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Alçak basınçta ve vakumda elektriksel boşalma olaylarının incelenmesi

Başlık çevirisi mevcut değil.

  1. Tez No: 75552
  2. Yazar: ŞEYDA AYDINAY
  3. Danışmanlar: YRD. DOÇ. DR. ÖZCAN KALENDERLİ
  4. Tez Türü: Yüksek Lisans
  5. Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1998
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Elektrik Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 109

Özet

ÖZET Bu çalışmada, alçak basınçta gazlarda ve vakumda boşalma olayları kuramsal ve deneysel olarak incelenmiştir. Vakum, yalıtkan olarak temel fiziksel üstünlükleri nedeniyle birçok uygulamada giderek yaygın olarak kullanılmaya başlamıştır. Bu da vakumla ilgili araştırmaların artmasına yol açmıştır. Bu amaçla tezde öncelikle alçak basınçta ve vakumda boşalma olayları ayrı başlıklar altında kuramsal olarak Bölüm 2 ve 3'te incelenmiş, delinme karakteristikleri ve bağıntıları çıkarılmıştır. Çalışmanın 4. bölümü, bu konuda yapılan deneysel çalışmaları ve sonuçlarını açıklayan bilgileri içermektedir. Bu bölümde, yapılan beş ayrı yüksek gerilim deneyi açıklanmıştır. İlk deneyde, yüksek alternatif gerilimde, alçak basınçta, çubuk-düzlem elektrot sisteminin delinme geriliminin basınç ve elektrot açıklığı ile değişimi elde edilmiştir, ikinci deneyde, yine alçak basınçta, aynı çubuk-düzlem elektrot sisteminin pozitif ve negatif yüksek doğru gerilimde, delinme geriliminin basınç ve elektrot açıklığı ile değişimi çıkarılmıştır. Üçüncü deneyde, yüksek alternatif gerilimde, alçak basınçta, bu kez düzlem-düzlem elektrot sisteminin delinme geriliminin basınç ve elektrot açıklığı ile değişimi elde edilmiştir. Bir sonraki deneyde, yine düzlem-düzlem elektrot sisteminin pozitif ve negatif yüksek doğru gerilimde, delinme geriliminin basınç ve elektrot açıklığı ile değişimi çıkarılmıştır. Son deneyde ise düzlem-düzlem elektrotlar arasına yerleştirilmiş 1 kV'luk ve 3 kVluk döküm reçine mesnet izolatörlerinin ayrı ayrı alçak basınçta, alternatif gerilimde atlama gerilimleri saptanmıştır. Elde edilen sonuçlar, çizelge ve grafikler şeklinde verilmiş, deney sonuçlan yorumlanmış ve fiziksel bakımdan değerlendirilmiştir. Bölüm 5'te, vakum ortamında ark olayı ile katot-anot üzerinde oluşan olaylar ve vakumda arkın söndürülmesi irdelenmiştir. Aynı bölümde vakumun temel fiziksel özelliklerine dayanan, pratikte kullanılan vakum aygıttan ele alınmış ve bu aygıtlardan yüksek gerilim tesislerinde hem bir açma-kapama hem de bir koruma elemanı olan vakumlu kesicilerin tarihsel gelişimi, yapısal özellikleri, üstünlükleri ve pratik uygulamaları hakkında geniş bilgiler verilmiştir. Çalışmanın 6. Bölümünde ise bu vakum konusunda yapılan deneysel çalışmalara yer verilmiştir. Bu amaçla öncelikle alan emisyon akımı karakteristiği ile vakumun delinme gerilimi arasında bir ilişki olup olmadığını araştırmak için vakumun delinme gerilimi ölçümlerinin yapıldığı bir çalışma incelenmiştir. Daha sonra ise vakumun yüksek alternatif gerilimde delinmesinin elektrot sistemlerinin geometrisine ve elektrot açıklığına bağlılığını belirlemek amacıyla yapılan başka bir deneysel çalışmaya yer verilmiştir. Delinme ve atlama gerilimlerinin azalan basınçla hızla ve yaklaşık lineer olarak azaldığı görülmüştür. Bu durum özellikle izolatörlerde daha belirgin olarak ortaya çıkmıştır. Sonuç olarak çalışma, alçak basınçta gazların ve izolatörlerin davranışını incelemek bakımından yararlı olmuştur. XI

