Surface Degradation of polymer insulator under UV Stress

Posted: June 05, 2016

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Surface Degradation of polymer insulator under UV Stress

Abstract

This paper talks about the negative impacts that ultraviolet light rays have on the polymeric insulators used in the electrical transmission lines. These effects are vividly discussed in details; furthermore, other environmental factors are also discussed in the paper. The fact that the polymeric insulators are generally organic materials, they are subject to several environmental dereliction effects which result in considerable gradual aging. The sources of ultraviolet rays are Sunlight and corona formation. Sunlight being the chief source of ultraviolet rays has weighty annihilation of composite insulators and mostly measurements reveal that it has a range of 270nm to 320nm. Furthermore, the paper has statistical facts collected from various towns in Libya indicating the Sunlight characteristics in these parts of the town. The conducted research has significant result supporting the possibilities of severe influences of Sunlight on the composite insulator.

Keywords: Polymer/Composite insulator, Ultraviolet rays (UV), Corona effects, Environmental factors, Aging effects, Transmission lines

Table of Figures

Figure 1:  the maximum solar radiation incident on a horizontal surface by units kw.h/m2/day. 2

Figure 2:  the minimum solar radiation incident on a horizontal surface by units kw.h/m2/day. 2

Figure 3:  Monthly average of the maximum and minimum solar radiation incident on a horizontal surface by units kw.h/m2/day. 3

Figure 4: value of optimum angles monthly change during the year 4

Figure 5:  Yearly optimum angles for some areas of Libya. 5

Figure 6 Cross section of insulators. 8

Effects of Ultraviolet Thermal Stress on Polymer Insulators (Composite Insulators) Used in Electrical Transmission Lines

The Conducted Research on effects of UV radiation on Electrical Insulators

The research conducted in some parts of Libya reveals the minimum, maximum, and monthly average value of the solar radiation incident on the horizontal surface of some regions of the country. Libya possesses a good study region given the fact that it is in arid areas of Africa where the intensity of Sun is high (Aldali 346).

Figure 1:  the maximum solar radiation incident on a horizontal surface by units kw.h/m2/day

In consideration of the global system wholly, specific outlines are identified by discrepancies in features of the surface of the earth that influence insolation. The maximum isolations are established mostly in the subtropical and tropical regions of the globe. At this point, there are many inclination angles of the Sun, and, also, there is lack of clouds cover in regions of desert allowing abundant insolation on to the surface of the earth. The amount of solar reaching the earth’s surface reduces to a minimal amount at the poles. At the poles there is low Sun inclination angle, and, due to the fact, that the Sun does not directly rise above horizon for a period close to half a year, an aspect that decreases insolation annually.      

Figure 2:  the minimum solar radiation incident on a horizontal surface by units kw.h/m2/day

Figure 3:  Monthly average of the maximum and minimum solar radiation incident on a horizontal surface by units kw.h/m2/day

It is apparent from the preceding that the overall  value of the global incident insolation on a horizontal surface in desert cities are intense during May, June, July, and August, while  it significantly decreases during  December and January in coastal cities (Barry, Roger, and Richard 457). So we can say that the annual rate changes from 4.44KW.h/m2/day to 10.33KW.h/m2/day within a year. It means that the average annual total for the amount of radiation that reaches Libya is 2.967MW.h/m2/day, and the total annual diffuse radiation is 0.557MW.h/m2/day.

The in-depth analysis of solar radiation reveals that these radiations instigates from events of solar wares solar wind. Constant Sun emission results in solar wind. The solar wind has particles composed of low energy having 91.3% protons which are doubly or solely charged. The ions contain negligible amount of electrons having energy below 500 eV. Owing to magnetic field of the earth, it is not possible for the particles to get near surface of the earth excluding via regions of polar. The radiation bursts intensely interrelated with Sun’s spot activity cycles at the surface of the Sun. High Fluxes of flare are the cause of the serious effects on transmission lines insulators.

The results of the linear fitting for some locations are listed in table 1.

Table 1:  Value of constants a, b and R2 in some location of Libya.

 

City

Whole Year

Apr→Sept.

Oct→Mar

a

B

R2

A

B

R2

a

b

R2

Benina

0.59

0.39

0.85

0.30

0.85

0.73

0.70

0.31

0.80

Gadames

0.49

0.51

0.28

0.46

0.53

0.60

1.32

0.14

0.32

Jagbub

0.16

0.80

0.14

0.14

0.84

0.16

0.01

0.93

-

Sabha

0.25

0.72

0.12

0.29

0.67

0.29

1.04

0.12

0.50

Nalut

0.45

0.39

0.33

0.31

0.52

0.42

0.11

0.76

0.02

Ejdabia

0.36

0.54

0.53

0.26

0.63

0.54

0.44

0.46

0.28

Figure 4: value of optimum angles monthly change during the year

The above figure shows the optimum angles for incident solar radiation on an inclined surface in some areas of Libya with the method presented by Liu and Jordan. The annual optimum angles of incident solar radiation in any location in Libya can be obtained. The results can be used to find yearly angle in some locations of Libya as in the figures 5.

