Üstün iletkenli senkron makinalarda kararlılık analizi /
Yazar: LALE ZEYNELGİL
Danışman: PROF. DR. M. EMİN TACER
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü
Derece: Doktora, 1994
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering
Dizin: Kararlılık analizi = Stability analysis ; Senkron makineler = Synchronous machinery ; Süper iletkenlik = Superconductivity

Özet:
Bu çalışmada, üstüniletkenli senkron generatörlerinin kararlılık analizi, makinanın tek ve çift rotor ekranı olması durumu için yapılmıştır. Günümüzde, elektrik enerjisine olan gereksinimin gün geçtikçe artması, güç sistemlerinde, üretim birimlerinin (generatörlerin) güçlerinin gittikçe büyütülmesine, iletim ve dağıtım şebekelerinin gittikçe karmaşıklaşmasına yol açmaktadır. Bunun sonucu olarak, gittikçe büyüyen güç sistemlerinde kararlılık sorunu daha da önem kazanmaktadır. Diğer taraftan, artan elektrik gereksinimini karşılamak için, gittikçe daha büyük güçlü senkron generatörler gerekmektedir. Bu sorunların gelecekte de önemini koruyacağı görüşü ile bu çalışmada, son yıllarda üzerinde yoğun çalışmalar yapılan üstüniletkenli senkron makinanın kararlılık açısından simülasyonu yapılmıştır. Tezin birinci bölümünde, önce güç sistemlerinde artan üretim gücü ve kararlılık sorunu hakkında bilgi verilmiş ve üstüniletkenli generatörlerin üstünlükleri belirtilmiştir. Daha sonra, üstüniletkenliğin tanımı yapılmış ve üstüniletkenler konusunda yapılmış olan çalışmalar hakkında bilgi verilmiştir. Son olarak, üstüniletkenlerin uygulama alanları, özellikle güç sistemlerindeki uygulamaları konusunda yapılmış olan çalışmalar anlatılmıştır. Tezin ikinci bölümünde, üstüniletkenlerin temel özellikleri anlatılmış, sınıflandırılması yapılmış, Tip-1 ve Tip-2 üstüniletkenlerin özellikleri incelenmiştir. Daha sonra, üstüniletkenlerin kararlılığı açıklanmıştır. Tezin üçüncü bölümünde, üstüniletkenli senkron makinaların yapısal özellikleri tanıtılmış, alışılagelmiş senkron makinalardan olan farklılıkları verilmiştir. Sonra, tek ve çift rotor ekranına sahip üstüniletkenli senkron makina için matematiksel modeller elde edilmiştir. Son olarak, düzeltilmiş merdiven tipi eşdeğer devre kullanılarak, çift rotor ekranına sahip üstüniletkenli senkron makina için bir matematiksel model oluşturulmuştur. Tezin dördüncü bölümünde, tezde yapılmış olan simülasyon çalışması açıklanmış ve elde edilen sonuçlar verilmiştir. Son bölümde ise, sonuçlar değerlendirilmiş ve gelecekte bu konuda yapılabilecekler ile ilgili önerilerde bulunulmuştur. XI

Abstract:
In this thesis, it is investigated stability of the superconducting synchronous machines. Requirement of electrical energy increases more and more, therefore, in the power systems, ratings of generating units must increased and distribution networks must become relatively complex. Consequently, the stability problem of the power systems which are increased more and more is very important. Stability of power system is defined as ability of synchronously operating of the generators in the system when any small or big perturbation occurs. On the other hand, to provide the increasing requirement of electrical energy, it is necessary to increase ratings of generating units. However, ratings of generators can not increase because of technological constraints as electrical, mechanical and transportation problems. Therefore, it is seem that maximum rating value for synchronous generators is 2000 MW in today. Development of superconducting synchronous machines is very important to overcome the maximum rating problem for synchronous generator. It must be given some knowledge about superconductor before superconductor synchronous machines are investigated. Superconductors are metals or alloys which exhibit the phenomenon of zero electrical resistance with direct current below a certain temperature, known as the transition temperature or the critical temperature. Superconductivity was first discovered by Heike Kammerlingh Onnes, the Dutch physicist, in 1 91 1. He had observed that mercury was not have resistivity to electric current when it was cooled about absolute zero temperature. For cooling mercury to absolute zero, he had bathed the mercury bar in liquid helium and then, he has observed that mercury had been superconductive at temperature of 4.2 Kelvin. The critical temperature at which the transition between the superconductive state and the normal state occurs is, in the absence of a xumagnetic field, independent of the shape or size the specimen. It is a characteristic of the material. The zero resistance of the superconductive state is only strictly true for direct currents of constant value. If the current is changing then a non zero resistance does exist although it may be still much less than that in the normal state. For AC currents of a given frequency this resistance depends upon both frequency and the specimen temperature. When a specimen enters the superconductive state it is magnetic properties undergo a radial change. The magnetic properties of a superconductive specimen are quite distinct from those of a normal specimen. Indeed, the magnetic properties of the superconductive state