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A REVI EW OF NEW I NNO VATIO NS I N P O W ER SYSTEM CO NTRO LD. SUTANTO

Department of Electric Power Engineering,Sc hoo l of Electrical Engineering,The University of Ne w South WalesSydney, NSW 2052, Australia.

Period

A B S T R A C T Financial and environmental constraints haveaffl icted power system s throughout the world. Environmentalpressures have forced the ret i rement and non-replacement ofpower station close to urban load centres as well as making it moredifficult and e xpensive t o establish add itional transmission tines tosupply these large load centres. All these factors have altered themethod of power system operat ion from the manner original lyplanned. Larger amounts of power are being conveyed over theinterconnections. This results in transmission bottlenecks, under-utilisation and sometimes unwanted extra loading of transmissionfacilities.Recently, devices known as FACTS (Flexible AC Transmissionsystems) specially designed to introduce more "flexibility" both innormal and emergency operation of an electric power system havebeen proposed. T he flexibility is related t o the direct control (bothlocal and centralised) of both active and reactive power. The mainaim is to increase the utilisation of existing facilities even to itsthermal limits (without sacrificing reliability) in order to reduce thedifficulties in installing new transm ission lines and to permit largertransfer of low-cost electrici ty through interconnec ted areas orcountries.This paper provides a review of the basic concepts of FACTS, theposs ib l e FACTS op t ions cu r ren t ly avai l ab le , t he recen tdevelopment and demonstrat ion efforts in USA and Europe andthe cost benefit analysis of such an installation.The paper concludes that FACTS technology is now available foruse. although the cost may be prohibitive initially, but as financialand environmental constraints increase in the future, the use ofFACTS technology may be unavoidable.

1. INTRODUCTIONChanging condit ions associated with the evolut ion of powersystems have created a continuing sequence of fresh problems. Inrecent years , the power system has become increasingly morecomplex to operate and system have become more insecure.Serious col lapses have occurred in greater number, e.g in theUnited States, Cana da, France, Belgium , Sweden and Japan.The circumsta nces which ha ve created this condition include:

-- the increasing reliance o n interconnections between powerutilitiesthe effect of the 1973/4 oi l crisis on the relat ive costs ofdifferent methods of electrici ty product ion. This hasconsiderably reduced the emphasis on oil-fired plant.- the reducing an d erratic rates of electricity growth,- the financial pressures that this has imposed on electricityutilities,- the increasing environmental constraints which also add tofinancial problems- all these factors have inhibited the construction of newpower stations.- the permanent ret i rement of power stations within largeurban load centres- f inancia l cons t ra in t s have cu r t a i l ed the add i t ion ofredundant system elements , so important for high levels ofsystem reliability.

All these factors have radically altered the method of power systemoperation from the manner originally planned. Added to this is thelack of the power stations which had provided an important back-up supply to many large urban load centres . Further, i t isincreasingly difficu lt to acquire new transmission line easements torelieve loadings on the pow er grid. To illustrate this, Table 1 showsthe aggregate EH V transmission additions placed in service in USA

EHV Additions345 .500 and

during the five-year periods from 1968 to 1987 and the projectedadditions through I987 [ I ] .Larger amounts of power a re being conveyed, not only over theinterconnections but into the important load centres. Financialpressures have minimised the production from oil-fircd generation.Consequently some ut i l i t ies maximise the import of power bykeeping tie lines at their limits some 90%of the time. These higherpower flows throughout the network resul t in t ransmissionbottlenecks, where som e transmission line s are overloaded andsome are either underutilised or used in an undesired manner. Thehigher power flows also produce a large increase of series reactivepower losses in the process of supplying these important loadcentres.

