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IMPROVEMENT OF TRANSIENT STABILITY USING UNIFIED POWER FLOW CONTROLLER

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  • IEEE T " S on Power Delivery. Vol. 11, No. 1. January 1996

    IMPROVEMENT OF TRANSIENT STABILITY USING UNIFIED POWER FLOW CONTROLLER

    R. MihaliE and P. hnko, Member IEEE Faculty of Electrical Engineering and Computer Science,

    Tt2aSka 25, 61000 Ljubljana, Slovenia

    ABSTRACT

    The aim of the paper is to analyze the effect of an Unified Power Flow Controller (UPFC) on transient stability margin enhancement of a longitudinal system. To utilize the UPFC possibilities fully, the three controllable UPFC parameters were determined during the digital simulation process performed by the NETOMAC simulation program. The basis for determination of the suitable damping strategy and for determination of the optimal UPFC parameters is a mathematical model, which describes the interdependence between longitudinal transmission system parameters, operating conditions and UPFC parameters in the form of analytical equations. On the basis of the mathematical model, the theoretical UPFC limits were also detected, and their appearance explained.

    Kevwords: FACTS, unified power flow controller, AC transmission, transient stability,

    1. INTRODUCTION

    One direction of large AC system development is the transmission of large amounts of power over long distances by high voltage transmission lines from remote power sources to load centers. Because of growing public impact on environmental policy, the building of new transmission facilities, in general, lags behind the increased needs of power transmission. As a consequence, some transmission lines are more loaded than was planned when they were built. With the increased loading of long transmission lines, the problem of transient stability after a major fault can become a transmission power limiting factor. In some cases, this factor may be considerably lower compared to other limiting factors. Power electronic equipment, including appropriate control, offers effective solutions to this problem. Such equipment, including advanced control centers and communication links, is the basis of the so called Flexible AC Transmission System (FACTS). Some FACTS devices controlled by power thyristors such as, for example, Static Var Compensator or Controlled Series Compensation, are already widely used, or their prototypes have been put into operation. The development of power semiconductor devices with turn-off capability (GTO, MCT) opens up new perspectives in the development of FACTS equipment. The universal and most effective device is expected to be the Unified Power Flow Controller (UPFC).

    95 WM 269-1 PWRD A paper recommended and approved by the IEEE Transmission and D i s t r i b u t i o n Committee of the IEEE Power Engineering Society f o r presentat- i o n a t the 1995 IEEE/PES Winter Meeting, January 29, t o February 2 , 1995, New York, NP. Manuscript sub- mi t ted July 19, 1994; made ava i l ab le f o r p r i n t i n g January 9 , 1995.

    D. Povh, Fellow IEEE SIEMENS AG

    Box 3220, 91050 Erlangen, Germany

    485

    This device can independently control more parameters, thus combining the properties of a static condenser (STATCON), controlled series compensation, and phase angle regulator.

    The objective of this paper is to answer the question of how to utilize the UPFC properties fully, or in other words: how should the UPFC parameters be controlled in order to achieve maximal desired effect when solving transient stability problems which appear when bulk power is transmitted by long transmission lines.

    2. UPFC OPERATING PRINCIPLES

    Basically, the UPFC structure is similar to that of the phase shifting transformer. As shown in Fig. 1, it consists of a parallel and series branch, each consisting of the transformer, the power electronic converter with turn-off capable semiconductor devices (GTOs for example) and the DC circuit. The parallel branch transformer is connected parallel with relation to the transmission line, while the series branch transformer is functionally a boosting transformer. In order to make the explanation of the basic operating principles easier, at first let it be assumed that the parallel and series branches are not connected. The parallel branch DC circuit consists of the capacitor Cp and the series branch DC circuit of the capacitor Cs.