Figure 5:  Yearly optimum angles for some areas of Libya

The results analysis attained show that the annual rate changes from 4.44KW.h/m2/day to 10.33KW.h/m2/day during the whole year, implying that the average annual total for the amount of radiation that reaches Libya is 2.967MW.h/m2/day and the total annual diffused radiation is 0.557MW.h/m2/day. It shows the high value of solar radiation over Libya (Hassan et al. 220).  Using the international standards (IEC standard) and its modified version for polymer insulators in coastal and desert areas, early collapse of insulators can be avoided.

Various regions recorded different values in the yearly optimal changes as a result ultraviolet light rays have varied effects as per every area. According to the chart indicated in figure 6, Gadames recorded the highest optimum angle while Sabha recorded the lowest optimum angle of 27.5 (Naser 167).

Mechanical and Chemical Effects of the UV Light

UV Radiation is characterized by very explicit and serious ecological settings for electrical insulators. The effects entail the influence of radiations of electromagnetic and the corpuscle charges. These factors typically cause physical features changes and degradation. UV Radiation changeover via a matter occurs due to its interface with substance atoms, transferring energy to the insulator. The excitation and ionization of the molecules and atoms consumes this energy. The mechanism of interaction of the radiations with other material depends on the internal structure of the material and types of bond. Outcome of the relations is change in electron structure of the material, disorders of the interstitials, and the vacancy defect type formation. Bombardments of particles having high energy with atoms present in the grid results lead to the formation of grid opportunities. Atoms that are reflected habitually possess sufficient vigor to generate extra vacancies. The capture of atoms that are ejected to interstitial position in non-equilibrium, the result is a type known as interstitial defect.

The varied internal structure of the insulator materials substances makes them experience varied resistance to ionization. The interaction of UV radiations of high energy interacts with inorganic materials often causes distortions resulting in deformation of crystalline structure. This is replicated in deteriorating of physical - mechanical properties. Covalent bonds presence in the composite materials predisposes them to distortion. The most conspicuous changes in properties include; polymerization, cracking, and condensation. The resultant eccentric of vicissitudes are gauged by the composition in structure and chemical composition of the substance. Moreover, the external irradiation physically also determines the resultant changes of the polymers. Variations in the property of the polymers during irradiation ominously depend on structure of chemical composition.

The structure of the polymer changes due to continual radioactivity of the ultraviolet light radiations. The high irradiations on the polymers undergoes low rate of absorption though the effects are enormously high. Major radiation of ionization has commonly extra energy more than that needed to confiscate an electron from a single atom, nevertheless, the better portion of it is transformed into the dynamic/kinetic energy of the released free electrons, sufficient to fragment chemical bonds.

The irradiation period causes partial alterations in the structure of the matter during which, the original structure resumes its normal structure after the radioactivity. These variations results in generation of electrical conductivity, which disappear after irradiation. Irreparable chemical progressions are instigated by deviations in illuminated systems. The effects associated with the process include: crosslinking- the crosslink generation between the molecules, destruction- the main chain tearing of polymer molecules; oxidation; release of gases; polymerization; and generation of the intermolecular links (Lean 107).

The changes produced by radiations are replicated on exterior appearance of polymer, which entail the mechanical and physical conditions. The dimensions of the polymers changes due to effects emanating from radiation. There is possible emission of gases as a consequence of irradiation of polymeric materials. Polymers are vulnerable to emissions of the gases via property changes even at smaller radiations. Gas formation causes polymer swelling and reduction in density. During incessant polymers irradiation, the process of crosslinking would be successful and so do the formation of three dimensional cross-linked molecules with high firmness and stability.

Demolition process advances opposing radiation effects. Radiation exposure has a significant effect on the polymers.  The quarter value carbon atoms, which form polymer, have a predisposition for total destruction to the polymeric insulators. The composites having a minimum of an atom of hydrogen in a group of ethylene have a predisposition to crosslinking.

