati ng objectives was not regarded r egarded as oppressive. oppresaive . differing Other utilities had diffe ring views on the need for cloae close synchronous time. t1lrle. Supplementary Automatic Supplelrlentary Autolrlatic Control

One, New England Power, had embraced embr aced the policy of selling lel1ing time tillle to their customers, custolllers, and assigned sssigned continuing manual adjustment to one of their stations for time tillM! regulation.

This proved to be an arduous task, particularly activities lsrly considering the many other activitiel for which the operators were responsible. relponsible. Thil led to the installation inltallation by New England This Power in 1927 1921 of what is regarded as the first use of automat automatic i c frequency control on a power system.

That step, to which reference will additionally be made shortly, was followed in the period 1928 to 1934 by comparable frequency control installations by other compaco.panies. For some, aome, objectives went beyond close frequency for tilDe time regulation, and incloae cluded additional objectivea objectives of simultaneous simultaneoua control cont rol of several aeveral generators generatora within a staItation to achieve appropriate automatic division aion of loading between them, and regulating frequency to assist asaist in control of tie line loading when interconnected. It think it is appropriate say that the sppropriate to ssy pioneering work done in the east and middle west weat in this thil period provided the fundamental fundamentll bases for moving IIIOving on in subsequent years yelra to the fully coordinated control of bulk power transfers between interconnected areas. Let's now see what some lame of these individual control developments were. Frequency Control As Aa above noted, the first system systelll to under-

take automatic frequency regulation regulstion was New England Power. Fower. Two types of controllers were installed at Harriman Marriman Station. Station . One was waa an adaptation by L&N of the Wunsch frequency recorder. Rather than regulate with simple "on" "off"" contacta, contacts, it was recognized Hon and "off that a relationship between control action and the extent of frequency deviation was desirable. The instrument was accordingly equipped to provide "lower" or "raise" concontrol impulses proportional to the deviation of the instantaneous frequency from fro~ 60 Hz. Nz. Contact closure clos ure operated the governor synchronizing motor to lower or raise generation respectively. Sketch (a) of Fig. 1 shows the control characteriatic characteristic of such a controller, drawn as ss the control balance points on a plot of frequency freque ncy versus tie line flow. The controller would endeavor to hold scheduled frequency, F Fa, o ' regardless of tie line flow. M

The other frequency controller was by Warren Telechron Company (later GE), GE) , and was a Warren Master clock with a mechanism arranged to provide contact closure related to the integration of frequency deviation over the previous two seconds seconds.. The results were apparently satisfactory for the both types of controllers as reported by Brandt (1929). (1929) . A year's operation confirmed the validity of automatic autolll8tic frequency and time control, and additionally indicated that closely regulated frequency would contribute con tribute to regulation of bulk power transfers. Shortly after construction of the New England recorder controller controller,, L&N L6N devised a simplified controller which had a fixed but

Evol ution of Real Re a l Time Cont r o l Applications Ap pli cat i ons Evolution Control adjustable balance point thereby eliminating the balancing slidewire of the recorder and the time required to operate it. Raise and lower impulses were proportionate to instantaneous frequency deviation and were made on a two-second cycle basis. The first of these units was installed at Wallenpaupack Station of Pennsylvania Power and Light in 1928. This type of unit remained standard for many years and was used on most of the installations in the United States and Canada Canada.. A general discussion of frequency control, including descriptions of the L&N Southern California Edison Big Creek installation, and of the available Warren type equipment as well as comments on operating experiences and problems encountered are contained in Hunt (1930 ) . Another report of that same (1930). general period which is of interest in refle cting the understanding of the problems flecting and needs related particularly to interconnected systems, systema, for which solutions had not yet been provided, provided , and which were not to be available until several years later, is Fitch (1930). Frequency Control with Time Correction

instantaneous" frequency Regulation from ""instantaneous" might or might not result in a precise synchronous time, depending on the calibraaynchronous tion of the controller and the overall effectiveness of the control system. syatem. This was recognized at an early date and for those who preferred the type of frequency regulation that was based on "instantaneous" values, an automatic time correction feature was added. one One technique for achieving this was to have an automatic vernier adjustment of the frequency control aet set point from accumulated system time deviation, Heath (1929). An early execution wa wass added by Southern California Edison to their Long Beach frequency controller. The arrangement worked very well, and in effect served as an overall corrective unit for both the calibration of the frequency controller and the integrated control responaes responses of the system. Commercial assemblies for such operat operation i on were supplied by L&N using a Warren clock as aa the master time standard. A number of installations of this thia type were made in the 1930-34 period, including the City of Vernon, Calif. Diesel Station, City of Seattle Diablo Station and Hoover Dam Station. Multi-Unit Control As Aa systems grew, it was in many cases caaea not feasible for a single unit in a multiple unit station to undertake the swings essential eaaential to regulate frequency. Further, better economy could be achieved by dividing total atation station load among units of the station in accordance with efficiency considerations. Three approaches were devised for such operation.

5

Proportionate loading. An early approach was to divide the load among participating units of the station in accordance with predetenD.1ned ratios, Doyle (1928, 1929). predetermined This technique originally proposed the use uae of shunts in the secondary secondsry ccircuits ircuits of generator current transformers, appropriately phased to be responsive to unit kw output, and distributing control pulses from the frequency regulator to maintain the desired machines, ratio of outputa outputs of the individual machines. I do not believe any installations were made using such shunta. shunts. Later, however, when Lincoln thermal converters became available, they were used in place of shunts to measure the outputs of the individual generators and the converter outputs were balanced through individual load controllers controllera to maintain the preset ratioa. ratios. This general technique has been very widely used.

Thia technique, intended Economic loading. This for hydro stations, divided the load between the units of a station in accordance with their incremental input/output input / output curves to match incremental water rates. Equipment for such application was jointly engineered by the I. P. Morris and De La Vergne Inc., and L&N and is described in Kerr (1930). Installations were made at Carolina Power & Light, Washington Water & Power and Montana Power & Light in 1930. The means of developing the incremental curves were cumbersome. They were comparable to the transmitters used for the Philadelphia Electric telemetering installation mentioned earlier, utilizing a Westinghouse Type R wattmeter driving an L&N slidewire rated to match the incremental characteristic. Valve point loading. This technique seeks to take advantage of the fact that steam units have their best efficiencies when steam controlled inlet valves are not in a position. Load diviaion throttling position.division within the plant is accordingly based on programming all controlled units, aave save one, to operate to have fully opened valves. Description of a pioneering system for such load programming is described in Purcell and Powel (1931). This paper is of additional interest and importance because of other material it contains, to which reference will be made later. Valve point loading continues to be regarded by many operators as important, despite operating difficulties in precisely determining when a given valve is or is not in a throttling position. INTERCONNECTED SYSTEMS CONTROL EARLY EFFORTS

By 1930-31 automatic frequency control was well established. estsblished. It was in use in many locations, some equipped with time error correction, many with multiple units participating within the regulating station. Summaries of experiences with supplementary control up to this time are contained in Henry and others (1929) and Fitch (1931). Major questions then were, "now that we

6

N. Cohn eohn

have frequency control, how do we spread the regulation and control power flow on interconnecting tie lines?"

It waa was clear that frequency control, by ita its very nature, meant absorbing load changes on the regulated unit or units regardless of where they originated. When the load cchanges hanges were in a remote area, it was waa absorbed by the local frequency regulating atation, station, resulting in undesirable changes In in tie line flow. "How to avoid this?" That was the question. There was full agreement that each company should endeavor to absorb its own load changes. Two separate techniques were explored in In the 1930-31 period: (I) (1) Parallel frequency control on interconnected companies, and (2) Constant tie line control on a system already under company connected to a ayatem control . Both will be reviewed. frequency control. Parallel Frequency Controllers Controllera

1930- 31 by A major program was undertaken in 1930-31 West Penn Power and American Gas and Electric (now American Electric Power) utilizing concurrently operating frequency controllers at three different stations: at their jointly owned Windsor station, at the West Penn Springdale Station and at the AG&E Phllo Station. This was a really "noble Philo experiment" involving cooperative effort by experiment" the two power companies, their station operating personnel and two manufacturers. I say two manufacturers, because it was decided to conduct the experiment with two complete sets of control equipment, the one being the proportional step instantaneous type frequency control by L&N and snd the other being the Warren short period time deviation integration type by G.E. The tests established that controllers could operate in parallel, but differences in operste calibration and sensitivities resulted in more deviation from schedule of power flow on interconnecting tie lines than was considered desirable or acceptable. Further indi cated. work was indicated. The experiment did di d establish, however, preference on the part of the operating people forr the instantsneous instantaneous type regulators, and fo thereafter these became theresfter becsme the essential standard in the United States and Canada.