    PARALLEL BRANCH U A TRANSMISSION LINE

    CONTROL,

    FIRING WLSES I I

    Fig. 1. General UPFC scheme

    The parallel branch is in fact a static condenser (STATCON). The operating principles are described in 111. At this stage the most important fact is that the parallel branch can act (in the greatest part of the operating area) as a reactive current source (current b), because the maximal output reactive current is independent of terminal voltage. The current phasor I (Fig. 1) is perpendicular to the input terminal voltage phasor IA (in this instance the losses - active and reactive - are neglected). The series branch represents the so called advanced controllable series compenaation (ACSC). The injected voltage of the boosting transformer UT (Fig. 1) is perpendicular to the line current 121. From this point of view, the series UPFC branch

    0885-8977/96/$05.00 0 1995 IEEE

  • 486

    UT (UT and e) and the reactive parallel branch current 4, and thus offers the possibility of independent control of the three electrical system parameters.

    acts as a series condenser. Nevertheless, from the system point of view, ACSC differs considerably from the series capacitor. The main difference between them lies in the operating characteristic. While a series capacitor is a reactive impedance,' an ACSC acts as a controllable voltage source whose voltage magnitude can be controlled independently of line current (the voltage phase, of course, being shifted by 90' with regard to the line current). By changing the ACSC voltage polarity, the effect of a controlled series reactor is achieved. Therefore an ACSC is superior to the series capacitive compensation in the oscillation damping efficiency. Additionally, an ACSC does not produce series resonance with the line reactance and thus cannot cause subsynchronous resonances; with suitable control they can be even damped [2].

    Each of the two just described UPFC branches can generate or absorb the reactive power independent of each other. The described properties do not change if the DC circuits of both devices are connected and if the common DC circuit consists of capacitor C (Fig. 1).

    Additionally, the possibility of a controllable phase shift between phasors UA and UB appears, due to the fact that between the series and parallel branch real power can be exchanged. The injected series branch voltage U, can theoretically be in any phase with regard to UA and can have any magnitude ranging from 0 to some maximal magnitude U,-. The operating area becomes the region limited by a circle with the radius UT-. The top of the phasor U, (and thus of the phasor UJ can take any position inside that area (Fig. 2). The operating point can be continuously changed (continuous change of U hase and magnitude). The component of the voltage U, which IS in phase with the current le represents the real part of the injected power. This is provided by the UPFC parallel branch (current 1,). The component of the voltage U, which is rectangular to the current le represents the reactive power component. It is generated internally (ACSC) and is independent of the real component. The injection of real and reactive power by serial branch of UPFC can be denoted as the result of voltage U, injection.

    T .p

    Fig. 2. UPFC phasor diagram and its operating area

    Assuming that UPFC is not connected directly to the stiff voltage source, the voltage of the UPFC shunt transformer terminal can further be controlled by regulating the reactive parallel branch current 6. The actual UPFC operating area becomes the circle denoting the series branch operating area, which can be shifted in phase by the input terminal voltage phasor UA (by &AU - Fig. 2).

    From the above explanation of the UPFC operating principles it is evident that, in contrast to other FACTS devices, UPFC has three independently controllable parameters, which are the magnitude and the phase of the series branch injected voltage

    3. THE UPFC MATHEMATICAL MODEL

    The mathematical UPFC model was developed with the aim of being able to study the relations between the electrical transmission system and UPFC in steady state conditions and during the electromechanical transient conditions. The model makes it possible to determine the three controllable UPFC parameters in various operating conditions, so that the UPFC impact on the state of the system is optimal according to the defined requirements. The model should contain the UPFC model included into the transmission system model. The basic scheme of the proposed model is shown in Fig. 3, and in this paper is referred to as the "basic model". ,

    ....................

    Fig. 3.

    The transmission system is represented by WO voltage sources and two l3 sections. This can be a representation of a longitudinal system consisting of generators, transformers, loads, series and/or parallel reactive elements etc. The parameters of the basic model can be achieved by the methods of "classic" network impedance reduction or by parameter identifying techniques. The UPFC model is positioned in the middle of the transmission system model. It consists of a series voltage source representing the UPFC series branch, the susceptance BT representing the UPFC parallel branch reactive compensation effect and of the current source !, representing the UPFC parallel branch active current. The modeling of the parallel branch reactive current by susceptance was performed in order to make the analytical deduction possible. The voltage source voltage UT magnitude and phase, and also the susceptance BT magnitude, can thus be controlled. The phase and the magnitude of the current source IT are determined by UPFC physical properties explained in sec. 2, i.e. iT balances the real UPFC power injected into the system by the series branch and is thus in phase with the terminal voltage &,.

