A Methodology for Solving VAr/Voltage Problems in the Western Algerian Transmission System

Transcription

1 Volume 52, Number 1, A Methodology for Solvng VAr/Voltage Problems n the Western Algeran Transmsson System M. KHIAT, Z. FRIOUI, A. CHAKER and D. REHIEL Abstract: Ths paper descrbes the methodology adopted to deal wth the VAr dspatch and voltage volaton problems. Ths flexble methodology combnng heurstc and numercal technques has been successfully used to determne a sutable strategy for the operaton of the western Algeran transmsson/sub-transmsson system. The njecton of shunt capactors at target buses, optmzaton step and correctve actons at specfc transformer taps and synchronous generators wth consderng heurstc rules allows to mantan the voltages n the whole network wthn the requred lmt and to reduce the actve power losses. Fnally, feld results correspondng to the worst case reported up to date are analyzed to show the valdty of the proposed procedure. Keywords: Voltage problems, heurstc rules, optmzaton. 1. INTRODUCTION Qualty and securty of electrc energy supply requres that proper attenton be pad to VAr dspatch and voltage control. Reactve power management can be defned as the control of the generator voltages, the varable transformer tap settngs, the compensaton and swtchable shunt element, the VAr njecton n synchronous and statc VAr compensators (eg, Facts devces). Addtonally when securty s not concern there s stll an economc ncentve to reduce actve power losses by reschedulng reactve power sources and shftng transformer taps. In Algera, the electrc power supply s based n three regons: the western, the eastern and the central regons. Because of ts large dstance from the man poles, the southern regon s not connected to the man network. Presently, the control of the reactve power and voltage s one of the most mportant concerns of the Algeran electrcty company (Sonelgaz). The Algeran system (220 and 60 kv system) possesses many buses radally connected to the man grd, certan consumpton areas beng remotely located from njecton buses. It s also characterzed by long transmsson lnes, an uneven dstrbuton of reactve power reserves among avalable generators, and an nsuffcent number of shunt capactors. As a consequence, operators are routnely facng severe voltage problem, and the VAr dspatch has become one of the most relevant concerns n the control center for the western Algeran transmsson system. In ths paper, we focus our nterest on the western Algeran network (220 and 60 kv system) that s connected to Morocco by the Oujda staton (bus n 12) at the west and by the Oued Sly staton (bus n 13) at the center. In ths network, severe voltage problems are tradtonally encountered (especally n the 60 kv electrcal network for whch operators are routnely facng low voltages), as well as local reactve power shortages. In order to prevent these problems, very effcent optmzaton programs can be used [1], [2], [3]. Besdes expert systems [4], [5], [6], heurstc (.e. knowledge-based) methods [1], [5], Metaheurstc methods [10] and other branches of AI technology (e.g. fuzzy logc) have also been proposed regardng VAr/voltage control. Our methodology has conssted n the development of an optmzaton program that allows two-fold control of the system and the ntegraton the heurstc rules. Frst, f one or several emergency voltage lmts are volated, then dentfy the most effectve shunt capactors and compute the amount of reactve power that should be njected, n steps of 5 MVAR, wth consderng the heurstc rules. Second, reschedule generator reactve powers and transformer taps so to mnmze actve-power losses. If other volatons occur, fne tunng s acheved through addtonal approprate control steps. The advantage of our user-frendly methodology combnng heurstc and numercal technques s to establsh a practcal number of control varables, to solve effcently the problem of voltage volatons and to reduce loss powers. In secton 2, we present the methodology adopted. Secton 3 and 4 descrbe n more detal how voltage problem are solved by heurstc method and actve power losses are reduced respectvely. Fnally, mplementaton and some results obtaned on the western Algeran network are presented and Manuscrpt receved February 03, Medamra Scence Publsher. All rghts reserved.