West Penn Power, the latter station already being under frequency control. Different reg ulators were used, one involving types of regulators Westinghous e, another a solenoid assembly by Westinghouse, utilizing a Westinghouse Kelvin balance balanee unit, and the third an L&N L6N proportional sstep tep controller, similar to the one used for frequency control except that instead of an ac Wien measuring bridge, it made a dc comparison s o n of the output of a Lincoln thermal converter that metered the tie line power flow and a dial set to the desired tie line flow. Sketch (b) of Fig. 1 is the control characteristic of constant tie line control. The control would act to hold tie line at scheduled flow, To, regardless regsrdless of frequency. snd Powel ((1932), 1932), previously rerePurcell and t o is a report of this installation. ferred to st least from the point of It notes that, at view of Duquesne itself, the constant tie line controller provided a solution to the problem of a fixed interchange with aB neighboring utility. As can be seen from the discussions discussionB of the paper, psper, however, there i nterconnected comwere differing views by interconnected panies. They noted that in holding a conconline, Duquesne frequently contristant tie lin~, buted adversely to system frequency, by opposing its own self-regulating forces that responded to remote load changes. Thus it failed to provide assistance to remote areas in their time of need and indeed aggravated prevailing conditions. The discussions reflect a considerable difference of opinion as to the relative relstive virtues of frequency control versus constant tie line control. In retrospect, that paper and its discussions reflect a watershed period in the evolution of real time controls for the effective regulation of bulk power transfers trsnsfers on interconnected systems. It was the first to introduce constant tie line control as an sn Its discussions ininoperating technique. ItB clude purposeful comments by Fitch, Sporn, Srsndt Brandt,, Hunt snd and Juncke, all then very aetive active iinn the work being done on power syscontrol. tems cont r ol. Most Host had formulated formulsted in papers and presentations what they felt were the prevailing needs to insure effective control of scheduled bulk power transfers trsnsfers on interconnected systems.

Constant Tie Line Control

Sporn's discussion, in reporting on the parallel frequency control experiment (which he control" described as "distributed con trol" -- probsbly the earliest use of a term and techably nique now frequently encountered) and on the rectly identified the work of others, cor correctly limitations of flat frequency control and tie line control as being unable to distinto guish as to whether the load changes to which they were responding occurred in or out of their respective areas. sreas. He also recognized the problems introduced when there were multiple ties and not just single ties between areas. aress.

At approximately that same time, Duquesne Light installed a constant tie line controller at their Colfax Station to regulate power flow on their tie to Springdale of

As Aa a less than significant autobiogrsphical autobiographical note I might add that, being then stationed ststioned in San Francisco, I was wss present at the Lake Tahoe presentation of this paper. Electri-

At about sbout this same time, there were independent tests on parallel frequency controllers controllera at Washington Water Power and Montana Power. Results, reported by McNair and others (1932), were comparable to those experienced in the Midwest, namely, too much variation of tie line flow.

Evolution j,;vo lution of Real Time Control Applications

ca.! 1) in its iu report of the cal West (193 (1931) meeting, commenting ~eeting , quotes me De as co~men ting at the meeting on the merits Derits of both frequency and tie line load control, and the probable uniqueness of each installation. A pertinent reference of that period, Sporn and Marquis (1932), provides an excellent summary of frequency and tie line load resystem changes,, a summary of sponses to sys t em load changes experiences with Windsor Station regulating frequency for the entire interconnection, interconnect ion, and the analysis that led to t o the parallel frequency control experiment at Windsor, Philo Phllo and Springdale. Springdale . It outlines achievements, ~ents, but also defines remaining re.aining unsolved problems as of that time. time . The paper emphasi~ed emphasized the need of proper overove rall coordination for economy purposes and loading . It reiterated the need for plant loading. each aarea rea to t o absorb its own load changes. changes . The importance of close frequency for more than time control was again stated. stated . It no ted most particularly the need for "proper "pr oper noted coordination coordina ti on of tie t ie line and frequency controllers so that the two function toward the ssame sme end with a minimum adnillUlll load swing sring and so that the functioning of one does not vitiate some prime function of the other or of the system at a time when such functioning f unctioning is harllv system performance". hllrllv needed for proper pr ope r syatem performance" .

Such commen t s const ituted a clarion call for comments constituted significant signifi cant additional ssteps teps in the evolution of power systems controls. cont r ols . That's how things were in 1933. There followed the development deve lopment work of 1934-38 out Out of which came major significant steps in the regulation t i on of bulk power powe r transfers. trans fers. INTERCONNECTED SYSTEM CONTROL INITIAL FREQUENCY-TIE FREQtJENCY-TlE LINE COMBINED CONTROLLERS

Steps taken in the Midwest and East to fulfill bulk power transfer objectives included t r ansfer objectivea contro l techcombined frequency-tie line control niques identified as "selective frequency", f reque ncy" , "tie line zoned" zoned" and "frequency zoned" controls. All were intended to be used in controlling concert with a master frequency cont r olling rea. Each will now be briefly examined. aarea. Selective Frequency

The first development of a combined frequency and tie line controller was, I believe, at Crawford Station of Commonwealth Edison in 1934. Its control con trol characteristic chara cte rhtic is shown in sketch (c) of Fig. 1. IItt is arranged so that control action is based on frequency deviation from schedule, but concontrol is i s permitted to act only when whe n it will also slso correct prevailing deviation of tie line flow, as in the first and third quadrants of the figure. There is no control action in the second and fourth quadrants fou rth quad r ants ssince i nce at that time control action needs to be taken t aken in one or more remote reeote areas to syste~ frequency and the tie correct both syste~ line flow of those areas. areas . This was an

7

develop~ent and was was a move in in interesting development the right directi direction. on . It added an element of sstability t ability when operated with master frequency control elsewhere elsewhere,, but there were i n calibras tll 1 questiona still questions of differences in tion and sensitivities between cont controllers, rollers , uncont r olled and the non-specific nature of uncontrolled deviation of tie line flow from schedule. The use of selective frequency control was quite limited, however, and it was superceded within a relatively short time by the introduction of the "tie line bias" controller, troller , which will be discussed later, "~oned controls". cont r ols " . after a brief review of "zoned

Zoned Controls Cont rols controls shortly These cont rols were introduced aho rtly after introduc tion of the selective selec ti ve frequency the introduction controller. controlle r. I am uncertain as to their extent original formulation f onnulation or ext ent of use. uae . They were a sstandard tandard option on L&N L6N installations after the introduct introduction i on of the tie line bias type of control. They were of two types. t ypes. cont r ol characteristic charac t e ri st ic is The one whose control shown in sketch ske tch (d) (d ) of Fig. 1 was identified as "tie line zoned by frequency deviation" deviation".. It would normally operat operatee to maintain a constant s tant tie line loading at scheduled flow To but would automatically shift to frequency control cont r ol at preset high and low frequency f requency levels when respective high and low frequency deviations occurred. occur r ed. other, The othe r , designated "frequency zoned by tie line deviations", deviations" , is illustrated in sketch (e) of Fig. 1. IItt would normally be operated as a constant frequency controller Operated to maintain frequency at FFo o '. but at preset high and low tie line deviations, deviations , it would woul d automatically shift to constant tie line control at the respective preset values. additional These controls represented addi tional steps in combining frequency and tie line flow into single controller i nto a aingle cont r oller to help overcome the limitat limitations ions of constant frequency or constant tie line controls. controls . They represented sen t ed additional steps in the evolution to a fully coordinated combined frequency-tie f requency-tie line controller.

INTERCONNECTED SYSTEM CONTROL -BIAS CONTROLS WITH CENTRALIZED FREQUENCY FREQtJENCY CONTROL A major contribution in the evolution evolut i on to present-day interconnected system control practice prac ti ce was the development and introduction of "tie line bias control" con trol" devised by Williams WllUams and Morehouse Horehouse (1935). (1935) . In my view this was was the genesis of current Current practice. pract i ce . It combined instantaneous ins tantaneous frequency and prevailing tie line flow in manner illustrated by the solid line control cont rol characteristic of sketch (f), Fig. 1. ske tch {n, I. In effect, the control control characteristic char acteristic is a linear curve of frequency versus ve rsus tie line, passing passiog through the point defined by scheduled frequency FFo To' The o and scheduled tie line To. effective operating tie line schedule is shifted automatically in the con controller troller

N. Cohn

8

along the control characteristic by the magnitude of frequency deviation from schedule. The amount of the shift is t a defined by the c urve, the inslope of the characteristic curve, bia." , verse of which is the "frequency bias", usually specified in 1n MW per 0.1 Hz. Ht. This technique has been optionally referred to as "tie line bias control" or "frequeocy "frequency biased tie control" or just plain "bias tle line control" "bi •• control". It went vent through three phases before evolving into the deslg" design and applications practice that remains standard to this chi. Irlll be briefly sumday. Each of these will marized, noting that in 1n all three of these theae phases one area was retained on constant frequency control to insure a constant frequency for the interconnection.