    The relations between the electric quantities shown in Fig. 3 can be written in complex form as follows:

    _V, = G, + &; Y', = & + jB,; 41, = & + jB3; _V, = G, + jB, z1 = R1 + jx, z, = R, + jx, go = Uoe+ = U0(cos(6) + jsin(6)); (1 ) U , = U,e'* = U,(cos(cp,) + jsin(cp,)) 1, = 4, + ill, The phase of the voltage phasor U2 is the reference phase. The aim of the mathematical deduction is to find the relations between source powers % and S2, the basic model impedahces and the UPFC controllable parameters U,, and BT' In order to simplify the mathematical derivation, the powers are expressed by the currents !, and &, as follows:

    Scheme of the "basic model"

    ;

    U, = U,

  • 487

    The following set of equations, which describes the basic model, can be derived from the basic relations between electric quantities by taking the already described electric UPFC properties into consideration. The eq. (7) describes the fact that 1, balances the real power injection of the series branch.

    U, - U2 !2s = ___

    R2 + i x 2

    From the set of equations presented, it is possible to find analytical solutions for the real and imaginary components of the current I , in the form of the followinu euuations.

    (9)

    The factors A, to A, and DET are functions of the basic model impedances, the generator voltage magnitudes, the transmission angle S and the controllable UPFC parameters (UT (pT and ST). They have no physical significance, and were introduced for practical reasons because the expressions which are the result of the mathematical deduction of the relation (9) from eq. (3) to (8) are very comprehensive in the expanded form. The factors mentioned are presented in 131. From (2) it is not difficult to obtain the source powers Po, Q,, P, and Q,.

    From the form of the solution (9), it is evident that in cases when the factor DET is negative, the values of currents IIR and 11, are not real. This means that the system cannot be brought into such operating conditions. In other words, if the terminal voltages % and u2 of the transmission system are defined (Fig. 3), the voltage phasor UT can not be arbitrarily rotated and/or "stretched". The areas where the factor DET is negative are referred to in the paper as undefined areas. Physically the appearance of the undefined areas can be explained by the fact that arbitrary amounts of power cannot be transmitted between sources and the UPFC terminals. It is not always possible to take enough real power from the system by the parallel branch in order to cover the real power injection of the series branch. The numerical calculations in the mathematical model have shown that on one of the boundaries of an undefined area the transmission system between voltage source and UPFC reaches its static stability limit of power transmission. On the other boundary of the undefined area the voltage Ug collapses. From the above explanation it is also evident, that there is no appearance of undefined areas if UPFC is connected to the stiff system, and that the undefined areas extend if connection between UPFC and the sources is weak.

    The problem of the undefined areas is demonstrated in the test system shown in Fig. 5, UPFC being positioned in the middle location. The aim of the demonstrating example is to answer the following question: if the transmission angle 6 is for instance 90 (or 120), and if UPFC parameters UT and 'pT have certain values, IS factor DT positive or negative? This calculation was repeated for UT values reaching from 0 to 1.2 P.U. (1 P.U. being phase-to-earth voltage of the test system) and (pT values

    reaching from 0 to 360. The results are presented in Fig. 4. The black painted areas represent the undefined areas. Of course, undefined areas change with the system operating point as well as with the UPFC location.

    t 1.2 UT /P.U.

    0 --i, pT/deg./ 360

    t 1.2 l$ m.u./

    Fig. 4. Undefined areas presented in the U, - 4 plane and a) S =90 , b) 6 =120

    4. TEST SYSTEM AND DIGITAL SIMULATION MODEL

    The possibility of transient stability enhancement by UPFC was studied on a longitudinal transmission system. The test system and its parameters are shown in Fig. 6. The transmitted pre-fault real power (Pgen - Fig. 6) is equal to 1350 MW (90% of generator rated power). The generator can be a representation of an electrically concentrated subsystem. The lines are modeled as series-connected Il sections (1 section per 100 km). Both the generator and the excitation control are modeled in detail. The turbine and governor are modeled in simplified manner using constant turbine power P,. The three possible UPFC locations were studied (UPFC connected to generator, to stiff source and between the lines).

