Power Flow Control and Voltage Profile Improvement Using Unified Power Flow Controller (UPFC) in a Grid Network

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1 Power Flow Control and Voltage Profile Improvement Using Unified Power Flow Controller (UPFC) in a Grid Network Takkolu Kalyani and T. Ramesh Kumar Department of Electrical Engineering, Bapatla Engineering College, Bapatla, India {kalyani.takkolu, rameshparota}@gmail.com G. Siva Prasad Department of Electrical Engineering, KORM College of Engineering, Kadapa, India sivaprasadgodina@gmail.com Abstract Power flow control is the critical factor affecting power transmission system. Unified Power Flow Controller (UPFC) is used to regulate the power flow control in power transmission line. The UPFC is represented with two voltage sources named Voltage Source Model (VSM), which is used to study the behaviour of the UPFC in regulating the active, reactive power and voltage profile in the system. These VSM is incorporated in Newton Raphson (N-R) algorithm. The equations of UPFC and the power balance equations of network are combined in to one set of non-linear algebraic equations by employing Simultaneous method and is calculated according to the Newton raphson algorithm and Performed on the IEEE 30-bus system. Simulation is done in Matlab. The results are compared with and without UPFC in terms of active and reactive power flows in the line and check bus voltages to analyze the performance of UPFC. Index Terms Newton-Raphson algorithm, load flow, unified power flow controller, voltage source model I. INTRODUCTION Electrical power systems are a large interconnected network that requires a careful design to maintain the system with continuous power flow operation without any limitations. Flexible Alternating Current Transmission System (FACTS) is an evolving technology used to help electric utilities fully utilize their transmission assets. This concept was first introduced by N. G. Hingorani [1]. Many types of FACTS devices have been proposed, among them Unified Power Flow Controller (UPFC) is a versatile and flexible device in the FACTS family of controllers which has the ability to simultaneously control all the transmission parameters of power systems i.e., voltage, impedance and phase angle which determines the power flow of a transmission line. This device was proposed by Gyugyi [2] and Gyugyi et al. [3]. The UPFC seem to be consists of two Voltage Source Converters (VSCs), one VSC is connected in series to the transmission line through a series transformer, similarly Manuscript received October 21, 2015; revised March 23, doi: /ijeee the other is connected in shunt to the transmission line through a shunt transformer and both are connected back to back through a DC storage capacitor (Gyugyi et al. [3]). In this paper the performance of UPFC is investigated on power systems effectively, to this it is required to formulate their appropriate model. In the area of power flow analysis the UPFC models have been published (Fuerete-Esquivel and Acha [4], [5]; Noroozian et al. [6]; and Nabavi-Niaki and Iravani [7], [8]) and consider the UPFC as one series voltage source and one shunt current source model or both the series and shunt represented by two voltage sources. In the area of power flow concept the UPFC is represented by two voltage sources called Voltage Source Model (VSM) (Fuerete-Esquivel and Acha [9]) also introduced another model called the Power Injection Model (PIM). The Voltage source model of UPFC is incorporated in N-R algorithm in to estimate the performance of UPFC in power flow control. Generally there are ways of solving power flow solutions, the Sequential and the simultaneous method: In the first method, the equations of UPFC are separated from the power flow equations and both the set of equations are solved separately and sequentially. In simultaneous method, the equations of UPFC and the power flow equations are combined in to one set of non-linear algebraic equations which find less complexity. A Jacobian matrix is then formed and are in non symmetric in nature. Here in this paper the simultaneous method is used. II. OPERATING PRINCIPLE OF UPFC The UPFC consists of two voltage source converters, one connected in series to the transmission line through a series transformer and the other in shunt to the transmission line through a shunt transformer, both are connected back to back through a DC link and can modelled as two ideal voltage sources between the two busses (Fuerete-Esquivel and Acha [9]; and Fuerete-