B18. Control With Bias B18S Withdrawal Tie-Line Bias In the original originsl design of bias control, the operating philosophy was that assistance aSSistance should initially be provided -- but only - - to an area or to areas in temporarily -need. The bias characteristic of the conassistsnce, troller would provide for such assistance, since for for.a decrease in frequency, for tle line schedule would autosutoexample, the tie highe r level of matically be shifted to a higher "out to provide the desired assispower "out" tance. It was further felt, however, that the assistance should be rendered for only a limited time, by which time the offending area or areas should have adjusted sdjusted their own ~ generation gene r ation to correct co rrect their deficiencies. If they did not do so, however, then a time tillle dependent mechanism in the tie line bias biaa controller would rotate the controller characteristic from the initial bias diagodiagonal shown in sketch (t) (f),, Fig. 1 toward the vertical or constant tie line character characterisi stic, shown as a broken line. This would withdraw the bias assistance that had theretherebefore been provided. M

In I~ other words, a constant tie line flow for the area ares was w.s given higher priority than thsn rendering sustained s ustained assistance to others. The initial installation of this type of equipment was wa s made I1ISde in 1935 at Harriman Station of New England Power Co. followed by one at Carolina Power & Light ~ight in 1936, 1936 , and one in early 1937 at Twin Branch Station of Indiana & & Michigan ~ichigan Electric, Elec tri c , a property proper t y of American Gas and Electric Co., now American Electric Power. Power . All three units were of the bias withdrawal type.

Though the controllers at New England and Carolina were presumably presu1ll8bly providing acceptable performance, satisfactory perforaance, getting sa tisfac tory operation with the bias controller at Twin Branch proved to be very difficult, and in fact we didn't achieve it. it . What we noted after long hours of adjustment sdjustment and observation was that at the start of a significant frequency change caused by remote load losd changes, the controller would cooperate aasist the remote re~te nicely, would act to assist penait its own tie line to areas, and would permit accordsnce with the bias go off schedule in accordance Alter a few minutes, however, however. depencurve. After controller , the ding on the setting of the controller, assistance ass i stance to the remote r emote areas would be autoautomatically withdrawn, creating a further departure in the already off-schedule frequency. Altogether, since remote load and generation changes were occurring all the time, time , this resulted in excessive tie line hunt i ng between the Twin Branch area and hunting other areas, areaa , a totally total ly unacceptable situation. tion .

r eflection that what was It aeemed seemed to me on reflection needed was waa to retain the bias response reaponse of the controller, but to delete the bias withdrawal action •. . This would permit the bias assletance to the remote area to t o continue assistance r e~o te area or uninterruptedly until the remote areas did what had to be done by generation changes within their areas. Twin Branch would not move back toward constant tie line control with all of its limitations, li~itations , while frequency f r equency remained off normal. When the frequency did move back toward t oward normal as remote areas a r eas made the requisite changes ~hanges to their generation, the bias controller ~ontroller would, in accordance with its bias characteristic, c haracteristic, re~ove assistance asaistance that was no smoothly remove longer required. The area would be back on normal tie line schedule achedule when frequency had been returned by the remote areas areaa to normal. normal . It seemed aeemed to me that such cooperation with wIth prevailing system needs prevaiUng ayatelll needa should ahould have higher priority than the area desire to t o maintain constant tie line transfer. tranafer. Such an arrangement, I felt, would lead to t o stable and non-hunting operat i on for the area. a rea. non- hunti ng operation We accordingly arranged to rebuild rebuil d the controller to remove the bias withdrawal function. done,, it worked, as prediction . With that done ted. It was waa another a nothet example exaDlple of using the system as aa our test tes t simulator. simulato r.

Tie Line Bias Control With Sustained Bias

auatained bias biaa ddid i d thus prove to be the A sustained solution solu tion to the problem. Thereafter all of the tie line controllers ~ontrollers we built, to this day biaa type. day,, have been of the suatained sustained bias

At the end of 1936 1I was transferred from frolll the L&N San Francisco Office to Chicago. It fell to my lot in 1937 to place into service the tie line bias control at Twin Branch StaStation. I was joined there by Clark Nichols Nichola from Company Headquarters, who was familiar famillsr with the two earlier installations at New England Power and Carolina Power & Light.

The control characteristic ~haracteriatl c of this controller then became beca..e sketch (f) (t) of Fig. 1 with the dash daah line and the time-dependent arrow removed. We were not yet at sketch re~oved. (g) Fig. 1, I , however, since s i nce we were on a "cascade" "caacade" not a "net interchange" interc hange" basis, basis , and a central station was ~entral frequency regulating regulsting atation waa still atil l in use. uae.

Evolution rime Contro Controll Applications I::vo lution of Real Real Time Net Interchange Versus eascade Cascade In relating the transition to sustained bias at Twin 8ranch, Branch, I think it is appropriate to identify one of the pioneers pioneera of that period who con contributed tributed significantly to the subsequent expanded use of bias control in In the middle west. 1 I refer to Jack Girard, at that time, and for many years thereafter, thereafter , the Operations Operationa Chief at Indiana and Michigan Electric. Girard had a clear concept of the potential benefits of interconnected operation, and in preparing new contracts with neighbors neighbor& he stipulated their use of tie line bias control to optilllze optimize reliability rel1abil1ty and economy. For many years after our initial meeting at Twin Branch 1I met regularly with Girard at analyze operations Twin Branch 8ranch or Chicago to analyte and explore new and expanded needs. We sent many restaurant tablecloths to the laundry laden with exploratory and tutorial sketches.

193ij one of the I&M interconnected neighIn 1938 bors, bora, Public Service Co. of Indiana, was to install tie line bias biaa control at the company's Dresser Station. They had six Rboundary- ties with 16M, "boundary" I&M, Cincinnati G&E, Louisville G&E and Northern Indiana Public queation arose as to which tie Service. The question or ties should serve as the basis basia for the control. cont r ol. This was resolved by Girard and Joe Trainor, Operating Chief of PSC of I. They decided to take the net of all six ties. I was invited to layout the control system sys tem and accordingly arranged for fo r Dresser to operate on a basis of area net interchange with all neighbors. I believe this was the first use of net interchange bias control on as multiple tie aarea. rea. At this thia time I&M I&H was regulating on the basis bas is of their two ties to t o the frequency regulating area, Ohio Power. In effect. effect, all areas beyond their own were, from the point of view of control at Twin Branch, Branch. effectively a part of the I&M area. The postulate defining this was simply to draw a line through whatever ties were serving the bias control and then complete complet e the line as a "circle" -c ircle" back on itself itaelf without crossing any ties.. All of the system other ties .ystem encompassed in .uch such a "ci "circle" r cle" constituted the area for which whi Ch that particular parti cular company was regulating. Where companies within this enlarged "circle" "circ le- were themselves themaelves regulating, such as PSC of I and Commonwealth C01nlllOnwealth Edison for the I&M I&H case, case. they were in effect independent control sreas areas within the overall larger I&M area. Any deficiency, def i ciency, however, howeve r, in their fulfilling their regulating responsibilities would become a regulating burden for I&M. I&H. Further, the bias setting for I&M 16M would have to include, include. in addition to a bias biaS for itself, the summation of bias settings of all of these additional areas, a procedure referred to as "cascad "cascading". ingR .

9

A ar ea would be A preferred technique for each area to net all of its own boundary ties and set ita own bias independently of the bias setits ita neighbors tings of its neighbors.. With this practice, each area would follow its own load swings. It would depart from its net interchange schedule when there were frequency deviations to provide assistance to areas in need in an amount ~ount related to its own responsibilisnd not the cumulative assistance due ties, and from itself and from frOlll its self-contained control areas. Girard agreed that there would be potential improvement to I&M I&H operations with such arrangements. In 1942 I&M arranged arrsnged to have snd integrated additional telemetering added and with the bias controller to shift Twin Branch operation "caacade" to net interope r ation from "cascade" change control. cont rol. As AB time went on, such auch operation became standard throughout the United Uni ted States and Canada for multiple-tie areas. INTERCONNECTED SYSTEM CONTROL --FULLY DISTRIBUTED AREA CONTROL Just as stimulated interconaa World War I had sti-ulated nections to serve the power needs of the time, so World War 11, on a much larger acale , caused expansion and extension of the scale, then existent interconnections. The expansions in the Southwest and their use of net interchange tie line bias bisa control are sre described (1945).. sc ribed by Morehouse (1945) "Midwest Interconnection", later known The "H.1dwest as the Interconnected Systema Systems Group (ISG), (ISG). had also alao grown and expanded. Philo Station of Ohio Power continued to t o provide pr ovide the master frequency regulating regulati ng function. funct ion. Its control area, ar ea, in effect, was waa the entire interconnection. Generation variations on the station were great. IInn addition to picking up Ohio Power load l oad variations, variationa, it would endeavor to absorb the regulating deficiencies of all of the other areas a reas of the considerably expanded interconnection. interconnection . While I had no direct contacts with Philo, my friends at I&M, I&H. a sister company of Ohio Power, Powe r, reported that the normal regulating assignment at st Philo was plus or minus 80,000 80 ,000 kW, and that frequently station generation would fluctuate beyond these theae high and low extremes and correspondingly interrupt frequency control. We discussed this aituation situation a lot at I&M. I&H. It seemed to me that the solution aolution rested reated in removing frequency control from Ohio Power, and placing it like all of the other areas of the interconnection, on net interchange bias biaa control. control . In other words, words. it seemed quite in order to use distributed bias control throughout the system .y'tem rather than on all areas area, but one, and simply have no master frequency regulating station. station . My Hy friends at I&M felt that t hat this might someday come, but that first Ohio and others would have to be convinced that system frequency could be maintained at 60 Hz Kz even if no station atation had the assignment asaignment to achieve this.

