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IEEE Transactions on Power Delivery, Vol. 10, No. 2, April 1995                                                           ...
1086injected in quadrature with the line current I .                                   it, or absorbed from it, by the sen...
1087is able to establish 0.5 p.u. power flow, in either direction, without        defined by the transmission angle on the...
1088                                                                                                       angle is zero (...
1089secondary voltages are in quadrature with respect to the corresponding      flow independently of the real power.prima...
1090rent vectors.) Because of its electronic controls, the UPFC can cause                 of series injected voltage to ma...
109113 shows the results obtained. As seen,the UPFC is positioned                                                         ...
and superior dynamic performance. Further studies are planned which                                                       ...
1093Summer Meeting, Paper No. 92 SM 467-1 PWRD, Seattle, WA, July               Scott L. Williams was born in Torrance, Ca...
Pd 1995 10-2-the upfc a new approach to power transmission control
Pd 1995 10-2-the upfc a new approach to power transmission control
Pd 1995 10-2-the upfc a new approach to power transmission control
Pd 1995 10-2-the upfc a new approach to power transmission control
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Pd 1995 10-2-the upfc a new approach to power transmission control


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Pd 1995 10-2-the upfc a new approach to power transmission control

  1. 1. IEEE Transactions on Power Delivery, Vol. 10, No. 2, April 1995 1085 THE UNIFIED POWFBFLOW CONTROLLER: A NEW APPROACH TO POWER TRANSMISSION CONTROL L Gyqyi . T R Rietman . A Edris . C. D. Schauda D. R T o ~ o s. L willirms . Westinghouse Science Western Area Electric Power t Technology Center i Power Administration Research Institute Abstract - l& paper s h m that the Unified Power Flow Controller l power demanded by Inverter 2 at the common dc link. This dc link(UPFC) is able to control both the transmitted real power and, power is converted back to ac and coupled to the transmission lineindependently, the reactive power flows at the sending- and the receb- lnnrmirrlon Une -I w " su@y Transformering-end of the transmission line. The unique capabilities of the UPFCin multiple line Compensation are integrated into a generalized power ___-- Inverter 1 Inwrler 2flow controller that i able to maintain prescribed, and independently scontrollable, real power and reactive p e r f o in the line. The paper lwdescribes the basic concepts of the proposed generalized P and Qcontroller and compares it to the more conventional, but related L T u<power flow c o n t r o l e ~ such as the l%y&br-Controlled Series Capaci-tor and Thyristor-ControlledPhase Angle Regulator. The paper alsopresents results of computer simulations showing the performanceof the UPFC under different system conditions. Measured Variables cnntml 0IW ZRef Re1 Keywords - AC transmission, FACTS, power flow controller, line Panmeter Satlngr Refcompensation, seriescapacitor, phase angle regulator, thyristor, GTO, Fig. 1. - Basic circuit arrangement of the Unified Power Flow Controllerpower converter, inverter. via a shunt-connected transformer. Inverter 1 can also generate or INTRODUCIION absorb controllable reactive power, if it is desired, aild thereby it can provide independent shunt reactive compensation for the line. The Unified Power Flow Controller (UPFC) was proposed for It is important to note that whereas there is a closed "direct" pathreal-time control and dynamic compensationof ac transmission sys- for the realpower negotiated by the action of series voltage injectiontems, providing the necessary functional flexibility required to solve through Inverters 1 and 2 back to the line, the corresponding reactivemany of the problems facing the utility industy. power exchanged is supplied or absorbed locally by Inverter 2 and The Unified Power Flow Controller consists of two switching therefore it does not flow through the line. Thus, Inverter 1can beconverters, which in the implementations considered are voltage- operated at a unity power factor or be controlled to have a reactivesourced inverters usinggate tzun-off (GTO) thyritorvalves, as illustrated power exchange with the line independently of the reactive powerin Fig. 1. These inverters, labelled "Inverter 1"and "Inverter 2" in exchanged by Inverter 2. This means that there is no continuousthe figure, are operated from a common dc link provided by a dc reactive power flow through the capacitor. This arrangement functions as an ideal a c t o ac Viewing the operation of the Unified Power Flow Controller frompower converter in which the real power can freely flow in either the standpoint of conventional power transmission based on reactivedirection between the ac terminals of the two inverters and each shunt compensation, series compensation, and phase shifting, theinverter can independently generate (or absorb) reactive power at UPFC can fulfill all these functions and thereby meet multiple controlits own ac output terminal. objectives by adding the injected voltage vp4,with appropriate ampli- Inverter 2 provides the main function of the UPFC by injecting tude and phase angle, to the terminal voltage v,. Using phasor repre-an acvoltage vw with controllable magnitude Vm (OsV,sV& and sentation, the basic UPFC power flow control functions are illustratedphase angle Q ( O S Q S ~ ~ ) ,at the power frequency, inserh with line in Fig. 2.via an insertion transformer. This injected voltage can be considered (a) Volta e @) Series (c) Phase angle (d) Multi-functionessentially as a synchronousac voltage source. The transmission line regulAion compensation regulation power flow controlcurrent flows through this voltage source resulting in real and reactivepower exchange between it and the ac system. The real powerexchanged at the ac terminal (i.e., at the terminal of the insertiontransformer) is converted by the inverter into dc power which appearsat the dc ln as positive or negative real power demand. The reactive ikpower exchanged at the ac terminal is generated internally by theinverter. The basic function of Inverter 1 is to supply or absorb the real94 SM 474-7 PWRD A paper recommended and approvedby the IEEE Transmission and Distribution Committee Fig. 2. - Basic UPFC control functions: (a) voltage regulation, @) seriesof the IEEE Power Engineering Society f o r presentat- compenastion, (c) angle regulation,and (d) multi-functionpower f o control lwion e t t h e IEEE/PES 1994 Summer Meeting, SanFrancisco, CA, July 24 - 28, 1994. Manuscript sub- Terminal voltage regulation, similar to that obtainable with a trans-mittedDecember 28, 1993; made available f o r printing former tap-changer having infinitely small steps, is shown at (a) whereMay 9, 1994. Vm=AV(boldface letters represent phasors) is injected in-phase (or anti-phase) with V, . Series capacitive compensation is shown at (b) where Vm=Vc is 0885-8977/95/$04.00 0 1994 IEEE
