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Katı yalıtkan malzemelerde elektriksel ve sulu ağaçlanma

Electrical and water treeing in solid insulating materials

  1. Tez No: 68913
  2. Yazar: AHMET REFİK MARANGOZ
  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: 1997
  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: Elektrik Mühendisliği Bilim Dalı
  13. Sayfa Sayısı: 152

Özet

ÖZET Katı yalıtkanların delinme dayanımına etkiyen etkenlerden biride ağaçlanma olayıdır. Özellikle yüksek gerilim sistemlerinde kullanılan katı yalıtkanlarda şekli ağaca benzeyen oyuk ve kanallar oluştuğundan, olay ağaçlanma olarak isimlendirilmiştir. Ağaçlanmaya yalıtkan içindeki boşluklar, yabancı maddeler, yarıklar, çatlaklar, kablonun yalıtkanı ve iletkeni arasındaki yüzeyin durumu, sıcaklık, nem, basınç, frekans ve elektriksel alan gibi pek çok etmen etki eder. Ağaçlanma, başlangıç koşullarına bağlı olarak elektriksel ağaçlanma ve sulu ağaçlanma olarak iki ana gruba ayrılabilir. Elektriksel ağaçlanma yalıtkanın iç veya dış yüzeyinde veya içinde başlayabilir. Başlangıcı için pek çok teori ileri sürülmüştür, ama hemen tüm teorilerin ortak noktası yüksek elektrik alana ihtiyaç duyulmasıdır. Bunun yanında üstte sayılmış olan diğer faktörler gelir. Herhangi bir zorlama ile başlangıç ya da gelişim safhasındaki ağaç delinmeye yol açabilir. Sulu ağaçlanma elektriksel alan varlığında nemin etkisiyle ouşan ağaçlanma türüdür. Nemin etkisinden dolayı daha alçak gerilimlerde de ortaya çıkabilir. Alan ve su etkisi kaldırıldığında görüntüsü kaybolur. Ancak tekrar grilim uygulanırsa, yeniden görünürlük kazanır ki buda yapısının sürekli olduğunu gösterir. Nem arttıkça başlangıç gerilim değeri düşer. Ağaçlanmanın önüne geçilmesi ile ilgili olarak yapılan çalışmalarda ağaçlanmanın önlenmesinden çok, ilerlemesinin yavaşlatılması üzerinde durulmuştur. Bunun için gerilim regülatörü olarak isimlendirilen katkı maddelerinden faydalanılmaktadır. Ayrıca frekansın azalmasıyla ağaçlanma başlangıç geriliminin artmasıda bir diğer özelliktir. Bu çalışmada ağaçlamaya etki eden faktörler, başlaması, ilerlemesi ve önlenmesi ile ilgili olarak bu güne dek yürütülen teorik ve deneysel çalışmaların ışığında konu incelenmeye çalışılmıştır. XV

Özet (Çeviri)

SUMMARY Keywords : Treeing Electrical treeing Water treeing Treeing is an electrical pre-break-down phenomenon. The name is given to that type of damage which progresses through a dielectric section under electrical stress so that, if visible, its path looks something like a tree. Treeing can occur and progress slowly by periodic partial discharge, it may occur slowly in the presence of moisture without any partial discharge, or it may occur rapidly as the result of an impulse voltage. While treeing has been generally associated with ac or impulse voltages, it has been observed recently with high dc voltage stresses in wet experimental conditions. Treeing may or may not be followed by complete electrical breakdown of the dielectric section in which it occurs; but in solid organic dielectrics it is the most likely mechanism of electrical failures which do not occur catastrophically, but rather appear to be the result of a more lengthy process. As a result of research extending over many years, much is known about treeing. It was observed during the early development of paper/oil insulation systems and has been observed in inorganic materials. However, much remains to be investigated and understood. In this survey of selected literature on treeing the list of references is certainly not complete and it is weighted toward publications in English. Yet the main aspects of our present knowledge are believed to be fairly presented in terms of; experimental observations, proposed explanations, possible cures, and test methods available. To maintain perspective during this study of dielectric damage, it must be emphasised at the outset that the service record of ethylene polymers and copolymers, which comprise most modern solid insulations used for commercial power cables, has been extremely good. A very small fraction of the failures in electrical systems is due to purely electrical deficiencies in the insulation. However, in view of the increasing demands made upon electrical power transmission and distribution systems, and the unique combination of properties already offered by polyethylene and related materials, it is appropriate to make every effort to fully optimise them for complete reliability in insulation systems. XVIA survey of the literature on the subject of treeing in solid dielectrics is presented. The purpose is to provide an introduction to the subject, some background for the current research work, and a list of references to some of the more rigorous treatments which in most cases have a narrower scope. The trees which grow in insulating materials can be considered in two classifications; electrical trees and water trees ( electrochemical trees ). They are all initiated at sites of high and divergent electrical stress and may be aggravated by the presence of moisture, chemical environment, and contaminants. It has been shown that trees will grow in any organic dielectric under appropriate conditions. However, the major portion of the work summarised here has been carried out on polyethylene. This is not because polyethylene is most subject to treeing, but because it is the most popular modern insulating material. It has excellent electrical properties, and enviable service record. Recent development of improved constructions for electrical insulating systems, including high voltage power cables, and research on formulation of materials and voltage stabilising additives have shown that improved resistance to treeing is possible. The discovery that electrical discharges can damage and destroy dielectric materials is not a recent one. It has been suggested by others that the earliest recognition of this fact probably followed the observation that lighting discharges can penetrate very thick sections of insulation on their way to ground. Fig. 1. Electrical trees The great variety in apperance of the patterns of stems and branches which comprise an electrical tree, in addition the circum stances of its initiation and growth, combine to explain the large number of descriptive names applied to trees. Included are dendite, string, fan, plume, delta, bush, broccoli, and bow - tie. Until recently the dictinction between electrical trees and water trees was often made on the basis of visibility and permanance alone. xvuElectrical trees, Fig. 1., which consist of hollow channels resulting from the decomposition of material, show up clearly in poyethylen and other translucent solid dielectrics when examine with with an optical microscope, while fresh, unstained water trees, Fig. 2., appear diffuse and temporary. There is international activity in research to overcome the tendency of all solid organic insulting materials to dielectric failure following tree growth from localized regions of high electrical stress. Elimination of the high stress regions by very smooth, tightly bonded construrctions, and materials completely free of containments and voids should render the formation of trees impossible at normal operating stresses. Water trees However, commercial power cables are not always examples of perfect construction and dielectric materials somtimes contain contaminants and voids. The function of voltage stabilizers is to minimize the effects of these mechanical imperfections by chemically or physically resisting the initiation and growth of trees. It is clear that electrical treeing can be controlled, at least in laboratory experiments by the use of voltage stabilizers. Successful demonstration of voltage stabilizers in finished cables have been reported in Japan. In Europe, efforts to improve cable performance by improving construction have made possible higher operating stresses. Water treeing ( electromechanical treeing ) have not been studied as long as electrical treeing. For this reason, although voltage stabilizers against water trees have not been publicly reported, we can not conclude yet that the problem is significantly more difficult. Some work on such systems, still considered proprietary, gives promise of success. Certainly water trees are impossible in isulation system where moisture is excluded. xviuIn this study it has been tried to give detailed explanations about electrical trees and water trees ( electromechanical trees ). I have tried to give sufficient information about the initiation, growth and cure for treeing. As I have stated above the two kinds of treeing are electrical treeing and water treeing. As for electrical treeing; it may start on the inner or outer surface as well as inside the insulator, for the initiation of treeing many theories have been suggested. But the common aspect of all these theories is that a high electrical field is needed. After this aspect comes the others ( temperature, humidity, frequency and etc.). An electrical treeing at the initiation or growing face might cause holing in the insulator in case of a high electrical stresses. When it comes to the water treeing; we can define it as trees forming by the effect of water with the presence of an electrical field. They may take place at lower voltages due to the negative effect of humidity. As far as the water and electical field effects disappear it also disappear. But ifan electrical field is applied again the water treeing shows up, which proves that it has a continiues structure. As humidity increases water treeing initiation voltage decreases. Concerning the curing of treeing, we can not talk about the prevention of initiation of treeing but growing of it. We have stated above that many experiement about treeing have been made. These experiments have been performed both on insulators and on the systems where insulator are used, such as power cables. These experiments give us to chance to have information on the behaviours of the power cables against treeing. We can examine these experiments under two groups; electrical treeing experiments and water treeing experiments. Electrical treeing experiments are usually based on point plane specimens. In figure three the experiment installation is seen. 7 ' ' ' '.I.' ' ' ' 'V Fig. 3. Experiment installation The experiment period depends on the property of the insulator. But this period is usually in the rank of hours. In this experiment the dimensions and growing of the treeing forming at the end of the needle can be observed. XIXIn the water treeing there are three kinds of experiments. In these three experiments the growing of the treeing both in the insulator and on the surface of the semi conductor can be observed. The studies made for the prevention of treeing are mostly based on demolishing the effects which cause treeing. For this purpose it has been tried to produce insulators with minimum structural faults and with best electrical, mechanical and chemical properties. Besides by using chemical additives; it has been tried to fill in the holes in the insulators to make them stronger against treeing. XX

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