ELECTRICAL PROPERTIES 2

Assignment ELEC9712 Topic 3: Electrical Breakdown of Polymeric Insulating Materials

Z3443312 Guan LUO Z3480903 Ying JU Z5003202 Dong ZHANG Z5014044 Tong HUANG

Content 1. Introduction ................................................................................... 2 2. Theories of Electrical Breakdown.................................................... 3 2.1 Solid material breakdown ................................................................................................... 3

3. Polymer structure........................................................................... 4 3.1 Polymer molecular structural features[4]............................................................................. 4 3.2 Molecular Structure ............................................................................................................ 4

4. Aging of polymeric materials .......................................................... 5 5. How electrical breakdown happens in polymeric materials ............ 7 6. How to avoid Electrical breakdown ................................................ 8 7. Conclusion...................................................................................... 9 8. References ................................................................................... 10

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1. Introduction Under AC or DC stress, space charge is one of main causes of aging and electrical breakdown in polymers. Space charge generally refers to trap charge, which is captured by a trap and then maintain staying on the dielectric medium. Trap charge also includes distributed charge which is informed by the non-uniform polarization. According to previous research work, space charge injection and accumulation in dielectrics could seriously transfigure dielectric internal electric field, and eventually resulted in aging or breakdown. Corresponding interpretive models for these phenomena took applied field as prerequisite. However, based on latest research achievement, it finds that applied field is not a necessary condition. Electrical breakdown (or dielectric breakdown) can also occur during the detrapping process of space charges without applied field. This assignment aims to clarify that space charge is a vital condition for dielectrics breakdown and detrapping process of space charge can lead to electrical breakdown.

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2. Theories of Electrical Breakdown Electrical breakdown, also called dielectric breakdown, is a phenomenon that occurs when solid medium operates under the strong electric field and some of the freely carriers move strenuous to collision with atom on the lattice, it will rapidly expand the movement which may cause the space charge to electrical breakdown. The features of electrical breakdown are showing as high breakdown voltage, very short voltage effect time and related to the density of electric field. However, the environment factors such as the temperature and operating voltage will not cause electrical breakdown. Breakdown mechanisms can be divided into: 

Solid materials breakdown



Liquid materials breakdown



Gas materials breakdown

Polymeric insulation material is one kind of the solid material, thus electrical breakdown of polymeric insulating materials can be classified into solid material breakdown. In this part, we will emphasis on introducing the solid material breakdown. 2.1 Solid material breakdown In solid materials breakdown, it consists of electrical, heat and electrical chemical breakdown. In this field, it may lead to high internal heat accumulation and over temperature which may result in the closing of insulating ability to insulations and cause the short circuit to destroy the electrical equipment. Thus, all of these actions will lead to chemical reactions of materials under such high thermal rating environment. The higher over-voltage, operating time and the number of voltage function, the easier possibility to cause electrical breakdown. A standard of lowest critical voltage caused the breakdown is called breakdown voltage, ratio of breakdown voltage and medium thickness is determined as dielectric strength[1]. Therefore, the dielectric strength can be used to judge the quality of electric materials and avoid for fusion and scorch the cables or machines.

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3. Polymer structure A polymer is a very large molecule comprising hundreds or thousands of atoms. Poly- means "many" and -mer means "part" or "segment", which is first used in Greek. With low conductivity, low dielectric loss, high breakdown strength and other excellent electrical properties, make it an indispensable material in electronic and electrical technologies. Most polymers have long been utilized to isolate and protect the current due to its inherent electrical insulation, such as cable sheathing, and capacitor dielectric material. 3.1 Polymer molecular structural features[4] Monomer: polymer can be synthesized by small molecules, such as polyethylene monomer is CH2 = CH2. Mer: recurring structural unit of the polymer, such as polyethylene is the structural unit -CH2-CH2Polymerization degree: n value in polymer molecular structure represents the number of repetitions in the polymer chain link, the greater n value is, the bigger relative molecular mass is. Molecular chain: A very long polymer molecules constituted by many links 3.2 Molecular Structure Linear polymer: Most polymers are linear polymers, molecular chain was linear one by one; the chain doesn’t have to be a straight one, it could be any shape without branches. Network polymers: macromolecular chains cross-linked to a three-dimensional network space; initial heating may melt, after hardened, neither dissolve nor melt, and therefore could not be molded again; high hardness and high brittleness. Branched polymer: Similar properties with linear polymers.

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Figure 1

4. Aging of polymeric materials From the previous part, we introduce the basic structure of polymer and also mentioned that polymeric materials are broadly applied in the field of insulation, because of their excellent processing, dielectric and thermal performance. However, every coin has two sides. The aging and breakdown of polymer has been the focus of attention from the day of its application. In order to find out the relationship between polymer aging and polymer electrical breakdown, a large number of experiments and research are conducted by scientists. All polymers aging phenomenon, in the end is the changing structure of the materials. Therefore, the reasons of polymeric materials’ aging can be summed up to three kinds of theories. 

The charge injection and extraction theory [5]

This theory was first put forward by Tanaka in 1978. Due to the different thermal elongation coefficients between foreign materials and polymer (assume in AC electric field), the Maxwell stress acts on the medium, resulting in the micro cracks, gas absorption and parcel in polymer. Although this theory is not precise enough and relatively rough, it still can explain some organic dielectric application process under low pressure. In fact, in the role of DC high voltage and the ultraviolet irradiation, electrical branch also exists [3]. 

