Power Flow Control through Transmission Line with UPFC to Mitigate Contingency

Power Flow Control through Transmission Line with UPFC to Mitigate Contingency Amit Shiwalkar & N. D. Ghawghawe G.C.O.E. Amravati E-mail : amitashiwalkar@gmail.com, g_nit@rediffmail.com Abstract This paper is based on practical contingency condition of a transmission line, over which a study using UPFC is done without sacrificing the available generation capacity. The Unified Power Flow Controller (UPFC) is the most versatile and complex power electronic equipment that has emerged for the control and optimization of power flow in electrical power transmission system. Installing the UPFC makes it possible to control an amount of real power flow through the line. This paper presents control of power flow through overloaded/under loaded line under study. Simulation were carried out using MATLAB software to overcome the contingency condition by using UPFC. Keywords Contingency, FACTS (UPFC), Power flow, WPCL (Wardha Power Co Ltd). I. INTRODUCTION Today s power system is highly complex and requires careful design of new devices taking into consideration the already existing equipments. Now a day, numbers of private generating units are getting commissioned due to power generation policy and open access to transfer power. But, due to variety of environmental and regulatory concern, the expansion of electric power transmission facilities is restricted. Power Transmission and Generation utility would be benefited if they could increase line power capability while being able to delay the construction of new transmission lines. So as to meet, additional power transfer capability, beyond the present limit, will be needed to meet the requirement and to provide adequate margin. Power transfer capability is constrained by facility overloads and reactive power deficiencies. For optimum utilization of available transmission facilities, FACTS (UPFC) can be used to mitigate the problems [1][2]. It is well known that the power flow through transmission line is a function of line impedance, magnitude and phase angle of bus voltage. If these parameters can be controlled, the power flow through the transmission line can be controlled in a predetermined manner. Flexible AC Transmission System (FACTS) uses advanced power electronics to control the parameters in the power system in order to fully utilize the existing transmission facilities. Modern power systems are becoming increasingly stressed because of growing demand and variety of factors. In recent years, energy, environment, right-of-way, and cost problems have delayed the construction of new transmission lines, while the demand for electric power has continued to grow. This situation has necessitated a review of the traditional power system concepts and practices to achieve greater operating flexibility and better utilization of existing power systems. Representative of the FACTS devices are SVC (Static Var Compensators), TCSC (Thyristor Controlled Series Capacitor), STATCOM, the Unified Power Flow Controller (UPFC), a phase shifters, etc. These devices may be connected in shunt/series or a combination of shunt and series. Variety of advantages like improvement in steady state power flow, transient stability, voltage stability etc. can be derived. Performance of an existing transmission system can be improved. The UPFC is the most versatile and complex of the FACTS devices, combining the features of STATCOM and the SSSC. The UPFC can provide simultaneous control of all basic power system parameters viz., line voltage, impedance and phase angle [1]. II. UNIFIED POWER FLOW CONTROLLER A. Basic Concepts Submit your The UPFC consists of two series and shunt converters with AC transmission systems. It is a combination of STATCOM and SSSC. These parts are coupled together with common DC link (capacitor) to allow the real power flow between the series and shunt converter output terminals bi-directionally, and to provide real and reactive line compensation without an external energy source. UPFC is able to control the 64

series voltage injection by the means of unconstrained angle. The transmission line with UPFC, which is extended in AC transmission network, is shown in the Figure 1 [7][10]. The series inverter is controlled to inject a symmetrical three phase voltage of controllable magnitude and phase angle in series with the line to control active and reactive power flows on the transmission line. So, this inverter will exchange active and reactive power with the line. The reactive power is electronically provided by the series inverter, and the active power is transmitted to the dc terminals. The shunt inverter is operated in such a way as to demand this dc terminal power (positive or negative) from the line keeping the voltage across the storage capacitor Vdc constant. So, the net real power absorbed from the line by the UPFC is equal only to the losses of the inverters and their transformers. The remaining capacity of the shunt inverter can be used to exchange reactive power with the line so to provide a voltage regulation at the connection point. (2.1) (2.2) The real and reactive power flow of the transmission line at the receiving end with UPFC is given by following equations (2.1) and (2.2). Real power of the system with UPFC: From equation (2.1) and (2.3) Reactive power of the system with UPFC (2.3) (2.4) (2.5) From equation (2.2) and (2.6) (2.6) (2.7) Fig. 1 The voltage represented by a phasor V is injected by SSSC in series with the transmission line. The magnitude of ΔV is limited from 0 to 0.5 pu and its angle Ø is constrained from 0 to 360 [2]. This voltage can be added vectorally to the sending end voltage (Vs δ), and the transmission line voltage can be seen as Vs + V, as shown by the Figure 1. Thus, the sending end voltage (V ef ) is effectively on the line voltage. According to this situation, UPFC affects both the magnitude and the phase angle of the line voltage on the transmission line. Hence, it is reasonable to expect its ability to control the transmittable real and reactive power demand by varying the magnitude and phase angle of the line at any given transmission phase angle between the sending and receiving end voltages. The transmitted real (P 0 ) and reactive power (Q 0 ) of the transmission above without UPFC (from sending to receiving end bus) are given by the expression (2.8) To provide the required real and reactive power, the magnitude and phase angle of the series-injected voltage from equation (2.5) and (2.8) are given by following equations: Phase angle of injected voltage, (2.9) (2.10) The equations (2.9) and (2.10) give information about V and the phase angle of series-injected voltage to get the demanded real and reactive power by the system within the specified range defined by the maximum limit of injected V. The phasor diagram based on the mathematical derivation above, which gives the magnitude and phase angle of injected voltage 65