  2. 2. 1086injected in quadrature with the line current I . it, or absorbed from it, by the sending-end generator. The voltage Transmtrswn angle regulation @hue sh$ing) is shown at (c) where injected by the UPFC in series with the line is represented by phasorV,=Vo is injected with an angular relationship with respect to Vothat V,havingmagnitude Vw(O<V,<O.Sp.u.) and angle p(O<p<36@)achieves the desired CJ phase shift (advance or retard) without any measured from the given phase position of phasor as illustratedchange in magnitude. in the figure. The line current, represented by phasor I, flows through M d t i - j i " powerflow control, executed by simultaneous terminal the series voltage source, V,, and generally results in both reactivevoltage regulation, series capacitive line compensation, and phase and real power exchange. In order to represent the UPFC properly,shifting, is shown at (d) where V,=AV+V,+V,. The powerful, hitherto unattainable, capabilities of the UPFCsum- -marized above in terms of conventional transmission control concepts,can be integrated into a generalized power flow controller that isable to maintain prescribed, and independently controllable, realpower P and reactive power Q in the line. Within this concept, theconventional terms of series compensation, phase shifting etc., becomeirrelevant; the UPFC simply controls the magnitude and angularposition of the injected voltage in real time so as to maintain or varythe real and reactive power flow in the line to satisfy load demandand system operating conditions. Fig. 4. - Two machine system with the Unified Power Flow Controller. In the following sections, the paper describes the basic conceptsof the generalized P and Q controller, compares it to the more con- the series voltage source is stipulated to generate only the reactiveventional power flow controllers, such as the Thyristor-Controlled power Q, it exchanges with the line. Thus, the real power P, itSeries Capacitor, and Thyristor-Controlled Phase Angle Regulator, negotiates with the line is assumed to be transferred to the sending-delineates the principles of the electronic control used in the imple- end generator as if a perfect coupling for real power flow betweenmentation, and presents results of computer simulations showing the it and the sending-end generator existed. This is in agreement withperformance of the UPFC under steady-state and dynamic system the UPFC circuit structure in which the dc link between the twoconditions. constituent inverters establishes a bi-directional coupling for real power flow between the injected series voltage source and the sending- BASIC PRINCIPLES OF P AND Q CONTROL end bus. As Fig. 4 implies, in the present discussion it is further assumed for clarity that the shunt reactive compensation capability Consider Fig. 3. At (a) a simple two machine (or two bus ac inter- of the UPFC is not utilized. That is, the UPFC shunt inverter istie) system with sending-end voltage V,, receiving-end voltage V,, and assumed to be operated at unitypower factor, its sole function beingline (or tie) impedanceX(assumed, for simplicity,inductive) is shown. to transfer the real power demand of the series inverter to the send-At (b) the voltages of the system in form of a phasor diagram are ing-end generator. With these assumptions, the series voltage source,shown with transmission angle S and IV, I= IV, l=V. At (c) the trans- together with the real power coupling to the sending-end generatormitted power P (P={ V?/HsinS)and the reactive power Q=Q,=Q, as shown in Fig. 4,is an accurate representation of the basic UPFC.(e={ U-cosS))supplied at the ends of the line are shown plotted It can be readily observed in Fig. 4 that the transmission line "sees"against angle S.At (d) the reactive power Q=Q,=Q, is shown plotted V,+V, as the effective sending-endvoltage. Thus, it is clear that theagainst the transmitted power P corresponding to the "stable"values UPFC affects the voltage (both its magnitude and angle) across theof S (i.e., 0 ~ 6 1 9 0 " ) . transmission line and therefore it is reasonable to expect that it is able to control, by varying the magnitude and angle of V, the trans- mittable real power as well as the reactive power demand of the line at any given transmission angle between the sending-end and receiv- ing-end voltages. In Figs. 5a through 5d the reactive power Q,supplied by thesend- ing-endgenerator, and Q, supplied by the receiving-end generator, are shown plotted separately against the transmitted powerP as a function of the magnitude V, and angle p of the injected voltage phasor V, at four transmission angles: S=O, 30", 604 and 90". At Vw=O, each of these plots becomes a discrete point on the basic Q-P curve shown in Fig. 3d, which is included in each of the above figures for reference. The curves showing the relationships between Qs and P, and Q, and P, for the transmission angle range of 0 ~ 6 1 9 @ when the UPFC , is operated to provide the maximum transmittable power with no reactive power control (V,=V,,- and p=pP+,,,), are also shown by a broken-line with the label "P(S)=MAX" at the "sending-end Fig. 3. - Simple two machine system (a), related voltage phasors (b), real and reactive power versus transmission angle (c), and sending-endlreceiving- and, respectively, "receiving-end" plots of the figures. end reactive power versus transmitted real power (d). Consider first Fig. 5a, which illustrates the case when the transmis- sion angle is zero (S=O). With V,=O, P, Q , and Q, are all zero, The basic power system of Fig. 3 with the well known transmission i.e., the system is at standstill at the origins of the Q, ,P and Q, ,Pcharacteristics is introduced for the purpose of providing a vehicle coordinates. The circles around the origin of the 1Qs,P}and { Q, ,PIto establish the capability of the UPFC to control the transmitted planes show the variation of Qs P, and Q, and P, respectively, andreal power P and the reactive power demands, Q, and Q,, at the as the voltage phasor V with its maximum magnitude V is rotated , Fsending-end and, respectively, the receiving-end of the line. a full revolution (O<pS360"). The area within these circles define Consider Fig. 4 where the simple power system of Fig. 3 is expand- all P and Q values obtainable by controlling the magnitude V anded to include the UPFC. The UPFC is represented by a controllable angle p of phasor V,. In other words, the circle in the { Q, ,d andvoltage source in series with the line which, as explained in the previ- { Q, ,P}planes define all P and Q, and, respectively, P and Q, valuesous section, can generate or absorb reactive power that it negotiates attainable with the UPFC of a given rating. It can be observed, forwith the line, but the real power it exchanges must be supplied to example, that the UPFCwith the stipulatedvoltagerating of 0.5 p.u.