10 IQ

N. Cohn

anslYles to t o demonstrate, to I had developed analyses my satisfaction,, that 60 cycles would .y own satisfaction auto_tically result if certain criteria automatically •• , were followed. For all participating are areas, power flow on each boundary tie should be metered at a common point and telemetered tele~tered to one of the two involved area. Nout R areas 88 as power "out" and to the other as aa power "in", ''In''. so 80 that the algebraic sum Bum of net interchanges would would be zero. zero . Similarly, schedules between each pair of areas should be of equal magnitude lgn, 80 ln summation but of opposite asign, so that in area. the net interchange schedules for al alll areas would .1so also be equal to zero. zero . With these criteria criterla fulfilled, fulfilled , and with If1 th all areas are •• regulating eeffectively, ffectively, system frequency would auto_tlcally automatically be maintained at 60 Hz Kz and individual area a rea net interchange schedules would be met. ~t.

Shouldd one or more of the Shoul t he areas not regulate effectively, there would be a corresponding departure of frequency from normal, and all re_ining areas, acting act ing on their respective remaining u acteristics, would automatically auto_tically bias ch, characteristics, shift s hift their net interchange schedules and assist the area or areas then in need. Assistance from each of these areas would be Asaistance a function of the area bias setting and the frequency fr equency deviation and would persist on a sustained areas at sus tained basis until the area or areaa fault adjusted generation to restore frefrequency to normal. I was located at that time in Chicago, and although there was skepticism skepticis~ on the part of some of my associates at Headquarters in Philadelphia that such an arrangement was practical, pr actical, I felt that the technique of no master frequency control would work, and anxiously awaited an opportunity to try it it out. An apparent opportunity arose in the Chicago area in Chicsgo i n 1944 when we were invited to to recommend re co_nd control arrangements for interCarnegie-Illinois steel mills change between Carnegie-Illtnois at Gary and South Chicago Works, but the control project did not materialize. materi al1te.

The following year, yesr, 1945, responding to an inquiry from Iowa & Light at Des I owa Power & concerning Moines concer ning a prospective tie with Iowa Co.. at Cedar I owa Electric Light Ligh t and Power Co Rapids, a letter was prepared prepsred and sent by the L&N Chicsgo Chicago Office recommending tie line bias control at each end, outUning outlining the benefits of having neither nei the r area operating on constant constsnt frequency control and defining such fin i ng the criteria that would make auch operation satisfactory. satiafacto ry. That project was deferred. The following year, 1946, however, a major maj or new interconnection, identified as the United Pool, later designated Interchange Power Servicea, Services, Inc., Inc ., was planned plsnned to include inc lude Iowa I owa Power & Light at Des Dea Moines, Hoines , IowalowaIllinois Gas & Electric Company, Kansas City Illinoia Power & Light Light,, and St St.. Joseph Light & Power Company. I was invited to meet with reprer epresentatives of these companies and of United Light & Railway Se Service rvice Company which was to

do the engineering for the project, pr oject , to recomme nd telemetering teleme t ering and control equipment for mend the interconnection, and of course did ao so.. t o apply appl y the new concept of An opportunity to no central frequency frequen cy controlling con trol ling station had arrived. I recommended fully distributed diatributed frequency frequen cy biased net interchange interchsnge control for each of the four control areas areas.. In discuss discussions ions with representat i ves of the several participating representatives companies, I described desc ribed the way in which the proposed equipment would work and why no master frequency regulating regula ting area would be required. required . Equipment to operate in accoraccordance with these concepts was ordered in 1947, and initial operation occurred in late 1948. Operations were fully in accord with expectations. We now now had operation oper at ion in Fi g . 1. I. accordance with sketch (g) of Fig.

1I would like to point out that in that same genera l time ti~ period Brandt 8randt (1947) ( 1947) described general r i ved considerations considerat i ons for the t he independently de derived same sa~ concept of eliminating the master I113ster frequency regulating area. ares . The subsequent s ubsequent application of his ideas idess to the Northeast interconnection is i s described in McCormack HcCormack and Metcalf (1949) (1949).. In Cohn (1950), I reviewed and analyzed inter-area inte r-area control techniques that had until then been developed and used, summarizing s ummarizing their characteristics and limitations. limitations . Incombinations of a master ccluded luded were the co~binations mas ter frequency f r equency control in one area a rea and constant tie line controls in other areas areas,, a master frequency control in one area a rea and frequency f requenc y biased net interchange inte r change controls in other othe r areas, and finally, distributed frequency biased net interchange cont control r ol in all areas Without without a master Cllaster frequency regulating area. area . f reque ncy biased net The fully distributed frequency interchange i nterchange control technique in all areas, without a central frequency regulating area, area , has, has , for close to t o 35 years, been the standard inter-area control practice on all USCanada interconnected SYBtema. systems. MAGNITUDE HAGNInrDE OF BIAS SETTINGS With the basic technique of frequency biased net interchange tie line control in all areas well established, questions arose in various varioua operating areas as to just what the proper magnitude of area bias settings sshould hould be. In Cohn (1950), ( 19 50) , I had suggested that the settings se t tings be equal to the area govergove rning characteristic characteristic.. Questions developed concerning methods of determining this characteristic. were also questions charact eristic. There vere related to its variability, and to the effects ef fects of the frequency frequen cy coefficient of area load. It was reported that there t he re were strong differences of opinion within ISG on the proper magnitude of bias settings, culcul minating in an unresolved debate deba te at the t he group's group ' a 1955 annual meeting ~eeting in Corpus Christi.. To help resolve disagreement,, ChrisU resol ve the disagreement the Interconnected Interconnec t e d Systems Committee (ISC)

Contro l Applications Evolution of Real Time Control Test Committee headed by L. V. Leonard of PSC of Indiana was asked aaked to investigate the 1I.atter and report to the ISG 1956 annual matter meeting !Deering scheduled for Des Moines. Hoines. The coamittee committee invited representatives represen tatives from froa four manufacturing companies to present meet ing in CincinCincintheir views at a two-day meeting nati. nat1. I1 was invited to speak for L&N UN and ao . At the end of the Cincinnati Cincinnatt sessesdid so. sions I was asked aaked by A. L. Richmond RichMOnd of Ohio Edison, Chairman of lSC, ISC, to "save my lily notes", and present the same aalll! analysis to the entire ISC group at Des Moines. Moines . This latter presentation is transcribed in Cohn (1956a) and formali%ed and expanded into an AlEE AIEE was formalized paper, paper. Cohn (1956b). (1956b) . Equations developed in the analysis analyais demonsettinga lower than the comcomstrated that settings bined governor-load governing characterischaracteristics resulted reaulted in undesirable withdrawal of assistance aaaistance to areas in need. Such withdrawal is appreciably greater in relative aasistance that magnitude than additional assistance would be provided if aettinga settings were above the combined governing characteristic. charac teriatic. Clearly, a setting higher than the anticipated governor-load governing characteristic characteriatic is much more .ore preferable, and system cooperative, characteristic . than a setting below the characteristic. The Mollman discuasion of this AlEE AIEE paper Hol lman discussion included an analysis, analysis , based bsaed on the paper's equations, equations , of a then recent large load drop. It showed that system bias settings averaged about one-half the system governing characteristic, teriatic , accounting for the observed adverse response of the system ayatem to the load drop. The Des Moines basis Haines presentation provided aabasia for the operating guideline adopted by ISC at that meeting. The guideline provided that area bias setting se tting be at least as aa high as the area governing gove rning characteristic characteriatic at peak load, load , and not less leas than 1%. Most Host of the time, therefore, the refore, bias settings aettinga would be higher than the area governing characteristic. tic . This same guideline later became a part of the North American Power Systems Interconnection Committee (NAPSIC) Operating Manual, successor Hanual , and is now a part of the aucceasor North American Electric Reliability Council Counci l Operating Manual, NERC (1982). INADVERTENT INTERCHANGE AND TIME DEVIATION When one or more areas ares, do not fulfi11 fulfill their regulating obligations, obligations , there will be frequency deviations deviationa and net interchange schedule deviations consistent cons istent with the bias biaa characteristics characteristi cs of the assisting aaaiating areas. There will be corresponding accumulations accUlllulationa of inadvertent interchange for both the offending areas and the assisting aaaiating areas, areaa, as aa well as an accumulation of system aystelll time deviation. Techniques for correcting for these theae deviations have been developed over the years year s and present practices are summarized aumma ri%ed in NERC HERC (1982).