2 32 ACTA ELECTROTEHNICA compared to those of a results Sonelgaz company n secton OVERVIEW OF METHODOLOGY ADOPTED VAr/Voltage optmzaton and control s one the most mportant securty and servce qualty ndexes. The system behaves dfferently dependng on whch lmt s exceeded. Several papers have presented methods for solvng VAr/voltage problems: knowledgebased, senstvty-based and heurstc rule- based [1], [4], [5], [6], [10]. In the present work, the basc methodology for solvng VAr/ voltage problems s as follows: 1. Determne the buses that undergo the most severe voltage volatons (target buses). 2. Identfy the most effectve shunt capactor (s) and compute the amount of reactve power that should be sutably njected, n steeps of 5 MVAR, to solve those volatons wth consderng heurstc rules. 3. Optmzaton process for reactve power of generators and transformer taps wth fxed shunt capactors. 4. If other less mportant voltage volatons occur at buses, use approprate control steps for the adjacent transformer tap and/or the nearest reactve power of generator,.e wth consderng heurstc rules. new t optmal t t (1) g, new g, optmal Q Q Q (2) Where t s the control step appled of transformer taps and Q the control step appled for generator reactve powers. Typcal and ntal Fg. 1. Overvew of the methodology adopted. values of t and q are 0.01 p.u. 5. Actualze new voltages wth a load flow. 3. HEURISTIC RULES FOR VOLTAGE PROBLEMS CORRECTION To facltate the resoluton of voltage volatons and to assst the operator, one can ntroduce rules that one call «heurstc rules». These rules are establshed n a partcular or general manner accordng to a partcular experence. The correcton process depends on the voltage volatons n the upper or lower lmt and network explotaton state (normal or contngency). Accordng to the experence acqured, some rules of correcton of the voltage volaton have been determned Rules correcton for voltage volaton IF there s no voltage volaton THEN the process ends. IF there exsts a voltage volaton. to dentfy the largest voltage volaton to select voltage accordng to ther level of volaton THEN to correct voltage accordng to the upper or lower lmt. When the lower lmt s exceeded, the correcton for voltage volaton n order of preference s as follows: to swtch off lnes that were formerly swtched, to dsconnect the reactances, to ncrease the voltage of producton groups, to connect the condensers, to ncrease voltage on transformer taps. If, the voltage volaton perssts, a dverson of load has been made. When the upper lmt s exceeded, the correcton for voltage volaton n order of preference s as follows: to decrease the voltage of producton groups to connect the reactances. to open the lnes whose dsconnecton s permssble to decrease voltage on transformer taps Once the process of adjustment s selected, the new voltage has to be actualzed. IF t remans no voltage to correct THEN the process ends. Moreover, the treatment of the varous controls offers an effectve and quck soluton to the followng questons: - What s the most adequate control to correct a gven lmt volaton? - What are the most effectve control varables? - What s the necessary varaton of the control varable, n order to obtan the desred correcton wthout nducng new volatons?