  3. 3. 1087is able to establish 0.5 p.u. power flow, in either direction, without defined by the transmission angle on the Q versus Pcurve that char-imposing any reactive power demand on either the sending-end or acterizes the basic power transmission at V,=O.the receiving-end generator. Of course,the UPFC, as seen, can force Considering next the case of S=3ff (Fig. Sb), it is seen that thethe generator at one end to supply reactive power for the generator receiving-end control region in the { Q, ,PIplane is again definedat the other end. (In the case of an intertie, one system can be forced by a circle, however, the sending-end control region boundary in theto supply reactive power for the other one.) I Q,,PI plane becomes an ellipse. As the transmission angle S is further increased, for example, to 60" (Fig. Sc), the ellipse definingthe control region for P and Qs in the { Q, ,PIplane becomes narrower and finally 15 . at 90" (Fig. Sd) it degenerates into a straight line. By contrast, the w Qrt 10 . control region boundary for P and Q, in the ( Q, , plane remains a circle at all transmission angles. f} r M Figs. Sa through 5d clearly demonstrate that the UPFC, with its > unique capabilityto control independentlythe real and reactivepower flow at any transmission angle, provides a powerful new tool for transmission system control. In order to establish the capabilities of the UPFC in relation to other, related power flow controllers, such as the Thyristor- ControlledSeries Capacitor (TCSC), and the Thyristor-Controlled <+. Phase Angle Regulator (TCPAR), a basic comparison between the power flow control characteristicsof these and the UPFC is presented Plot (b) in the next section. The basis chosen for this comparison is the capabilityof each type of power flow controllerto vary the transmitted real power and thereactive power demand at the receiving-end.The receiving end var demand is usually an important factor because it significantly influences the variation of the line voltage with load demand, the overvoltage at load rejection, and the steady-statesystem losses. Similar comparison, of course, could easily be made at the p- sending-end, but the results would be quite similar, at least at practical transmission angles. -0.s 0 10 . P 15 . -0.5 0 y q i oy 1.5 COMPARISON O F THE UPFC TO THE TCSC AND TCPAR -03 4.5 Consider again the simple two machine system (twobus intertie) shown in Fig. 4. This model can be used to establish the basic trans- mission characteristic of the Thyristor-Controlled Series Capacitor and Thyristor-Controlled Phase Angle Regulator by using them in place of the UPFC to establish their effect on the Q versus Pcharac- teristic of the system at different transmission angles. Comparison of the UPFC to the TCSC Thyristor-Controlled Series Capacitor scheme^^.^ typically use a thyristor-controlled reactor in parallel with a capacitor to vary the effective series compensating capacitance. In practice, several capacitor banks, each with its own thyristor-controlled reactor, may be used to meet specific application requirements. For the purpose of the present investigation, the TCSC, regardless of its practical implementa- tion, can be considered simply as a continuouslyvariable capacitor 3 t Qr whose impedance is controllable in the range of OSX&X- The 10 . controllable series capacitive impedance cancels part of the reactive line impedance resulting ina reduced overall transmission impedance 0.5 (i.e., in an electrically shorter line) and correspondingly increased transmittable power. The Thyristor-ControlledSeries Capacitor is an actively controlled c- capacitive impedance which can affect only the magnitude of the O I -0.1 J . 0.5 1.0 --c 1.5 P -0.5 4.5 1 current flowing through the transmission line. At any given impedance setting of the TCSC, a particular line impedance is defined at which the transmitted power is strictly determined by the transmission angle (assuming a constant amplitude for the end voltages). Therefore, Fig. 5. - Attainable sending-end reactivepower vs. transmittedpower(left the reactive power demand at the end-pointsof the line is determined hand side plots) and receiving-end reactive power vs. transmitted power (right hand side plots) values with the UPFC at S i @ , S=3@, 6=W, 6=W. by the transmitted real power in the same way as if the line was uncompensated but had a lower line impedance. Consequently, the In general, at any given transmission angle 6,the transmitted real relationship between the transmitted power P and the reactive-powerpower P, and the reactive pawer demands at the transmission line demand at the receiving-end Q, be represented by a single Qr-P canends, Q,and Qr , can be controlled freely by the UPFC within the curve for the TCSC compensated line in the two machine systemboundaries obtained in the { 12, ,PI and { Q, ,PI planes by rotating shown in Fig. 6 at any series capacitive impedance in the range ofthe injected voltage phasor Vp4 with its maximum magnitude a full OsX,=X- (The same curve, of course, also describes the Q,-Prevolution. The boundary in each plane is centered around the point relationship.) This means that an infinite number of Q,-Pcuwes can
