Steady State Analysis of Unified Power Flow Controllers

Transcription

1 Helwan University From the electedworks of Omar H. Abdalla Winter February 15, 2009 teady tate Analysis of Unified ower Flow Controllers Omar H. Abdalla Mohamed A. E. Ghazy Lotfy M. Lotfy Nermeen A. M. Hassan Available at:

2 1 teady tate Analysis of Unified ower Flow Controllers O. H. Abdalla (1), enior Member, EEE, M. A. E. Ghazi (2), L. M. Lotfy (2), and N. A. M. Hasan (2) (1) Oman Electricity Transmission Company, (2) University of Helwan, Egypt. Abstract The paper presents a tutorial review of the basic operation, control functions and steady state performance of a Unified ower Flow Controller (UFC). A typical circuit arrangement of a UFC is given and principles of controlling active and reactive powers are described. The main functions of UFC are analyzed; including voltage regulation, series reactive compensation, phase compensation, and combined actions. A simplified two-bus power system is considered to demonstrate the main effects of the UFC. tudies and results are presented to show the wide range capabilities of the UFC in controlling transition active and reactive powers simultaneously and/or independently. ndex Terms Unified ower Flow Controller, Transmission ystems, Control of Active and eactive owers. T. NTODUCTON HE Unified ower Flow Controller (UFC) concept was proposed by Gyugyi in 1991 [1]. The UFC is used for the real time control and dynamic compensation for AC transmission systems, providing more flexibility in power system today [2-4]. Moreover, the UFC has the unique capability to control simultaneously and/or independently, all the parameters affecting power flow in the transmission line which are voltage, impedance, and phase angle [5, 6]. Also it can independently control both the real and reactive power flows in the line under transmission system constraints [7-9]. This paper demonstrates the UFC main configuration with the detailed operation for its series and shunt branches. Also each branch control functions and the effect of the injected voltage in the active and reactive power control are discussed in more details.. CCUT AANGEMENT The UFC circuit as illustrated in Fig. 1 consists of a shunt and a series branches. Each branch is considered as a voltage source converter using semiconductor devices of fully controlled type, such as the nsulated-gate Bipolar Transistors (GBT) and the Gate Turn-Off (GTO) thyristors. The GTO is a more advanced version of the conventional thyristor, where the GTO unlike ilicon-controlled ectifier (C), can be turned off by applying negative gate signal. Also it has fast turn off, lower cost, weight, volume and noise. The GBT is a faster switching device than the GTO which is limited to the order of 1 khz. But the higher the switching frequency the higher the harmonic order present which is not required here, Therefore, the UFC with GTO is preferable than UFC with GBT. The shunt converter and series converters are connected to the transmission line through step down transformers. They are operated from a common DC link provided by a DC storage capacitor which has a capacitance value specified for better dynamic performance but with cost consideration. These back to back converters can independently generate (or absorb) reactive power at its own AC output terminal [1-3]. hunt Fig. 1. UFC circuit arrangement. eries. OEATON OF UNFED OWE FLOW CONTOLLE The shunt converter basic function is to provide the real power demanded by the series converter from the AC power system. t can also, if desired, generate or absorb reactive power at the UFC connected bus, which is independent of the active power transfer to (or from) the DC terminal. Therefore it can provide reactive power compensation for the transmission line and thus provide indirect voltage regulation at the input terminal of the UFC [3]. The main function of the UFC is provided by the series converter by injected an AC voltage in series with the line with controllable magnitude E (0 E Emax ) and phase angle α (0 α 360 o ), at the power frequency, through a series connected transformer. The converter output voltage injected in series with the line can be used for direct voltage control, series compensation, phase shifter and their combinations which will be discussed in details in the next section. The real power exchanged at AC terminal of the insertion transformer is converted by the series converter into DC power which appears at the DC link as positive real power