3 Volume 52, Number 1, Should we use one control or varous and smultaneous controls? - What coordnaton between the controls s needed n order to correct the same volaton? An answer could be provded to these questons by usng heurstc and/or numercal rules, thus: - Always use the geographcally nearest control, untl the voltage s corrected. If ths s suffcent, one can move to the next control. - Always grant prorty to the use of generators, to the transformers, and then to the batteres, n that order. - Establsh a crteron, whch would allow us to classfy and compare the effectveness of the varous controls. - Establsh an optmsaton process under certan constrants Heurstc rules on the means of control Producton groups Producton groups are well stuated to satsfy needs n reactve energy compensaton. Furthermore, ther dynamc performances allow them to face the abrupt demand fluctuatons. However, they can only partally compensate the reactve loads by reason mportant voltage falls n order to create reactve energy transts. Ther adjustment of the tenson and management of the reactve power are nsured by groups. Ther acton s coordnated n tme (prmary adjustment) and n the space (secondary adjustment) Backup generators The man objectve of the backup generators s to provde energy durng rush hour or n case of emergency. However, connectng the backup generators to the network must be undertaken n extreme stuatons n order to produce reactve power to ncrease voltage. The general rules for connectng or dsconnectng the backup generators are as follows: Connecton IF V k V mn k (emergency) AND load tendency s favourable, AND generator s not n servce, AND generators, transformers, and condenser batteres cannot resolve the volaton, AND when t connected and does not consume reactve power, THEN connect the backup generator. If the backup generator s connected to the network, t functons n the same way as any other generator. Dsconnecton IF Vk V k (emergency) AND mnmum tme has elapsed, durng whch the generator has been connected, AND load tendency s favourable, AND t has been started n order to ncrease the voltages, THEN dsconnecton the backup generator Transformer taps Network voltage varatons are closely lnked to the fluctuatons of the reactve power n the system of producton, transport and dstrbuton. These voltage varatons are set out of sources of reactve power, and by the transformer taps. A transformer taps s a connecton pont along a transformer wndng that allows the number of turns to be selected. By ths means, a transformer wth a varable turn rato s produced, enablng voltage regulaton of the secondary sde. Selecton of the tap n use s made va a tap changer mechansm. A mechancal on-load tap changer (OLTC), also known as under-load tap changer (ULTC) desgn, changng back and forth between tap poston 2 and 3. Because nterruptng the supply s usually unacceptable for a power transformer, these are often ftted wth a more expensve and complex on-load tap-changng mechansm. OLTC may be generally classfed as mechancal, or as electronc, whch n turn may be ether asssted or sold state. The proposed rule determnes the tap poston of the ULTC transformer to mantan the voltage wthn the permssble lmts as much as possble. IF V V mn AND generators s not capable to solve the volaton, AND load tendency s favourable, AND tap poston s not adequate, THEN change the tap poston of the ULTC transformer. IF V V AND load tendency s favourable, AND change the tap poston does no provoke other voltage volaton, THEN change the tap poston of the ULTC transformer Condensers They allow to nsure the mantenance of the tenson and to lmt actve losses. General rules obeyng the connecton or the dsconnecton of these elements are as follows: Connecton IF V V mn AND generators and transformers utlsaton are not capable to solve the volaton AND load tendency s favourable, AND connecton does no provoke other voltage volatons THEN connect the condensers.

4 34 ACTA ELECTROTEHNICA Dsconnecton IF V V AND load tendency s favourable, AND the dsconnecton does no provoke other voltage volaton, THEN dsconnect the condensers Reactances They are used to compensate reactve energy provded at slack hours by lnes. The general rules that govern these devces are: Connecton: IF V V AND regular control devces cannot solve the volaton, AND load tendency s favourable, AND ts connecton does not gve rse to a lower volaton more severe than the upper volaton beng solved, THEN connect the reactance. Dsconnecton IF V V mn AND load tendency s favourable, AND ts dsconnecton does not lead to new volatons, THEN dsconnect the reactance Lnes There exsts a very strct condton for the openng of some lnes. It s the last resort to use. Durng weak demands, the openng of the lne s made only near the knot where the voltage s superor to ts lmt. Dsconnecton IF V V AND load tendency s favourable, AND the other controllers cannot solve the volaton, AND the transt of a lower power than a percentage of ts total capacty, AND the tme from the last swtchng exceeds a threshold, THEN open the lne. Connecton IF the lne was dsconnected to correct voltage volaton, AND load tendency s favourable, AND the placement n servce does not provoke a great voltage to the upper lmt tensons, THEN connect the lne FACTS devces An attempt wll be made to assess whether the use of ths equpment s nterestng and proftable n some cases as there are, n ths doman, no standardsed solutons, except the FACTS whch are commonly used. The two prncpal reasons that justfy the nstallaton of FACTS devces n electrc networks are: - Increase of dynamc stablty lmts (transtory stablty, voltage stablty). - Effectve masterng of the low of energy and very VAr/voltage problems. 