Özet (Çeviri)

ELECTRICAL DISCHARGES IN GASES OF LOW PRESSURE AND VACUUM SUMMARY In this study, discharge phenomena in gases of low pressure and vacuum are investigated by theoretical methods and by experimental tests. High voltages can be applied to gases at low pressure because of different reasons. It is therefore important to know the behaviours of gases under this conditions for the system assurance and quality. Vacuum has many relatively good insulating and basic pysical characteristics with respect to other insulators. Therefore, It has been used in many applications recently. For example, for the equipment such as X-ray tubes or vacuum circuit breakers, vacuum has a specific purpose. For this reason, lots of theoretical and experimental developments on vacuum have been achieved for a long time. In addition, vacuum circuit breakers have been used in practice instead of other circuit breakers to switch circuits and to interrupt shortcircuit currents. The word vacuum is derived from the Greek word meaning“empty”- In practical usage, It shows a state which comes closely to what the term“vacuum”means literally. There are several degrees of vacuum. Research into vacuum was nonexistent until the 17th century. From this century, lots of inventions and discoveries have been achieved in vacuum technology. Vacuum is required for a very wide variety of applications in science and technology. Some of them are radio and electronics, electrical industry, metal making, chemicals, medical instruments and drugs, foods, consumers industries, aircraft, optics and scientific research. These are explained in the first chapter (introduction) in details. In the second chapter, electrical discharges in gases of low pressure are explained. A gaseous dielectric always contains electrons. This is caused by gas molecules to be ionized by light, radio-active radiation or cosmic radiation. If an electrical field is applied, the electrons are accelerated in the direction of the electric field. In gases of low pressure the free path X is very small and the electrons collide with the gas molecules. Between two collisions electrons gain a kinetic energy W: W = eEX = f(e(E/p)) (1) With the collision, three different reactions occur a) The collision may be elastic. The electron gives a fraction of its energy to the gas molecule. XIIb) The collision may be inelastic. An atom is excited into a higher energy level by the collision. This is shown by A + e -> A + e (2) where A represents the atom, e the electron and A the atom at its higher energy level. c) The collision liberates an electron from the gas. This is called ionization and represented by A + e->A+2e (3) Ionization is very important for starting breakdown in gases. Here an ionization coefficient a is described. It is equal to the average number of ionizations per cm in the field direction. This is shown by a=pf(E/p) (4) and for many gases It can be shown by a = A.p.e"PP/E (5) where A and B are constants. These depend on the type of gas and on temperature. By ionization of gas molecules, the number of electrons increases. If N0 electrons leave the cathode, Nx electrons arrive at a distance x. In the dx distance, dNx new electrons occur. This is represented by dNx = Nx.a.dx (6) By integrating this equation from 0 to x, we obtain Nx= N0.eax (7) Consequently, Na electrons arrive at the anode. This Na electrons is equal to XIIINa = N0.e1, an infinite number of electrons occurs and the gas breaks down. Other feedback mechanism is created by photons. Photons from the excited atoms in the gas arrive at the cathode and release new electrons. The Townsend mechanism described here requires: a) The creation of starting electrons. b) The occurrence of an avalanche. c) Feedback processes that create new electrons. The condition for breakdown can be shown by Ud= f(p.a) (10) This expression is called Paschen's law and also written by Ud = B.p.a / (In (A.p.a / In (1 + 1/T))) (1 1) where Ud is the breakdown voltage. With these (Ud and (p.a)), Paschen's curve can be drawn. Paschen's curve has a minimum. At the left hand side this minimum, the number of atoms is very small. An electron has little oppurtinity to collide with gas atoms. Therefore, little ionization occur. At the right hand side the minimum, a lot of collisions occur but the electrons do not gain sufficient energy. Therefore, the number of ionizations is small. xivIn the third chapter, electrical discharges in vacuum and the properties of vacuum as an insulator are investigated. Vacuum is self-repearing and its breakdown stress is very high. Therefore It is a good insulator. Electrons that cause vacuum breakdown originate from the electrodes. This is called electron emission. The electron emission has two types. Field emission and heat emission. There are two mechanisms which can