I11I

General practice insofar as aa bulk power transfera are concerned is to pay for scheduled fers transfers tranafers only, and pay back inadvertent peak- or "off accumulations in kind at "on peak" perioda consistent conaistent with the period of peak" periods their respective accumulations. Time deviations are corrected by appropriate frequency schedule offset, of het, with participation by all areas. a reas. Difficulties have been encountered at times in some aspects aapects of the present payback procedures and interest intereat has haa been expressed by some operators in a technique of payback in dollars. It has been felt that this would be an incentive for better area regulation. Equitable payback in dollars dollar. would require a technique for determining how much of each area's inadvertent interchange had been caused cauaed by itself and how much by each of the other areas of the interconnection. i nterconnect i on . Such a technique has not been available. In Cohn (1971a) there are developed equations to define the area-caused area-cauaed components of system aystem time deviation and of each area's area ' s inadvertent interchange. These Theae equations have not had practical use since they include nonmeasur able parameters. More recently, howmeasurable ever, eve r, in Cohn (1982), (1982) , these equations have been more fully developed and their understanding atanding expanded. expanded . They now contain only known or measurable parameters. They can be used as aa a basis for a penalty/reward, credit/ debit dollar payback technique for inadvertent interchange, and also for unilateral correction of area-caused time deviation and inadvertent interchange to replace present NERC NERe corrective techniques. techniquea. These new techniques, niquea, not yet supported aupported by practical use, will not be further discussed discuased in this thia presentation. They will be separately summarized in Cohn (1983) to be presented in the Carpentier invited session aeasion on Real Time Applications later in this thia symposium. symposium . AREA CONTROL CONtROL --ECONOMY DISPATCH DISPATCll Thus far, this presentation has concerned itself primarily primari ly with bulk power and energy transfers between areas of an interconnected system. aystelll. Maintenance Kaintenance of bulk power transfer schedules while While area load l oad is varying, and simultaneously silllultaneously contributing to areas in need, as programmed pr ogrammed by the bias characterischaracteriatic tie when system frequency departs from normal, dictate the total generation requirement III!nt for an area. The next step is to distribute the total required generation among alternative available area sources to optimize economy and fulfi11 fulfil1 other operating criteria such as aa security and environmental obligations. Techniques for computing desired generation distribution diatribution within a control area, taking into consideration incremental efficiencies of units, fuel costs coats and transmission losses, were developed over the years by a

12

N. Cohn

number of individuals, individual. , aB as were techniques for controlling plants and units to achieve these theae desired generation levels. I'll I ' ll identify some IOIM! of these theae contributions. Transmission Teaoami •• lon Loss Lo •• Computation

Refe r ence. de.c r ibing contributions to deterReferences describing mining incremental coat po~er delivered cost of power from sources f r om alternative al ternat i ve sour ces include Steinberg 6.& George (1943);; War Ward, Smith (1943); Gent ge (1943) d , Eaton & 6. (1951 , Hale (1950) (1950);; Kirchmayer 6.& Stagg (1951, FerguBon, Jacobs 6.& Harker 1952); Harder, Ferguson, (1954), War_on (1956); Watson War_on & 6. (1954); Early 6.& Watson Se.dUn Stad1in (1959) (1959).. These and related contribucontributions provided a basis ba, is for achieving optimum econoay by loading stations 8tatlon8 to [0 equal costs economy of power delivered, i.e. i . e. equal -lambd."lambda".. In the years yeara up to about 1950, general practice was for comp_oiea companies to prepare charta charts or tables or their equivalent to guide operaoperatablea tors tora in achieving, generally with manual control,, the deaired desired generation allocationa. allocations. control On-line operation of multiple-plant control aoon followed . soon followed. Multiple Mult iple Station Control With One Station Refe r ence Ser ving As The Reference Serving The earliest used for multiple earlieat technique techniq ue uaed mult i ple a t at i on cont r ol was waa based on proportioning propo r tioning station control the load dis distribution tr ibu tion between stations in in ~nner that t hat had previously pr eviously been much the t he manner used for proportioning propo rt ioning load l oad between units. One station served as the "master" "master" in in receiving rece i ving control control pulses from f r om the central cent ral dispatching additionally di spatching office and served addit i onal ly as the aa t he reference against agai nst which other othe r plants were loaded. loaded . Other stations stat i ons received control master in pre-set t rol pulses to follow the maater patterns. The central load dispatcher had load~ and manually set adjustments for -baae "base load" " r at i o~ for fo r each source and uaed "ratio" used these adjustments to establish the loading relationrelation"follow" plants to the master ship of the "followmaater plant. Initial installations, installationa , about 1950, included Virginia Electric Power and Detroit Edison. des-Edlson . The latter installation insullation is des Campbell (1952) (1952).. cribed in CSIIpbell Multiple M ul t iple Station Control With Area Required Generation As Ceneration Aa The Reference

A reference of total area required generation, developed with feedforward from the parameter area control requirement (later (later renamed area control error error,, or ACE) and a feedbsck from total area a r ea generation would feedback gener ation and represent total required generation baae would be a auperior superior reference. With base setter s it would point and participation setters provide full and completely flexible ranges plant . Furthermore, Furthermor e , for the loading of each plant. refe rence would elithe feedforward/feedback reference minate iinteraction nteraction between plants planta regardless of their respective rates of response to control assignments. aaaignments. This technique, identified as Area-Wide Ceneration Control and described in Cohn Generation (1953), was installed at all three campanies companies well . Many simiin 1952 and performed very well. elaewhere lar systeals systems have been installed elsewhere over the year. years.. An interesting installation for the regulation of the Columbia River hydro aystem system is described in Benson, ia deacribed Senson , Johannson & McNair (1963) (1963)..

ita feedforward-feedback feed forwa r d-feedback reference From its circuit , there is ia derived a unique feature, feature , circuit, later used for both analog and digital decontrols. Compusired generation computer control s. Computed area control error can be automatically decomposed into source control errors erro r s whose SUIII is equal to the t he area control error. The sum economy dispatch computer then searches for fo r source algebraic aource control errors whose algeb r aic sum matches prevailing area control error. These source control eerrors Theae aource r rors are a r e then used uaed for non-interactive source control control to achieve ~ero zero area control errors er r ors and economy dispatch diapa t ch for prevailing pr evailing area generation. generation . A summary of control techniques techniquea prevalent in this thia period is contained in Nichols ((1953). 1953) . The Early Bird In 1953, 1953 , Don Early of Southern Services devised an on-line computer capable of propr oviding continuoua continuous real-time data on the cost of power delivered from various operating sources.. An operating unit is described in sourcea Early, Phillips (1955).. It was an Phillipa & Shreve (1955) important forward step in economy dispatch.. econo~y dispatch Desired Ceneration Generation Computer

In 1951, Union Electric, Illinois Power, Power , and conCentral Illinois Public Service Company contracted electritrac t ed to supply large amounts of elect r ical energy to t o the atomic energy plant in the r ea . I was invited to meet with Kentucky aarea. representatives rep r esenta t ivea of their engineering and operating departments depa r tments to layout control cont r ol to satisfy sat i sfy the new new operating needs. needs . It was waa planned to make a transition, transition , in a single step at each company company,, from relatively little atep control (although Union Electric already had extensive telemetering) to full scale on-line control for virtually vi r tually all of each area's units. master station within srea'a units . Using a lII8ster the area as aa a reference as aa on previous previoua iinnlimitations.. Allostallations had, had , I felt, lillitationa cation cat i on assignment aaaignment ranges were limited, limit ed, and interaction between atationa stations was probable probable..

The next step in the advance to complete automatic economy dispatch was proposed by Miller (1954), who suggested the combination of the onon-line line cost of power delivered computations of Early Ea r ly with the feedforward-feedfeedforward-feedback allocation computations of Cohn Cohn.. He on-line thereby provided pr ovided an on-l i ne multi-station multi-sta t ion desired generation control system serving deaired servi ng the purposes of both area control and econamy dispatch complete with real-time r eal- time nomy computation incremental transmission loss comput ation for operating aources sources.. Many such units were installed, inatalled, one of which installed at the Co.. is Central Illinois Public Service Co (1965).. described in Derks Derka and Preston (1965)

Evolution of ns of Real Time Control Control Applicatio Applications Dispatch Lambda Dispatch

In 1961 the Pennsylvania-New Pennsylvania-New Jersey-Karyland Jersey-Maryland 12-company system, that hsd had operated (pJM) 12-company then as sn an isolated single area with until then free flowing ties between companies, planned establish permanent permanent ties north to the New to establish and York, New England, and Canada system snd west to the Interconnected Syatems Systems Group Group.. weat PJM had not used used,, nor from their point of needed,, automatic ffrequency view needed r equency control during its many years of operation as an independent single area system. Operating practice pr actice had been to manually dispatch a desired incremental des ired increme ntal cost of power delivered (system lambda) to each of its several operating opera ti ng entities who in i n turn would adjust the generation sources of their respecti ve respective systems to operate at this common lambda value, and thereby achieve optimum operating economy for the pool. From time to time, on instructions inatructions from the control center at Philadelphia, Philadelphia , manual adsystem justments would be made to sys tem frequency to keep system time within desired limits considered adequate. adequate .