EHV T ransmission Line Additions in the US.Aetual (1968-1987) and Projected (1988-1997)Sources: EEIStatistical Yearbooks of the Electric UtTtv

765kV

' 7 . 7II

L IzuDIo~ .cBL IoEuIo*~

Fig. 1 Universal Transmission t in e Curvelncrease of Net Reactive Power Losses with Line Loading

IEEE CatalogueNo. 95TH80250-7803-242344/95/$4.0001995EEE

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To gain a better appreciation of this influence, it is useful toexam ine the reactive power characteristic of a typical transmissiontine.For this purpose Fig. 1provides a universal react ive powercharacteristic of transmission lines which is applicable to lines ofany voltage level once the no load charging (B ) and surgeimpedance loading (SIL) are known for the line. It can then beused to quan tify the net reactive power losses of that transmissionline, which can be of any voltage or length. A numerical examplehas been evalua ted in Tab le 2 to provide a basis for understandingtransmission line net losses on reactive power demand. For thisevaluation, in Table 2, a 160km. 00kV transmission line with a noload charging of 133MVAr and SIL of 850MVA is used. As bothl ine charg ing and SIL are proport ional to the square of thetransmission line voltage level the effect of 90%. 95% and nominalvoltage levels on the net reac t ive power losses have beenevaluated in the Table 2.

TAR1.E 7

2550MVALine Load2975MVA

-._---NET REACTIVE POWER LOSSES N 160km5OOkVTRANSMISSIONLINE

3.3 I2003.7 13703.5 14633.88 1650

I 1 Line Loading I Series Reactive Power I

Voltage (kV)2 3 03 4 550 0165I100

TypicalSIL (MW ) Thermal Ratings1 5 0 4 0 04 0 0 I 2 0 08 5 0 2 6 0 02200 54005 2 0 0 2 4 0 0 0

These difficulties have provided the motivation to investigatenovel ways to better utilise and control existing transmissionsystems. Specifically, Flexible A C Transm ission Systems (FACTS)are designed to provide the ability to direct the ac tive and reactivepower flow along pre-determined corridors within large ACinterconnections. This is achieved by ap plication of fast switchingof modern power semi -conducto rs , advanced con t ro l andprotection concepts similar to those developed for HVDC systems.Consequently, ful l benefi t can be made of exist ing but under-utilised thermal capacity with the intention of avoiding or delayingnew construction.This paper reviews a limited set of possible FACTS options that arecurrently being investigated in USA and Europe.

2. FACTS OPTIONSThe concept of FACTS was initiated in early 1988 by EPR l inUSA [2]. The technology is envisaged to offer the powerauthorities the following possibilities:1. Rapid control of power flows on their transmission routes,2. Secure loading of transmission lines to their full thermalcapacitiesThe central technology of FACT S involves high power electronicssupported by advances in digital control using microprocessors.digital protective relays, integrated communications (includingfibre optics) and advanced co ntrol centres.Som e of the power controllers that have been considered are [3]:a)b) Static VAR Compensatorc) Thyristor control phase-shifterd) Controllable series capacitore) Dynamic Load brakef) High Energy Arresterg) Modular series reactorh) Fault current limiteri) Circuit breakerj) Load tap changingk) Ferro resonance dampingI Combination of the aboveTwo of these, the SSR dam ping device and the s tatic VARcompensator, are comm ercially a vailable. Current research work inUS and Europe tends to concen t ra t e on the nex t twotechnologies, i .e. power angle regulator and modular seriescapaci tor[3]. Recently bat tery energy storage schemes 141 an dnovel unified controller sch eme [5] comb ining some of the FACTSoptions have also been repon ed in the literature.2.1 Power Transfer EquationsThe power transfer across a transmission facility is determined bythe relat ive magnitudes and phase angle of the sending andreceiving terminal voltages and the electrical characteristics of thenetwork faci l i t ies connect ing the sending and receiving endterminals as shown in Figure 2. The equat ion given is a goodapproximation for EHV systems where the induct ive reactancecomponent is much greater than the resistance component.

