OPTIMAL POWER FLOW USING UNIFIED POWER FLOW CONTROLLER (UPFC)

Int. J. Elec&Electr.Eng&Telcomm. 01 G Svaprasad and K Ramamohan Reddy, 01 Research Paper ISSN 319 518 www.jeetc.com Vol. 1, No. 1, October 01 01 IJEETC. All Rghts Reserved OPTIMAL POWER FLOW USING UNIFIED POWER FLOW CONTROLLER (UPFC) G Svaprasad 1* and K Rama Mohan Reddy 1 *Correspondng Author: G Svaprasad, svaprasadgodna@gmal.com A crtcal factor effectng power transmsson system s the power flow control. To regulate the power flow control n power transmsson lne unfed power flow controller (UPFC) s used. The UPFC s represented wth two voltage sources named Voltage Source Model (VSM), whch s used to study the behavour of the UPFC n regulatng the actve, reactve power and voltage profle n the system. These VSM s ncorporated n Newton Raphson (N-R) algorthm for load flow studes. The equatons of UPFC and the power balance equatons of network are combned n to one set of non-lnear algebrac equatons by employng Smultaneous method and s calculated accordng to the Newton raphson algorthm; Performed on the IEEE 30-bus system. Smulaton s done n Matlab. The results are compared wth and wthout UPFC n terms of actve and reactve power flows n the lne and actve and reactve power flows at the bus to analyze the performance of UPFC. Keywords: Newton-Raphson algorthm, Load flow, Unfed power flow controller, Voltage source model INTRODUCTION Electrcal power systems are a large nterconnected network that requres a careful desgn to mantan the system wth contnuous power flow operaton wthout any lmtatons. Flexble Alternatng Current Transmsson System (FACTS) s an evolvng technology used to help electrc utltes fully utlze ther transmsson assets. Ths concept was frst ntroduced by N G Hngoran, n (1988). Many types of FACTS devces have been proposed, among them Unfed Power Flow Controller (UPFC) s a versatle and flexble devce n the FACTS famly of controllers whch has the ablty to smultaneously control all the transmsson parameters of power systems.e., voltage, mpedance and phase angle whch determnes the power flow of a transmsson lne, ths devce was proposed by Gyugy n (199); and Gyugy et al. (1995). 1 Department of EEE, KSRM College of Engneerng, Kadapa. 105

The UPFC seen to be conssts of two Voltage Source Converters (VSCs), one VSC s connected n seres to the transmsson lne through a seres transformer, smlarly the other s connected n shunt to the transmsson lne through a shunt transformer and both are connected back to back through a DC storage capactor (Gyugy et al., 1995). In ths paper the performance of UPFC s nvestgated on power systems effectvely, to ths t s requred to formulate ther approprate model. In the area of power flow analyss the UPFC models have been publshed (Fuerete-Esquvel and Acha, 1998; Noroozan et al., 1995; and Nabav-Nak and Iravan, 1996a and b) and consder the UPFC as one seres voltage source and one shunt current source model or both the seres and shunt represented by two voltage sources. In the area of power flow concept the UPFC s represented by two voltage sources called Voltage Source Model(VSM) (Fuerete- Esquvel and Acha, 1998) also ntroduced another model called the Power Injecton Model (PIM). The Voltage source model of UPFC s ncorporated n N-R algorthm n to estmate the performance of UPFC n power flow control. Generally there are ways of solvng power flow solutons, the Sequental and the smultaneous method: In the frst method, the equatons of UPFC are separated from the power flow equatons and both the set of equatons are solved separately and sequentally. In smultaneous method, the equatons of UPFC and the power flow equatons are combned n to one set of non-lnear algebrac equatons whch fnd less complexty. A jacoban matrx s then formed and are n non symmetrc n nature. Here n ths paper the smultaneous method was used. UPFC OPERATING PRINCIPLE The UPFC conssts of two voltage source converters, one connected n seres to the transmsson lne through a seres transformer and the other n shunt to the transmsson lne through a shunt transformer, both are connected back to back through a DC lnk and can modelled as two deal voltage sources between the two busses (Fuerte-Esquvel and Acha, 1997; and Fuerte-Esquvel et al., 000). The UPFC allows smultaneous control of actve power flow, reactve power flow, and voltage magntude at the UPFC termnals. Alternatvely, the controller may be set to control one or more of these parameters n any combnaton or to control none of them. The actve power demanded by the seres converter s drawn by the shunt converter from the AC network and suppled to bus m through the DC lnk. The output voltage of the seres converter s added to the nodal voltage, at say bus k, to boost the nodal voltage at bus m. The output of the seres voltage source V se and se are controllable magntude and angle between the lmts V se max V se V semn and 0 se respectvely and of the shunt voltage source s V sh and sh controllable between the lmts V sh max V sh V shmn and 0 sh. The voltage magntude of the output voltage V se provdes voltage regulaton, and the phase angle se determnes the mode of power flow control. Fgure 1 shows the voltage source model of the UPFC. Z se and Z sh are the mpedances of the two transformers between 106

