Power System State Estimation with Interline Power Flow Controller

Transcription

1 Power System State Estmaton wth Interlne Power Flow Controller C.Venkatesh Kumar PG Scholar M.E (Power Systems Engneerng) College of Engneerng,Anna Unversty Gundy,Chenna, V.Gomath Power Systems Engneerng College of Engneerng,Anna Unversty Gundy,Chenna, Abstract: Wth the flexble of AC transmsson systems (FACTS) technque, the power flow n the nterconnected power systems can be controlled flexbly. Ths paper s concerned about the state estmaton of system, whch contan Flexble AC Transmsson System (FACTS) devce. Interlne power flow controller (IPFC) s one of the versatle FACTS devce whch s consdered for State Estmaton. Based on the conventonal power system state estmaton model, a knd of model for state estmaton wth IPFC s ntroduced n ths paper, n whch power njecton model s used and the affect of IPFC on the power flow s transferred to the lnes whch s connected. Ths method can be ntegrated to the conventonal state estmaton program wth the consderaton of IPFC. The results demonstrate that the model s effectve for practcal use. The Operaton and workng of Interlne Power flow Controller was done and the results are smulated and presented usng MATLAB/SIMULIK. I. ITRODUCTIO After the establshment of power markets wth transmsson open access, the sgnfcance and use of FACTS devces for manpulatng lne power flows to releve congeston and optmze the overall grd operaton have ncreased. As a result, there s a need to ntegrate the FACTS devce models nto the exstng power system applcatons.ths paper wll present an algorthm for state estmaton of networks coordnated wth a FACTS devce. Due to the enlargement of nterconnected electrc power system and the ncreasngly complexty of electrc power system structure, hence energy management system(ems)s crtcal for modern power system State estmaton plays an mportant role n EMS, whch provdes a relable and consstent system data by processng real tme redundant telemetered and pseudo measurements. These measurements typcally consst of bus voltage magntudes, real and reactve lne flows and power njecton. Processng these real tme data, dfferent knds of advanced applcaton software n EMS are derved, such as voltage stablty analyses, securty constrant and transent stablty analyss et al. Snce the concept of flexble AC transmsson systems (FACTS) was proposed by Hngoran n the 1860s,many varous FACTS devces have been utlzed to meet a growng demand of the transfer capabltes due to developng wheelng transactons n the deregulaton envronment. Some nterestng applcatons of FACTS devces can be found to economc dspatch(ed), AC/DC optmal power flow(opf), avalable transfer capablty(atc), contract path based electrcty tradng, and transmsson congeston management. IPFC, a versatle VSC-based FACTS controller for seres compensaton wth the unque capablty of power flow management among mult-lnes of a substaton and has the capablty to equalze both real and reactve power flow between the lnes, transfer power from overloaded to underloaded lnes, compensate aganst reactve voltage drops and the correspondng reactve lne power, and to ncrease the effectveness of the compensatng system aganst dynamc dsturbances. So,there has been ncreasng nterest n the analyss of IPFC n power system.however, very lmted efforts have been made to study the mpact of FACTS devces on power system state estmaton. A new method s ntroduced to ncorporate IPFC devces nto the power state estmaton. Ths paper attempts to deduce the model of state estmaton wth IPFC usng the conventonal power system state estmaton model. A power njecton model that transfers the affect of IPFC towards the power flow to the transmsson lnes s presented. Ths method can be ntegrated to the conventonal state estmaton program wth the consderaton of IPFC. Furthermore, the results demonstrate that the model s effectve for practcal use. The Operaton of Interlne Power flow Controller was done and the results are smulated usng MATLAB/SIMULIK. II. THE STATE ESTIMATIO PROBLEM A. Formulaton WLS state estmaton mnmzes the weghted sum of squares of the resduals. Consder the set of measurements gven by the vector z : Z= z1 z2 z m = h1 ( x1, x2,.., xn ) h2 ( x1, x2,.., xn ) hm ( x1, x2,.., xn ) + e1 e 2 e m =h(x)+e (2) h T =[h 1 (x),h 2 (x),,h m (x)] (3) ISS: ISB:

2 h (x) s the nonlnear functon relatng measurement to the state vector x x T =[x 1,x 2,,x n ] s the system state vector e T =[e 1,e 2,,e m ] s the vector of measurement errors. Let E(e) denote the expected value of e, wth the followng assumptons: E(e)=0, =1,2,,m E(e e j )=0 Measurement errors are assumed to be ndependent and ther covarance matrx s gven by a dagonal matrx R : Cov(e)=E[e.e T ]=R=dag{ 1 2, 2 2,.., m 2 } (4) The WLS estmator wll mnmze the followng objectve functon: m Mn J(x)= = 1 (z -h (x)) 2 /R =[z-h(x)] T R -1 [z-h(x)] (5) The objectve of weghtng the squared dfferences n (5) s to provde a mathematcal way of descrbng the accuracy of the meters. More precsely, the standard devaton a of a meter s a statstcal value that descrbes how tghtly the measurements taken are clustered around the true value. Thus, f the standard devaton s large, the measurement s relatvely naccurate; whle a small standard devaton value ndcates a small error range. B. The Measurement Functons Accordng to the prevous dscusson, the measured quanttes are represented by the vector z, and h(x) represents a set of functons that depend on the values beng estmated. These functons are used to calculate the estmated values correspondng to measured values z. For ths study, only the bus voltage magntudes, the njected real and reactve powers, and the real and reactve branch power flows wll be used as the quanttes beng measured. Wth excepton of the bus voltage magntudes, the correspondng h(x) functons are nonlnear and are calculated as follows: Real and reactve power njecton at bus : where: V, V j s the voltage magntude at bus, δ j G j +jb j g j +b j g s +b s s the dfference between the voltage phase angles at buses and j, s the j th element of the complex bus admttance matrx, s the seres admttance of the branch connectng buses and j, s the shunt admttance of the branch connectng buses and j, s the number of buses n the system. II. ITERLIE POWER FLOW COTROLLER It s common that the Interlne Power Flow Controller employs a number of dc to ac nverters n order to offer seres compensaton for each lne. As a new concept for the compensaton and effectve power flow management, t addresses the target of compensatng a number of transmsson lnes at a gven substaton. 2.1 Confguraton of Interlne Power Flow Controller Generally, the Interlne Power Flow Controller (IPFC) s a combnaton of two or more ndependently controllable statc synchronous seres compensators (SSSC) whch are sold-state voltage source converters whch nject an almost snusodal voltage at varable magntude and couples va a common DC lnk as shown n Fgure1.Conventonally, seres capactve compensaton fxed, thyrstor controlled or SSSC based, s employed to ncrease the transmttable real power over a gven lne and to balance the loadng of a normally P = V V ( cos sn ) 1 j G j δj + B j δ j = j Q = V V ( sn cos ) 1 j G j δ j B j δ j = j Real and reactve power flow from bus to bus k (3) (4) P j =V 2 (g s +g j )- V * V j (g j cosδ j +b j snδ j ) (5) Q j =-V 2 (b s +b j )- V * V j (g j snδ j -b j cosδ j ) (6) ISS: ISB:

3 encountered mult-lne transmsson system. They are controlled to provde a capablty to drectly transfer ndependent real power between the compensated lnes whle mantanng the desred dstrbuton of reactve flow among the lne [1],[3].Consder smplfed schematc of IPFC model n fgure1, each compensatng nverters s lnked together at ther dc termnals. Wth ths scheme, n addton to provdng seres reactve compensaton, any nverter can be controlled to supply real power to the common dc lnk from ts own transmsson lne. Thus, an overall surplus power can be transferred from the underutlzed lnes whch can be used by other lnes for real power compensaton. Evdently, ths arrangement mantans the overall power balance at the common dc termnal by approprate control acton. An elementary IPFC scheme consstng of two back-to-back dc to ac nverters s used as a tool to compensate a transmsson lne by seres voltage njecton. Two synchronous voltage sources, wth phasors V1pq and V2pq, n seres wth transmsson lne1 and 2 respectvely, represent the two back-to-back dc to ac nverters as llustrated n fgure Equvalent Crcut IPFC s a knd of VSC-based FACTS devce. Just lke UPFC, IPFC s also called combned compensator because It conssts of at least two statc synchronous seres compensators (SSSCs) whch are connected va a common dc voltage lnk that can be represented by a capactor. For smplcty, ths paper deals wth IPFC combnng only two SSSCs, as shown n Fg. 1. However, followng dervatons can be appled to IPFCs consstng of more than two VSCs wthout much dffculty. Usually, n the steady state analyss of power systems, the VSC s represented as a synchronous voltage source njectng an almost snusodal voltage wth controllable magntude and angle [9]. As for IPFC, the two VSCs are connected n seres wth two lnes as shown n Fg. 2. V, Vj and Vk are complex voltages at buses, j and k, respectvely,defned as VbLθb (b=, j). Vsej and Vsek are the controllable complex voltages of the two synchronous voltage sources,defned as VsenLθsej, (n=j, k). zsej and zsejk are the seres transformer mpedance. Pcal, and Qcal, are the transmtted actve and reactve powers through the two branches of IPFC leavng bus. PcalJ (Pcal,k) and Qcal,j (Qcalk) are the transmtted actve and reactve powers through one branch of IPFC leavng bus j (k), respectvely. Actve power can be transferred from one lne to the other va the common dc lnk. Based on the above equvalent crcut, the power flow equatons at each bus are : Assumng lossless converter valves, the actve power suppled to one converter equals the actve power demanded by the other, f there are no underlyng storage systems; that s where the superscrpt * denotes the conjugate of a complex number. ISS: ISB:

4 Step6: Check for convergence. If max{ Av( k+1 >, Ae( k+1 > }> go to Step 3; Otherwse, set k = k + 1 and go to Step 7; Step 7: The process s fnshed and prnt results. FLOW CHART: So, the msmatch power equatons consderng generaton Pgm, Qgm, and load PIm, Qlm at each bus are as follows: where m=,j, k Here PneJm and Qlne,m are conventonal transmtted actve and reactve power only through transmsson lnes leavng bus m. Equaton (6) stll apples here. If the seres transformers are assumed to contan no resstance, the followng equaton can be derved. IV. CO-ORDIATIO ALGORITHM FOR STATE ESTIMATIO WITH ITERLIE POWER FLOW COTROLLER The detaled soluton steps of the proposed algorthm can be summarzed as follows: Step 1: Input system data and telemetered measurements load flow; Step 2: Set teraton count k = 0; Step3: Intalze the state vector v (0),e (0) and calculate system measurements; Step 4: Wth IPFC Compute Jacobn matrx H (x (k) ); Step5: Obtan V (k+1) AD θ (k+1). V (k+1) = V (k) + V (k+1), θ (k+1) = θ (k) + θ (k+1) V.RESULTS OF SIMULATIOS In ths secton, IEEE 14 bus and 30 bus system has been used to test the effectveness of the ntroduced method. The data for testng the modfed state estmaton are obtaned usng the results from power flow analyss. The soluton accuracy, convergence behavor and computatonal effcency of the proposed method are verfed by the test results and compared wth those obtan from tradtonal state estmaton method. For all smulatons, the tolerance used to defne convergence s ISS: ISB:

5 TABLE I STATE ESTIMATIO RESULTS FOR IEEE 14 BUS BUS O. TRUE VALUES WITHOUT IPFC WITH IPFC V/pu δ( ) V/pu δ( ) V/pu δ( ) [5] H. Sngh, F.L. Alvarado, "Weghted Least Absolute Value State problem was Estmaton Usng Interor Pont Methods," IEEE Transactons on Power Systems, 9(3), 1994, [6] B. Xu, A. Abur, "State Estmaton of Systems wth Embedded FACTS Devces," n Proc IEEE Power Tech Conf, 5 pp [7]A. abav-ak, M. R. Iravan, " Steady State and Dynamc Models of Unted Power Flow Controller (UPFC) for Power System Studes ", IEEE Transactons on Power systems, Vol. 11, o. 4, ovember [8] M. oroozan, L. Angqust, M. Ghandhar, and G. Anderson, " Use of UPFC for Optmal Power Flow Control ", IEEE Trans. On Power Delvery, Vol. 12, o. 4, pp , October [9] John J. Granger, Wllam D. Stevenson, Jr., " Power System Analyss", McGraw- Hll, J (x) Iteraton number 4 4 REFERECES: [1]A.J. Wood, B.F. Wollemberg, Power Generaton, Operaton and Control, 2nd. Ed. (ew York: Wley, 1996, ). [2] J. J. Granger, W. D. Stevenson Jr., Power System Analyss, (ew York: McGraw-Hll, 1994, ). [3] 0. Alsac,. Vempat, B. Sttot, A. Montcell, "Generalzed State Estmaton," IEEE Transactons on Power Systems, 13(3), 1998, [4] A. Abur, M.K. Celk, "Least Absolute Value State Estmaton wth solvng SE n Equalty and Inequalty Constrants," IEEE Transactons on Power Systems, 8(2), 1993, ISS: ISB: