Modelling and Controlling of UPFC for Power System Transient Studies

Transcription

1 Moelling an Controlling of UPFC for Power System Transient Stuies 29 Moelling an Controlling of UPFC for Power System Transient Stuies Ali Ajami 1, S.H. Hosseini 2, an G.B. Gharehpetian 3, Non-members ABSTRACT The unifie power flow controller (UPFC) is a typical FACTS (Flexible AC Transmission Systems) evice that is capable of instantaneous control of transmission line parameters. This paper presents the transient moel an control system of UPFC. The presente control system enables the UPFC to following the exchanges of line active an reactive power reference values, regulating the DC link an bus voltage. The presente control scheme has the fast ynamic response an hence is aequate for improving transient behaviour of power system after transient conitions. The presente simulation results emonstrate that the presente control system acts properly in the steay an transient states. It is shown that by aing a supplementary control system to the shunt inverter control system, it is possible to balance line current too. The presente control an power system of UPFC was simulate an teste with the PSCAD/EMTDC simulation program. Keywors: FACTS, UPFC, Transient Moeling, Power Flow, Line Current Balancing 1. INTRODUCTION The high rating power electronic equipments have mae it possible to consier new technologies such as the FACTS [1 for power flow control, secure loaing an amping of power system oscillations. FACTS equipments are an attractive alternative for increasing the transmission capability of existing transmission lines. The UPFC is one of the typical FACTS evices that can provie simultaneous control of all or selectively basic parameters of power system [2, 3 (transmission voltage, line impeance an phase angle) an ynamic compensation of AC power system. The UPFC can fulfill the functions of STATCOM [4, 5, SSSC [6, 7 an phase shifter meeting multiple control objectives. Manuscript receive on January 10, 2007 ; revise on May 15, The author is with Electrical Engineering Department of Azarbaijan University of Tarbiat Moallem, Tabriz, Iran, ajami@azaruniv.eu 2 The author is with Electrical & computer Engineering Faculty of Tabriz University, Tabriz, Iran, hosseini@tabrizu.ac.ir 3 The author is with Electrical Engineering Department, Amirkabir University of Technology, Tehran, Iran, grptian@aut.ac.ir The general structure of the UPFC contains two back to back voltage source converters using insulate gate bipolar transistor (IGBT) or Integrate Gate Comutate Thyristor (IGCT) with a common DC link (Fig.1). First converter is connecte as parallel an another converter as series with transmission line. The shunt converter is use to provie active power emane by the series converter through a common DC link. The series converter provies the main function of the UPFC by injecting an AC voltage with controllable magnitue an phase angle. The transmission line current flows through series converter an therefore, it exchanges the active an reactive power with the AC system. Since the converters are connecte to a common Dc link, they exchange only active power an there is no reactive power flow between them. It means that reactive power coul be controlle inepenently at both converters. Generally, this structure (Fig.1) enables voltage control by the shunt inverter an inepenent active an reactive power flow control by the series inverter. Fig.1: General configuration of UPFC In the parallel branch of UPFC the active power is controlle by the phase angle of the converter output voltage. In the series branch of UPFC the active an reactive power flows in the transmission line are influence by the amplitue as well as the phase angle of the series injecte voltage. Therefore, the active power controller can significantly affects the reactive power flow an vice versa. In orer to improve the interaction between the active an reactive power control, a so calle ecouple Watt-Var control algorithm base on -q axis theory was use [8, 9. UPFC moels have been presente by several authors [10. In [11 the UPFC moel consists of a controllable voltage source connecte in series with the transmission line an two current sources ae in shunt. The presente moel in [12 consists of two ieal synchronous voltage sources connecte in series an shunt with transmission line. The UPFC moel given in [13 the DC link of UPFC is consiere but

