Enterkonnekte elektrik güç sistemlerinde kapalı çevrim güç akışlarını önlemeye / düzenlemeye yönelik bulanık karar verme temelli bir yöntem

Abstract

Seksenli yıllara kadar tüm dünyada elektrik enerji sistemlerinin tamamı; üretim, iletimve da˘gıtım devletlerin kontrolünde idi. Bunun ba¸slıca nedeni çok büyük yatırım vei¸sletme maliyetlerinin olması idi. Kalitenin ve maliyetlerin ancak rekabet ortamlarındaolu¸saca˘gına inanılan kapitalist ekonomilerde, özellikle hava yolu i¸sletmecili˘ginde tekelsisteminin kaldırılıp serbest piyasa ko¸sullarına geçilmesi ile ula¸sım maliyetlerindekiciddi dü¸sü¸s ve yaygın ekonomik servislerin gelmesi ile, benzer bir ba¸sarının elektriksektöründe de olu¸saca˘gına olan inançla elektrik güç sistemlerinde de özelle¸stirmefuryası ba¸sladı. ˙Ingiltere, ˙Izlanda ve ABD bu i¸sin öncülü˘günü yaparak sistemlerininneredeyse tamamını özelle¸stirerek sektörü özel te¸sebüse bıraktı. Daha do˘grusu elektrikenerjisinin üretimi, iletimi ve da˘gıtımını farklı farklı firmalara satılarak ba¸ska türlü birtekelin olu¸sması da engellenmeye çalı¸sıldı. Devlet kendisi, ya da kurdu˘gu düzenleyiciotoriteler aracılı˘gı ile sektörde yanlızca kontrolör olarak görev üstlendi.Devlet tarafından i¸sletilen sistemlerde güç kayıplarının yüksekli˘gi, verimin dü¸süklü˘gü,veya enerjinin çok uzun yollardan (iletim hatlarında dola¸sarak) tüketim noktalarınavarması sorununa ciddi bir ¸sekilde yakla¸sılmadı. Maliyetlerin artması do˘grudantüketiciye yansıtıldı˘gından, yada maliyetlerin bir kısmının devlet tarafından kar¸sılanmasınedeni ile enterkonnette sistemlerin do˘gasında var olan kapalı çevrimler içindedola¸san enerji (yada bu dola¸sım sonucu hatlarda olu¸san ısı kayıpları) pek sorunolarak görülmedi. Özelle¸stirme sonucunda; hangi miktarlarda enerjinin hangi iletimhatlarında iletildi˘gi, hangi firmanın hangi iletim hattını ne kadar süre ile ve hangisaatlerde kullandı˘gı, enerji iletim sisteminin hatlarının dolulu˘gu nedeni ile e˘gersistem bir arıza durumuna geçerse bundan kimin sorumlu oldu˘gu konuları çok önemkazanmaya ba¸sladı. Elektrik enerjisinin izleyece˘gi yol fizik kurallarına göre yani,Kirchhoff'un akım yasası ve Ohm yasasına göre belirlenir. Enerji akı¸sı kontratyapılan güzer˘gah yerine empedansı en dü¸sük olan iletim hatlarını seçer. Bu durum,enerjiyi satınalan ile satan arasına üçüncü birinin girmesi sonucunu do˘gurur. Enerjiakı¸sının böyle olması sonucu bazı i¸sletim problemlerini ortaya çıkardı. Þöyleki, enerji alı¸s-veri¸s planında sınırlamalar, iletim kapasitesinde de˘gi¸simler, hatlarına¸sırı yüklenmesi, güvenlik ve güvenirlilik konuları ve enerjinin serbest piyasadafiyatlandırılması komplikasyonları, enerjiyi satınalan ile onu satan arasına üçüncübirinin (sistemi kullanılan taraf) girmesi sonucu, üçüncü birinin sisteminin kullanımbedelinin kimler tarafından kar¸sılanaca˘gı gibi.Kapalı çevrim güç akı¸sı, i¸ste bu üçüncü birimin i¸sgal edilen hatlarında olu¸san enerjikayıplarının nasıl ücretlendirilece˘gi, kime faturalandırılaca˘gı, sistemde hatların doluolması durumunda tıkanıklı˘ga sebep olunmasından, sistemin çöküntüye gitmesindenkimlerin ne derecede sorumlu tutulaca˘gı gibi konuları içerdi˘ginden çok önemkazanmı¸stır. Halen aktif bir ara¸stırma konusudur.Bu çalı¸smada amaç; enterkonnekte elektrik güç iletim sistemlerinde kapalı çevrimgüç akı¸slarını önlemeye/düzenlemeye yönelik bir yöntem geli¸stirmektir. Bunugerçekle¸stirirken ilave amaçlar da probleme dahil edilmi¸stir. Bunlar; sistembaralarındaki gerilim profilinin bozulmaması ve düzeltilmesi (gerilim genliklerininarzu edilen de˘gerlerde tutulması), iletim hatlarındaki hem aktif hem de reaktifkayıpların en aza indirilmesi ve temel durum de˘gerlerinden fazla uzakla¸sılmaması,hatlardaki güç akı¸slarının iletim hatlarının akım ta¸sıma kapasite limitleri dahilindetutulmasıdır. Görüldü˘gü gibi problemimiz çok amaçlı ve çok kısıtları olanbir problemdir. Bu çoklu amaçlı ve kısıtlı problemde bulanık karar vermedenyararlanılmı¸stır. Bunun sonucunda elde edilen tek amaçlı en iyileme problemineevrimsel arama tekniklerinden genetik algoritmalar uygulanmı¸stır.Bu yöntemi düzenlerken i¸sletim ko¸sullarını etkileyen elektrik güç sistemi kontrolve durum de˘gi¸skenleri ve bunların üzerlerindeki kısıtlamalar da göz önündebulundurularak, bulanık ortamda her birisi için uygun üyelik fonksiyonlarının tespitedilmeleri ile modellenmi¸slerdir.Genetik algoritmaların uygulanması sırasında GA operatörlerininde problemeuyarlanması açısından çe¸sitli benzetimler yapılmı¸s ve litaratürdeki uygulamalar daincelenerek çalı¸smaya en uygun olanının tespiti yapılmı¸stır.Problem günlük bir i¸sletme problemi oldu˘gundan gün içerisinde defalarca çözümüyapılmaktadır. Bundan dolayı çözüm süreside önem te¸skil etmektedir. Çözümsüresinin azaltılması için hem genetik algoritmalarda paralel algoritmalar seçilmi¸shem de kurumdaki yüksek ba¸sarım labratuvarlarından yararlanılarak yüksek ba¸sarımlıbilgisayarlarda paralel ortam hesaplama tekniklerinden yararlanılmı¸stır.

After the liberalization of the electricity markets, the operation of the power system hasfundamentally changed. How much power flows on which transmission lines, whichcompany uses the other's transmission lines and/or the amount and duration of thetransmission line usage have all become very important issues.When the actual power flow exceeds system-operating limit for a transmission path, thetransmission system operators must immediately mitigate the transmission overloadsto reduce the actual power flow across the path. The deviation of actual electricpower flows in transmission circuits from the scheduled (expected) power flows iscalled closed loop flows, and/or unscheduled flows, and these flows may cause thetransmission limit violations. The unscheduled flows have been seen as a seriousproblem, since these flows may result in blackouts and affect cross border tradingin the electricity markets. Therefore, such flows in an interconnected power systemshould be kept under control to improve both the operation conditions and the marketconditions.In interconnected power systems there are many parallel transmission lines and closedloops causing electrical energy to follow different routes to reach load centres. Thus,when for a reason a transmission line becomes out of service, there are alternativeways of feeding loads at the end of that particular transmission line. Otherwise, lossof electrical energy may cause interruption of all types of production, transportation,communication, entertainment, education and the other life related activities to beaffected partially or totally.Up until eighties all activities of electrical power systems, such as production,transmission and distribution, were under the control of governments. The reasonfor this was that both investment and operation costs were too high. With thebelief that both the quality and reduced cost would be possible in a free market,especially considering the success of deregulated aviation systems, electrical powersystems were deregulated. England, Iceland and USA were the first to deregulate theirpower systems. Production, transmission, and distribution systems were privatized todifferent companies. Either government herself or via regulation authorities took theregulation role.High power loses, low efficiencies, or the long paths electrical energy travels through(using transmission systems) in a government controlled system were not taken intoconsideration seriously. Since the rising costs are directly adjusted to the customers'bill, or part of the cost is subsidized by government, the loop flows, which is a naturalresult of an interconnected power system, were not considered as a problem. As aresult of deregulation the following questions have become quite important. How muchpower flows on which lines? Who is using a certain transmission line and how long theline is being used? At which hours a transmission line is used? If, due to overloadedlines a fault occurs who is responsible for this fault? Electrical power does not followa prescribed line, it rather follows ohm's law. That is, energy does not follow froma contracted path, it flows through the lines which has the least resistance. This factbrings a third party into a picture when there is an energy seller and a buyer. This, isturn, creates operational problems, such as limitations on power transactions, changesin transmission line capacity, lines being overloaded, security and reliability problems,and complications in pricing the electricity in the market (who pays the transmissionline usage, when there is third party between a buyer and seller on a contracted path).Closed loop flows, tries to answer the question as to who will be responsible for the lineusage in the third party's system, how the pricing shall be done, who is and how muchresponsible for the congestion of transmission lines especially on the third party'ssystem. This field is still a heavily researched area.The main purpose of this study is to design a method to control power flows inclosed loops of interconnected electrical power systems. This problem has becomesignificant due to deregulation in power systems. By this method we aim to preventand/or regulate unwanted loop flows. There are some additional purposes neededto be handled besides the main purpose. These additional purposes are the voltageprofiles of each bus of the system should not be destroyed, active and reactive powerlooses of transmission lines must be minimized and should not be far away from theone of basic case solution. The values of the transferred power must be kept underthe current carrying limits of the transmission lines. As seen the problem has multiobjectives and multi constraints that are needed to be handled. The multi objectivesand multi constraints problem is handled by using fuzzy-decision-making techniques.The finalized single objective problem is optimized by using genetic algorithms whichis one of the evolutionary search algorithms.The power system control variables, state variables and the constraints that areeffecting the operational conditions of the system are also taking under considerationswhile arranging the method. A membership function is determined to each objectivesand constraints in fuzzy environment. The break-points of the fuzzy membershipfunctions are tested. After a set of simulations the best form of the membershipfunctions are determined to improve the success rate of the solution of the problem.In fuzzification step, the objectives and constraints were fuzzified by using designatedmembership functions, where these functions should be properly constructed toguarantee the overall performance and improve the success rate of the solution of theproblem. After a set of simulations the best form of the membership functions aredetermined by arranging the break-points of the fuzzy membership functions. Thevalue or value of a fuzzy membership function defines the degree of membership thata crisp value has in a fuzzy set. The closer the value of membership is to one, the betterthe solution is for that objective or constraint. In fuzzy decision-making, the optimalsolution is defined to be the one with the highest degree of membership, and thus theoptimization problem becomes that of maximizing the satisfaction with the solution;subject to the crisp and fuzzy constraints. In the proposed approach, the objectivefunctions were minimization of the line flows of the closed loop on the contractedpaths, and minimization of both total active losses and total reactive losses. Theconstraints (voltages remaining within the limits, line flows remaining within the limitsetc..) were also handled in fuzzy environment by using designated fuzzy membershipfunctions.Two different shapes of fuzzy membership functions are tested for the main objectivethat is the line flows for the closed loops. One of the shapes is trapezoidal function.The other shape is exponential function. The study showed that more satisfying resultscan be obtained by using exponential form of the membership functions. It is describedby four parameters that are the break-points of the function.The reason handling the objectives and constraints in fuzzy environment is that thesmall variations of power systems variables (bus voltages, line currents etc.,) fromtheir limit values can be tolerated, and this can help to obtain one of the best solutionsto the problem. Furthermore, one may reach to a better objective function by allowingsmall violations of limits. To what degree toleration and/or on which variable ofpower systems it is allowed depends on systems operating conditions. In practice,to determine the break-points of each fuzzy membership function, the experience ofsystem operators are also taking in considerations.In optimization step, first, the applications of genetic algorithms in literature arechecked. One of the good characteristics of evolutionary search algorithms is thatthey could be easily adapted from one problem to another by modifying relatedparameters and the way of application techniques of operators. Then some simulationsare performed to determine the best variety of each operator of genetic algorithms thatthey fit better to our problem.The problem is an operational problem in power systems that is required to solve manytimes as a daily routine. That is why the time spent for a solution is gained importance.To reduce the computational time some parallel computing techniques and some typesof parallel genetic algorithms are also searched. The simulations belong to largersystems among the test systems are used are performed using the utilities providedby the high performance computing lab in computational science and engineeringdepartment of informatics institute.The main disadvantage of genetic algorithms is the high CPU time execution. Thiscan cause heavy computation demands in a large-scale power system case, resultingin large dimensional matrix operations. One way to overcome these computationdemand is to use parallel algorithms in multi-processor computers or in distributedenvironments.To solve a computational problem, parallel computing simply involves thesimultaneous use of many computing resources. Computing resources come in theform of either a single computer with multiple processors or multiple computers(computing nodes) connected by a fast network.In most cases, there are both paralleling and unparalleled parts in the problemformulations. The speed up of a program using multiple processors in parallelcomputing is limited by the time needed for the serial part of the program. Therexxviiare also several factors that limit the speed up such as communication time for sendingmessages.Genetic algorithms are very powerful search and optimization techniques that can beapplied to problems in many different areas. They can produce an acceptable solutionto a problem in a reasonable time. However when they are applied to very hard andlarge problems it takes quit a long time. In that case there might be some effort toreduce the time into a reasonable range. One way of reducing execution time is toimplement genetic algorithms in parallel environment.Master-slave parallelism among the types of parallel genetic algorithms was chosen.The algorithm was implemented in MATLAB environment using toolboxes such asgenetic algorithm toolbox, parallel computing toolbox, distributed computing server,and MATPOWER which is an open source power flow program was also used.The large portion of the computation time is spent for the function evaluation in theproblem. This is due to the nature of the problem solved. Master-slave parallel geneticalgorithm is perfectly matched with the needs. It is concluded that by using a largersystem, we would gain more benefit from using a high performance computing facilitythat allows parallel implementation of our program to get the system solution fasterthan the serial corresponding.As a result of this work can be concluded as follows. A fuzzy set theory basedgenetic algorithm procedure is proposed to control and/or prevent loop flows. Theproblem is formulated as a multi-objective problem subject to operational and electricalconstraints and it is handled in fuzzy environment. The proposed method works welland very satisfactory results for the objectives such as scheduled path flows, active andreactive power losses are obtained. When the classical methods are used to controlloop flow in a rigid manner a feasible solution may not be found. However, using theproposed method one can find a feasible solution. This can be better adjusted usingdifferent membership functions, since the shape of the membership function affectsthe satisfaction level appreciably.