Yüksek doğru gerilimle enerji iletiminin modellenmesi

Abstract

Yüksek doğru gerilimle enerji iletimi, uzun mesafeler sözkonusu olduğunda, gerek havai hat, gerekse kablo ile iletimde, alternatif gerilimle iletime kıyasla daha ekonomik olmaktadır. Sadece mali tabloyu değiştirmesi açısından değil, beraberinde getirdiği teknik özellikler ve üstünlükler nedeniyle de, bugün özellikle batı ülkelerinde yaygm uygulama alanları olan ve güç elektroniği, yan iletken teknolojisinde meydana gelecek ilerlemeler doğrultusunda gelişmeye açık bir yöntemdir. Bir enerji iletim yöntemi olarak taşıdığı avantajlar dışında, farklı işletme frekanslarına ve gerilimlerine bağlı sistemlerin bağlantılarında kullanılması doğru gerilimle iletimin en önemli özelliklerinden biridir. Bu çalışmada Türkiye'de henüz uygulanmayan ve batıda 1882 yılından bu yana yaygınlaşarak kullanılmasına rağmen, ülkemiz için yeni bir kavram olma özelliğini koruyan yüksek doğru gerilim ile enen i iletiminin, gerçek zaman modelini oluşturmada gerekli olan ön çalışmalar tamamlanmış ve bir bilgisayar modeli kurulmuştur. Bu amaçla, önce yüksek doğru gerilimle iletimin prensipleri açıklanmış ve modelin oluşturulmasında kullanılan, EMTDC/PSCAD güç sistemleri simülasyon programı hakkında bir ön bilgi verilmiştir. Daha sonra çevirici üniteleri için harmonik analizleri yapılmış ve bu harmonikler için filtre tasarımı gerçekleştirilmiştir. xı

H.V.D.C. TRANSMISSION MODELING H.V.D.C. transmission has always been considered as an alternative to AC power transmission. In the early development of power generation and distribution, many discussions are made in selecting a method for power transmission as a basis for an initial decision. In this thesis, the methods of h.v.d.c. transmission are discussed and the modeling of h.v.d.c. transmission is analyzed. Simulations are done by using EMTDC/PSCAD power system simulation software which was produced by Manitoba H.V.D.C. Research Center. The main claims generally made in favour of the d.c. alternative are: 1. D.C. transmission results in lower losses and costs than equivalent a.c.lines, but the terminal costs and losses are higher. 2. Since the capacitance of a cable limits ac power transmission to a few tens of kilometers, A.C. transmission via cable is impractical over long distances. Because beyond that limit, the reactive power generated by cable capacitance exceeds the rating of the cable itself. Since capacitance does not come into play under steady- state dc conditions, such restriction does not exist. As a result, power can be transmitted by cable under large bodies of water, where the use of ac cables is unthinkable. Furthermore, underground dc cable may be used to deliver power into large urban centers. Unlike overhead lines, underground cable is invisible, free from atmospheric conditions, and solves the problem of securing the rights of way. 3. D.C. constitutes an asynchronous interconnection and does not raise the fault level appreciably. 4. The power flow in a d.c. scheme can be easily controlled at high speed. For example, power in the megawatt range can be reversed in a d.c. line in less than one second. This feature makes it useful to operate dc transmission lines in parallel with existing ac networks. When instability is about to occur (due to the disturbance on ac system), the dc power can be changed in amplitude to counteract and dampen out the power oscillations. Thus with appropriate controls, a d.c. link can be used to improve a.c. system stability. Quick power control also means that dc short-circuit currents can be limited to much smaller values than those encountered on ac networks. Xll 5. D.C. stations, with ör vvithout transmission distance, can be justified for the interconnection of a.c. systems of different frequencies ör different control philosophies. Although the economic advantage of d.c. power transmission was understood from the early days of the electrical technology, its practical application had to wait for the development of a suitable rated electronic valve. Among the various svvitching principles used in early days of the power electronic industry, mercury-arc rectification was found the most suitable for handling large currents. in parallel with the amazing development of the micro-electronic technology of recent times there has been an impressive revolution in the h.v.d.c. schemes. in spite of the successful operation of the mercury-arc schemes, the incidence of arc-backs, considerable maintenance and voltage limitations encouraged the development of the solid state technology. By the mid-eighties some 20 000 M W of thyristor schemes were in operation and the total installed capacity of h.v.d.c. links reached at 56 000 MW by 1992. in the fırst chapter of mis study, a general view över this concept is introduced and historical background of h.v.d.c. transmission is given. in chapter 2, the advantages and disadvantages of h.v.d.c. transmission are discussed in detail and different types of transmission are given including monopolar and bipolar implementations. The static conversion of power from a.c. to d.c. and from d.c. to a.c. which constitutes the central process of h.v.d.c. transmission is given in the third chapter. Utilization of h.v.d.c. transmission system in New Zealand is presented as an example of computing commutation reactances in both receiving and sending ends of the d.c. link. Rectifıer and inverter operations of h.v.d.c. converters are analyzed in detail and steady-state voltage-current relationships are derived and formulated. The voltage-current characteristics are given for both rectifier and inverter. The operation point at full load is determined. After expressing reactive power demand depending on the firing angle (a) and commutation angle (u) as parameters, the necessity of reactive povver compensation is emphasized. in chapter 4, harmonics which are produced by the converters from both aç point of view and de point of view, are presented. Excessive levels of harmonic current must be prevented since they vvill cause voltage distortion, extra losses and overheating as well as interference with external services like telephone signals. The obvious place to eliminate the harmonics is the source itself. in theory, characteristic harmonics could be eliminated either by some complex converter confıguration (which would be uneconomical), ör by the use of a series filter preventing the harmonics from arising. Therefore, accepting that the appearance of harmonics is an inherent property of the static power conversion process, it is necessary to reduce their penetration into the aç and de systems. These are considered in this section and harmonic fılters for both de and aç side are designed. The ideal conditions, used to calculate the characteristic harmonics produced by h.v.d.c. converters, are not met in xiii practice and, as a result, relatively small quantities of non-characteristic harmonics are always present. Possible causes of non-characteristic harmonics are: a. Firing errors. b. AC voltage unbalance and/or distortion c. Unbalance of converter components All these effects cause the converter to generate non-characteristic harmonics, for example orders 1, 2, 3 etc. on dc side and 2, 3, 4 etc. on ac side. By way of example the results of measurements, during back-to-back commissioning tests at the Benmore terminal of the New Zealand scheme is given. In chapter 5, the power system simulation software, EMTDC/PSCAD is presented. The EMTDC program has been developing since 1976. It helps study dc transmission systems in a reasonably modular fashion. One advantage to the user is the FORTRAN coding which must be used to develop models. In the sixth chapter simulations concerning the operation of h.v.d.c. transmission system are done and the criteria regarding to design of filters which should be used in that type of power transmission system are inspected. In chapter 7, the results of discussions and suggestions for future works are aggregated.