Elektronegatif gaz karışımlarında elektriksel yalıtkanlık analizi
Analysis of electrical insulation properties in electronegative gas mixtures
Yazar: ZEHRA ÇEKMEN
Danışman: PROF. DR. M. SEZAİ DİNÇER
Gazi Üniversitesi / Fen Bilimleri Enstitüsü / Elektrik-Elektronik Mühendisliği Ana Bilim Dalı
Derece: Doktora, 2010
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering

Özet:
Yüksek dielektrik dayanımı ve kimyasal kararlılığı nedeniyle Sulfur Hexafluoride (SF6), yüksek gerilim mühendisliğinde ve elektrik güç uygulamalarında izolasyon ve ark söndürme ortamı olarak yaygın bir kullanıma sahiptir. Ancak SF6 gazı sera gazları arasında anılmaktadır. Bu problemin çözümü saf SF6 yerine farklı tampon gazlarla oluşturulmuş karışımlarının kullanılmasıdır böylece GIS'lerde kullanılan SF6 miktarı sınırlanmış olur. Öte yandan son yıllarda argon düşük ark empedansı, yüksek akımları iletebilmesi ve yüksek sıcaklıklarda kimyasal kararlılığı nedeniyle plazma işleme ve darbe güç anahtarlama uygulamalarında önem kazanmaktadır. Ancak düşük dielektrik dayanımı argonun en büyük dezavantajıdır. Bu dezavantajı gidermek için SF6 argon karışımı argonun iyi özellikleri kaybedilmeden dielektrik dayanımının artırılması için iyi bir çözüm sunar.Efektif iyonlaşma katsayısı, ortalama elektron enerjisi, elektron sürüklenme hızı ve difüzyon katsayısı gibi çığ parametreleri SF6, Argon ve SF6-Ar gaz karışımlarında gaz molekül yoğunluğuna indirgenmiş elektrik alan E/N'nin 10Td ile 600Td değerleri arasında Boltzman transport denklemi kullanılarak incelenmiştir. Boltzman Transport denkleminin sayısal çözümü için iki terimli küresel harmonik yaklaşım kullanılmış ve elde edilen denklemler sonlu elemanlar metodu Galerkin formülasyonu ile kararlı durum Townsend koşullarında çözülmüştür.Geliştirilen model ve kodun doğruluğunu test etmek amacıyla Reid gaz modeli değerlendirme testi olarak kullanılmış ve elektron enerji dağılım fonksiyonları (EEDF) ve çığ parametreleri hesaplanmıştır. Doğrulanan kod sırasıyla SF6 ve argon gazları için uyarlanmış ve yine EEDF ve çığ parametreleri elde edilmiştir. BTE son olarak %1SF6-%99Ar, %5SF6-%95Ar, %10SF6-%90Ar, %25SF6-%75Ar, %50SF6-%50Ar, %70SF6-%30Ar karışım oranları için 100Td? E/N ?600Td aralığında ve %0,1SF6-%99,9Ar, %0,3SF6-%99,7Ar, %0,5SF6-%99,5Ar, %0,7SF6-%99,3Ar karışım oranları için 10Td? E/N ?300Td aralığında çözülerek EEDF'ları elde edilmiş ve iyonlaşma, yapışma ve efektif iyonlaşma katsayıları ile ortalama elektron enerjileri, elektron sürüklenme hızları ve difüzyon katsayıları hesaplanmıştır. Ayrıca iyonlaşma ve yapışma katsayılarının eşit olduğu kritik/limit alan değerleri de incelenen tüm SF6-Ar karışım oranları için saptanmıştır.