SSR (Sub Synchronous Resonance) damping

Fig. 2 Power Transfer Equation

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Examinations of the P ower Tra nsfer Equation provides insight intothe techniques for power flow control. The only viable options tocontrol the power t ransfer across a t ransmission faci l i ty is tomodify one or a combination of the following parameters: theterminal voltages (E1 or E2), the inductive reactance ( X l 2 ) or therelative phase a ngle (612)between the two terminals.Stat ic shunt VAR compensator at tempts to regulate vol tagemagnitude at the point of connection (usually at the receiving end)to increase the transmission capacity of the power system. In thisway, the static shunt VAR compensator provides both voltagesupport and increased stability margin.While the inductive reactance of the transmission line can bereduced by physical changes to the conductors, the emphasis ofFACTS devices is to introduce external components having acapacitive or inductive reactance characteristics to change thetransmission effect ive reactance. For example, the insert ion ofseries reactors increases the inductive reactance between the twoterminals, thereby reducing the power flow, and the insertion ofseries capacitors reduces the inductive reactance between the twoterminals, thus increasing the power flow. Th is is the basis for thecontrollable series compensators, where series capacitance andinductances are rapidly inserted or bypassed.The final part of the power transfer equation includes the term sin(612). The magnitude of the power flow wil l vary in directproport ion to the s ine of 612 and wil l be maximum at 90'. It isimportant to note that the direction of power flow is determined bythe relative ph ase angle between the terminals. As a result. alteringthe effective reactance of the transmission circuit will not alter thedirection of power flow. However, a power flow control deviceut i l is ing phase angle adjustment techniques can produce bi-directional control. This establishes the fundamental concept of asol id-state phase shift ing device. Such a device continuouslyadjust the power angle 61 2 to produce a rapid control of powerflow.2.2 Implementation of some FACTS controllers [5]The following descriptions of FACTS implementations are takenfrom Reference 5. Other applications are described in recent EPRI"FACTS" C onference[6].2.2.1. Static VAR CompensatorA typical shunt-connected static VAR compensator, composed ofthyris tor-switched capaci tors (TSCs) and thyris tor-control ledreactors (TCRs), is shown in Fig. 3. With proper co-ordination ofthe capacitor switching and reactor control, the VAR output canbe vaned continuously between the capaci t ive and induct iveratings of the equipment. The compensator is normally operated toregulate the magnitude of voltage of the transmission system at aselected terminal, often with an appropriate modulation option toprovide damping if power oscillation is detected. The static VARcompensator c an also provide transient stability improvement.2.2.2 Control lable Series CompensatorThyris tor-con trol led series com pensators consist of thyris tor-switched capacitors, or a fixed capacitor in parallel with a thyristor-controlled reactor. The development of such compensators havebeen recently reported in the literature [1,7]. These schemes areshown in FigA(a) and 4@).In the thyristor-switched capacitor scheme of Fig.4(a). the degreeof series compen sat ion is controlled by increasing or decreasingthe number of capacitor banks in series. To accomplish this, eachcapacitor bank is controlled by a thyristor bypass switch, or valve.The opera t ion of the thyris tor switches is to co-ordinate withvoltage and current zero crossings: the thyris tor switch can beturned on to bypass the capaci tor bank when the applied acvoltage crosses zero, and its turn off has to be initiated prior to acurrent zero at which it can recover its voltage blocking capabilityto activate the capacitor bank.In the fixed-capacitor. thyristor-controlled reactor schem e of Fig.4@). the degree of series compensation in the capacitive operatingregion (the admittance of the TCR is kept below that of the parallelconnected capacitor) is increased (or decreased ) by increasing (ordecreasing) the current in the TCR. The TCR may be designed for

a substant ial ly higher maximum admit tance at ful l thyris torconduction than that of the fixed shunt-connected capacitor. Inthis case, the TCR , with an appropriate surg e current rating, can beused essentially as a bypass switch to limit the voltage across thecapacitor during faults and other system contingencies of similareffect.I 1r.n.ril.hnulu

r i

~ q x l o r llnh Reactor LnU

Fig 3 Static Var Co mpensa tor employing thyristor-switchedcapacitors and thyristorcontrolled reactors

Fig. 4 Controllable series comp ensator using (a) thyristor switchedcapacitors, and (b) a thyristor-controlled-reac tor with a fixedcapaci tor

Fig. 5 Thyristor-controlled tap-changer fo r phase angle control.