Thermal electron theory

The core of the thermal electron theory is, when electrons are injected into the medium, electrons are captured by traps due to the scattering, while the excess energy is transferred to another electron in the form of the radiation, making it a thermal electron. The thermal electron makes polymer molecules decomposition. It can be expressed as the following formula:

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AB + e (thermal)→(decomposition)A + B + e (cold)→A + B + e (traps) + energy With the decomposition of large polymer molecules, low density area of Small molecules is formed, and thus low density area expands unceasingly, created for the occurrence of impact ionization and further caused aging and breakdown of the polymer medium. 

Theory of photo-degradation[2]

Some scientists hold the idea that thermal electron has little effect on electric branches, and thus he put forward the photo-degradation theory. This theory point out that under the action of AC electric field, for the tip-plate electrode structure, when the tip is in the negative half cycle, electrons are captured by different shades of electrons’ trap. When the needle is in the positive half cycle, the injection of holes and electrons recombine and glows. And with the increase of the applied voltage and carrier injection, some localized states which are original trap states, transform into recombination center. This newly formed recombination center will produce light with shorter wavelengths. In addition, the higher the voltage get, the greater the intensity and energy. The role of ultraviolet radiation and oxygen make polymer degradation rapidly, also produce electric branches. The starting points of above several theories are all about high-energy particle or ray what are caused by the charge injection make large molecules polymer chain rupture, without considering the directly effect of space charge on material structure. Furthermore, because of the complexity of the structure of the polymer and the complexity of trap charge, there is no unified theory to explain all the phenomenon of aging and breakdown. In this case, through the analysis related to trap energy, looking for a new way of interpretation is very important. The latest study found that the formation of electrical tree branches is not in charge of the injection, but accompanied by trap charge to take off the trap formation.

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5. How electrical breakdown happens in polymeric materials Electrical breakdown of polymeric insulating materials commonly caused by electrical affect, thermal affect, chemical affect, mechanical force affect and humidity affect. These affects may result in insulation aging, thus lead to electrical breakdown. a) Non-uniform electric field degree Normally, the breakdown strength in uniformed and dense solid dielectric can reach 1~10MV/cm. It is determined by the internal structure of materials, and slightly relate to external factors. However in homogeneity of dielectric materials, the breakdown strength will decrease when the insulation thickness increase. But when the thickness is very small approximately 3~

cm, breakdown voltage will increase

again. Moreover, partial increase strength of electric field may cause the partial discharge that may lead to the chemical dielectric breakdown when partial discharge operates for a long time. b) Function time and types of voltage Three forms of solid dielectric breakdown: electrical breakdown, thermal breakdown and electrical chemical breakdown are all related to voltage function time. Under the electric field distribution, the lightning impulse breakdown voltage is generally greater than the power frequency voltage. The partial discharge will be more stronger and dielectric losses will be more huge when increase the frequency of AC current, which may increase the possibility of thermal and chemical breakdown. c)

Temperature

When during the comparatively low temperature and in the electric breakdown range, the breakdown strength is unrelated to temperature. However, when temperature is very high, it may lead to thermal breakdown and thermal hard to dissipation, the breakdown strength will be much lower. d) Dielectric performance and structure Solid dielectrics are usually heterogeneous and densification, air gaps inside the materials may cause the variation of electric field, thus destroy the materials. In

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addition, over thickness will make the electric field non-uniform, hard dissipation and reduce the breakdown strength. Since solid materials have good heat transfer function, the dielectric losses will be small and strength voltage will be improved. e) Mechanical load When withstand the mechanical load, the breakdown voltage will decrease when the materials have cracks. f)

Humidity

The breakdown voltage will decrease when solid dielectric insulations are affected with damp.

6. How to avoid Electrical breakdown According to the influence factors of solid electrical breakdown, the methods of avoiding and minimizing the electrical breakdown can be found from: 1) Make the electrical field more uniform. Non-uniform electrical field is easy to cause partial discharge. This is one of the reasons which lead to electrical breakdown. 2) Reduce the frequency of AC current. If the polymeric material is carrying AC current, to reduce the frequency is a good way to reduce the chance of electrical breakdown. 3) Lower the temperature. For a certain low level of the temperature, the influence of the temperature the breakdown will be on a low level. When the temperature becomes higher, it may increase the chance of breakdown. 4) Do not overload. Try to ensure the mechanical load in a certain range because too much mechanical load will lead to the decrease of breakdown voltage. 5) Try to avoid using insulations in the dump environment. If the humidity of surrounding is on a high level, it will also lower the breakdown voltage of the polymeric insulation material.

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7. Conclusion In conclusion, the theory that space charge in polymer now plays an important role in the process of aging and breakdown has been accepted by most people. Due to the complexity of the polymer structure and the difference of test method and experimental condition, different researchers often put forward diverse views from dissimilar angles and a lot of different ways of understanding. But one thing never change is that — all aging and breakdown phenomenon, in the final analysis is the medium structure change. Furthermore, on what is the relationship between aging and breakdown to material structure is still not very clear. Under the action of electric field, the chemical changes of the material and how these changes affect the insulation performance of a material also did not have enough data. A lot of works should be done in this respect.

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8. References 1. Zener, Clarence. "A theory of the electrical breakdown of solid dielectrics."Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character (1934): 523-529. 2.

Dissado, Len A., and John C. Fothergill. Electrical degradation and breakdown

in polymers. No. 9. IET, 1992. 3.

Kim Li, Zoey Peng, Process of space charge in polymer aging and breakdown

[J], Chinese Science Bulletin, 2000, 45(23): 2469-2475. 4.

Chen C. K, Raimond Liepins. Electrical properties of polymers: Chemical

Principles. Cambridge University Press, 2005. 5.

Tanaka T, Greenword A. Effects of charge injection and extraction on tree

initiation in polyethylene. IEEE Trans Pow Appr Sys, 1978, PAS-95: 1749~1757

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