    UPFC

    I ,location alternatives

    , , :- i UPFC ,. . . . . . . . . . . ,

    GENERATOR xd"=.182 P.U. LINES: SYSTEM: Pn=1500 MVA xd'=.270 P.U. r=0.03 Wkm Un=500 kV Ta=8s xd=1.47 P.U. x=0.33 Wkm f=60 Hz Td'=0.034 s xq"=.211 P.U. c=12 nF/km Pg=1350 MW Td=2s xq'=.636 P.U. Tq'=0.04ls xq=xd

    Fig. 5. Test system

    UPFC for dynamic simulation purposes is modeled as shown in Fig. 6. Basically, the model is comparable to the one applied in the mathematical model. The voltage source and the two current sources are controllable. The UPFC coordinator controls the sources, so that the model, shown from outside, acts as an UPFC (the real powers of the voltage source UT and of the current source lT are balanced, thus the current source IT is held in phase with the terminal voltage 4, while the phase of source 4 is shifted by 90'). The UPFC controls the three controllable UPFC parameters in order to fulfill the required damping strategy explained later. The current 4 can be freely selected; therefore let it be limited so the power of the series branch defined by maximal U, and current corresponding transmission of 1350 MVA power.

    that the readbe power of the parallel branch dees net exceed

  • 488

    I I I I I I UPFC Network Represerttstian I UPFC MODU I

    Fig. 6.

    The disturbance in the test system represents a three phase fault on BUS1 (Fig. 5), followed by disconnection of the line. The maximal fault duration time at which the system maintains synchronism is 75 ms. The aim of the study was to determine minimal UPFC dimensions to assure synchronism after the faults- duration of 100 and 150 ms respectively - that is for cases when 25 and 75 ms margins are required. This is also an equivalent to increased power transfer at the same fault duration.

    5. DETERMINATION OF THE UPFC PARAMETERS AND

    UPFC model for dynamic calculation

    BASIC OUTLINES OF THE CONTROL STRATEGY

    In order to achieve the maximal UPFC effect on the test system transient stability margin enhancement, during the critical period the UPFC parameters have to be controlled so as to achieve maximal real power flowing from the generator (UPFC rating being, of course, limited: the injected voltage magnitude U, and parallel branch reactive current Iq should not exceed certain values). These parameters are referred to in the paper as "optimal UPFC parameters". The circumstances in the test system were first studied with the mathematical model. Therefore, the impedance scheme of the test system (Fig. 5) was transformed into the form of the basic model (Fig. 3). For this purpose, the generator impedances of the test system were taken into consideration as the generator transient reactance, and the lines were modeled as n sections. Naturaly, the parameters of the basic model change if the UPFC location or orientation (the positioning of UPFC as faced to the system) is changed. The basic model transmission angle F and power Po correspond to the angle between the generator main field voltage and stiff system voltage and the generator real power P en of the test system respectively. The "optimal UPFC paramepers' giving the extreme (maximum or minimum) transmission power must satisfy the following set of equations:

    The numerical calculations, with the data of the test system, have shown that a solution to the Set of equations (IO) does not exist. If the first equation of the set is satisfied, then the value of a p ~ / a u ~ is always positive and the value of aP0/8& is positive, if maximal source power is required and negative if minimal source power is required. This means, if extreme source powers are required, the parameters U, and BT should be set to maximal values, defined by UPFC dimensions. BT should be positive (capacitive character) if maximal source power is required and negative (inductive character) if minimal source power is required. Thus, two of the three UPFC controllable parameters are determined. Although it may seem to be obvious,

    that U, and BT are set to maximal values to achieve maximal transmitted power, theoretical cases have been found in the vicinity of undefined areas where this is not the case. The analytical solution of the first equation (10) could not be found, therefore for each operating condition it had to be found numerically.