Automatic area control was now to be instslled which would utilize the by then well stalled developed frequency biased net interchange technique for fo r inter-area bulk power transtrsnsfers. In addition, it was desired to supplement ssuch uch control with a system lambda dispatch which would closely emulate, automatically, the previously prevailing manually I118nually system. operated dispatch syst em. I was invited to suggest su~~est suitable control arrangements. arrangements . Data on total generation gene ration at each of the companies was available at the control center. center . There it was arranged to be combined with feedforward feed forward from prevailing prevsiling area srea control error e rr or to provide the parameter total pool required generation. r equired gener a tio n . An adjustable function generator was pre-programmed to to simulate, from availsble available pool data, dsta , the relationship between total required generagener ation and approximately the necessary necesssry lambda to achieve it. it . This initial value of lambda, lsmbda, itself closely c l osely matching the real resl need, was automatically refined to the precisely pre cisely requisite value by control action of the prevailing area control error. error . The computed lambda was continuously broadcast brosdcast to all participating companies which, it was intended, would install distributed station and unit control cont r ol of their own choices to maintain their own respective generation generat i o n outputs at the broadcast common lambda value. Area control error was forced forced to zero by lambda signal initiated station generation generat i on responses, with feedback feedbaCk from the boundary tie lines and system frequency. frequency . In this way, loIay, the pool would loIOuld fulfill fulfi11 its obligations to its interconnected neighbors, while IoIhl1e achieving optimum op timum economy for for its it s own internal internal operations. operat i ons. This basic basic lambda disdiapatch system is described in Cohn (1962). (1962) .

]3 13

It performed r the rs . Though performed loIell well ove over the yea years. now replaced with with digital equipment, it has been retained for standby standby use at the pJH PJM Valley Forge Control r. Control Cente Center. Advances in Telemetering I should note that paralleling the progreas progress in area control systems, there were advances 1n in telemetering, permitting ready data monitoring and collection over the greater disdistances involved. My own experiences were largely with impulse impulse,, impulse duration, and frequency techniques developed primarily by r oE. D. Doyle, W. E. Phlll1ps Phillips and J. B. Ca CaroIus, and utilizing microwave transmission. Control Execution Before going on to the last of the pre-dipre-dir ect digital power system control techrect niques, let me say a few words about control execution. Thus far I have limited my rerevielol of evolution to commarks in this review menting on the levels of total gene ration generation that are required within an area, and on the desired ge neration levels at stations oorr at generation units withi n the aarea. res. That ' s of course within That's only part of the story. Achieving the desired limit limitss by application of control signals is 1s the oother ther and equally important requirement . I will loIill part of the control requirement. comment on three aspects of this, namely, comment processing of the area contro controll signal, supplementing the desired generation control sction with ancillary action when the rate action of response of the desired generation assignment is not adequate to satisfy the area generat ion control demand, and coordinating generation cont rol signals sent to fossil fuel units control with loIith related boiler controls.

contro l signal. Power system Processing the control cont r ol engineers and operators are familiar control with the rapid random variations in system frequen cy and comparably rapid "synchroni»synchronifrequency srea net interchange. These zing swings" in area a rea conare the two parameters that make up area trol error, which correspondingly itself has r elated short-term swings, in the t he order of related seconds , in addition to longer term "sus"susseconds, tained" variations. The rapid swings are cont r ollable , certainly not at the prenot controllable, art . An objective object ive sent state of the control art. in control cont rol operation operat i on is to use suitable suitabl e adapt ive means in processing filtering or adaptive the area control signal to secure a truly effective signal for the initiation of control action. te chnique has found appreciable a ppr e c iable One such technique use . It is the "error "e rror adaptive computer use. controller» (EACC) described in Ross (1966). controller" Assist action. When the units currently ares control signals lack the the receiving area to respond at st a rate adequate to ability to fulfill prevailing area control requirement, assistance from other o ther operating operating units is assistance desirable.

14

Cohn N. eohn

One early technique for securing such assistance wss was designated "base load backup". It wss IoM Twin Branch Station in was first used at I&M 1944, and at Des Dea Moines Holnes Ststion Station of Iowa case, a new Power && Light in 1948. In each csse, efficient generating unit having capability of rapid response was available, but the understandable operating desire wss was to keep it as close as 8a possible to a selected base load load.. This apparent contradiction was resolved by permitting the new unit to act as 88 the basic regulating source. In addition, its load recorder carried contacts which, ita base whenever the unit moved away from its load setting in response to regulation demands, initiated supplementary control unite which it was inaction to the slower units tended should ultimately absorb the regulation. These slower units were moved IDOved up and IDOst rapid rate, down, as required, at their most to provide generation changes that would cause the area controller, recognizing these changes, to move the new unit back toward its base load setting. This was a sort of "hare and turtle" arrangement, which served its purposes very well. A later technique, still in use, is identified as "normal assist action". Assistance from units not otherwise at that '!me time assigned a regulating function, is triggered by deviation of area control control error from zero, either at preset deviation devistion limits or in direct preset proportion to area control error deviation. Descriptions DeacriptionH of such techniques are included in Cohn (1966).

Another early installation was made at Canton Canton,, Ohio for American Electric Power, utilizing for that application an IBM 1710 st AEP's computer. The latter was utilized at request to provide capsbility capability for later adaptation to Direct Digital Control. This installstion stallation is described in Kinghorn, McDaniel and Zimmerman (1965) and Morgan and others ((1965). 1965 ). The step to DDC Doe was indeed aa described in Stagg later taken by AEP AEP,, as and othera others (1967) (1967).. Direct Digital Control I have noted above the adaptation by AEP of Cont rol the Canton Digitally Directed Analog Control rol. As planned in to Direct Digital Cont Control. the introduction to this paper, that is where I will stop with evolutionary history. Ce rtai nly what has occurred since the introCertainly duction of digital computers in electric power systems controls represent modern msgnitude . The many advances of mammoth magnitude. great direct digital control center installations that have been made in recent years and the additional ones currently being made, with their expanded capabilities for l oad flow, contingency studies, atudies, security securit y load determinations and snd many other new applicarelisbility tions add great cubits to the reliability and economy of system operation. As earlier noted, we will hear more of those important snd advanced sdvsnced facets of real time t ime evolution and systema controls later at this in power systems conference.. conference Scheduling Practices

Coordinated control. It was recognized early that it was one thing for the control to demand generation changes from a fossil fuel fired unit. It was another to achieve required ene energy r gy conversions to fulfill the control demand. Early steps included coordinating boiler steam pressure and elements of boiler combustion control with generation control signals. A fully comprehensive coordination technique including tie-ins of the generation control signal with all significant controlled variables of the boiler and its is described in Bristol ita auxiliaries ia extensive use, (1956). It has found extenaive uae, particularly on once-through units. Digitally Directed Analog Control All of the control techniques I have thus far discussed have been of the analog type. With the advent of digital technology and digital computers, interest developed in capability adapting their flexibility and capabi lity to power system control. Reliability and programming technology had not yet, however, been fully developed and a first step was to replace the analog anslog desired generat ion compugeneration tations with digital computation, and use anslog allothe results automatically to set analog cation equipment. A pioneering installation of this type, utilizing an L&N 3000 compu computer ter was made at Detroit Edison in 1961. It is described in Blodgett, Hissey, Falk and Schultz (1962).

Discussions of inter-area practices that had snd been developed and applied for short and long term scheduling of various types of bulk power transfera transfers are contained in Mochon and others other s ((1972). 197 2) .

CONCLUSION I have always regarded the int interconnected erconnected electric power aystem, system, with its hierarchical,, multi-variable, multi-level cal multi- level parameters, its geographically wide ranging extent, its ita parallel operation of literally hundreds of energy sources, its ita extensive and non-linear aelf-regulating self-regulating forces, forcea, and its many objectives of operating economy, security and environmental influences, as a most challenchallencontrol ging systems cont r ol problem, perhaps one of the most challenging of all large scale systems control cont r ol problems. Much has been done in providing operable solutions during the past sixty years, at paces that at times may rettospect , though with seem very slow in retrospect, increasing rapidity in later periods, and with highly accelerated accelersted activity in current digital and microprocessor microprocesaor environments. Although a lot has indeed been done, there t here is much yet to be done. In Hunt (1931), ia (1931) , my old friend from Southern California Edison Company said, "Load frequency control is in a state of evolution". Now, more than fifty years later, I think it very proper to borrow and repeat his phrase with just a slight variation namely, "Power systems concontrol remains in a state of evolution".