2 Esquivel et al. [10]). The UPFC allows simultaneous control of active power flow, reactive power flow, and voltage magnitude at the UPFC terminals. Alternatively, the controller may be set to control one or more of these parameters in any combination or to control none of them. The active power demanded by the series converter is drawn by the shunt converter from the AC network and supplied to bus m through the DC link. The output voltage of the series converter is added to the nodal voltage, at say bus k, to boost the nodal voltage at bus m. The output of the series voltage source Vse and se are controllable magnitude and angle between the limits Vsemax Vse Vsemin and 0 se 2π respectively and of the shunt voltage source is Vsh and sh controllable between the limits Vshmax Vsh Vshmin and 0 sh 2π. The voltage magnitude of the output voltage Vse provides voltage regulation, and the phase angle se determines the mode of power flow control. Fig. 1 shows the voltage source model of the UPFC. Zse and Zsh are the impedances of the two transformers between the line and UPFC. III. MODELLING OF UPFC The two ideal series and shunt voltages source equations of the UPFC from Fig. 1 are: (1) Vse V se (cos se j sin se ) Vsh Vsh (cos sh j sin sh ) (2) Based on the voltage source model of UPFC the active and reactive power equations are: At node k: Pk V 2 k Gkk VkVm (Gkm cos( k m ) Bkm sin( k m )) VkVse(Gkm cos( k se ) Bkm sin( k se )) (3) VkVsh (Gsh cos( k sh ) Bsh sin( k sh )) Qk V 2 k Bkk VkVm (Gkm sin( k m ) Bkm cos( k m )) (4) VkVse (Gkm sin( k se ) Bkm cos( k se )) VkVsh (Gsh sin( k sh ) Bsh cos( k sh )) At node m: Pm V 2 mgmm VmVk (Gmk cos( m k ) Bmk sin( m k )) VmVse (Gmm cos( m se ) Bmm sin( m se )) Qm V 2 m Bmm VmVk (Gmk sin( m k ) Bmk cos( m k )) VmVsh (Gmm sin( m se ) Bmm cos( m se )) (5) (6) Series converter: Pse V 2 se Gmm VseVk (Gkm cos( se k ) Bkm sin( se k )) (7) VseVm (Gmm cos( se k ) Bmm sin( se m ) Qse V 2 se Bmm VseVk (Gkm sin( se k ) Bkm cos( se k )) Figure 1. Voltage source model of the UPFC VseVm (Gmm sin( se m ) Bmm cos( se m )) In to providing a supportive role in the active power exchange that take place between a series converter and the AC system, the shunt converter may also generate or absorb reactive power in order to provide independent voltage magnitude regulation at its point of connection with the AC system. Shunt converter: Psh V 2 shgsh VshVk (Gsh cos( sh k ) Bsh sin( sh k ) (9) Qsh V 2 sh Bsh VshVk (Gsh sin( sh k ) Bsh cos( sh k )) (10) where: Ykk Gkk jbkk Z se Z Ymm Gmm jbmm Z Ysh Gsh jbsh Z Figure 2. Simultaneous control of voltage, impedance and angle sh sh (11) (12) se Ykm Ymk Gkm jbkm Z se (13) (14) Assuming the UPFC converters were loss-less in this voltage source model, which implies that there is no absorption or generation of active power by the two converters for its losses and hence the active power supplied to the shunt converter Psh equals the active power demand by the series converter Pse at the DC link. Then the following equality constraint has to be guaranteed. Pse+Psh=0 (15) The converter output voltage was used to control the mode of power flow and voltage regulation at the nodes as follows: The bus voltage magnitude can be controlled by injecting a voltage Vse in phase or anti-phase has shown in Fig. 2. Power flow can be controlled by injecting a voltage V se in quadrature to the line current ( se = m±90, m is the angle between Vm and Im) Fig. 2. Power flow can be controlled by injecting a voltage of magnitude V se in quadrature to node voltage m. Fig. 2. (8) Further more if the coupling transformers are assumed to contain no resistance then the active power at the bus k matches the active power at bus m, then: Psh+Pse=Pk+ Pm = (16)