3 2 demanded by the shunt converter. The reactive power exchange at the AC terminal is generated internally by the inverter.. UFC FUNCTON The operation of the UFC from the standpoint of conventional power transmission is based on reactive shunt compensation, series compensation, and phase shifting [1-3]. The UFC can fulfill these functions and thereby meet multiple control objectives by adding the injected voltage E, with appropriate amplitude and phase angle. The basic UFC Fig. 4. With no series compensation, the function of the shunt branch is to regulate the line voltage by injecting a shunt reactive current into the transmission line. The compensation type inductive or capacitive is depending on whether the shunt voltage source H is higher or lower than the sending end voltage as shown in Fig. 4. H H L H E E L H H H H H H UFC H < nductive compensation H H > Capacitive compensation power flow controller functions are illustrated in Fig. 2. A. Uncompensated simple power system A simple two machine system connected through a transmission line whose reactance is x L with its voltages phasors shown in Fig. 3 and. The equation of power transfer is given as follows: = sin δ (1) L Fig. 2. UFC schematic diagram. Fig. 4. hunt compensation. C. eries converter operation The series branch provides the main function of the UFC by controlling the three parameters affect the power flow in a transmission line simultaneously and independently. t is represented by a series AC variable voltage source with controllable magnitude E and phase angle α measured from the reference voltage and connected to the sending end by a reactance E as illustrated in Fig. 5. E α E L 1 δ Fig. 3. Uncompensated system. Circuit oltage phasors ( Fig. 5. eries branch schematic diagram. B. hunt converter operation The shunt branch is represented as a variable voltage source H with variable magnitude and phase angle and connected to the sending end by a reactance H as shown in ( α ) + j ( + ) + 0 δ = E E L (2)

4 3 = δ + E j ( α ) 0 Therefore,.. E = in ( δ ) + in ( α ) (3) Where, = L + E 1 1 (nductive) E2 The series converter has the following four operation modes: oltage regulation t is considered as a terminal voltage regulator similar to that obtainable with a transformer tap-changer having infinitely small steps [1, 2] and [9]. The phasor diagram is shown in Fig. 6 and. When the series voltage E is injected in-phase or out-of-phase (substituting angle α = ± δ in Equation (3)), the voltage 1 at bus 1 in Fig. 5 is increased (or decreased). Fig. 6 (c) illustrates the effect of the injected series voltage on the transmitted power. 1 (Capacitive) eries reactive compensation t is used to control the current or power flow by regulating the transmission line s effective reactance by connecting the series voltage E in quadrature with the line current as shown in Fig. 7, where the series voltage E may be replaced by a reactance CE (0 CE ). Therefore, Equation (1) can be written as follow:. ( ± = (4) CE E1 sin δ ) Note: the equivalent reactance ( + CE ) means inductive compensation and the equivalent reactance ( CE ) means capacitive compensation. Fig. 7. eries reactive compensation hasor diagram - δ curves hase angle compensation This is used for regulating the phase angle of the line voltage by a series connected compensating voltage E, in quadrature with respect to the voltage, keeping the voltage amplitude of the sending-end nearly constant as shown in Figure E1. =. sin ( δ ± θ ) (5) Where, θ is the resultant phase shifting angle. Combined compensation: t can be achieved by simultaneous voltage regulation, series compensation, and phase shifting, as shown at Fig. 9. Where, T = E1 + E2 + E3 (c) Fig. 6. oltage regulation 1 = + E 1 = - E (c) - δ curves when E varied

5 4 +θ -θ + - E3 and the reactive powers Q s and Q against the transmitted power for (0 δ 90 o ), respectively. Where;. =.sin δ (6) 2. Q =.cos δ (7) Q 2. =.cos δ (8) Where, the negative sign for the opposite direction shown in Fig. 8, assuming that = = 1 pu and = 1 pu. + - p Q Q Fig. 8. hase angle compensation. hasor diagram - δ curves E1 δ Fig. 10. imple two machine system oltage phasors T E2 Qs,Q T E3 Fig. 9. Combined compensation phasor diagram.. NCLE OF EAL AND EACTE OWE CONTOL A. Basic two-bus system The basic power system of Fig. 10 with the well known transmission characteristics is introduced to provide a guideline to establish the capability of the UFC to control the transmitted real power and the reactive power demands, Q s and Q r at the sending end and receiving end of the line, respectively. A simple two bus system with sending end voltage, receiving-end voltage, line impedance (assumed, for simplicity inductive) and the phasor diagram with power angle δ are shown in Fig. 10 and. Fig. 11 and show the relation of the transmitted power, and the reactive power Q and Q against angle δ,