Evo lut ion of li cat ions Evolution of Rea Reall Time Contro Controll App Applications Llle ongoing ongoing need to challenge the •..;ll:l. 0..:. "e ...1..::)" ~"e continuing continuing validity of present day day practices of distributed distributed frequency frequency biased net net interof change control r eady in its 35th year, control,, al already need for a dynamic realthere remains the need time on-line computation of generating unit incremental heat rates for better conCurrent concurrent economy dispatch dispatch,, there is a need for IDOre more effective adaptive and predictive technology for greater effectiveness in control execution. And there remains the need of the education of system operators in digital and technology,, and the comparable need control technology software speciaof educating computer and aoftware lists in the technology of power system Who Will will do these things, and operation. \./ha the many others that obviously II can't begin to identify? The late Nobel Laureate, Harold Urey, said it very well well,, in another context,. but equally applicable to Our our context "We must leave something for the field. ··We young people to solve solve.. It would be most disappointing if we older people solved all the problems, problems , which of course none of us do", (Urey, 1976). will ever do··, .L

To youth, then -- with confidence -- my best Wishes for success in the cont inued evoluwishes continued fields . And to tion of these and related fields. age, this admonition: Let them do it. ACKNOWLEDGEMENT II should like to express appreciation to L&N Management who authorized company personnel to assist in the preparation of this paper. I extend particular thanks to S. L. Peirce, Peirce , in whose area the paper was processed, to Eileen Robinson who typed it, and to Adins Adina Zupanick for her assistance with the references.

REFERENCES NOTE: NOTE;

Patent references carry the filing date.

Benson, A. R., Johannson, D. E. and McNair, HcNair , H. D. (1963). (1963) . Centralized load-frequency control for the United States Columbia River power system. system . IEEE Winter Meeting, Meeting , paper CP63-230-.--B10dgett, Blodgett , D. G.; Hissey, T. W.; Fa1k, Falk, A. K. and Schu1tz, Schultz , W. B. s. (1962). Application of an on-line digital computer for dispatch and control of the Detroit Edison system. IEEE Winter Meeting, N.Y., N.Y. , Paper Paper CP62-247. Brandt, Robert Robert (1929). (1929) . Automatic frequency control. Electrical World, 93, No. 8, 385- 388 , 385-388. -Brandt, Robert (1947). Theoretical Theoreticsl approach approsch to speed and tie line control. cont rol. AIEE Trans., Trans ., 66, 24-29. Brandt, Robert Robert (1953). (1953) . Historical approach to speed and tie line l ine control. control . AIEE AI EE Transactions Transsctions Pt. Pt. Ill, Ill, 72, 7-9. Bristol, Bristol , E. E. S. (1956). (I956) . Control systems for electrical generating gene rating units. U. U. S. Patent 2,861,194. 2,861,194.

]5 15

Campbell, W. J. (1952 ) . New system control (1952). allocates changing load among 29 generating units. Electrical Electrical World, ~, 137, No. 1,32-33. 1, 32-33. Csrpentier, Carpentier, Jacques (1983). Digital automatic generation control. IFAC Sympome Digital sium on Real Ti Time Digital Control Applications cations,, Guadalajars, Guadalajara, Hexico Mexico,, January. eohn Cohn,, Nsthan Nathan (1950). Control of power flow on interconnected systems. Proceedings of the American Power Conference Conference,, Chi Chicago 12, 159-175. Electric Light and cago,, ll, Power, 2B, 28, Nos. 8, 9. Cohn, Nathan(l95 3). A new method of areaNathan-(1953). wide generation control for interconnected systems systems.. Proceedings of the American IS, 316 344. Power Conference, Chicago, 15, 316-344. Electric Light and Power, 31, 31: Nos. 77,, 8, 99.. -Cohn, Nathan (1956a). A step-by-step analyanalysis of load-frequency con trol showing the control system regulating responses associated with frequency bias. Minutes of the Annual Meeting of the Interconnected Systems Committee, Des Moines, Hoines, Iowa. tieCohn, Nathan (1956b). ( 1956b) . Some aspects of tieline bias control on interconnected systems. AIEE Trans., ~ 75 Pt. Ill, 1415-1428. Cohn, Nathan Nathsn (1962). Economic Loading of Power Systems Systems.. U U.. S. Patent 3,270,209. Cohn, Nathan ((1966). 19 66). Control of generation and power flow on interconnected sysWiley and Sons, New York. tems. John Wi1ey ""S"eCOnd Second edition 1971. Cohn, Nathan (1971a). Techniques for improving the control of bulk power transfers on interconnected systems. IEEE Trans. on Power Apparatus and SystemS, Syste;;, 2409 2419. PAS-90, 2409-2419. (1971 b). Power system control Cohn, Nathan (1971b). practice. Proceedings of the Ninth Alle rton Conference on Circuit Annual Allerton 719 727. and System Theory, 719-727. (1982) . Decomposition of time Cohn, Nathan (1982). deviation and inadvertent interchange on interconnected systems. I: Identification, separation and measurement of components. 11: Utilization of components for performance evaluation and corrective control. IEEE Trans. on Power Apparatus and Systems, PAS-I0l, No. 5, 5. 1144-1169; No. 8, 2711-2720. Ar ea control perforperforCohn, Nathan (1983). Area cor rective conmance measurement and corrective trol in interconnected systems. IFAC Symposium on Real Time Digital Control Guadalajara, Mexico, Applications, Guada1ajara, January. ( 1979). A Stroke of Luck. Conot, Robert (1979). Seaview Books, N.Y. A. and Preston, Preston , E. H. H. (1965). ( 1965). Derks, R. A. Cen tral Unique operating features of the Central Illinois Public Public Service Co. dispatch Proceedings of the the computer system. Proceedings 27, American Power Conference, Chicago, 27, 1111-1118. --111l - ll18. Doyle, E. D. D. (1928). (1928) . System of distribution. distribution. Doyle, Patent 2,054,121. 2 ,054,121 . U. S. Patent

16

N. Cohn

Doyle, Doyle. E. D. (1929). ApparatuI Apparatus for controlling load l oad distribution. U. S. Patent Re. 20,548. 20, 548. Early, E. D., Phillips, Shreve, Phl11ipa, W. E., E o, and Shreve. W. T. (1955). An incremental inc remental cost of AIEl!: Trans. Tranl. power delivered computer. AIEE Pt. Ill, 74, 529-535. Watlon, R. E. E . ((1956). 1956). A Early, E. D. and Watson, ~ethod of determining deteraining conscaotl new method constants for the general loss gene r al transmission 10 •• equation. equatton . AIEl!: Tranuctions Pt . Ill. AIEE Transactions Pt. Ill, 74, 1417-1423. Electrical West wut (1931). Report-of Reponof 8e8l100 session on [ranamia.ion transmission and distribution, AIEE T8hoe ."'"'6'77 Pacific tou Coastt Meeting, Lake Tahoe,~ No. 4, 194-195. 194 195. S.. (1931). Fitch, H. S (l9JI) . The Pennsylvania-OhioPennaylva nia-OhioWeat i nte r connect i on . PresenPreaenWest Virginia interconnection. red at the che AIEE AIEl!: Summer Convention, AshAahted viIIe , N. C. , June. June . ville, N.C., George, George. E. E. (1943). (1943) . Intrasystem In trasyeceIII transmistranslliasion losses.. AIEE 62,, 153-158. alon 106181 AIEl!: Trans., 62 153-158 . Harder, E. L. and others (954):(1954):- Loss La . . evaluation, 11 - current-power-foTa current-power-form loss loas AIEl!: Tranll., 73 , 716-73l. formulae. AIEE Trans., 73, 716-731. Heath, L. o. (1929). Apparat;;'a Appara~s for speed Ipeed control. U. S. Patent Re. 19,157. Henry, R. T. and others otherl ((1929). 1929 ). Symposium Sympollum on frequency control. control . Annual meeting of the Electrical Section Section,, Empire State Gas and Electric Association, Briarcliff Manor, N.Y., December 6. Humphrey, G. S. (1927). The interconnection of power Iys systems tems surrounding lurrounding the PittsPlttlburgh area. The Electric Journal, XXIV, 25 1-258 . 251-258. ---Hunt, L. L . F. (1930). Automatic Automatic. control in hydro-electric plants. plantl. Electrical West, Welt, 64, No. 6, 6 , 337-354. Hydro-Electric Commission of Ontario HydrO=Electric Power Powe r Commllsion (1926).. The Toronto load totllizer totalizer.. (1926) The company Bulletin, October. Courtesy the late P. A. Borden. IEEE SYltems Systems Controls Subcommittee ((1977). 1977). Bibliography on automatic sutomatic generation control and boiler-turbine-gove boiler-turbine-governor rnor controls. 1971-74. 77CHI211-2-PWR 77CH1211-2-PWR IEEE Working Group (1981). (1981) . Description Delcription and bibliography of major economy-security economY-lecurity functions. IEEE Trana Trans.. on Power ApparaApparatus and Systems, PAS-100, PAS-lOO, 211-235. 211 - 235. IEEE Power Engineering SOCiety Society (1982). ( 1982). Pearl Street. IEEE Center for the History of Electrical Engineering. Insull, Samuel (1921). Speech at Peoria, Illinois. Purdue University Collection. I Courtesyy Prof. M. E. Van Valkenburg, U. Courtel of Illinois. Hl1noh. Jollyman, J. J . P. ((1927). 1927 ) . The speed control system. The Electric of a transmission tranlmllsion syltem. Journal, XXIV, XXIV , 250-251. Kerr, S. L. (1930).. Autolllltic Automatic operator for L . (1930) economy control. cont r ol. Presented at the AIEE Middle Eastern District Meeting NO.~ No.~ Philadelphia, Philldelphia, October. Oct ober. Kinghorn, Kinghorn , J. H.; McDaniel, HcOaniel, G. H. and snd Zimmerman, ma n, C. P. (1965). Development of coorcoordination and control of generation generat i on and power flow on American Electric Elect ri c Power System.. Proceedings of the American SYltem Power Conference, ~, 27, 1068-1078.