Abstract:
Because of its high dielectric strength and chemical stability, Sulfur Hexafluoride (SF6) has been widely used as a gaseous insulating medium and arc quenching medium in many applications in the field of high voltage engineering and electrical power applications. However, this superb gas medium has been shown to be a greenhouse gas since it is an efficient absorber of infrared radiation and cannot be removed rapidly from the Earth?s atmosphere due to its inertness. One of the solutions to the greenhouse problem is the use of SF6 mixtures with other gases instead of pure SF6 thus limiting the amount of SF6 used in Gas Insulated Systems. On the other hand, argon has gained importance in recent years in the plasma processing and pulse power switching applications because of its low arc impedance, its ability to conduct high electric currents and its chemical stability to higher temperatures. However, the low dielectric strength is a major drawback of argon for some of its applications. To eliminate this drawback SF6 and argon mixture has offered a good solution in terms of increasing the dielectric strength of argon without losing any of its good properties.The effective ionization coefficients and transport parameters such as electron mean energy, drift velocity and transverse diffusion coefficient in binary and ultradilute SF6-Ar gas mixtures have been calculated for density reduced electric field strength E/N values from 10 Td to 600 Td. These calculations have been performed by using the two term spherical harmonic expansion to obtain the numerical solution of the Boltzmann transport equation based on finite element method under steady-state Townsend condition.In order to confirm the model and the code developed in the present study the Reid ramp model has been used as a benchmark test and then electron energy distribution functions, effective ionization coefficients and transport parameters have been evaluated for pure SF6, pure argon and with mixtures of SF6 contents of 1, 5,10, 25, 50, and 70 percent in the binary mixture. Finally SF6 contents in the ultradilute mixtures of 0,1, 0,3, 0,5 and 0,7 percent are taken into account with the evaluated effective ionizations and transport parameters of electron mean energy, drift velocity and transverse diffusion coefficients. Moreover the limiting E/N values at which ionization and attachment frequencies are in balance have been calculated for all of the SF6-Ar mixture ratios.Because of its high dielectric strength and chemical stability, Sulfur Hexafluoride (SF6) has been widely used as a gaseous insulating medium and arc quenching medium in many applications in the field of high voltage engineering and electrical power applications. However, this superb gas medium has been shown to be a greenhouse gas since it is an efficient absorber of infrared radiation and cannot be removed rapidly from the Earth?s atmosphere due to its inertness. One of the solutions to the greenhouse problem is the use of SF6 mixtures with other gases instead of pure SF6 thus limiting the amount of SF6 used in Gas Insulated Systems. On the other hand, argon has gained importance in recent years in the plasma processing and pulse power switching applications because of its low arc impedance, its ability to conduct high electric currents and its chemical stability to higher temperatures. However, the low dielectric strength is a major drawback of argon for some of its applications. To eliminate this drawback SF6 and argon mixture has offered a good solution in terms of increasing the dielectric strength of argon without losing any of its good properties.The effective ionization coefficients and transport parameters such as electron mean energy, drift velocity and transverse diffusion coefficient in binary and ultradilute SF6-Ar gas mixtures have been calculated for density reduced electric field strength E/N values from 10 Td to 600 Td. These calculations have been performed by using the two term spherical harmonic expansion to obtain the numerical solution of the Boltzmann transport equation based on finite element method under steady-state Townsend condition.In order to confirm the model and the code developed in the present study the Reid ramp model has been used as a benchmark test and then electron energy distribution functions, effective ionization coefficients and transport parameters have been evaluated for pure SF6, pure argon and with mixtures of SF6 contents of 1, 5,10, 25, 50, and 70 percent in the binary mixture. Finally SF6 contents in the ultradilute mixtures of 0,1, 0,3, 0,5 and 0,7 percent are taken into account with the evaluated effective ionizations and transport parameters of electron mean energy, drift velocity and transverse diffusion coefficients. Moreover the limiting E/N values at which ionization and attachment frequencies are in balance have been calculated for all of the SF6-Ar mixture ratios.Because of its high dielectric strength and chemical stability, Sulfur Hexafluoride (SF6) has been widely used as a gaseous insulating medium and arc quenching medium in many applications in the field of high voltage engineering and electrical power applications. However, this superb gas medium has been shown to be a greenhouse gas since it is an efficient absorber of infrared radiation and cannot be removed rapidly from the Earth?s atmosphere due to its inertness. One of the solutions to the greenhouse problem is the use of SF6 mixtures with other gases instead of pure SF6 thus limiting the amount of SF6 used in Gas Insulated Systems. On the other hand, argon has gained importance in recent years in the plasma processing and pulse power switching applications because of its low arc impedance, its ability to conduct high electric currents and its chemical stability to higher temperatures. However, the low dielectric strength is a major drawback of argon for some of its applications. To eliminate this drawback SF6 and argon mixture has offered a good solution in terms of increasing the dielectric strength of argon without losing any of its good properties.The effective ionization coefficients and transport parameters such as electron mean energy, drift velocity and transverse diffusion coefficient in binary and ultradilute SF6-Ar gas mixtures have been calculated for density reduced electric field strength E/N values from 10 Td to 600 Td. These calculations have been performed by using the two term spherical harmonic expansion to obtain the numerical solution of the Boltzmann transport equation based on finite element method under steady-state Townsend condition.In order to confirm the model and the code developed in the present study the Reid ramp model has been used as a benchmark test and then electron energy distribution functions, effective ionization coefficients and transport parameters have been evaluated for pure SF6, pure argon and with mixtures of SF6 contents of 1, 5,10, 25, 50, and 70 percent in the binary mixture. Finally SF6 contents in the ultradilute mixtures of 0,1, 0,3, 0,5 and 0,7 percent are taken into account with the evaluated effective ionizations and transport parameters of electron mean energy, drift velocity and transverse diffusion coefficients. Moreover the limiting E/N values at which ionization and attachment frequencies are in balance have been calculated for all of the SF6-Ar mixture ratios.