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2.2.3 Phase- ShifterAlthough there is no high power, non-mechanical phase-shifter inservice. the principles for using a phase-shifting transformer with athyris tor tap-changer are well establ ished [8]. Just as theconventional phase-shifter with a mechanical tap-changer, thethyristor-controlled counterpart also provides quadrature voltageinjection. The magnitude of the quadrature voltage injected couldbe varied continu ously by thyristor firing-angle control at theexpense of harmonic generat ion. However, a s tep-l ike control ,generat ing no harmonics, appears to be bet ter sui ted for highpower applications.A thyristor-controlled phase shifting transformer arrangement isshown in Fig. 5 . It uses three non-identical transformer windings inproportions of 1:3:9 and. with a switch arrangement that canbypass a winding or reverse its polarity, it can produce a total of27 steps using only 12 thyris tor switches (of three differentvoltage ratings) per phase.The phase angle requirements for power flow control can bedetermined from generator rotor-angle m easurements, if possible, orfrom power measurements. With these, the thyristor-controlledphase-shift ing t ransformer could be applied to regulate thetransmission angle to maintain balanced power flow in mult ipletransmission paths, or to control it so as to increase the transientand dynam ic stabilities of the power system.2.2.4 mplementation of a unified power controller [fi]A unified power control ler can be described by the circui t inFigure 6.The power flow controller consists of two controllableelements. a voltage source, inserted in series with the line and acurrent source, connected in shunt with the l ine. Both themagnitude and angle of the voltage source can be freely regulated,whereas only the magnitude of current is variable with its phaseangle is fixed at 90 egrees with respect to voltage at the point ofconnection. The current source, therefore, provides reactive powercompensat ion and the vo l t age source p rov ides the ser i escomp ensation and phasc shifting ci ibility

m

Fig. 6 Unified Power ControllerThere are a numbe r of feasible solid-state impleme ntations for theu n i f i e d p o w e r f l o w c o n t r o l l e r d e s c r i b e d a b o v e . T h eimplementation considered feasible and econom ical with presentlyavailable pow er sem iconductors is similar to those proposed for theadvanced s t a t i c VAR compensato r , con t ro l l ab le ser i escompensato r , and phase-sh i f t e r . I t employs vo l t age sourceinverters (i .e. inverters fed from a d c vol tage source) composed ofgate-turn off (GTO)hyristor valves, in appropriate harmonicneutral ised configurat ions which ensure almost dis tort ion-freeoutput , economic manufacturing and inherent redundancy formulti-level partialavailability.The proposed implem entation of the unified power f low controllerusing two voltage-source inverters operated from a common dclink capacitor, is shown schematically in Fig. 7.This arrangement isactually a practical realisation of an AC to AC power converterwith independently controllable input and output parameters.Inverter 2 is used in the arrangement shown to generate a voltagesource at the fundamental frequency with variable amplitude (0 5Vp q S Vpqmax) and phase angle (0 S 6 < 2%).which is added tothe ac system terminal vol tage vo(tf8, the series connected

coupling transformer. In this way, the inverter output voltageinjected in series with the line can be used for direct voltagecontrol, series compensation, and phase-shift.