    In order to illustrate the UPFC impact on the test system, the system transmission characteristics (generator real power vs. transmission angle 6) were calculated with optimal UPFC parameters, UPFC being connected to the voltage source terminal. For each 6, first the optimal 4 angle was calculated from 1. eq. (lo), then the transmitted power was calculated applying (2) and (9). The set of "optimized" transmission characteristics (positive values) for various maximal U, values is shown in Fig. 7a. In Fig. 7b the corresponding UPFC DC link powers are plotted. In this special case (UPFC connected to the stiff system - &=k',=&=O - c.f. Fig. 3) the optimal 6, if maximal transmitted power is required, (e-) can be calculated analytically and amounts to:

    If minimal transmitted power is required, optimal q+ (I+,,,,,,) amounts to: ' P T m n = 'PTmax + (11)

    '(MAX)'-maximal power required "(MIN)"-minimal power required

    -2000 -7 Fig. 7. a) Transmission characteristics - different U,, 4 for

    each 6 being determined so as to achieve maximal "(MAX)" or minimal "(MIN)" transmitted real power.

    b) corresponding UPFC DC link powers

    If the UPFC is located at the voltage source terminal, the undefined areas do not appear, because there is no impedance

  • 489

    From point F on, the operating point is held on characteristic "Pomi; until back swing limit (point G) is reached. Because the decelerating energy is reduced by the transition of the operating point from the 'Pomm" to the "Pomi," Characteristic, the back swing is relatively small, and goal 'II" is reached. From point G on, the power swing damping strategy takes place (not shown in the Figure). During this period, the UPFC injected voltage magnitude UT is modified in proportion to the power swing magnitude. Depending on the sign of the real power flow time derivative, the parameter 4 is chosen so that the generator real power flow is accelerated or decelerated in a proper manner by the UPFC to damp oscillations.

    6. SIMULATION

    Modeling and digital simulation are carried out by the program system for digital simulation. In order to achieve maximal UPFC effect on lSt swing stability margin enhancement - i.e. to answer the basic question: "what can be achieved by UPFC?" - the three UPFC controllable parameter must be optimized on-line, that is, during the calculation within each particular digital simulation integration step. The parameters U, and Iq are known from the preliminary static model study based on mathematical model calculation (section 5), and only the optimal parameter q must be determined on-line. This is realized in the UPFC control module (Fig. 6).

    between the UPFC and voltage source. If UPFC is shifted "inside' the transmission corridor, the appearance of the undefined areas is possible during dynamic conditions, especially in the domain of large transmission angles.

    With the UPFC, the problem of lst swing instability of the test system (sec. 4) is one which should primarily be solved. Additionally, the damping of the system should be improved. The specific UPFC global control strategy was applied to achieve the following goals:

    I) to maintain the system in synchronism during the lSt swing, 11) to reduce the rotor back swing to small extent, 111) to prevent the system from persevering near the maximum

    of lst swing and IV) to damp as effectively as possible the following swings.

    The problem of the generator rotor persevering near the maximum of the lst swing is not problematic if only one generator is present. If the generator of the test system is a representation of a part of the system it may lead to local oscillations between various generators and thus to instability, therefore goal "111" was introduced.

    Po/Pm*x

    Fig. 8. Representation of the UPFC damping strategy

    The UPFC lSt swing damping strategy is schematically presented in fig 8. The fault occurs in point A. During the fault, the rotor of the generator accelerates to point B. At that point, the fault is cleared. The control acts so as to lead the operating point from B to the characteristic of the maximal transmittable power - point C (Pow - U, and I on their limits, 'pr is "optimal"). The operating point is then held on this curve until point D (lSt swing limit) is reached. Of course, the UPFC has to be rated so, that the operating point does not exceed the limit of the lst swing stability margin E (otherwise the system loses synchronism). If this is valid, goal "I" is reached. Because of goal "Ill", the control acts so that the UPFC does not change its acting strategy until the chosen real power on the back swing is reached, which exceeds the turbine power level PT by AP. At this moment, the operating point is in position K and the procedure for the transmission of the operating point from the characteristic of the maximal transmittable power "Poms; to the characteristic of the minimal transmittable power "Porn,;, i. e. to point F, takes place. If the operating point is led from point D to the characteristic "Pomi;, the decelerating energy will be small and the rotor of the generator will persevere in the area of high transmission angles. In tllis -se, goal "111" would not be achieved.

    q

    The momentary system quantities, which are the result of the digital simulation process in the present integration step, are introduced into the UPFC control module. On the basis of a known test system structure, the parameters of the basic model are calculated, and are valid for the present state of the system (impedances, transmission angle 8). On the basis of this calculated data, 1. eq. (10) is solved to determine the optimal parameter q (to achieve maximal or minimal real power flow). According to the general damping strategy (Fig. 8), the suitable UPFC parameters are selected depending on the position of the system operating point (Fig. 8). As in the UPFC control module this procedure is finished; the three UPFC control parameters are sent to the UPFG coordinator (Fig. S), which calculates the parameters of the controllable voltage and current sources of the UPFC network representation so as to assure the proper (UPFC - like) behavior. Then the simulation process can go one integration step forward with new UPFC parameters. The whole procedure is repeated in each integration step.