4. OPTIMIZATION: FORMULATION AND SOLUTION 4.1. Formulaton of the problem Assumng that the actve power generaton has been schedule accordng to the economc dspatch soluton, t s possble to mnmze actve power losses by properly adjustng, wthn acceptable lmts, generator voltage magntudes, transformer taps and swtchable VAr sources. In ths process, allowable bounds on certan dependent varables, lke the load bus voltages and the generator reactve powers, should be enforced as well. Although t s customary n conventonal load flow to consder generator voltage magntudes as control varables and the respectve reactve powers as dependent varables, the roles of these two varables can be nterchanged n the optmzaton process, as long as the power flow equatons are satsfed. The advantage of ths procedure s twofold: - All bus voltages magntudes belong to the state vector. There are no present PV buses and no need to dstngush between both types of buses. - Reactve power lmts are more easly modeled when the assocated varables are explctly handled. Otherwse, functonal nequalty constrant would appear. Mathematcally, for a network wth n buses, the actve power loss mnmzaton problem can be stated by resortng to the followng set of vectors: - Dependent or state vector: composed of the n bus voltage magntudes v and of the n 1 bus phase angles relatve to the slack bus: x [ v, ] : (2n 1) 1 (3) - Independent or control vector composed of the n g controllable reactve power njecton and of the nt transformer tap changer ratos: u [ Q, t] : ( ng nt ) 1 (4) - Parameter vectors: comprse the ( nn g ) 1 load bus, the reactve power njecton vector, Q and the ( n 1) 1 actve power njecton, p : 4.2. Method of soluton The optmzaton process consdered n ths work s the mnmzaton of the total actve power losses, wth shunt capactors fxed whch can be formulated as: Mn : P L ( V,, t) (5) Subject to: (1) The net actve and reactve power flow equatons:

5 Volume 52, Number 1, n g P ( cos sn ) - c j V V j G j j B j j P P 0 (6) j 2 n g Q sn cos c j V V j (G j j B j j ) Q Q 0 (7) j 2 (2) The physcal lmts on the reactve power of generators: g g g Q Q Q, 1,... n,mn g (8) (3) The physcal lmts on transformer taps: t t t mn 1,...n t (9) Where: nm P.( 2 2 L G j V V j 2. V. V j.cos j ) (10) j We use the Reduced Gradent (the constraned Gradent s obtaned from the augmented Lagrangan functon L and the Kun-Tuker condtons for the mnmum) together wth an teratve method. The nverse transpose of the jacoban matrx can be calculated by usng sparse vectors technques. The actve power productons are consdered as fxed (except for the slack bus). From vector Gradent L u of and we obtan the new dfferent values of the reactve power generated and the transformer tap. At each teraton, wth the new values of reactve power and the transformer tap, we actualze the new voltages wth the Newton Raphson power-flow soluton. Ths teratve procedure termnates as the rate of decrease of the objectve functon, evaluated by the load flow, and s found to be less than a certan tolerance. 5. IMPLEMENTATION AND RESULTS The one-lne dagram of the western Algeran 220/60kV transmsson/sub-transmsson system s shown n Fg. 2 (appendx). Its man data and operatonal lmts are summarzed n Table1 and Table 2. Among the several scenaros tested, the worst case has been, selected to better llustrate the capabltes of the methodology adopted. The weakness of the 60 kv porton, predomnantly radal n the lower rght-hand sde area, leads to a voltage profle whch, at peak loadng, to close to voltage collapse for certan buses. Table 1. Man data of the Western Algeran system. Parameter VALUES Load buses 64 Generator buses 4 Lnes 78 Transformer taps 12 Shunt capactors 8 Actve load demand MW Reactve load demand 541.1MVAR Total actve losses MW Table 2. Lmts of control varables and bus voltages. Magntude Lower Upper Voltage / p.u 1.11 p.u Voltage/ p.u 1.10 p.u Taps 0.90 p.u 1.10 p.u Q shunt MVAR Q g 1-250MVAR 500 MVAR Q g 9-90 MVAR 180 MVAR Q g MVAR 70 MVAR Q g MVAR 36 MVAR For the consdered power system, two cases are provded. The frst one corresponds to the actual data of the Sonelgaz company (case 1), whle the second one s related to our smulaton results (case 2). For each case, we provde the varaton of the voltage for the dfferent buses (Fg. 3). The desred voltage for the 60 kv network has to be n the range of 0.95 to 1.1 p.u., whle for the 220 kv network, ths range s 0.99 to 1.11 p.u. Fgure 3 shows how the voltage magntudes evolve through the dfferent phases of the proposed Fg. 2. Western Algeran 220/60kV transmsson/sub-transmsson system by Khat.