Fig. 7 Implementation of the unified power flow controller usingtwo-voltage-sourced inverters with a direct voltage link.Inverter I (connected in shunt with the ac power system via acoupling transformer) is used primarily to provide the real powerdemand of Inverter 2 at the common dc link terminal from the acpower system. Since Inverter 1 can a l so genera t e o r absorbreactive power at its ac terminal. indep endently of the real powerit transfers to (or from) the dc terminal, it follows that, with propercontrols, it can also fulfil the function of an .independent advancedstat ic VAR compensator providing react ive power compensat ionfor the t ransmission l ine and thus executing indirect vol tageregulat ion at the input terminal of the unified power flowcontroller.2.2.5 Battery Energy Storage Plant (BESP)Research work is currently being carried out at the University ofNew South Wales to investigate the use of battery energy storageplant as an extension of the unified power flow control ler. InFigure 7, the power to provide the real power demand comes fromthe power system itself, negating some of the benefits of reducedlosses. Clearly if a battery energy plant is av ailable, it can replacethe DC link capacitor in Figure 7 . It can provide all the activepower requirements of the device and in time of emergency (lineoutage). inverter I can supply emergency load for a short duration(4 hours or so). The advantages of Battery Energy Storage Plant tosystem planning and operation have been reported by Lachs andSutanto[4].In ou r proposal, the GT O s in Figure 7will be replaced by eitherIGBTs o r MCTs. A m icroprocessor provides the interface betweenthe energy stored in the batteries and the AC system. It is theversatility of this microprocessor controlled electronics that canal low the Battery E nergy S torage Plants to have the fol lowingfeatures:

- With IGBT or MC T control , the AC output of the BSPcan be varied in a fraction of a second- This output of both real and reactive power can be variedextremely quickly between maximum output and input- With i ts s tored energy, the nominal output could bedoubled for short periods- Its react ive power output can provide the necessaryshunt and series reactive power compensation and henceit can provide a rock steady control of voltageThis react ive power control does not appreciably tap i tsstored energyIts maximum react ive power output is independent ofvoltage levels on the AC network.With a fault on the AC network, the contribut ion doesnot exceed its rated output.Battery Energy Storage Plant (BESP) is most useful ifcontinuously run in parallel with the AC network

----

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--

BESP can also maxim ise the utilisation of grid-connectedrenewable energy plants, such as solar or wind generatorsIf external supply is temporarily lost and later restored, aBESP can automatically be re-synchronised.System Operational and Planning Features of a BESP usedas a Unified Power C ontroller2.2.6

Control l ing the outputs of groups of BESP is useful for theoperation of the main transmission system which links the urbancentres with the system's generators as described in the followingsections.2.2.6.1 Fast Load Pick-upWear and stress occurs on turbines, boilers and their control attimes of rapid load change. Fast load pick-up of numbers of BE SPacting as unified power flow controllers would allow much slowerrates of load change on turbo-generators as well as aiding theoperation of the power grid.2.2.6.2An alternative to Pumped Storage Plant.Hydro pumped storage schemes, which are connected to thetransmission grid, require substantial capital outlay by electricutilities. However, by pumping at off-peak periods to raise waterstorage levels, the energy can then meet peak demands or difficultoperating conditions. BESP acting as a unified power controllerwould be more effect ive as their control assis ts dis tribut ionnetworks as well as he transmission grid.2.2.6.3Transm ission Grid EmergencyWhen an emergency causes a sudden increase of loading ontransmission lines supplying cities, receiving end voltage woulddeteriorate and system voltage instability could be threatened. Themajor factor leading to system voltage instability is the sharpincrease of series reactive power losses of the suddenly moreheavily loaded transmission lines. Rotating unit field forcing wouldprovide access to extra reactive power resources to gain a periodof respi te [l l ] . With mainly shunt capaci tors for react ive powercompensat ion (as c i ty power s t a t ions are re t i red ) , t he i rcharacteristics would exacerbate the voltage reduction in urbanareas, BESP acting as a unified power controller can instantlyproduce maximum react ive power output and their MW outputwould lower transmission line loadings, so significantly reducingthe series reactive power losses. Like rotating unit field forcing.this can provide the initial calm which is so valuable in regainingoperational control at the first stage of system voltage instahilityand so prevent collapse.2.2.6.4 Summary of advantages and disadvantagesIn summ ary, some of the main advantages are as follows:- Battery Energy Storage Plant can supply some of the loadat heavy demand periods (maybe 1 or 2 hours a day)- Battery Storage Plant can be charged at night and usedduring the day providing a better load factor- Fast load changes can be met by BESP- BESP can raise the levels of operat ional securi ty andreliability- Reduction of losses at heavy load- Improved Voltage control at point of connection- Reduction of spinning reserve- Allow new and renewable generation, in small blocks, tobe effectively connected- By proper design of inverter I, the unified controller canac t an emergency supply