    Additionally, the logic is integrated into the UPFC control module, which perceives undefined areas and determines UPFC parameters so as to achieve a certain safety margin between them, and an actual operating point. In such situations the UPFC parameters cannot, of course, allways be "optimal".

    I GENERATOR

    Fig. 9. Oscillograms and P(8) characteristic - fault duration 75 ms, without UPFC

  • 490

    The control procedures described could also be implemented in real UPFC control, the system parameters needed for the calculation of the "optimal UPFC parameters" being derived from local electric quantities (the system structure of which is known).

    The results of the digital simulation (oscillograms and dynamic transmission characteristics) are shown in Fig. 9 and 10. In the Fig. 9, the dynamic behavior of the test system quantities is shown for the case of a 75 ms fault duration and without UPFC. The system is at the limit of transient stability (fautt duration of 76 ms causes loss of synchronism in lSt swing). The system is also at the limit of oscillatory stability.

    The circumstances in the case of a 100 ms fault duration and UPFC being included between "BUSY and the stiff system - the series UPFC branch being connected to the stiff system - are presented in Fig. 10; the UPFC is dimensioned so that the synchronism is just maintained. In the oscillograms and in the dynamic transmission characteristic, the significant points related to the control strategy are marked.

    Fig. 10. Oscillograms and P(6) characteristic - fault duration 100 ms, UPFC located at "BUSS"

    The simulation tests were made in all three location alternatives and for both orientation alternatives (parallel branch on the 'left sideM and parallel branch on the "right side" with relation to the series branch"). The favorable location (the smallest UPFC dimensions to assure stability) is location at 'BUS1" (Fig. 5), with the series branch being connected to the generator terminal.

    The minimum injected voltage magnitudes U,, which assure the system stability in cases of fault durations of 100 and 150 ms, are 0.075 P.U. and 0.507 P.U. respectively. The corresponding UPFC series branch rated powers are 110 and 735 MVA. The rated power is calculated according to the pre-fault circumstances (the product between maximal injected voltage and the pre-fault current). During the transients the powers can, for a short period, be considerably higher. The MVA ratings of other FACTS devices (SVC, STATCON, series compensation, phase shifting transformer) should be considerably higher in

    order to achieve the same transient stability margin enhancement of the system studied [4]. This was expected because the UPFC control 'searches" for the best combination between parallel and series compensation effect and the angle shift effect on-line, depending on momentary system conditions. The device with the same MVA rating and one controllable parameter cannot be as effective.

    7. CONCLUSIONS

    The UPFC can provide simultaneous, fast and independent control of all three basic system parameters (terminal voltage, transmission line impedance and phase angle) and thus acts as a combination of STATCON, ACSC and controllable phase shifting transformer. The smooth transition between various operating modes is possible without any mechanic actions. However by acting as one of the mentioned FACTS devices (although it is the most appropriate one in certain conditions) the UPFG possibilities are not fully utilized. To achieve optimal UPFC behavior during the dynamic phenomena, the most effective combination of the three operation modes must be determined on-line. Only in this way is it possible to evaluate UPFG efficiency and make comparisons with other devices.

    The developed mathematical model describes the interdependence between longitudinal transmission system parameters, operating conditions and UPFC parameters in the form of real analytical equations. The model reveals, among other things, shows where theoretical UPFC limits are in the form of nondefined areas. In the transient stability study of a heavily loaded transmission system, the optimal UPFC parameters can be determined on-line on the basis of the mathematical model integrated into the UPFC control module. On the basis of static calculations by the mathematical model, the determination of an effective damping strategy is possible. With the inclusion of an UPFC into a meshed system, the determination of the optimal UPFC parameters is expected to become a crucial question, especially when dynamic phenomena need to be controlled.