  4. 4. 1088 angle is zero (S=O). Since the TCSC is an actively-controlled, but functionallypassive impedance, the current through the line with the compensated impedance (X-XJ remains invariably zero, regardless of the of the actual value of X,. Thus both P and Q, are zero, the system is at standstill, which condition cannot be changed by the TCSC. By contrast, since the UPFC is an actively-controlled voltage source, it can force up to 0.5 p.u. real power flow in either direction and also control reactive power exchange between the sending-end Fig. 6. - Two machine system with the Thyristor-Controlled Series Capacitor and receiving-end buses within the circular control region shown in the established by using the basic transmission relationships, i.e., The Q,-P characteristic of the TCSC at 6=3@ is shown in Fig.P={ V/(X-XJ}sin8 and e,=( V/(X-X,)} (l-cosS} , with X, varying 7b. As seen, the relationship between Q, and P,for the total rangebetween 0 and X - Evidently, a given transmission angle defines of series compensation, OSSSW, defined by a straight line connect- isa single point on each Qr-P curve obtained with a specific value of ing the two related points on the lower and upper boundary curves,X,. Thus, the progressive increase of X, from zero to Xcm could (Q,-P),owe, (Q,-P),p,, which represent the power transmission at andbe viewed as if the point, defining the corresponding P and Q,values S =30",with zero and, respectively, maximum series the given transmission angle on the first Q,-P curve which represents It can be observed that, as expected, the TCSC controls the real powerthe power transmission with no compensation (X,=O), moves through by changing the degree of series compensation (and thereby thean infinite number of Q,-P curves representing progressively increasing magnitude of the line current). However, the reactive power demandseries compensation until it finally reaches the last Q,-P curve that of the line cannot be controlled independently; it remains a directrepresents the power transmisjion wt maximum series compensation. ih function of the transmitted real power obtained at S=3@ with valyingThe first Qr-Pcurve, representing the uncompensated power transmis- X, ( O s X C a & . It is also to be noted that the achievable maximumsion, may be considered as the lower boundary curve and identified increase in transmittable power attainable with the TCSC is a constantby ( ,P,, Q ). -- The last Q,-P curve, representing the power transmission percentage (defined by the maximum degree of series compensation)with maximum series compensation, may be considered as the upper of the power transmitted with the uncompensated line at the givenboundary curve and identified by (e,-P),,,. Note that, as before, transmission angle. In other words, the maximum increase attainableall Q,-P curves are considered only for the "stable"range of the trans- in actual transmitted power is much less at small transmission anglesmission angle ( 0 ~ 6 ~ 9 0 " ) . than at large ones. This is due to the fact that the TCSC is a series The plots in Figs. 7a through 7d present the range of Q,-Pcontrol impedance and thus the compensating voltage it produces isfor TCSC, having the same 0.5 p a . maximum voltage rating stipulated proportional to the line current, which is a function of angle S. Byfor the UPFC, at the four transmission angles considered: S=P, 304 contrast, since the UPFC is a voltage source, the maximum604 and 90". In each figure the two boundary curves, (Q,-P),owe, and compensating voltage it produces is independent of the line current(Q,-P),+,, representing zero compensation (X,=O) maximum and (and of angle 8 ) . Therefore the maximum change (increase orseries compensation (X,=X&, are shown by a broken-line for decrease) in transmittable power, as well as in receiving-end reactivereference, together with the previously derived circular control region power, achievable by the UPFC is not a function of angle S and isof the UPFC shown by a dotted-line for comparison. determined solely by the maximum voltage the UPFC is rated to inject in series with the line (assumed 0.5 p.u. in this paper). This Plot (a) 6=0" Plot (b) k,W characteristic of the UPFC is evident from the circles of identical radius defining the control regions of the UPFC for the four different transmission angles considered. Figs. 7c and 7d, showing the Q,-Pcharacteristics of the TCSC and Control the UPFC at S=60" and S=90",respectively, further illustrates the Region far UPFC above observations. The range of the TCSC for real power control / 6=30° , , I remains a constant percentage of the power transmitted by the uncom- pensated line at all transmission angles (OSSSW), the maximum but actual change in transmitted power progressively increases with in- -0.5 0 , 0.5 4.0 1.5 creasing S and it reaches that of the UPFC at &=go". However, it should be noted that whereas the maximum transmitted power of Control 4.5 - p Interval 1.5 P.u., obtained with the TCSC at full compensation, is associated for TCSC with 1.5 p.u. reactive power demand at the receiving-end, the same Plot (e) 6=60 Plot (d) 1.5 p.u. power transmission is achieved only with 1.0 p.u. reactive power demand when the line is compensated by a UPFC. From Figs. 7a through 7d, it can be concluded that the UPFC has superior power flow control characte&tics compared to the TCSC it can control independently both real and reactive power over a broad range, its control range in terms of actual real and reactive power is independent of the transmission angle, and it can control both real and reactive power flow in either direction at zero (or at a small) transmission angle. 0.5 1.0 1 1.5 7 4.5 0.5 1.0 7 1.5 Comparison of the UPFC to the TCPAR Control 4.5 Intrrvd for TCSC Phase Angle Regulators (PARS)provide controllable quadrature voltage injection in series with the line. The commercially available Fig. 7. - Attainable Q, and P values with the TCSC (heavy line) and those PARS employ a shunt-connected evcitatwn transformer with appropri- with the UPFC (inside dashed-line circle) at S=Oq 6=304 6=604 6=904 ate taps on the secondary windings, a mechanical tap-changing gear, and a series insertion transformer. The excitation transformer has Consider Fig. 7a which illustrates the case when the transmission wye to delta (or delta to wye) windings and thus the phase to phase