to meet the demand of real and reactive power of the system, is obtained, as shown in Figure 2 [2]. Fig. 2 The transmittable of the real and reactive power will be specified by the maximum limit of injected ΔV.The wide range of controlling the transmitted power independent from the transmission angle (δ ) indicates not only superior capability of UPFC in power flow applications, but also promises powerful capacity for transient stability improvement and power oscillation damping. The controllable of the real power (P) and the reactive power (Q) at the receiving end between any transmission angle (δ) can be seen in the following mathematical equations. The powerful capabilities of UPFC are discussed above in terms of conventional transmission control concepts. It can be integrated into a generalized power flow controller that is able to maintain prescribed controllable real power and reactive power Q in the line. B. Controlling the Real and Reactive Power Flow When Considering the equations (2.9) and (2.10), the real and reactive power change from the uncompensated values P O (δ) and Q O (δ)as a function of the magnitude and the phase angle of the injected voltage ΔV As the angle is an unrestricted variable 0 O < <380 O, the attainable boundary of the control region for P(δ, ) and Q(δ, ) is obtained from a complete rotation of the phasor with its maximum ΔV max. Fig. 3 Fig. 4 The equations mentioned above verify that this control region is a circle with a center defined by the coordinates defined by P O (δ) and Q O (δ), and the radius is given by following equation: (2.11) The circular control regions are defined by the equation (2.11). These regions are shown in the Figure 4.3 and 4.4. The center of these circuses are Po(δ) and Qo(δ) at angles δ= 0, δ = 30, δ = 60 and δ = 90 respectively. The focus of the centers are indicated by the + sign as δ varies between 0 and 90. When the transmission line angle (δ) and amplitude are zero, the real power Po(δ) and reactive power Qo(δ)flows are zero. According to this situation, the system is at standstill at the origin of the P and Q coordinates. It is illustrated as a case in the Figure 3. The circle around the origin of the plane is the locus of the corresponding Q and P values (at receiving end). It is obtained as the voltage phasor V which is rotated around itself by a full revolution (0 O < <380 O ) with its maximum value V max.the boundary of the circle in the (P, Q) plane shows all P and Q values obtainable with UPFC in accordance with given rating. The UPFC with the specific voltage rating of 0.5 pu is able to obtain 0.5 pu power flowing into either direction without imposing any additional reactive power demanded on either the sending or the receiving end bus, as seen from Figure 3 and 4. The skills of UPFC demonstrated in the Figures 3 and 4 shows the capability of controlling the real and reactive power flow independently at any transmission angle provide a powerful tool for AC transmission control. 66