cause vacuum to break down. Breakdown caused by field emission and by particles are examined in detail with breakdown characteristics. At short electrode spacings under vacuum which are under 2 mm breakdown occurs because of electron emission. At large electrode spacings under vacuum breakdown occurs because of particles. Here, voltage is more dominant than field strength. Breakdown voltage is shown by Ud = KVa (12) where K is a constant which is in the order of 30 kVT per Vmm and a is the electrode spacing. In the forth chapter, five experimental tests and their results are investigated. In the experiments, plane-plane and rod-plane electrode sysytems have been used. At plane-plane electrode system, 75 mm diameter, 6 mm width plane electrodes have been used. At the rod-plane electrode system, the same plane electrode and the rod electrode that has hemisphere ended with 2 mm diameter have been used. In the experiments, 664 mm high, 120 mm diameter a sylindirical test chamber which are sufficent for 10~3 bar- 6 bar gas pressures have been used. In the first experiment, ac high voltage is applied to the plane-plane electrode system for different pressures at different electrode spacings. Then, the breakdown voltage variation according to the different pressures at different electrode spacings is obtained. After this, as a second experiment now positive and negative DC high voltages are applied to the same electrode system and breakdown voltage values are measured. In the third experiment, the electrode system is a rod-plane system. And ac high voltage is applied to the rod-plane electrode system for different pressures at different electrode spacings. For the forth experiment, positive and negative DC high voltages are applied to the same electrode system and breakdown voltage values are recorded. At the last experiment, ac high voltage is applied to the 1 kV and 3 kV cast resin support insulators, respectively. All values that are obtained from the five experiments are used to draw the graphics which contain the breakdown voltage variation according to the different pressures at different electrode spacings. In the experiments, electrode spacing varies from 5mm to 25 mm (5,10,15,20,25) and pressures used here are vary from atmospheric pressure (1013 mbar) to 100 mbar. xvFor the plane-plane electrode system (at 10 mm and 20 mm electrode spacings) at ac voltages, the breakdown voltage increases with increasing pressure or decreases with decreases pressure. The breakdown voltages at lower pressures have almost the same values. But If the pressure increases, they have different values. For the plane-plane electrode system (at 10 mm and 20 mm electrode spacings) at DC voltages, the breakdown voltage increases with increasing pressure. At negative DC voltages, the breakdown voltages are higher than those at positive DC voltages. The breakdown voltages that are obtained at ac voltages for the rod-plane electrode system decrease with decreasing pressures. Also the breakdown voltages at DC voltages decrease with decreasing pressures. For 1 kV and 3 kV cast resin support insulators, the flashover voltages decrease with decreasing pressures. In the fifth chapter, arcing phenomena in vacuum and the arc distinguish mechanisms are examined in detail. Because vacuum has relatively insulating and specific physical characteristics, it is used in many applications such as vacuum circuit breakers. Vacuum circuit breakers are not only switching devices but also protecting devices. Thus, these are very critical for the systems and circuits. Their historical background, construction characteristics and applications are explained as well. The possibility of interruption in vacuum was first proposed in the nineteenth century, but the serious study of vacuum interruption was undertaken at the California Institute of Technology in 1923-1926. At that time, It was very difficult to maintain a good vacuum in a interrupter. Therefore, It took a lot of time to design the vacuum circuit breakers. The General Electric Company of the USA announced the development of the first power interrupter which could interrupt system fault currents. This interrupter rated was 12,5 kA, 650 A and 15,5 kV. In the last chapter, experimental studies related to vacuum are explained. First of all, the relationship between field emission current characteristic and electrical breakdown in vacuum are examined. Secondly, the breakdown of vacuum gaps is investigated at ac voltages with different hemisphere-plane configurations and spacings of up to 20 mm. Breakdown and flashover voltages have been shown strongly influenced by decreasing pressure in gas and at insulators. In order to research the entire physical mechanism, investigations will be continued. XVI

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