are basic to the understanding of the superconductive state. An important magnetic property of the superconductive state is diamagnetism. The forces which a diamagnetic body experience in a non uniform magnetic field can be pictured in terms of "induced magnetization currents". The lines of magnetization currents are closed. This means that magnetization currents do not bring electric charge into or out of the body. Although these magnetization currents are certainly real we can not measure them directly. These magnetization currents produces a magnetic field. These magnetic field is known as the "induced magnetic field". This induced magnetic field has opposite direction that of the applied magnetic field. Therefore, nearly all of the total magnetic flux lines is excluded from the specimen. The exclusion of magnetic flux from a specimen in the superconductive state was first discovered in 1933 by Meissner and Ochsenfeld and is known as the "Meissner effect". Although Onnes' discovery of the first superconductive material, mercury, occurred over seventy years ago, studies on superconductivity is constraint because to obtain the temperatures which is required for superconductivity it was used liquid helium. Liquification of helium is expensive and requires considerable energy. Furthermore, unless the liquid helium is tightly sealed in a heavily insulated container, it quickly warms and vaporises away. In 1987, physicists have stumbled on an unusual class of the ceramic compounds that change everything. They too must be cooled to become superconductors, but only to a temperature of 98 K. Firstly, Paul C. W. Chu and et al was discovered that the compounds Y-Ba-Cu-0 has become superconductor at 98 Kelvin. This development brought superconductivity into the range of the practical; liquid helium can be replaced as a coolant by liquid nitrogen, which makes the transition from a gas at the easily produced temperature of 77 Kelvin. Moreover, liquid nitrogen is cheaper than liquid Xlllhelium and so long - lasting that scientists carry it around in ordinary thermos bottles. Also, the ceramics may be able to generate even more intense magnetic fields than metallic superconductors. However, the goal of superconductivity researchers is to push the phenomenon of superconductivity into a practical temperature range and researches are continued. The application of superconductors to rotating electric machines was proposed as soon as it was feasible to make practical solenoids. The first experiments with homopolar machines were successful because the duty imposed on the superconductor wasn't different from that for a simple DC solenoid. On the other hand, the first experiments with AC machines experiences several problems. But, many groups of researches around the world have developed several superconducting synchronous generator solving these problems. At the present time, the optimal configuration of superconducting synchronous machines is the superconducting field windings rotating inside a stationary armature windings which are normal conductor. These machines have following advantages: - A reduction in size and weight - The potential for higher ratings - Higher efficiency compared with conventional machines - Elevation of terminal voltage - A reduction in capital plant cost - A reduction in capitalised cost of loses - An improvement in power system stability Both of field winding and armature winding include no magnetic iron. Iron is used only as a magnetic shield to minimize to alternating magnetic fields external to the machines. Therefore, even with a laminated iron environmental magnetic shield, the machine is essentially air-cored. Because of its air-cored nature, it is necessary one or more cylindrical, conducting shells in the annular space between the field and armature windings, rotating with the field windings. These shells are also named as screens. XIVThe rotor screening system must be perform several function, some of them in apparent conflict: - It must provide thermal isolation to the superconducting field winding - It must provide a high degree of isolation for the rotor from rapidly time varying magnetic fields components due to space harmonics of armature fields, the harmonics on the system, system imbalance resulting in negative sequence currents, and faults. - It must provide minimum rotor flux leakage. - It must serve as an amortisseur winding, providing machine and system damping. - The screening system must be able to withstand the strong mechanical loads imposed by faults. Because of the conflicting requirements, it is used two or more screens in superconducting synchronous machines. The double-screen machine has two screens having following properties: - Low-temperature, lightweight inner screen having a long diffusion time constant to give good electromagnetic and thermal shielding. - Strong outer screen, able to withstand short-circuit forces and having a shorter diffusion time constant to provide good damping. Basic configuration of the superconducting synchronous machine which has two rotor screens is shown in Figure 1. MAGNETIC SHIELD LOW VACUUM SEAL. I/IM<*>0