Fgure 1: Voltage Source Model of UPFC the lne and UPFC. In addton to provdng a supportve role n the actve power exchange that take place between a seres converter and the AC system, the shunt converter may also generate or absorb reactve power n order to provde ndependent voltage magntude regulaton at ts pont of connecton wth the AC system. The converter output voltage was used to control the mode of power flow and voltage regulaton at the nodes as follows: a. The bus voltage magntude can be controlled by njectng a voltage V se n phase or ant-phase has shown n Fgure. Fgure : Smultaneous Control of Voltage, Impedance and Angle b. Power flow can be controlled by njectng a voltage V se n quadrature to the lne current ( se = m ± 90, m s the angle between V m and I m ) Fgure. c. Power flow can be controlled by njectng a voltage of magntude V se n quadrature to node voltage m. Fgure. MODELLING OF UPFC The two deal seres and shunt voltages source equatons of the UPFC from Fgure 1 are: V V cos j sn...(1) se se se se V V cos j sn...() sh sh sh sh Based on the voltage source model of UPFC the actve and reactve power equatons are: At node k: P V kg V V ( G cos( ) B sn( k kk k m km k m km k m V Vse( G cos( B sn( k km k se) km k se V V ( G cos( ) B sn( k sh sh k sh sh k sh Q V kb V V ( G sn( ) B cos( k kk k m km k m km k m V V ( G sn( ) B cos( k se km k se km k se V V ( G sn( ) B cos( k sh sh k sh sh k sh At node m: P V mg V V ( G cos( ) B sn( m mm m k mk m k mk m k V V ( G cos( ) B sn( m se mm m se mm m se...(3)...(4)...(5) Q V mb V V ( G sn( ) m mm m k mk m k B cos( V V ( G sn( ) B mk m k m sh mm m se cos( mm m se...(6) Seres converter P V seg V V ( G cos( ) B sn( se mm se k km se k km se k V V ( G cos( ) B sn( ) se m mm se k mm se m...(7) 107

Q V seb V V ( G sn( ) B cos( se mm se k km se k km se k V V ( G sn( ) B cos( se m mm se m mm se m Shunt converter:...(8) P V shg V V ( G cos( ) B sn( )...(9) sh sh sh k sh sh k sh sh k Q V shb V V ( G sn( ) B cos(...(10) sh sh sh k sh sh k sh sh k where Y G jb Z Z...(11) 1 1 kk kk kk se sh 1 Ymm Gmm jbmm Z se...(1) 1 Ykm Ymk Gkm jbkm Z se...(13) 1 Ysh Gsh jbsh Z sh...(14) Assumng the UPFC converters were lossless n ths voltage source model, whch mples that there s no absorpton or generaton of actve power by the two converters for ts losses and hence the actve power suppled to the shunt converter P sh equals the actve power demand by the seres converter P se at the DC lnk. Then the followng equalty constrant has to be guaranteed. P se + P sh = 0...(15) Further more f the couplng transformers are assumed to contan no resstance then the actve power at the bus k matches the actve power at bus m, then P sh + P se = P k + P m = 0...(16) NEWTON-RAPHSON ALGORI- THM AND FLOWCHART WITH INCORPORATION OF THE UNI- FIED POWER FLOW CONTROLLER From the mathematcal modellng pont of vew, the set of nonlnear, algebrac equatons that descrbe the electrcal power network under the steady state condtons were solved for the power flow solutons. Over the years, several approaches have been put forward to solve for the power flow equatons. Early approaches were based on the loop equatons and methods usng Gauss-type solutons. Ths method was laborous because the network loops has to be specfed by hand by the systems engneer. The drawback of these algorthms s that they exhbt poor convergence characterstcs when appled to the soluton of the networks. To overcome such lmtatons, the Newton-Raphson method and derved formulatons were developed n the early 1970s and snce then t became frmly establshed throughout the power system ndustry (Gyugy et al., 1995). In ths project a Newton Raphson power flow algorthm was used to solve for the power flow problem n a transmsson lne wth UPFC as shown n the flow chart n Fgure 3. Steps to Solve the Newton-Raphson Algorthm Step 1: Read the nput of the system data that ncludes the data needed for conventonal power flow calculaton,.e., the number and types of buses, transmsson lne data, generaton, load data and locaton of UPFC and the control varables of UPFC,.e., the magntude and angles of output voltage seres and shunt converters. Step : Formaton of admttance matrx Y bus of the transmsson lne between the bus and j. Step 3: Combnng the UPFC power equatons wth network equaton, we get the conventonal power flow equaton: 108