  5. 5. 1089secondary voltages are in quadrature with respect to the corresponding flow independently of the real power.primary phase to neutral voltages. These voltages, via the tap-changing The plots in Figs. 9a through 9d present the Ql-P relationship forgear and the series insertion transformer, are injected in series with the TCPAR in comparison to that of the UPFC at S=P, 30", 604the appropriate phases of the line. As a result, the two sets of three- and W. It can be observed in these figures that the control rangephase voltages, obtained at the two ends of the series insertion trans- centers around the point defined by the given value of angle S=So(=O",former, are phase shifted with respect to each other by an angle a, 30", 604 90") on the Q,-P curve characterizing the uncompensatedwhere o=tan-(V, w; Vuis the magnitude of the series injected volt-age and Vis the magnitude of the line voltage (phase to neutral). power transmission (V,=O for the TCPAR and VM=O the UPFC). for As the phase-shift angle a is varied in the range of -30"Sa~30"Since the voltage injection can be of either polarity, amay represent (which corresponds to the maximum inserted voltage of V,=O.S P.u.),either phase advance or retard. this point moves on the uncompensated Qr-Pcurve in the same way The Thyristor-Controlled Phase Angle Regulators (TCPARs) as if the fixed angle Sowasvaried in the range of S,-30"SS0~S,+30". are functionaUy similar to the conventional PARS, except Consequently, the TCPAR, in contrast to the TCSC, can controlfor the mechanical tap-changer which is replaced appropriate effectively real power flow, as illustrated in Fig. 9a, when the anglethyristor switch arrangement. Like their mechanically-controlled between the sending-end and receiving-end voltages is zero (S=O).counterparts,they alsoprovide controllable,bi-directional,quadraturevoltage injection. However, their control can be continuous or step- Plot (a) 6=0 Plot(b)like, but the continuous control is usually associated with some har-monic generation. For the purpose of the present investigation, thepractical implementation of the TCPAR is unimportant. For simplicity,the TCPAR is considered as an ideal phase angle regulator, whichis able to vary continuously the phase angle between the voltages Contml 1.0 t - - - - _ _ _ _ _ I I I I I COOtrOl t - - - - - - - - -J I I at the two ends of the insertion transformer in the control range of -am~asa,,without changing the magnitude of the phase shiftedvoltage from that of the original line voltage. A basic, and in the present investigation important, attribute ofall conventional (mechanical and thyristor-controlled) phase angle + * .: 4.5regulators, including the ideal one stipulated above, is that the total VA (i.e., both the real power and the vars) exchanged by the series (-ws a 5 30-)insertion transformer appears at the primary of the excitation trans- Plot@) 1=60" Plot (d) k..w former as a load demand on the power system. Thus, both the real and the reactive power the phase angle regulator supplies to, or absorbs from, the line when it injects the quadrature voltage must be absorbed from it, or supplied to it by the ac system. By contrast, the UPFC itself generates the reactive power part of the total VA it exchanges as a result of the series voltage injection and it presents only the real power part to the ac system as a load demand. Consider Fig. 8 where the previously considered two machine system shown again with a TCPAR assumed to function as an ideal phase angle regulator. For this, the transmitted power, and the reac- 4.5 0 I 0.5 1.0 7 1.5 4.5 0 1 0.5 I 1.0 ---c 1.5 tive power demands at the sending-end and receiving-end, can be described by relationships analogous to those characterizing the un- 4.5 1 compensated) system: P={V/XlsinS and Qr=Q,={ V/XlU-cosS3, Fig.9.-AttainableQ,andPvalueswiththeTCPAR(heavy1ine)andthose Qr with the UPFC (inside dashed-line circle) at S=Oq S=3U, S=604 8=9@. The Qr versus P plots in Fig. 9 clearly show the general superiority of the UPFC over the TCPAR for power flow control: The UPFC has awider range for real power control and facilitates the indepen- VS dent control of the receiving-end reactive power demand over a broad range. For example, it is seen that the UPFC can facilitate up to 1.0 I 1 I p.u. real power transmission with unity p e r factor at the receiving- end (Ql=O),whereas the reactive power demand at the receiving-end would increase with increasing real power (Q, reaching 1.0 p.u. at Fig. 8. - Two machine system with the Thyristor-Controlled P = L O P.u.), in the manner of an uncompensated line, when the power Phase Angle Regulator. flow is controlled by the TCPAR.where S=S-a. Thus, it is clear that the TCPAR cannot increase the IMPLEMENTATION O F P, Q CONTROL ANDmaximum transmittable power, P=V/X, or change Q, at a fixed P. THE DYNAMIC PERFORMANCE OF THE UPFCConsequently, the Q,-Prelationship, with transmission angle 8(OS~SW) controlling the actual power transmission, is identical The preceding discussion has demonstrated the superior capabilityto that of the uncompensated system shown in Fig. 3d. The function of the UPFC to establish an advantageous range of P, Q operatingof the TCPAR is simply to establish the actual transmission angle conditions on a transmission line for a given S angle between sending-S, required for the transmission of the desired powerP, by adjusting and receiving-endvoltages. The capability of the UPFC results fromthe phase-shift angle a so as to satisfy the equation S=S-a at a given its unique ability to inject an arbitrary voltage vector in series withS,the angle existing between the sending-end and receiving-end volt- the line upon command, subject only to equipment rating limits. (Theages. In other words, the TCPAR can vary the transmitted power term vector, instead of phasor, is used in this section to representat a fixed 6, or maintain the actual transmission angle 8 constant a set of three instantaneous phase v?riables9, voltages or currents,in the face of a varying 6, but it cannot control the reactive power that sum to zero. The symbols 0 and i are used for voltage and cur-