    8. REFERENCES

    [ l ] L. Gyugyi et al.: Advanced Static Var Compensator Using Gate Turn - Off Thyristors for Utility Applications, ClGRE 1990 Session 26th August - lSt September, Paris;

    [2] L. Gyugyi: Dynamic Compensation of AC Transmission Lines by Solid-state Svnchronous Voltaae Sources, IEEUPES Summer Meeting, Vancouver, July 18-22, 1993;

    [3] R. MihaliE, P. iunko: Streckenmodell zur Einstelluna eines univerasalen LastfluOrealers, Accepted for publication in Archiv fijr Elektrotechnik;

    R. MihaliE, I . PapiE, D. Povh, P. hnko: Improvement of transient Stability by Insertion of FACTS Devices, NTUNIEEE International Conf. on Modern Power. Systems, Athens, Aug.-Sept. 1993;

    [4]

    Rafael MlhaliE - was born in Gornja Radgona, Slovenia on Dec. 25. 1961. He received the Dip lhg. degree in 1986r MSc. in 1989, and Ph.D. degree in Power Engineering in 1993 from the University of Ljubljana, Slovenia. After finishing his basic technical education in 1986 he became a teaching assistant at the

  • 491

    Department of Power Systems and Devices of the Faculty for Electrical and Computer Engineering in Ljubljana. Between 1988 and 1991 he was a member of the Siemens Power Transmission and Distribution Group, Erlangen, Germany. His areas of interests are system analysis and FACTS devices. Dr. MihaliE is a member of the CIGRE Working Group 38-05-06.

    DuHan Povh (M83, S88, F94) received his Diplhg. degree from University Ljubljana/Slovenia in 1959, Dr.-lng. degree from TH DarmstadVGermany in 1972. He is now professor at the University in Ljubljana, and is active in a number of committees and working groups of ClGRE and IEEE. His areas of interest are system analysis, network planning, insulation coordination of EHV and HVDC transmission systems and development of HVDC and Static Var Compensators technique. Prof. Dr. DuSan Povh is Head of Department on system planning and system analysis in the Siemens Power Transmission and Distribution Group.

    Peter h n k o received his MSc. degree in 1974, and D.Sc. degree 1978, from the University of Ljubljana, Slovenia. From 1985 to 1990 he was Associate Professor. Now he is Professor at the Faculty for Electrical and Computer Engineering - Dept. for Electrical Energy Systems and Devices in Ljubljana. Presently he is a chairman of this dept., member of an advisory group at the Slovenian Board of Energy, and Research Fellow of the Institute Joief Stefan in Ljubljana. His study and research areas are Transformation and Transmission Equipment, Transients Analysis and Switching Devices. Mr. P. tunko is a member of IEEE

  • 492

    Discussion

    D.N. Kosterev (Oregon State University, Corvallis, OR): The paper presents a study of the transient stability en- hancement using the Unified Powerflow Controllers (UP- FCs). To represent the device adequately in transient stability studies, its capability characteristics should be considered. These capability characteristics determine the maximum range of the UPFC controllable parameters (here series voltage phase and magnitude, and shunt admittance) depending on bus voltage and line current at the device location. The capability characteristics should be estab- lished based on the device ratings. However, there is no discussion in the paper on the UPFC equipment ratings, and on how to relate them to the device control capabili- ties.

    In the paper, the UPFC operating regions are derived based on the device mathematical model. It will be im- portant to show how the operatin2 regions depend on the UPFC current and voltage ratings, and to demonstrate that the selected device ratings provide adequate control range for the system to meet transient stability and dis- turbance performance requirements.

    R. NlihaflE: I would like to thank to Mr. D.N.Kosterev for his comments and interest in the paper.

    I agree that the determination of the device bility characteristics should be the basis of future work concerning UPFC realization. In reality the UPFC operating area would probably not be ideal (circle that can be shifted - in the article). The circle represents only one o limitations. Other limitations dependent on the system operating point (terminal voltage, line current etc.) and the dynamic device behavior (short - time overload capability e.g.) would deform this circle.

    Another interesting question is the one about the e rating "needed" to achieve certain goal (system stabil example). This goal can be achieved by various combin series and parallel UPFC branch ratings. They are independent. The final answer on the question of rating may be derived from a cost analysis.

    Manuscript received April 14, 1995.

    Manuscript received February 24, 1995.


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