2 30 ECTI TRANSACTIONS ON ELECTRICAL ENG., ELECTRONICS, AND COMMUNICATIONS VOL.5, NO.2 August 2007 the losses of converters an coupling transformers are not consiere. This paper presents a etaile UPFC moel uner realistic conitions. In this paper the transient moel an control strategy of UPFC is presente. The presente control system is capable of controlling the line active an reactive power flows, regulation of the bus voltage an transient behaviour enhancement of power system after a transient conition. Also in this paper a supplementary control system is presente for line current balancing. The simulation results show the performance an effectiveness of the presente control strategy. 2. THE DETAILED MODEL OF UPFC Fig. 2 shows the etaile moel of UPFC. Where, L sh, L sr an R sh, R sr represent leakage inuctances of transformers an losses of inverters an transformers. The terms of N sh an N sr are the turn ratio of shunt an series coupling transformers. The series inverters act as SSSC. It injects voltage, Vser, in series with the transmission line. The KVL equations of series an shunt inverters are: V sr V sr1 = R sr I sr1 + L sr t I sr1 (1) V sh1 V sh = R sh I sh1 + L sh t I sh1 (2) For the DC link of UPFC we have: I c = C t V c = I shc I src G c V c (3) With applying this transformation to equations (1) an (2), the equations (6) an (7) can be obtaine, respectively. [ [ Vsr (Vo V = i )/N sr + R sr I srl + L sr t I srl ωl sr I srl V srq (V oq V iq )/N sr + R sr I srl + L sr t I srlq ωl sr I srlq (6) [ [ Vsh Vi /N = sh R sh I shl L sh t I shl + ωl sh I shl V shq V iq /N sh R sh I shlq L sh t I shlq + ωl sh I shlq (7) In balance conitions, the converters output voltages are basically a funamental frequency sinusoi, given the presence of harmonic filters. Hence, the converters can be accurately represente as voltage sources operating at funamental frequency for transient an steay state stability stuies, where transient oscillations are in the orer of 2-3 Hz uner balance operating conitions. Consequently, the UPFC can be moelle in the transient state as Fig CONTROL STRATEGY OF UPFC The control system of UPFC has two parts an is escribe in the bellow subsections Control System of Series Inverter By controlling the magnitue an phase of line current, line active an reactive power can be controlle. The line current is controlle by series inverter injecte voltage (V sr ). The Active an reactive powers are calculate from equation (8). [ Po = 2 [ [ Vo V oq Isr Q o 3 V oq V o I srq (8) The relationship between line sie an inverter sie currents is given by equation (9). Fig.2: Single phase equivalent circuit Consier the -q transformations [14 which are presente with the equations (4) an (5). V V q V 0 = T (θ) V a V b V c or V q0 = T (θ)v (4) I sr1 = N sr I sr (9) The reference currents of series inverter are obtaine from equation (10). [ I srl Isrlq = 2 [ [ 1 1 Vo V oq P o 3 N sr (Vo 2 + V oq) 2 V oq V o Q o (10) Consiering equations (10) an (6) the control system of series inverter can be forme as shown in Fig Control System of Shunt Inverter T (θ)= 2 Cosθ Cos(θ 2π 3 ) Cos(θ + 2π 3 ) The magnitue of bus voltage epens on reactive power flowing from shunt inverter. Thus, by controlling t the reactive power in shunt inverter, the magni- 1 / 2 1 / 2 1 / 2 tue of bus voltage can be controlle. The injecte Sinθ Sin(θ 2π 3 3 ) Sin(θ + 2π 3 ), θ = ω 0 (5) reactive power by shunt inverter is given as:

3 Moelling an Controlling of UPFC for Power System Transient Stuies 31 Fig.3: Control system of series inverter Q sh = 2 3 (V ii shq V iq I sh ) (11) The series inverter of UPFC controls the line active an reactive power flows by injecting the series controlle voltage. The exchange active power between series inverter an transmission line is cause to change the DC link voltage. Thus, for regulating the DC link voltage, the shunt inverter must supply emane active power by series inverter an losses of two inverters. We have: P sr = 3 2 (V seri sr + V serq I srq ) (12) P sh = P sr +P c +P loss = 2 3 (V ii sh +V iq Ishq) (13) The P loss an P c can be obtaine from equations (14) an (15). P loss = V 2 cg c + 3R sh I 2 sh1 + R sr I 2 sr1 (14) ( P c = V c I c = V c c V ) c t (15) From equations (11), (13) the reference currents of shunt inverter can be calculate as: [ Ish 1 (Vi 2 + V iq 2) [ [ Vi V iq Psh V iq V i Q sh = 2 I shq 3 (16) With consiering the relationship of shunt transformer an equation (16) we have: [ Ishl I sh1 = N sh1 I sh (17) = 2 [ [ 1 1 Vi V iq Psh I shlq 3 N sh (Vi 2 + V iq 2) V iq V i Q sh (18) Also, in this paper the shunt converter can be use to balance an harmonic compensation of line current. Therefore a supplementary control signal must be ae to the control system of shunt converter. To obtain this signal, the active, P, an reactive, Q, powers of the loa sie are calculate. P = P + P = 3 2 (V ii l + V iq I lq ) (19) Q = Q + Q = 3 2 (V ii lq + V iq I lq ) (20) The unbalance current of loa can be obtaine as follows: I unbal = 1 3 (I la + I lb + I lc ) (21) The esire value of Q, Q, P an I unbal is equal to zero. Therefore, the shunt branch must compensate these parameters. To extract unesire component of active power, i.e. P, the instantaneous active power signal must pass through a high pass filter with a cut off frequency of 10 Hz. The -q forms of the supplementary reference signal of shunt branch current are calculate using equations (22) an (23). I = P V i QV iq Vi 2 + V iq 2 Iq = P V iq QV i Vi 2 + V iq 2 Now, they must be ae to equation (18). [ Ishl I shlq = 2 3 N sh2 N sh1 { 1 Vi 2 + V iq 2 (22) (23) [ [ [ Vi V iq Psh I + V iq V i Q sh Iq (24) The reference currents of shunt inverter can be obtaine by: I sha I shb I shc = T (θ) 1 I Shl I Shlq 0 + I unbal I unbal I unbal (25) Fig.4 shows the control system of this inverter. To consier the losses of inverters the DC link voltage error must be ae to this control system. }

4 32 ECTI TRANSACTIONS ON ELECTRICAL ENG., ELECTRONICS, AND COMMUNICATIONS VOL.5, NO.2 August 2007 Fig.4: Control system of shunt inverter Fig.5: Generator circuit of gate pulses Fig.6: Configuration of simulation system V cerr = V c V c (26) Fig. 5 shows the gate pulse generator circuit of the series an shunt inverters base on SPWM technique. In the presente SPWM technique, the error signal is applie to a PI controller an the output signal of PI controller is the reference signal of SPWM technique. 4. SIMULATION RESULTS The two machine test system, whish is simulate by PSCAD/EMTDC [15, is shown in Fig.6. The inverters consist of IGCT base three phase voltage source converters. The transmission line configuration is illustrate in Fig. 7 an the parameters of this system are liste in tables 1 an 2. As mentione earlier, the UPFC can control line active an reactive power flows in steay state an transient conitions. Fig. 8 shows the reference an line active an reactive power flows. Fig.8-a reveals that the presente control system of UPFC not only responses to the step changing in the active power, but also is able to exchanging the irection of line active power flow. Fig.8-b shows the receiving en sie reactive power an its reference value. Fig.8-c inicates the reactive power flows with zero reference value when the active power is change as Fig. 8-a. These figures show the active an reactive powers are controlle as