  6. 6. 1090rent vectors.) Because of its electronic controls, the UPFC can cause of series injected voltage to maintain a demanded P, Q conditionthe series-injected voltage vector to vary rapidly and continuously on the line is an exciting new concept that could not be contemplatedin magnitude and/or angle as desired. It is thus not only able to estab- before the advent of the UPFC. It should be emphasized that thislish an operating point within a wide range of possible P, Q conditions P, Q controller is not intended to simply reproduce the actions ofon the line, but also has the inherent capability to transition rapidly conventional equipment such as, mechanical or thyristor-controlled,from one such achievable operating point to another. phase angle regulators, load tap-changers, and series capacitors (al- The UPFC control system may be divided functionally into intemal though the UPFC could undoubtedly do this). Instead, a generalizedand ertemal controls. The internal controls operate the two inverters automatic P,Q controller is envisioned by which P and Q at a pointso as to produce the commanded series injected voltage and, simulta- (terminal) on the line are forced to and maintained at strategicallyneously, draw the desired shunt reactive current. The internal controls chosen values by continuous closed-loop feedback control. The abilityprovide gating signals to the inverter valves so that the inverter output to define P and Q in this way opens up a range of new possibilitiesvoltageswill respond in accordance with the control structure illustrat- for power system control. Apart from the obvious advantages fored in Fig. 10. From this figure it can be seen that the series inverter power scheduling and automatic power limiting, the fast dynamicresponds directly and independently to the demand for series voltage response of the UPFC power controller will automatically preventvector injection. Changes in series voltage vector, UN, can therefore power oscillations from existing on the effected virtually instantaneously. In contrast, the shunt inverter Although it has been shown that a range of stable operating pointsoperates under closed-loop current vector control whereby the shunt exist for fixed values of UN, it is not self-evident that fast automaticreal and reactive power components are independently controlled. control can be used to determine UN. Firstly, PM is a two-dimensional N variable requiring a vector control system, and secondly, a suitable vP9 control structure must be devised to ensure closed-loop stability. TRAN SM l S S l 0 N LINE Considerable effort has been applied to this control problem and T* a unique and effective power controller has been designed to imple- ~ P H A S E I,+; i LOCKED 11 Lw ment the mathematical model shown in Fig. 11.A description of the control system is beyond the scope of this paper, but it is relevant here to present digital simulation results that show its effectiveness. To verify the proposed power controller, digital simulation studies were performed using EMTP. Figure 12shows the simplified trans- mission system that was modeled, comprising a sending- and receiving- end bus and a predominantly inductive transmission line, and Figure Pq REF Inlmotlon Fig. 10 - UPFC internal control systemThe shunt reactive power responds directly to an input demand. Theshunt real power is dictated by another control loop that acts tomaintain a preset voltage level on the dc link, thereby ensuring therequired real power balance between the two inverters. As mentionedpreviously, the inverters do not exchange reactive power throughthe link. The extemal controls are responsible for generating the I I I Idemancis for series voltage vector, Om, and shunt reactive current Fig. 12 - Simplified transmission system and UPFC modelvector, jq. These demands can be set to arbitrary constant values, 2 1 I i ~ " " " " " " " " " " " " " " " "by operator input for example, or else their values can be dictated 0by automatic control to meet specific operating and contingencyrequirements. Figure 11 shows, in block diagram form, a mathematicalmodel of the UPFC with internal and external controls. Automatic control of shunt reactive power, for the purpose ofvoltage regulation, is well known from conventionalstaticvar compen-sators, and the same principles can be applied here, as shown in thevoltage control block of Fig. 11. However, fast automatic control ...................... ...................... - % I UPFC k L > ~ ! ? TRANSMISSION 4 TRANSMISSION LINE POWER CONTROLLER I BL3CK MEASURED y L . c . .c 041E9L B L W , oLAE I.. ~ VOLTAGE i REGULATOR REFERENCE Fig. 13 - Simulation results for steplike changes in P and Q demands
  7. 7. 109113 shows the results obtained. As seen,the UPFC is positioned demonstrate the effectiveness of the power controller in dampingadjacent to the sending-end bus. In the first simulation, the sending- power oscillations. In this case the sending-end bus voltage is heldand receiving-end bus voltages are assumed to be constant, of the constant and the receiving-end voltage is set to swing in angle withsame 1p.u. magnitude, and displaced by angle S=20". As illustrated a frequency of approximately 2.5 Hz and an average S=-20". Figurein Fig. 13, the power controller is instructedto perform a series of 14 shows the result obtained with the UPFC inactive (i.e., vw=O),step changes in rapid succession. Firstly P is increased, then Q, and Fig. 15 with the UPFC power control activated. The results arefollowed by a series of decreases, ending with a negative value of dramatic, showing that the UPFC is able to eliminate the alternatingQ. The waveforms, showing the injected voltage vp4,the system voltage power component on the line and maintain constant P and Q withat the two ends of the insertion transformer, v,=v, and v,=v,+v,, a relatively small injected vw.and the line current, illustrate clearly the operation and fast Clearly there are practical limitations to the UPFC, operated as(approximately a quarter cycle) response time of the UPFC. a generalized power flow controller, associated with the rating of The second simulation, shown in Figs. 14 and 15, is intended to the equipmentinstalled and the possiblesystem imposed voltage limits and other practical constraints. These limits must