6 36 ACTA ELECTROTEHNICA methodology, for the 9 buses whose voltage s ntally below the lower lmt. From Fg. 3c, one can see that a severe volaton voltage occurs at target buses 46, 67 and 68 (0.759 p.u, p.u, p.u respectvely) for case 1. Rasng the tree voltages whose magntude s under 0.85p.u consttutes the most urgent task. The recommendaton at ths phase consst of connectng 10 MVAr, 5 MVAr and 10 MVAr at buses 46, 67 and 68, respectvely (as reported n Table 3). In ths case the applcaton of the heurstc rules s necessary. Usng these data (fxed capactors), we obtaned by optmzaton procedure descrbed n secton 4, reactve power of the generators (Table 4) and transformers tap change ratos (a) 2 Case Case2 Table 3. Shunt capactors n [MVAR]. Bus Case 1 Case Case Case2 Table 4. Reactve power of generaton n [MVAR]. Bus Case 1 Case 2 (MVAr) (MVAr) However, we have found that other less mportant voltage volatons are stll observed at buses 17, 24, 60, 64 and 66 (1.115, 1.131, 0.911, and 1,111, respectvely). To avod these volatons, the fnal procedure of our smulaton approach s used. In partcular, approprate control actons have been appled to 3-17, 9-23 and transformer taps wth consderng heurstc rules. The calculatons show that all voltages are then n the desred lmts (Fg. 1b, 1c, case 2) f these control actons are +0.02, and for 3-17, 9-23 and transformer taps, as compared to the fnal values gven n Table 5. Table 5. Transformer tap ratos. Buses Case n 1(p.u) Case n 2(p.u) Fnally, a comparson between case 1 and case 2 shows that ths operaton process also results n a decrease of the total actve power losses from to MW (20%). 6. CONCLUSION (b) (c) Ths paper s focused on the VAr/voltage control problems of the western Algeran transmsson/subtransmsson system. In the last few years, the operators have frequently faced wdespread voltage volatons, especally at the sub-transmsson level, and a tool capable of helpng them n systematcally solvng those problems s clearly needed. Ths tool combnes heurstc rules procedures, manly ntended to handle Case1 Fg. 3. (a) Voltages profle for buses 1-13, 220 kv; (b) Voltages profle for buses 14-40, 60 kv; (c) Voltages profle for buses 41-68, 60 kv Case2

7 Volume 52, Number 1, specal stuatons and dscrete devces, wth effcent numercal technques, n order to determne n whch sequence, and to what extent, the avalable controllers should be rescheduled. In case there are no voltage problems, a lst of feasble control acton are recommended n an attempt to reduce actve power losses. Ths procedure s based on methodology that manly ncludes three man aspects:. a sutable njecton of shunt capactors n target buses or crtcal buses wth consderng the heurstc rules;. an optmzaton process nvolvng two control varables, namely the reactve power of the synchronous generators and the transformer taps change ratos.. a fne tunng process consstng n the applcaton of approprate control steps for the transformer taps and/ or the reactve power of the generators wth consderng the heurstc rules. Accordng to the obtaned results, the proposed approach gves good results. The advantages of ths method are: optmal soluton, flexblty and customzed soluton. Ths procedure allow us to determne a strategy that results n keepng the voltage wthn the requred lmts as well as n a sgnfcant reducton of the total actve power