5 Moelling an Controlling of UPFC for Power System Transient Stuies 33 inepenently by UPFC. As it can be seen, uring transient conitions UPFC can limit the line active power an receiving en reactive power flows to the reference values. Fig.9 escribes the DC link capacitor voltage. Fig.10 shows the line mipoint an the sening en voltages. These figures inicate that the shunt inverter control system is regulating the DC link an bus voltages as properly. Now we set the line active an reactive power flow reference points to 100 MW an 0 MVAR, respectively. This time the loa angle of generator No.2 has been change as shown in Fig. 11. The simulation results are presente in Fig. 12 a an b. It is obvious that UPFC can easily regulate an compensate line active an reactive power flows. An attractive application of UPFC is loa current balancing. The control system shown in Fig.4 has ae this capability to the conventional UPFC. The unbalance loa has been moelle as shown in Fig.6. The simulation results are illustrate in Fig. 13. As it can been seen the sening en generator currents are balance. Table 3 lists the harmonics an THD of line current when the active an reactive powers are change as step. Fig.8-a: Reference an receiving en sie active power Fig.8-b: Reference an receiving en sie reactive power Table 1: Power system parameters V(KV) MVA f R+jLw(pu) Phase(eg.) Gen j0.4 0 Gen j Fig.8-c: Receiving en sie reactive power when Qref is zero Table 2: Parameters of UPFC Ccp Lep Rcp Ccs Lcs Rcs C 10µF 2 mh 5Ω 10µF 1 mh 5Ω 470µF Fig.9: DC link capacitor voltage (KV) Fig.7: Transmission lines configuration Table 3: Harmonics of line current Harmonics 3th 5th 11th 13th 13th THD orer Values % Fig.10: Voltage of the UPFC connection point an the sening ensie voltage

6 34 ECTI TRANSACTIONS ON ELECTRICAL ENG., ELECTRONICS, AND COMMUNICATIONS VOL.5, NO.2 August 2007 Fig.11: Step changes of loa angle of Gen.2 Fig.12-a: Reference (100MW) an receiving en sie active power Fig.12-b: Receiving en sie reactive power when Qref is zero Fig.13: Sening en an loa sie currents 5. CONCLUSIONS This paper presents the transient moel an control system of a UPFC. The presente UPFC control system can regulate line active an reactive power flow an voltage at line mipoint. The presente control system of UPFC not only responses to the step changing in the active an reactive power, but also is able to exchange the irection of line active power flows. In this paper an aitional control is ae to shunt inverter control system to compensate unbalance loa current. The simulation results inicate the fast ynamic response, valiity an effectiveness of the presente control scheme. In this paper the power circuit an control system of UPFC are simulate by using the PSCAD/EMTDC simulation program. References [1 L. Gyugi, A Unifie Power Flow Control Concept for Flexible AC Transmission Systems, IEE Proceeings, Vol.139, No.4, pp , July [2 L. Gyugyi, C.D. Schauer, et al, The Unifie Power Flow Controller: A New Approach to Power Transmission Control, IEEE Transactions on Power Delivery, Vol. 10, No. 2, pp , April [3 H. Fujita, Y. Watanabe, H. Akagi, Control an Analysis of a Unifie Power Flow Controller, IEEE Transactions on Power Electronics, Vol.14, No.6, pp , November [4 Pranesh Rao. M. L. Crow, Zhiping Yang, STATCOM control for power system voltage control applications., IEEE Trans. on Power Delivery, Vol. 15, No. 4, pp , October [5 S.H. Hosseini, A. Ajami, Transient Stability Enhancement of AC Transmission System Using STATCOM, TENCON 02, October 2002 Beijing, China. [6 L. Gyugyi, C.D. Schauer, K.K. Sen, Static Synchronous Series Compensator: A Soli-State Approach to the Series Compensation of Transmission Lines, IEEE Transactions on Power Delivery, Vol. 12, No. 1, pp , [7 S.H. Hosseini, A. Ajami, Dynamic Compensation of Power Systems Using a Multilevel Static Synchronous Series Compensator, the 8 th International Iranian Conference on Electrical Engineering, May [8 S. Roun, Q. Yu, et al, Performance of a Unifie Power Flow Controller Using a -q Control System, AC an DC Transmission Conference, April [9 D. G. Cho, E. Ho. Song, A Simple UPFC Control Algorithm an Simulation on Stationary Reference Frame, ISIE Conference, Pusan, Korea, pp , 2001.