necessarily be ob- served by the controller, and in some cases they will prevent the UPFC from achieving the mast desirable operating point. Consequent- ly an optimization strategy is required to ensure that the best possible operating point is always achieved, based on pre-determined criteria Real (P) an Reacl ve ( 0 ) Line Pouer for the particular transmission system. -1 STUDY FOR UPFC APPLICATION IN THE POWER SYSTEM I OF THE WESTERN AREA POWER ADMINISTRATION 0 The Western Area Power Administration is responsible for a -1 transmission system that consists of more than 16,500 miles of line over an area of some 1.3 million square miles. Major power and var controlfacilities presently used by Western include conventionalphase angle regdatoxs, static var compensators, series capacitoxs, and HVDC interties. The realization of a practical UPFC has several logistic and operational advantages based upon Westems diversity in power system facilities and equipment. With the UPFC being able to control all three line parameters (voltage,impedance, and angle), the concept of having one static piece of equipment in lieu of several distributed 1 equipment installationshas the potential for not only improved system performance, but reduced maintenance, less overall property for 0 facilities, and simplified operations for dispatchers. -1 ... In the past, Western has studied the UPFC using small test cases -2 . . ,!lI!l,. . . . ., . .Linp,Currsqt .cphap;A). . . . . . . . . . . . . . ..... ....... .... to simulate system transients and dynamic response. The present 0 0.1 0.2 03 . 0.4 0.5 t 1necsec) 0.6 0.7 0.8 0.9 1 full-sized planning system study examines whether the UPFC could provide multi power flow and dynamic compensation functions in Fig. 14 - Simulated power oscillation (UPFC inactive) a major application like the Mead-Phoenix project. This project involves the construction of a 500 kV line from the Phoenix area (Westwing) to the Las Vegas area (Mead and Marketplace) and on P to Los Angeles. The Westwing-Mead line has 70% series compensa- I tion and the Marketplace-Adelantoline 45%. The project also includes two phase angle regulators,each rated for 650 MVA, 0-16", for steady- 61 state requirements, and two static var compensators, one rated for I-. . . . . . . . . Real I .. I , I (P) , . .anp. Reactjve. (. 0.) . .... Line Pouer I . . . . ) . .. . I .... 8 . ... 400 Mvar at the Marketplace bus and the other rated for 375 Mvar at Adelanto, to damp power oscillations following system disturbances. Figure 16 shows the simplified one-line diagram of the 500 kV 4 MEAD 230 MCCULLOUa -2 . . . I . . . . , . . . . I . . ..... U1 Cphqse-A), Upq (ppase-A) I . . . . . . .I . . . . , . . . . , . . . . ADELANTO ?? Y*RKETPL / 1 0 -1 -2 .... I . . . . , . . U1 Cphyse-A),, I . . . . . ....., . . . . Uo. (ptpse-A> . ... 2 " " I " " I " " , ~ ~ ~ . I . ~ . . , . . . . , . . . . , . . .. I . . . . 1 . ... DEVERS -2 0 .... 0.1 , . . . . I . . . . , . . . . - 02 . 03 . L i n e , Curreql . (phaqe-A) 01 . .... ..... 0.5 time(sec) Fig. 15 Power oscillation damping with UPFC regulating P and Q 0.6 .. 07 . I . . . . I . . . . I . . . . 0.8 0.9 1 IMPERIAL - Fig. 16 - Simplified 500 kV system with the Mead-Phoenjx projeci
  8. 8. and superior dynamic performance. Further studies are planned which will focus on finding an effective combination of the UPFC and con- ventional equipment (e.g., series capacitors), and on the application of improved control algorithms, in order to come up with the best trade-off between cost and performance. CONCLUSIONS The Unified Power Flow Controller, from the viewpoint of conven- tional transmission compensation and control, is an apparatus that can provide simullaneou, real-time control of all or any combination of the basic power system parameters (transmission voltage, line impedance and phase angle) which determine the transmittable power. However, the UPFC can also be viewed as a generalized real and reactive power flow controller that is able to maintain a prescribed P and Q at a given point (bus) on the transmission line. As is shown in the paper, a broad range of P and Q control is achievable with reasonable UPFC rating. The computer simulations indicate that the attainable response of the control is very fast, almost instanta- neous, and thus the UPFC is extremely effective in handling dynamic system disturbances. The UPFC provides a flexibility for ac power transmission control that has been hitherto achievable only by HVDC transmission. Strate- gically chosen and adjustable P and Q values, representing steady-state and temporaxy p w e r system requirements and contingency conditions, can be forced to and maintained at a given point on the transmission system by continuous closed-loop feedback control. Apart from the I 2.00 . . . . I . 8 " I , " I " I " T " " ~ " I obvious advantages for power scheduling and automatic power limit- ing, the UPFC will also automatically counteract power oscillations and can adapt almost instantaneously to new P and Q demands to 1.75 - enhance the transient behavior of the system and optimize its perfor- ill Corlventlona! mance under transmission contingency conditions. t ACKNOWLEDGEMENTS The development of the Unified Power Flow Controller is being carried out under joint sponsorship by the Electric Power Research Institute (EPRI) and the Western Area Power Administration within //I WESTWING-READ 5 3 O X U POWER FLOW (giqauatts) 1 EPRIs Flexible AC Transmission System (FACTS) initiative. The authors would especially like to thank Dr. N. G. Hingorani and Mr. 11me(sec) S. L. Nilsson for recognizing the potential of, and providing support Fig. 17 - Real power oscillation dampmg with conventlonal equipment for, this project. (dashed line) and with the UPFC (continuous line) The authors would like to acknowledge the contributions of Mr. K. L. Parker of the Westinghouse Science & Technology Center, Figure 18 displays the Mead 500 kV bus voltage both for the who generated the programs for computing