3 IV. NEWTON-RAPHSON ALGORITHM AND FLOW CHART WITH INCORPORATION OF THE UNIFIED POWER FLOW CONTROLLER matrix is modified and the power equations were mismatched until convergence was attained. Step 8: If the convergence achieved in Step 7, the output load flow was calculated for PQ bus that includes the Bus bars voltages, gene-ration, transmission line flow and losses. From the mathematical modelling point of view, the set of nonlinear, algebraic equations that describe the electrical power network under the steady state conditions were solved for the power flow solutions. Over the years, several approaches have been put forward to solve for the power flow equations. Early approaches were based on the loop equations and methods using Gauss-type solutions. This method was laborious because the network loops has to be specified by hand by the systems engineer. The drawback of these algorithms is that they exhibit poor convergence characteristics when applied to the solution of the networks. To overcome such limitations, the Newton-Raphson method and derived formulations were developed in the early 1970s and since then it became firmly established throughout the power system industry (Gyugyi et al. [3]). In this project a Newton Raphson power flow algorithm was used to solve for the power flow problem in a transmission line with UPFC as shown in the flow chart in Fig. 3. A. Steps to Solve the Newton-Raphson Algorithm Step 1: Read the input of the system data that includes the data needed for conventional power flow calculation, i.e., the number and types of buses, transmission line data, generation, load data and location of UPFC and the control variables of UPFC, i.e., the magnitude and angles of output voltage series and shunt converters. Step 2: Formation of admittance matrix Ybus of the transmission line between the bus i and j. Step 3: Combining the UPFC power equations with network equation, we get the conventional power flow equation: n Pi jqi VV Y ( i j ij i j ) P i jq i ' ij ' (17) j 1 where: P 'i Q i Active and reactive power flow due to UPFC between the two buses th Pi jqi Active and reactive power flow at the i bus th Vi i Voltage and angle of i bus ' V j j =Voltage and angle at j bus th Step 4: The conventional Jacobian matrix are formed (Pik and Qik) due to the inclusion of UPFC. The inclusion of these variables increases the dimensions of the Jacobian matrix. Step 5: In this step, the Jacobian matrix was modified and power equations are mismatched (ΔPik, ΔQik for i = 2, 3,, m and ΔPiik, ΔQiik). Step 6: The busbar voltages were updated at each iteration and convergence was checked. Step 7: If convergence is not achieved in the next step the algorithm goes back to the step 6 and the Jacobian Figure 3. Flow chart for load flow by N-R method with UPFC V. TEST CASE AND SIMULATION Standard 30-bus network shown in Fig. 4 is tested with and without UPFC to investigate its performance. Flat voltage start is assumed for the two UPFC voltage sources. 484

4 Figure 4. Single line diagram of IEEE 30 bus system VI. RESULT OF SIMULATION The network was tested without UPFC and with UPFC. And it was observed that the UPFC parameters were within limits. It was also observed that the losses between buses 1 and 2 are more compared to the remaining. Hence the UPFC is introduced between buses 1 and 2 to reduce the losses. From Table I it is observed that the power flow for the line active and reactive powers will be regulated due to the UPFC and losses get reduced. The voltages of the buses with and without UPFC are also tabulated in Table II which indicates that the voltage got increased in each bus due to placing of UPFC. Waveforms without UPFC are shown in (Fig. 5-Fig. 9) and Waveforms with UPFC are in (Fig. 10-Fig. 14). Fig. 5 and Fig. 10 show the bus voltages without and with UPFC. From these figures it has been observed that the magnitude of the bus voltages have been increased due to placing of UPFC in the bus network. Fig. 6 and Fig. 11 show the phase angles without and with UPFC. From these figures it has been observed that the magnitude of the phase angles have been decreased due to placing of UPFC in the bus network. Fig. 7 and Fig. 12 show the active power flow without and with UPFC. From these figures it has been observed that the active power flow has been regulated due to placing of UPFC in the bus network. Fig. 8 and Fig. 13 show the reactive power flow without and with UPFC. From these figures it has been observed that the reactive power flow has been regulated due to placing of UPFC in the bus network. Fig. 9 and Fig. 14 show the total losses without and with UPFC. From these figures it has been observed that the total losses have been reduced due to placing of UPFC in the bus network. (MW) (MW) (MW) (Radians) Figure 6. Phase angle without UPFC Figure 7. Active power flow without UPFC Figure 8. Reactive power flow without UPFC Figure 9. Total losses without UPFC (Volts) (Volts) Figure 5. Bus voltages without UPFC Figure 10. Bus voltages with UPFC 485