The variations in characteristics is influenced by ratio of speed beyond stated antiphons contingent on nature of chemical compositions of structure of polymer with conditions of the reaction and conditions of irradiation doses. In existence of oxygen, the oxidization still advances, resulting in significant impacts on certain polymers. Bigger sized weight of the molecular polymer quickens crosslinking of polymer and the lower the weight of molecular polymer, the lower the cross linking. In addition, entire course is dependent on the energy of irradiation (Swift Heavy Ion Induced Modification of Aliphatic Polymers 339). The presence of mechanical strain in radiation, distresses the process of crosslinking and speed ratio of the degradation process. Additionally, it distresses the distribution of crosslinks and, thus, the structure of crosslinking. For instance, cited mechanisms of the degradation owed to irradiation are typical for systems of cables systems entrenched in reactor cores in atomic power stations (Chudnovsky 434).

Corona and UV effects

When intensity of the electric field surpasses the breakdown strong point of air ranging in the region of 15 KV per cm, the result is the discharge of corona on the insulators surface. Corona production depends on particular atmospheric conditions such as humidity and the density of air. Furthermore, geometrical orientation of the polymeric insulator has significant role in instigating corona activity. Corona produces UV light, gas product such as nitrogen (IV) oxide and ozone gas, and the heat (Abd and Richard 201).

These discharges causes considerable electro-mechanical strains in polymeric insulators, thus, resulting into chemical degradation. It is true beyond reasonable doubt that continual degradation may eventually render polymeric insulators useless and unusable (Chen 103). However, it is also true that when the polymers possess the right chemistry, they are capable of withstanding these adverse effects throughout the time prescribed for service. Corona also causes energy losses, generation of ozone, noise generation, generation of TVI and RI.

Corona can take place on the moist surface, and this activity is termed as wetting corona. This leads to increase in intensity of electric field along the transmission lines. Wetting activity of corona is dependent on the magnitude and type of the wetting activity and electrical field intensity on the insulators surface. The characteristics of the surface of polymeric insulators affect the magnitude of corona wetting, that is, hydrophilic or hydrophobic. Moreover, wetting type such as those caused by mist, rain, condensation, and the fog has significant influence on the magnitude of the corona wetting. The electrical field surface magnitude depends on the grading ring dimension, position of grading ring, end fittings, and the hard wares fitted at the live-end (Brezina and Ryszard 573).

Corona wetting is mostly experienced at the ground and live terminals. The discharge activity is more likely to occur due to presence of lower hydrophobicity (Kempe 57). Corona results in formation of nitric acid in the company of water (H2O + NO2 HNO3) which may lead to deterioration of the surface (Introduction to Atomic Physics 3. 35 – 36). Additionally, precipitation of salt, the rain, wind and dust particles may result in significant physical changes on the surface of the insulating material through cracks and roughening. The alteration of polymeric insulators occurs when salt, other soluble substances and acids react. (Manos et al. 603). Therefore, the polymeric insulator becomes hydrophilic and, consequently, water infiltrates the insulating material resulting into the failure of the material. The physical, electrical, and chemical properties of the polymer insulators are very basic in service lifetime of these insulators. Therefore, it is necessary to predict the effects well to analyze the aging effects.

Conclusion

Generally, insulators utilized in electricity transmission lines are very significant components portion in the transmission of electrical power systems. Detailed analysis and concrete expertise on the insulators is very fundamental for the Electrical Engineers. In simple terminology, insulator is a physical material or technique that constrains the transmission of either electricity or heat (Chudnovsky 114). The insulators are utilized to guard human beings from the hazardous electricity effects while flowing in the conductors. They are resources constituting contradictory influence on the electrons flow system. They hardly permit the electrons to smoothly flow from one atom to the next (Wagner 231). This is because they have covalent bonds thus they are not permitted to wander around to the neighboring atoms.

The advancements have been gradually made to achieve the most modern type of insulators as in fig 6 below.

Figure 6 Cross section of insulators

Polymeric insulators have several advantages, such as, performance contamination, reduction in the costs of setting up, the weight is light, resistance to vandalism, no or very low maintenance and it has a design which is compact (Heeger et al. 609). UV radiation is the part of electromagnetic spectrum found between the visible light and the x-rays. It is 40 to 400 nm translating to energy between 30 to 3 eV. The Sun is the principal source of the ultraviolet light rays. The UV rays have significant hazardous effect on materials contingent to the wavelength of emission.

Apart from UV radiations there are other several factors that may hinder the performance of the polymer insulators (Drobny 76). They include chemical pollutants, temperature, leakage currents, and rainfall effects on flash over the voltage, humidity, effects caused by the prolonged contamination and conductivity flow on the HDPE, service deterioration appearance, other parameters such as contact angle and roughness of the surface, mishandling of the insulators and erosion.                             

Works Cited

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Lean, Judith L. "Cycles and Trends in Solar Irradiance and Climate." Wiley Interdisciplinary Reviews: Climate Change. 1.1 (2010): 111-122. Print.

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