and plotting the P, Qconventional equipment (dashed line) and the UPFC (continuous relationships shown in the paper.line). It can be seen that the UPFC also improves significantly thedamping of the voltage swings. REFERENCES " " I " l " " I , " ~ " " 1 " " ~ 1.10 " 8 " 8 " ~ / [l] Gyugyi, L., "A Unified Power Flow Control Concept for Flexible AC Transmission Systems,"IEE PROCEEDINGS-C, Vol. 139, No. 4, July 1992. [2] Gyugyi, L., "Dynamic Compensation of AC Transmission Lines by Solid-state Synchronous Voltage Sources," IEEEPES Summer Power Meeting, Paper No. 93 SM 434-1 PWRD, Vancouver, B.C., Canada, July 1993. MEAD 500KU BUS UOLTAGE (per-unit) [3] Chamia, M., and Angquist, L., "Thyristor-Controlled Series Capacitor Design and Field Tests," EPRI Flexible ACTransmission 0.85 : . , . i , , , . I . . , I . . . . ) , , . . I . , . . I . . . . I . . . . I . , . . N . . , System (FACTS) Conference, 18-20 May, 1992, Boston, MA. 0 I 2 3 4 5 6 7 8 9 10 1 I me( sec) Fig. 18 - 500 kV bus voltage at Mead (dashed line: [4] Juette, G. et al., "Advanced Series Compensation (ASC) - Main conventional equipment: continuous line: UPFC) Circuit and Related Components," EPRI Flexible ACTransmission System (FACTS) Conference, 18-20 May, 1992, Boston, MA. The above hypothetical application study shows that a single UPFCcould replace the combination of phase angle regulators, series capaci- [5] Urbanek, J. et al., "Thyristor-Controlled Series Compensationtors, and static var compensators and provide equivalent steady-state Prototype Installation at the Slatt 500 kV Substation," IEEE/PES
  9. 9. 1093Summer Meeting, Paper No. 92 SM 467-1 PWRD, Seattle, WA, July Scott L. Williams was born in Torrance, California, in 1963. He1992. obtained a B.S.E.E. from Wilkes College, Wilkes-Barre, PAin 1985 and the M.S.E.E. degree from Clarkson University, Potsdam, NY[6] Larsen, E. V., et al., “Characteristics and Rating Considerations in 1986. He has seven years experience in the areas of closed-loopof Thyristor-Controlled Series Compensation,” IEEEPES Summer feedback control systems, real-time embedded microprocessorMeeting, Paper No. 93 SM 433-3 PWRD, Vancouver, B.C., Canada, applications, analog and digital circuit design, and detailed systemJuly 1993. analysis and simulation. He joined the Westinghouse Science & Technology Center in 1992 where he is a Senior Engineer in the[7] Baker, R. and Guth, G., “Control Algorithm for a Static Phase Power Electronics Department. He is currently developinga detailedShifting Transformer to Enhance Transient and Dynamic Stability hardware TNA scale model of the UPFC to rigorously test andof Large Power Systems,”IEEE Transactions on Power Apparatus evaluate control and protection strategies.and Systems, Vol. PAS-101, No. 9, September 1982, pp. 3532-3541. Thomas R. Rietman (M’) was born in central Minnesota. He[8] Wood, P., et al., “Study of Improved Load Tap-Changing for received his B.S.E.E. degree from the University of Colorado andTransformers and Phase-Angle regulators,” EPRI Report EL-6079, his M.S.E.E. degree from the Universityof Minnesota. After receivingProject 2763-1, 1988. the M.S.E.E. degree, he obtained a position at the B o n n e d e Power Administration in Portland, Oregon, where he worked for 17 years[9] Schauder, C. D. and Mehta, H., “Vector Analysis and Control in both the engineering and operations areas of the organization.of Advanced Static Var Compensators,” IEE PROCEEDINGS-C, In 1988he joined the Western Area Power Administration in GoldenVol. 140, No. 4, July 1993. where he is a staff engineer and engineering applications computer programmer. BIOGRAPHIES Laszlo Cyugyi (IEE P 1976) received his undergraduate education Duane R. Torgerson (SM’) received the B.S.E.E. degree inat the University of Technology, Budapest, further studied electronic engineering from the California Polytechnic Universitymathematics at the University of London, England, and electrical and the M.S.E.E. degree from the Purdue University. He is presentlyengineering both at the University of Pittsburgh, PA (M.S.E.E. 1967) an engineer in the Division of Substation Design at the Westernand at the University of Salford, England (Ph.D. 1970). He joined Area Power administrationand responsiblefor substation applicationsthe Westinghouse Science & Technology Center in 1963, where he involving solid-statepower technology. He is member of CIGRE andis now the manager of the Power Electronics Department and participates in IEEE, IEE, and IEC working groups.responsible for technology and product development in this area.Dr. Gyugyi has a broad experience in all areas of power conversion Abdel-Aty Edris (SM’1988) was born in Cairo, Egypt, in 1945.and control, and has been pioneering advanced power electroniobased He received his B.Sc. with honor from Cairo University in 1967, M.S.compensation and control techniques for utility applications,for which from Ain-ShamsUniversity,Egypt, in 1973, and Ph.D. tiom Chalmerse k r t he received the Westinghouse Order of Merit (the corporation’s University of Technology, Sweden in 1979. From 1981 to 1990 hehighest honor) in 1992. was employed by ASEA (now ABB) in Vasteras, Sweden, and from 1990 to 1992 he w s with the Transmission and Relaying Center of a Colin D Schauder (M’1981) was born in Port Elizabeth, South . A B B s Advanced Systems Technology in Pittsburgh, PA. His workAfrica, in 1952. He was awarded the B.Sc. Engineering and Ph.D. has involved power system analysis for HVDC and SVC projects.degrees by the University of Cape Town, South Africa, in 1972and In 1992 he joined EPRI as Manager of Flexible AC Transmission1978, respectively. From 1978 to 1983 he was employed by GEC Systems (FACTS).Industrial Controls in Rugby, England. In 1983 he joined theWestinghouse Science & Technology Center where he is now anAdvisory Engineer in the Power Electronics Department. His workhas man@ been involved w t high performance ac motor drives and ihthe design and control of advanced power conversion systems.