Some of the main disadvantag es of BE SP are:-- BESP c an be more expensive in its initial costBESP require a lot of storage areas to store the batteriesand associated protective equipments and watering system3. POSSIBILITIESOFFEREDBY ACTS [2]

The fol lowing examples are taken from reference [2 ] to show theplanning possibilities that can be obtained using FAC TS devicesdescribed in Section 2.

Figure 8shows AC Power Flow in Parallel Paths. Without anycontrol, power flow is proportional to the inverse of the varioustransmission line impedances. It is likely that the lower impedancel ine may become overloaded. I n this case it is not possible tocontrol the steady state power flow s in the two parallel lines.

AC Power Flow - Parallel Paths

Fig. 8 AC Pow er Flow in parallel paths without any controlFigure 9 shows AC and HVDC Lines in parallel. With HVDC,powercan be controlled freely and continuously. Further, because poweris electronically controlled, an HVDC line can be used to its fullthermal capacity if adequate converter capacity is provided. Ifnecessary, HVDC can also help to maintain stability. This is themain advan tage of the HVDC system. However, HVDC is usuallytoo expensive for widespread use and will only be consideredwhen long transmission lines are involved or if asynchronous tiesare required.

Fig 9 AC Power F low in Parallel Paths with HVDC lineFigure 1 0 shows one of the transm ission lines with additionalimpcdance in the form of a high speed controlled series capacitor.With this arrangement , one can obtain substant ial ly (but notcompletely) the same advantages as with HVDC but at a muchlower cost . This is because with high speed control of seriesimpedanc e, one can obtain appropriate stea dy state power (withinthe range of impedance control) yet changdmodulate impedanceas rapidly as required for stability considerations.Figure 9 also shows that one of the AC lines may have a high-speed phase angle regulator. Again, with this approach, one canobtain substantially th e same advantages as HVDC, but with lesscost. Whereas the series capacitor controls the impedance to levelsbelow the series inductance of the line, the phase angle regulatorwith plus-minus range, controls the apparent impedance bothways. The HVDC link is in effect, an electronic 360 degree phaseangle regulator.

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Series Capacitor 01 Phase Angle Regdator-Fi g 10 AC Po wer Flow in Parallel Paths with FACTS Devices4. RESULTS OF COMPUTER STUDIES CARRIEDOUTBY EPRI IN USA 91

EPRI sponsored a project in 1988 to assess the benefits of theF A C B technology. The fol lowing was taken from reference [91 todescribe the results obtained so far.Th e fmt result looked Fig. 11) at maxim ising the power transfer ona 300 mile corridor using various combinations of "FACTS" -typedevices. The figure shows increases of power transfer capabilityranging from 1500 MW to 4000 MW at costs ranging from SI00million to $750 million.