Fgure 3: Flowchart for Load Flow by N-R Method wth UPFC j 1 j j j j n P Q VV Y P jq...(17) where P jq = Actve and Reactve power flow due to UPFC between the two buses. P Q = Actve and Reactve power flow V < = V j < j = Y j = at the th bus. Voltage and angle of th bus Voltage and angle at j th bus Admttance of the transmsson lne between the bus and j Step 4: The conventonal jacoban matrx are formed ( P k and k Q ) due to the ncluson of UPFC. The ncluson of these varables ncreases the dmensons of the jacoban matrx. Step 5: In ths step, the jacoban matrx was modfed and power equatons are msmatched ( P k, P, Q ). k k k Q for =, 3,, m and Step 6: The busbar voltages were updated at each teraton and convergence was checked. Step 7: If convergence s not acheved n the next step the algorthm goes back to the step 6 and the jacoban matrx s modfed and the power equatons were msmatched untl convergence was attaned. Step 8: If the convergence acheved n Step 7, the output load flow was calculated for PQ bus that ncludes the Bus bars voltages, generaton, transmsson lne flow and losses. TEST CASE AND SIMULATION Standard 30-bus network was tested wth and wthout UPFC to nvestgate ts performance. 109

Fgure 4: Sngle Lne Dagram of IEEE 30 Bus System Flat voltage start was assumed for the two UPFC voltage sources. RESULT OF SIMULATION The network was tested wthout UPFC and wth UPFC. And t was observed that the UPFC parameters were wthn lmts. The smulatons show the power flow for the lne actve and reactve powers whch were tabulated below (Table 1). The voltages of the buses wth and wthout UPFC were also tabulated (Table ). Table 1: Lne Flows Wth and Wthout UPFC Lne No. Lne Flows wthout UPFC Lne Flows wth UPFC P(MW) Q(MVAR) Losses P(MW) Q(MVAR) Losses 1-1.733 -.754 0.05311 1.4088-0.59 0.038091 1-3 0.8774 0.0039 0.031001 0.7607-0.095 0.03493-4 0.4335 0.015 0.009839 0.406-0.005 0.008311 3-4 0.84 -.0769 0.00855 0.713-0.148 0.006387-5 0.838 0.0058 0.0984 0.5988-0.145 0.01555-6 0.6055 -.0189 0.019449 0.5485-0.054 0.015480 4-6 0.7453 -.1538 0.006609 0.613-0.1 0.004738 5-7 -0.147 0.107 0.001587 0.039 0.1695 0.001370 6-7 0.3807 -.011 0.003740 0.1975-0.094 0.001135 6-8 0.963 -.095 0.001107 0.965-0.095 0.001057 6-9 0.916 -.0133 0.000000 0.958-0.011 0.000000 6-10 0.1650 0.03 0.000000 0.1674 0.04 0.000000 9-11 -0.000 -.1570 0.000000 0.0000-0.157 0.000000 9-10 0.916 0.166 0.000000 0.958 0.188 0.000000 110