Kirchmayer, Stagg, G. W. Kirchlll8yer, L. K., and Ind SUgg, w. (1951). (I95I). Analysis losses Analyaia of total and incremental 10lsea in transmission Trans., tranamlllion systems. syltemll . AIEE Tranl ., 70, 1197-1204. Kirchmayer, K., Kirchmayer , L. K ., and Stagg, G. W. (1952). Evaluation Evalultion of methods methodl of coordinating incremental fuel COlltl costs and incremental tranl1l111ion 101sel. Trans . Pt. transmission losses. AlEE AIEE Trans. Ill, Ill , 71, 513-521. LeedS:-M.~. Leed"-H.E. (1912). (1912) . Electrical recorder. U. S. Patent 1,125,699. Lincoln, P. M. H. (929). (1929). Totalhing Totalizing of electric system loads. Trans.,, Pt. Ill, loadl . AIEE Trans. 48 , 775-782. 775- 782. 48, McCoriack, HcCOrm.Ck, J. E. and Metcalf, Met cdf , C. N. (1949). Load-frequency Load- frequency control of the Northeast interconnection. Electric Light and Power, Power , 24, 24 , February, February . 70-73. 70-73 . McDaniel, dis(1914). Evolution of dhHc:Danie1, G:-H. G:-H. (1974). techniquea and facilities. Prepatch techniques sented lented to t o a meeting of the IEEE Power SYltess Engineer ing Committee, Scotts Systems Engineering Scotts13 . dale, Az., Nov. 13. othen (1932). Operating McNair, J. S. and others e.periencel with the "Automatic Operaexperiences tor" tor in the Northwest. Presented to the Engineering Section Meeting of the Northwest Electric Light and Power Association, Portland, Oregon, Or egon, April iJ-1 5. 13-15. Miller, W. W. G., G. , Jr. (1954). Generation control system. system . U. S. Patent 2,836,731. Mochon and othera others (1972). Symposium Sympolium on scheduling and billing of bulk power Icheduling transfers. Proceedings of the American Power Conference, Chicago, 34, 904-967. Morehouse, S. B. (1945). Inter-system power coordination in the Southwest region. Electric Light and Power, 23, December, 62-68, 72, 105. 105 . Morehouse, hhtorical Morehoule, S. B. (1965). Some historical highlights in the operation of electric utility sYltems systems on an interconnected interconnect ed balis. Presented at and coordinated basis. the Annual Joint Meeting of the Northeast ealt Regional Committee of the InterSystem connected Systems Group and the Syatem Operation Committee Commi ttee of the Pa. Electric Association, A'lociation, Pittsburgh, Pittlburgh , Pa. Morgan, W. S. and othen others (1965). Facilities Facil1t1e1 for the American Electric Power System production and control center. Proceedings of the American Power Conference, 27, 1079-1085. NERC-r1981). Map of 230 kV and above transmission lines, power stations and substasubs tations. North American Electric Reliability Council, Princeton, N.J. NERC (1982). Operating manual. North American Electric Reliability Council, Princeton, N.J. Ni cho Is , C. (1953). Techniques in handling Nicholl, ( load-regulating load-regulat ing problems problema on interconnected power systems. Transactions sYltems . AIEE Transac tion. Part Ill, 72, 447-460. Preminger, J. (1960). TT960) . Bibliography Bibliogrsphy of load and frequency control literature 19221957. Gene ral Meeting, Paper 1957 . AIEE Winter General CP 60-489. Purcell, T. E. and Tieand. Powel, C. A. (1931). Tieline control of interconnected networks. netwOrka. AIEE Trana. Trans. Pt. 2l, 40-50. Pt . Ill, 1!., M

]177

Evolution of Real Time Control Applications Ross adaptive RoIlS.t C. W. W. (1966). ( 1966 ) . Error Err or adapt ive control cont r ol computer co mput e r for f or interconnected power systems. IEEE Trans. oonn Power Apparatus and Systems .t PAS-BSt PAS 85 . 742-749. 742- 7'9 . Sporn M. ((1932). Spo rn,t Philip Ph1l1p and Marquis Harqu is.t V. H. 1932 ) . FrequencYt Frequency. time and load control cont rol on ini nter co nn ected systems. Electrical Elect r ical Wo r ld t, terconnected World Harch 12 t, 495-500; April 2 t, 618-624. 6 18- 624 . March Stagg t, G. W. W. and snd others (1967). On-line comc omputer optimizes optlm i zes loading of 38 J8 generators. Instrumentation TechnologYt No.. It Technology . ~t ~. No I, 31 34 . 31-34. St e1 nberg,t M. H. J. and Smith, 943) . Steinberg Smith t T. H. (1 (1943). Economy loading of power plants and elecelec tric systems. WileYt tri c systema. WH ey , New New York. Urey, H. c. GeochiUl t, CosmochiGl. UreYt C. (976) (1976).. Geochim Cosmochim. Acta t 40 t, 570. ~,~

U. S S.. Dept. of Energy (1981). Map, Kap . North No r th American interconnected control areas. a r e ••• Wa rd, J. B. and others (1950). Total To tal and ininWard, remental losses lossel in power powe r transmission transm1lsion ccremental netwo r ks. AIEE AI EE Trans., Trens., 69, 69 , 626632. networks. 626-632. Watso n t, R. E. and Stadlin, Stadl1n , W.lD. W.O. (1959). Watson Calculation incremental Cal culat i on of inc remental transmission t ransmission losses and a nd general transmission transDlission loss l oss equation. lIlt equat i on . AIEE Trans., Trana . , 78, Pt. 11 1, 12-18. 12 -1 8 . Williams, Will1ams . A. J., Jr. Jr . and Morehouse, Ho rehouse , S. S . B. ( 193 5) . Electrical generating system. systeDl. (1935). U. S. Patent 22~t 124,725. 1 24.725. Wunsch (1925). Wunsch,t Felix Felb (19 25 . System SysteDl of of frequency f r equency control. oorr speed measurement measur ement and co ntr ol . U. s. S. 1 , 751,538 and 1,751,539. 1,751 , 539 . Patents 1,751,538 (b) •

( a)

LOWER FREQ

L OWER LOWER

RAISE

RAISE

To T.

TIE LINE FLOW

( d)· dl·

+O F +6F

LOWER

F F.

0

/', ,NO , I,?"CONTROL'·'·

RAISE

LOWER L OWER

RAISE_ RA I SE.

-AF - oF

,/ 'Ii': !//,- .' //// : I

T o (e)

To• T

•

(f) I II

LOWER

•

F F.o

RAISE

To (g)

To T. NET

Fig. Fig . 1

IINTERCHANGE NTERCHANGE

Evolution of real r ea l time tiDle control ccharacteristics. harac teriatics . Curves show balanc~ bslance. points controller of cont r o ller on plots of frequency frequen cy versus ve r s uS tie t i e line flow. Points Po ints not on characteristic result res ult in In "raise" ··rais e" or "lower" " l ower" control action. action . Fa scheduled FO is i s sch eduled frequency. To is i s scheduled tie line flow. flow . Increase in in outgoing power powe r is to the right of the th e tie line coordinate. coordinat e. Individual sketches apply to the following types of control: con tr ol: (b) Consta Constant (a) Constant frequency freq uency nt tie line (d) (c) Selective Selec tive frequency f r equency ( d ) Tie line zoned ((e) e) Frequency Freq uency zoned ((f) f ) Tie line bias bial -- withdrawal (g) ( g) Frequency biased net interchange i n ter change -- sustained. s ustained . In all a ll areas. aress • (b),, (c)t •* Control types t ypes (b) (c) , (d), (d) , (e)t (e). and (f) used in In concert with sa master _ster frequency controlling area, (a). ares , (a ).