5 Figure 11. Phase angle with UPFC Figure 13. Reactive power flow with UPFC (MW) (MW) (Radians) (MW) Figure 12. Active power flow with UPFC Figure 14. Total losses with UPFC TABLE I. LINE FLOWS WITH AND WITHOUT UPFC Line No. Line Flows without UPFC Line Flows with UPFC P(MW) Q(MVAR) LOSSES P(MW) Q(MVAR) LOSSES

6 REFERENCES TABLE II. BUS VOLTAGE WITH AND WITHOUT UPFC Bus No Voltage without UPFC V θ rad VII. N. G. Hingorani, High power electronics and flexible AC transmission system, IEEE Power Engineering Review, vol. 8, pp. 3-4, July [2] L. Gyugyi, Unified power flow controller concept for flexible AC transmission system, IEE Proc., vol. 139, no. 4, pp , July [3] L. Gyugyi, C. D. Schauder, S. L Williams, and T. R. Rietman, The unified power flow controller: A new approach to power transmission control, IEEE Trans. on Power Delivery, vol. 10, no. 2, pp , April [4] C. R. Fuerete-Esquiivel and E. Acha, Newton-Raphson algorithm for the reliable solution of large power networks with embedded FACTS, IEE Proc.-Ganev. Tvansnr. Distrib., vol. 143, no. 5, pp , September [5] C. R. Fuerete-Esquivel and E. Acha, Unified power flow controller: A critical comparison of Newton-Raphson UPFC algorithm in power flow studies, IEE Proc.-Gener. Transm. Distrib., vol. 144, no. 5, September [6] M. Noroozian, L. Angquist, M. Ghandhari, and G. Anderson, Use of UPFC for optimal power flow control, in Proc. IEEE ikth Stockholm Power Tech Conference, Stockholm, Sweden, 1995, pp [7] A. Nabavi-Niaki and M. R. Travani, Steady-State and dynamic models of Unified Power Flow Controller (UPFC) for power system studies, presented at 1996 IEEE ipes Winter Meeting, Baltimore, 1996, pp [8] A. Nabavi-Niaki and M. R. Iravani, Steady-State and dynamic models of unified power flow, IEEE Trans. on Power Systems, vol. 11, no. 4, pp , Nov [9] C. R. Fuerete-Esquivel and E. Acha, Incorporation of UPFC model in an optimal power flow using Newton s method, IEE Proc.-Gen. Transm. Distrib., vol. 145, no. 3, pp , May [10] C. R. Fuerete-Esquivel, E. Acha, and H. Ambriz-Perez, Comprehensive Newton-Raphson UPFC model for the quadratic power flow solution of practical power networks, IEEE Trans. on Power Systems, vol. 15, no. 1, pp , Feb [1] Voltage with UPFC V θ rad CONCLUSIONS In this paper the UPFC Voltage Source Model (VSM) is used to investigate the performance of the Unified Power Flow Controller (UPFC) and thereby the load flow studies are done by incorporating the Voltage Source Model of UPFC in the Newton Raphson (N-R) algorithm. The N-R algorithm is able to control the flow of power and voltage individually as well as simultaneously. The result for an IEEE-30 Bus system has been presented above with and without UPFC and are compared in terms of Real and Reactive power flow and the Voltage magnitude. It was observed that the UPFC regulates the real and reactive power of the buses and the lines and it also controls the voltage of the bus within specified limits, thereby reduces the total losses in the lines. Takkolu Kalyani is serving as a Lecturer in the Department of Electrical Engineering, Bapatla Polytechnic College, India. She holds her B. Tech in Electrical Engineering from KORM College of Engineering, Kadapa, AP. She obtained her M. Tech in Electrical Engineering with specialization in Power System from Acharya Nagarjuna University, AP. T. Ramesh Kumar is serving as an Assistant Professor in the Department of Electrical Engineering, Bapatla Engineering College, India. He obtained his M. Tech in Electrical Engineering with specialization in Power System from NIT Warangal. He authored and co-authored many papers in leading international proceedings and journals in Power System. His area of interest is Power System Planning. G. Siva Prasad is serving as an Assistant Professor in the Department of Electrical Engineering, KORM College of Engineering, India. He obtained his M. Tech in Electrical Engineering with specialization in Power System from KSRMCE, Kadapa. He authored and co-authored many papers in leading international proceedings and journals in Power System. His area of interest is Power System Optimization. 487