7 Moelling an Controlling of UPFC for Power System Transient Stuies 35 [10 K. K Sen, E. J. Stacey, UPFC- Unifie Power Flow Controller: Theory, Moeling an Application, IEEE Trans. on Power Delivery, Vol. 13, No. 4, pp , October [11 E. Lerch, D. Povh, R. Witzmann, R. Hlebcar an R. Mihalic, Simulation an Performance Analysis of Unifie Power Flow Controller, CIGRE, August [12 J. Bian, T. A. Lemak, R. J. Nelson an D. G. Ramey, Power Flow Controller Moels for Power System Simulations, Power System Technology, Vol. 19, No. 9, pp , September [13 Z. Huang, Y. Ni, C. M. Shen, F. F. Wu, S. Chen an B. Zang, Application of Unifie Power Flow Controller in Interconnecte Power System- Moeling, Interface, Control Strategy an Case Stuy, IEEE Trans. Power Systems, Vol. 15, No. 2, pp , May 2000 [14 P. Sh. Kunur, Power System Stability an Control, New York, McGraw-Hill,1994. [15 PSCAD/EMTDC V4.1, Power System Simulation Software User Manual, Manitoba HVDC Research Center, CANADA, Active Filters. He is IEEE member an has over 250 papers in journals an conferences. G.B.Gharehpetian was born in Tehran, in1962. He receive his BS an MS egrees in electrical engineering in 1987 an 1989 from Tabriz University, Tabriz, Iran an Amirkabir University of Technology (AUT), Tehran, Iran, respectively, grauating with First Class Honors. In 1989 he joine the Electrical Engineering Department of AUT as a lecturer. He receive the Ph.D. egree in electrical engineering from Tehran University, Tehran, Iran, in As a Ph.D. stuent he has receive scholarship from DAAD (German Acaemic Exchange Service) from 1993 to 1996 an he was with High Voltage Institute of RWTH Aachen, Aachen, Germany. He hel the position of Assistant Professor in AUT from 1997 to 2003, an has been Associate Professor since Dr. Gharehpetian is a Senior Member of Iranian Association of Electrical an Electronics Engineers (IAEEE), member of IEEE an member of central boar of IAEEE. Since 2004 he is the Eitor in Chief of the Journal of IAEEE. The power engineering group of AUT has been selecte as a Center of Excellence on Power Systems in Iran since He is a member of this center an since 2004 the Research Deputy of this center. Since November 2005 he is the irector of the inustrial relation office of AUT. He is the author of more than 250 journal an conference papers. His teaching an research interest inclue power system an transformers transients, FACTS evices an HVDC transmission. Ali Ajami was born in Tabriz, Iran in He receive B. Sc. an M. Sc. egrees from electrical an computer engineering faculty of Tabriz university, Iran in the electronic engineering an power engineering at 1996 an 1999 respectively. He receive the P.H.D. egree at 2005 in the electrical an computer engineering faculty of Tabriz university, Iran in the power engineering. In 2005 he joine the electrical engineering epartment of Azarbijan University of Tarbiat Moallem, Iran. His main research interests inclue the ynamic an steay state moeling an analysis of FACTS evices such as SSSC an STATCOM, UPFC, harmonics an power quality compensation systems such as Active Filters, UPQC. He has over 40 papers in journals an conferences. S.H. Hosseini was born in Maran, Iran in He receive the M.S. egree from the faculty of Engineering University of Tabriz, Iran in 1976, the DEA egree from INPL, France, in 1981 an Ph.D. egree from INPL, France, in 1981 all in electrical engineering. In 1990 he joine the University of Tabriz, Iran, as an assistant professor in the Dept. of Elec. Eng., from 1990 to 1995 he was associate professor in the University of Tabriz an since 1995 he has been professor in the Dept. of Elec. Eng. University of Tabriz. From Sept to Sept he was visiting professor in the University of Queenslan Australia; from Sept to Sept he was visiting professor in the University of Western Ontario Canaa. His research interests inclue Power Electronic Converters, Reactive Power Control, Harmonics an Power Quality Compensation Systems such as SVC, UPQC, FACTS evices an