Fig.1 Maxim ising Power TransferA second study looked at a transient stability limited radial systemand compared the al ternat ives of a new l ine versus thyris torswitched series capacitors to achieve an increase of 1000M W intransfer capability. Using the measure of years-to-payback. theresults show a 3 to 1 benefit in the series capacitors with a 0.3year-to-payback versus 2.5 years-to-payback for the new line.A third result examined increasing the transfer capability on atransient stability limited complex system. Three alternatives wcrcconsidered. The first two provided an increase of transfer capacityof 1200MW. one, with the addit ion of two new l ines. has apayback period of 3.2 years. The second, the addition of thyristor

switched series capacitors also provided 1200 MW of transfercapacity but with a payback period of only 0.7 years. The thirdaltemative, retrofitting five existing power angle regulators withthyris tor switched controls , provided an addit ional 500MWtransfer capacity with a payback period of 1. I years.5. CONCLUSIONA 2-volum e report was published in 1990 by EPR I and containsmore detailed discussions on the various study carried out on theeffectiveness of "FACTS"[IO]

The paper has provided a review of various FACTS devices.FACTS devices are shown to be rapid pow er flow control devicesfor application in a highly interconnected transmission network.Rapid control of the power flow on ac transmission lines toenhance the utilisation of the inherent capabilities of the networkis a significant departure from the present mode of electric powertransmission network operation. The successful application of thisconcept may significant ly change the complexion of futureplanning and operation of electric transmission.Although at present the initial cost of FAC TS devices is generallymuch more than that of installing a new transmission line, it willbecome increasingly more difficult if not possible to acquire newtransmission line easement, so that FACTS devices provide theonly realistic option. T he result of the computer studies carried ou tby EPRI. on he econom ic benefit of FA(JTS devices, suggests thatif the measure, years-to-paybac k is used, nstalling FACTS devicesis more favourable than installing new lines. Further, recentadvances in high power semiconductor technology have resultedin lower cost and higher power GTO hyristors and other powersemiconductor devices.

1.

2.

3.4.5.6.7.8.9.10.

11.

6. REFERENCESR.M. Maliszewski, B.M. Pastemack, H.N. Scherer Jr., M.Chamia, H. Frank, L. Paullson. "Power Flow control in aHighly Interconnected Transmission N etw ork , CIGRE Paper37-303, 1990Session, Paris.N.G.Hingorani, "High Power Electronics and Flexible ACTransmission System", Joint APC/IEEE Luncheon Speech,April 1988 at the American Power Conference 50th AnnualMeeting in Chicago. Printed in IEEE Power EngineeringReview, July 1988.N.G. Hingorani. "FACTS - Flexible AC Transm ission System",IEE Publication No. 345: AC & DC Power Transmission,1991,pp 1-7.W.R. Lachs and D. Sutanto. "Battery Storage Plant WithinLarge Load Centers". IEEE Trans. on Power System, May1992. Vol. 7. No. 2. pp. 762-769.L. Gyugi, "A U nified power flow control concept for flexibleAC Transmission Systems", IEE Publication No. 345: AC &DC ower Transmikion. 1991,pp. 19-26.EPRI "Flexible AC TRansm ission Systems (FACTS).". B oston,18-20 Mav 1992.Christl, N. et . al . , "Advanced Series Compensat ion withvariable Impedance", EPRI Workshop on FACTS, Cincinnat i.Ohio, No. 1990.Wood, P. Et. al., "Study of Improved Lo+,Tap Changing forTransforme r and Phase angle Regulators, EPRI Report EL-6079, Project 2763-1, 1988.Iveson, R.H., "Brief Progress Report on th e United Statesinitiative - 'FACTS' - Flexible AC Transmission System".CIGRE, 1990.Group 37, p.29.Ewart , D.N, et . al . , "Flexible AC Transmission Systems(FACTS): Scoping Study, Volume 1 and 2". EPRI Report EL-6943, Project 3022-1.1990.Lachs. W.R. and S utanto. D.. "Vo ltage Instabi l ity inInterconnected Power Systems: A Simulat ion Approach",IEEE Trans. Power System s, Vol. 7, No. 2, Ma y 1992. pp. 753-761.

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