III. SYSTEM UNDER STUDY A. Transmission Line under Study A 540MW WPCL generation plant and 480MW Koradi Power plant are interconnected by two separate 220kV line. One line directly connected to Ambazari substation, Nagpur and other through 220kV Wardha substation, a load center, through 160 km transmission line. The load center is modeled by a 150MW load at 220kV Wardha S/stn. The load is fed by the WPCL plant and remained generation fed to grid at Ambazari substation is shown in Figure 5. Fig. 5 B. Modelled Load Flow without UPFC The above said system with parameters given in Table I, is modeled in MATLAB and contingency condition is studied. In which it is found that the line WPCL-Wardha (L2) is getting overloaded. Whereas, the WPCL-Ambazari (L1) line remain underloaded. With the generation of 4x135MW at WPCL and no CTPS Generation, the power through L1 and L2 were 190MW and 257MW, as in Figure 6. Fig. 6a Similarly, the load flow using Gauss-Seidel and N-R methods were calculated for the system parameters give in Table-I and its results for the real power flow through the system under study is compared with simulation results. IV. SIMULATION STUDY In order to improve power flow, the transmission line is compensated by an UPFC. The UPFC is connected to line facing contingency condition i.e. line- L2 and adjacent to CTPS-WPCL station, as shown in Figure 7. UPFC Phasor Model has been used for this simulation. The existing system is naturally unbalanced without UPFC, for all Gen-Sets at WPCL as per simulation results shown in Figure 6. Fig. 6 Comparison with the realistic load flow data from field is done, where 02 generating sets evacuating the power through the system and the modeled system in MATLAB. With these 2 gensets practical power flow through line L1 and L2 were 76MW and 146MW respectively which is equal to simulated result as shown in Figure 6a. With 03 gensets the simulation result of power flow is 133MW and 202MW through L1 and L2 respectively. Fig. 7 At this location the UPFC is used to reduce the power flow in the line L2 to desired limit so as to push the power in underloaded line L1. The simulation result of the system configuration, in MATLAB Simulink is as shown in Figure 6. At 5sec the UPFC is brought into circuit, with required power flow reference, The line L2 is controlled to carry 235MW power through it and remaining 212MW power carried over by L1 [3][6][10]. V. EFFECT OF UPFC ON LINE PERFORMANCE The transmission line under study with parameters as given in Table I is considered. UPFC is placed in series with line L2 as shown in Figure 7. Bypass option blocks the UPFC for the first 5 second. Voltage, active 67

power, reactive power and current variations in the transmission line with UPFC and without UPFC are studied and compared. When the transmission line is without UPFC, the sending-end and receiving-end p.u. voltages, real power and reactive power are shown in first 5 seconds of Figure 8. When UPFC is placed across the same transmission line, after 5 seconds, the voltages at buses were improved, desired power flow through the line L2 by which the power through line L1 gone up. The Mw loss of line L2 without UPFC was 8.1MW, whereas with UPFC it reduced to 6.5 MW (instead of 6.75MW which the loss for reduced power flow) [6][9]. As well known, with the increase of power flow the reactive power also increases. With UPFC this reactive power is maintained to desired level so as to maintain voltage profile and losses due to reactive current through line. Fig. 8 VI. UPFC PARAMETER The real power flow through line L2 is regulated to 235MW from 257MW due to that the power flow in line L1 gone up from 190MW to 212MW, and the reactive power to 14.04MVAr. for this power flow adjustment it is found that series insertion voltage of 0.085 p u -70 Degree needs to be injected, as shown in Figure 9. VII. CONCLUSION In this study, the MATLAB environment using phasor model of UPFC connected to a three phase-three wire transmission system. This paper presents control and performance of UPFC intended for installation on a transmission line. Simulation results show the effectiveness of UPFC in controlling real and reactive power through the line. Fig. 9 Also, with UPFC in transmission line, results in improvement of transient stability of the system, which is an added advantage along with the power flow control [5], improved Plant Utilization Factor, better Voltage Profile. Line to line voltage Table-I Transmission rating Length of the transmission line Resistance of the line Inductive reactance of the line Capacitive reactance of the line VIII. REFERENCES 220 kv 260 MVA 160 Km 0.069947 Ω/Km 0.001355 Ω/Km [1] N.G. Hingorani and L. Gyugyi, Understanding FACTS: concepts and technology of flexible ac transmission systems, IEEE Press, NY, 1999. [2] Y.H. Song and A.T. Johns, Flexible ac transmission systems (FACTS), The Institute of Electrical Engineers, London, 1999. [3] M. Noroozian, L. Angquist, M. Ghandhari, and G. Andersson, Use of UPFC for optimal power flow control, IEEE Trans. on Power Delivery, Vol. 12, No. 4, pp. 1629-1634, 1997. [4] L. Gyugyi, Dynamic compensation of ac transmission line by solid-state synchronous voltage sources, IEEE Trans. Power Delivery, Vol. 9, pp. 904-911, Apr.1994. [5] R. MihaliEet.al, Improvement of Transient Stability Using Unified Power Flow Controller, IEEE Transactions on Power Delivery. Vol.11,No1. January 1996. [6] A. Edris, A.S. Mehraban, L. Gyugyi, S. Arabi, T. Reitman, "Controlling the Flow of Real and Reactive Power ", IEEE Computer Applications in Power, pp. 20-25, January 1998. [7] L.Gyugyi, C.D. Schauder, S.L. Williams, T.R. Reitman, D. R. Torgerson, A. Edris, "The Unified Power Flow Controller: A New Approach to Power Transmission Control, IEEE Trans. 68

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