Table 1 (Cont.) Lne No. Lne Flows wthout UPFC Lne Flows wth UPFC P(MW) Q(MVAR) Losses P(MW) Q(MVAR) Losses 4-1 0.4163 0.0670 0.000000 0.4075 0.0578 0.000000 1-13 -0.000 -.1044 0.000000-0.000-0.104 0.000000 1-14 0.0731 0.0155 0.000677 0.0719 0.0149 0.00063 1-15 0.1643 0.0315 0.0018 0.1600 0.088 0.001639 1-16 0.0668 0.0057 0.000418 0.0635 0.0036 0.000359 14-15 -.0105 -.0018 0.00005 0.0093-0.00 0.00000 16-17 0.0313-0.013 0.000095 0.081-0.015 0.000080 15-18 0.0580 0.0050 0.000369 0.0564 0.0041 0.000331 18-19 0.056 -.0047 0.000045 0.040-0.005 0.000038 19-0 -.0693 -.0388 0.0001-0.070-0.039 0.00019 10-0 0.096 0.0485 0.001005 0.0941 0.049 0.000986 10-17 0.0589 0.071 0.00076 0.061 0.0740 0.0008 10-1 0.1903 0.1413 0.00191 0.195 0.145 0.001865 10-0.0567 0.033 0.000305 0.0563 0.03 0.00086 1-3 0.0134 0.05 0.000010 0.0158 0.064 0.000011 15-3 0.039 -.00390 0.00011 0.09-0.005 0.000086-4 0.0564 0.0317 0.00048 0.0560 0.0316 0.000453 3-4 0.0143 0.0050 0.000031 0.019 0.0044 0.00004 4-5 -.0167 0.0119 0.00008 -.0184 0.0113 0.000086 5-6 0.0354 0.037 0.000475 0.0354 0.036 0.000449 5-7 -.053 -.0118 0.00033 -.0539-0.01 0.00035 8-7 0.1856 0.0607 0.000000 0.1870 0.0604 0.000000 7-9 0.0619 0.0168 0.000914 0.0619 0.0166 0.000863 7-30 0.0710 0.0168 0.00170 0.0709 0.0166 0.00163 9-30 0.0370 0.0061 0.000356 0.0370 0.0060 0.000336 8-9 -.0047 -.0140 0.000005 -.0044-0.016 0.000004 6-8 0.1910 -.1018 0.000615 0.191-0.109 0.000595 111

Table : Bus Voltage wth and Wthout UPFC Bus No. Voltage wthout UPFC Voltage wth UPFC [V] rad [V] rad 1 1.06 0 1.06 0 1.04659-0.0941 1.0647-0.0807 3 1.0738-0.1337 1.04596-0.11657 4 1.01997-0.16304 1.045-0.14310 5 1.01585-0.4716 1.06760-0.19006 6 1.01783-0.19447 1.04437-0.16909 7 1.0093-0.519 1.0460-0.18601 8 1.01805-0.0751 1.04467-0.18150 9 1.030-0.581 1.04836-0.533 10 1.00916-0.839 1.03531-0.553 11 1.0546-0.581 1.07958-0.533 1 1.00857-0.6683 1.03319-0.4011 13 1.0307-0.6683 1.04736-0.4011 14 0.99583-0.8357 1.0103-0.558 15 0.99388-0.861 1.01946-0.5811 16 1.0016-0.7944 1.0674-0.5169 17 1.0013-0.8654 1.0733-0.5801 18 0.98659-0.9850 1.0171-0.696 19 0.98555-0.30 1.01190-0.7301 0 0.99063-0.9874 1.01696-0.6963 1 0.9915-0.935 1.01856-0.6430 1.0008-0.9001 1.0670-0.6106 3 0.99139-0.937 1.01777-0.6437 4 0.98813-0.9654 1.01491-0.670 5 0.98737-0.8856 1.01467-0.593 6 0.96914-0.9635 0.99694-0.6660 7 0.99573-0.0609 1.0308-0.4970 8 1.01676-0.0609 1.04355-0.1805 9 0.9758-0.39473 1.0034-0.7117 30 0.96345-0.31779 0.99176-0.8659 11

Fgure 5: Shows the Bus Voltages wthout UPFC Fgure 8: Shows the Reactve Power Flow wthout UPFC Fgure 6: Shows the Phase Angle Wthout UPFC Fgure 9: Shows the Total Losses wthout UPFC Fgure 7: Shows the Actve Power Flow wthout UPFC Fgure 10: Shows the Bus Voltages wth UPFC 113

Fgure 11: Shows the Phase Angle wth UPFC Fgure 14: Shows the Total Losses wth UPFC Fgure 1: Shows the Actve Power Flow wth UPFC Fgure 13: Shows the Reactve Power Flow wth UPFC CONCLUSION In ths paper the UPFC Voltage Source Model (VSM) was used to nvestgate the performance of the Unfed Power Flow Controller (UPFC) and thereby the load flow studes were done by ncorpatng the Voltage Source Model of UPFC n the Newton Raphson (N-R) algorthm. The N-R algorthm s able to control the flow of power and voltage ndvdually as well as smultaneously. The result for a IEEE-30 Bus system has been presented above wthout and wth UPFC and were compared n terms of Real and Reactve power flow and the Voltage magntude. Hence t was observed that the UPFC regulates the real and reactve power of the buses and the lnes and t also controls the voltage of the bus wthn specfed lmts, thereby reduces the total losses n the lnes. REFERENCES 1. Abbate L, Trovato M, Beeker C and Handschn E (00), Advanced Steady- State Models of UPFC for Power Systems Studes, IEEE, pp. 449-454. 114

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