6 5 Fig. 11. Angle δ versus, Q and Q r B. Two-bus system with UFC E α + - (d) versus Q and Q r The above system is expanded to include the UFC, which is represented by a controllable voltage source in series with the line as shown in Fig. 12. The series branch provides the main function of the UFC. The phasor E has a magnitude E (0 E Emax ) and phase angle α measured from the reference voltage (where α = δ - ). The phase angle (0 360 o ) is measured from the given position of phasor, as illustrated in Fig.12 and. The UFC shunt inverter is assumed to be operated at unity power factor, as its reactive compensation capability of the UFC not utilized. ts main function is to transfer the real power demand of the series inverter to the sending-end generator. With these assumptions, the series voltage source, together with the real power coupling to the sending end generator is an accurate representation of the basic UFC. Fig. 12 shows that the transmission line + E as the effective sending end voltage. Thus it is clear that the UFC affects the voltage magnitude and phase angle across the transmission line. Therefore, by varying the magnitude and angle of E, the transmitted real power as well as the reactive power demand of the line can be controlled at any given transmission angle between the sending-end and receiving-end voltages. Q Q. EFFECT OF NJECTED OLTAGE ON AND Q CONTOL To illustrate the effect of the series injected voltage of the UFC, the reactive power at sending and receiving ends Q, and Q r are plotted separately against the transmitted power as a function of the power angle δ (0 δ 90 o ). The magnitude of E = 0.5 pu and phase angle which provides the maximum transmitted power at: d /d = 0, ( = =max., = -tan -1 (cot δ)) The results are shown in Fig. 13. Also, the -Q curve without UFC is included in the figure as a reference. Equations (6) to (8) can be modified to include the UFC as follow: E.. E = in ( δ ) + in ( δ ) (9) δ Fig. 12. Two bus system with UFC. chematic diagram hasor diagram n order to represent the UFC properly, the series voltage source is stipulated to generate only the reactive power Q E it exchanges with the line. Thus the real power E exchanged with the transmission line is assumed to be transferred to the sending-end generators if a perfect coupling for real power flow between it and the sending-end generator excited. The dc link between the two converters establishes a bi-directional coupling for real power flow between the injected series voltage source and the sending-end bus. Q Q 2 E = + cos( ) cos ( δ ) (10) = 2 E Cos( δ ) + Cos( δ (11) ) t can be observed from Fig. 13 that when the power angle is zero (δ=0). With E = 0 (without UFC),, Q, and Q are all zero. But when the UFC is operated with E = 0.5, pu it is possible to provide 0.5 pu power flow, without any reactive power demand on either the sending-end or the receiving-end. Also, at the same real power flow, the sending and receiving end reactive power is lower. For illustrating the wide boundaries of and Q due to the effect of the injected voltage magnitude E = 0.5 pu and the variations in the angle with full revolution (0 360 ), the

7 6 same plots in Fig. 13 is repeated but with definite values of δ (δ=0, 30, 60º), as shown in Fig. 14. n contrast, the control region boundary for and Q in the {Q, } plane remains a circle at all transmission angles. sending reactive pow er (Q s) ecieving reactive power (Qr) Transmitted eal ower () pq=0 pq=0.5,max Transmitted eal power () pq=0 pq=0.5, =max. Fig. 13. eactive power at sending and receiving ends versus transmitted real power with and without UFC. n Fig. 14, the circles in the {Q, } and {Q, } planes define all and Q and, respectively, and Q values achievable with the UFC of a given rating. These circles illustrate the wide range operational capability of the UFC. The UFC with voltage rating of 0.5 pu is able to establish 0.5 pu power flow, in either direction, without demanding any reactive power generator at either the sending-end or the receiving-end. The UFC can force the generator at one end to supply reactive power for the generator at the other end. n general at any given transmission angle δ, the transmitted real power, and the reactive power demands at the transmission line ends, Q and Q, can be controlled freely by the UFC within the boundaries obtained in the {Q, } and {Q, } planes by rotating the injected voltage phasor E with its maximum magnitude a full revolution (0 360 ). Wherever the transmission angle δ is increased, the control region in the {Q, } plane in Fig. 14 at δ=30 becomes an ellipse. t becomes narrower at δ = 60 until it degenerates into a straight line at δ = 90 as shown in Fig. 14 (c) and (d), respectively.. CONCLUON The Unified ower Flow Controller (UFC) can provide simultaneous, real-time control of all or any combination of the basic power system parameters (voltage, line impedance and phase angle) which affect the power flow in transmission networks. The series converter provides the main operation of the UFC, thus the UFC is represented by a controllable voltage source in series with the line. The shunt converter is assumed to be operated at unity power factor, when its reactive compensation capability of the UFC not utilized. n this case the main function of the shunt converter is to provide the real power demand of the series converter to the sending-end generator. The UFC provides flexibility for ac power transmission control that has been illustrated by representing the steadystate operation functions of the UFC. The shunt converter can regulate the bus voltage in which the UFC is inserted, and the series converter has the ability to provide voltage regulation, series reactive compensation, phase angle compensation and also combination of them. Therefore by controlling the magnitude and phase angle of the series voltage E, the power flows in the transmission line and hence the magnitude of loading can be controlled. The UFC can be viewed as a generalized real and reactive power flow controller that is able to maintain a prescribed and Q at a given point on the transmission line. Moreover, a wide range of and Q control is achievable with the UFC.. EFEENCE [1] L. Gyugyi, "A unified power flow control concept for flexible AC transmission systems," nternational Conference on AC and DC ower Transmission, ept [2] L. Gyugyi, T.. jetmu, A. Edris, C. D. chauder, D.. Torgoan, and. L. Williams, The unified power flow controller: A new approach transmission control, EEE Trans. ower Delivery, ol. 10 No. 2, [3]. apic,. Zunko, D. ovh, and M. Weinhold, "Basic control of unified power flow controllers," EEE Trans. ower ystems, ol. 11, No. 4, Nov [4] E. Wirth, and A. Kara, nnovative power flow management and voltage control, ower Engineering Journal, vol. 14, No. 3, June [5] K.. adiyar, and A. M. Kulkarni, Control design and simulation of unified power flow controller, EEE Trans. ower Delivery, ol. 13 No. 4, Oct [6] Z. Huaung, Y. Ni, C. M. hen, F. F. Wu,. Chen, and B. Zhag, Application of unified power flow controller in interconnected power systems Modelling, interface, control strategy, and case study, EEE Trans. ower Delivery, ol. 15 No. 2, April [7] C. D. chaider, L. Gyugyi, M.. Lund, D. M. Hamai, and A. Edris, Operation of the unified power flow controller (UFC) under practical constraints, EEE Trans. ower Delivery, ol. 13 No. 4, Oct [8] J. Y. Liu. Y. H. ing, and. A. Mehta, trategies for handling UFC constraints in steady-state power flow and voltage control, EEE Trans. ower Delivery, ol. 15 No. 2, April [9]. An, J. Condern, and W. Gedra, An ideal transformer UFC model, OF first-order sensitivities, and application to screening for optimal UFC locations, EEE Trans. ower ystems, ol. 22, No. 1, Feb

8 7 δ = 0 o, E =0.5 pu δ = 30 o, E =0.5 pu (c) δ = 60 o, E =0.5 pu (d) δ = 90 o, E =0.5 Fig. 14. ending and receiving reactive power against the active power