losses. ACKNOWLEDGEMENTS Acknowledgments are due to the operators of the western dspatchng Sonelgaz company, for provdng us wth the characterstcs of the western Algeran network 220/60 kv. REFERENCES 1. J.L. Martnez Ramos, A. Gomez Exposto, A Hybrd Tool Assst the Operator n Reactve power /Voltage control and Optmzaton, IEEE Trans, On Power System,vol10(2),1999, pp D. Thurakan, K. Parthasaraty, D.L. Pror, Improved Algorthm for optmum reactve power allocaton, Electrc power and energy systems. Vol 6, 1984, pp T. Tran-Quoc, J.C. Sabonnadere, N. hadj-sad, R. Feullet, Voltage-VAr control n the Vetnam power system, 3 th PSCC n Trodhem,1999, pp Y. Tamura, H. Sasak, Expert System Appled to Voltag-VAr Control Fnal Report, N 139, 1991, pp S.P. Sng, G.S. Raju, A.K. Gupta. Sensblty based expert system for Voltage control n power system, Electrcal power & Energy system, A. Gomez Exposto, J.L. Martnez Ramos, Senstvty-Based Reactve Power Control for Voltage, IEEE Trans, On Power System, vol8,n 3,1993,pp J.G. Rolm, L.J.B. Machado, M.R. Irvng, Swtchng to control Voltage problems: a hybrd approach Electrc power and energy system, vol19, N 1, 1997, pp W.M. Lebow and al, Optmzaton of reactve volt- Ampere (VAr) sources n system plannng, Vol1, EL-3729 research project , Fnal report, November Y. Wallah, Calculatons and program for power system networks Prentce-Hall, Inc., Englewoo clffs. 10. S. Cheth, M. Khat, A. Chaker, Means optmzaton of reactve Power compensaton usng the partcle swarm optmzaton PSO Method: Aplcaton n the Western Algeran transmsson system, Internatonal Revew of Electrcal Engneerng (IREE), vol. 4 n. 4, July-August 2009, pp Prof. Mounr KHIAT Z. FRIOUI Prof. Abdelkader CHAKER Prof. Djelloul D. REHIEL Department of Electrcal Engneerng ENSET Oran, Algera E-mal: Khat2_2000@yahoo.fr, mounr.khat@enset-oran.dz Mounr KHIAT s a Professor n the Department of Electrcal Engneerng at the ENSET, n Oran, Algera. He receved a doctorate degree n Electrotechncs from the Unversty of USTO, Oran. Member of the SCAMRE laboratory. Hs research actvtes nclude the control of large electrc power systems, multmachne, multconverter systems, Facts devces and HVDC systems. Hs teachng ncludes neural process control and real tme smulaton of power systems. Abdelkader CHAKER s a Professor n the Department of Electrcal Engneerng at the ENSET, n Oran, Algera. He receved a Ph.D. degree n Engneerng Systems from the Unversty of Sant-Petersburg. Drector of SCAMRE laboratory. Hs research actvtes nclude the control of large power systems, multmachne multconverter systems and the unfed power flow controller. Hs teachng ncludes neural process control and real tme smulaton of power systems. Djelloul RAHIEL s a Professor n the Department of Electrcal Engneerng at the ENSET, n Oran, Algera. He receved a M.A. degree n Electrotechncs from the ENSET. He receved a doctorate degree n Electrotechncs from the Unversty of USTO, Oran. Hs research actvtes also nclude the control of large power systems, multmachne multconverter systems and the unfed power flow controller. Member of the SCAMRE laboratory. Zoulkha FRIOUI He receved a Master degree n Electrotechncs, opton: Systems mantenance, from the ENSET. She prepares a doctoral thess. She s an author and coauthor of several natonal communcatons. Hs research actvtes also nclude the electrcal networks mantenance.