Factors influencing the electrical insulation of glass fiber composite tubes

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Niina Palmu

FACTORS INFLUENCING THE ELECTRI- CAL INSULATION OF GLASS FIBRE

COMPOSITE TUBES

Master’s thesis Faculty of Engineering and Natural Sciences Examiner: Associate Professor Mikko Kanerva Examiner: M.Sc. Nazanin Pournoori

September 2021

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ABSTRACT

Niina Palmu: Factors influencing the electrical insulation of glass fiber composite tubes Master’s Thesis

Tampere University

Degree Programme in Materials Science September 2021

Glass fiber composite tubes are used in high voltage insulators used in power transmission lines, and insulating tools used in the maintenance of live apparatus. Composite tubes can be hollow or foam-filled and they consist of glass fiber reinforced polymer. They are produced by pultrusion, which is a process used to manufacture continuous, constant cross section profiles.

Main functions of high voltage insulators are support the weight of the structures and insulate the live conductors. This thesis examines factors influencing the electrical insulation of glass fiber composite tubes. The main goal is to study how different materials and structures of the tubes affect electrical insulation and mechanical properties.

In the literature research, current materials used in the insulators and their important properties are explored. In addition, a survey of material selection was conducted for material suppliers. The main findings for glass fibers were the effect of boron content on strength and formation of brittle fractures, the effect of seeds in the glass fibers on leakage currents and the compatibility of sizing with the resin. In case of the resins, the most important property was found to be moisture absorption, which affect leakage currents and the formation of brittle fractures.

In the experimental part, the structure of the composite tubes is examined by microscopic examination and dye penetration test. Mechanical properties are studied with bending and compression test and electrical insulating properties by dielectric test. Studied materials consist of combinations of different glass fibers and resins. The glass fibers have varying boron content and sizings. The resins used were polyester, vinyl ester and epoxy based. Tubes have three different lay-ups.

Test results showed that the glass fiber sizing has clear effect on structural integrity and mechanical properties, but not in electrical insulation. Boron-containing glass was found to be inferior to boron-free in both mechanical and electrical insulation properties. Of the resins, epoxy performed best both in electrically and mechanically. Polyester cannot be recommended on electrical insulation applications because of its low resistance to moisture. The number of layers does not affect electrical insulation, but the smaller layer number have degrading effect on mechanical properties. The addition of glass tissue impaired the electrical insulation properties after the moisture exposure, but its effect on the mechanical properties would need further studies.

Keywords: electrical insulation, dielectric properties, composite insulator, composite, glass fiber, glass fiber sizing, polymer

The originality of this thesis has been checked using the Turnitin OriginalityCheck service.

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TIIVISTELMÄ

Niina Palmu: Lasikuitukomposiittiputkien sähköneristävyyteen vaikuttavat tekijät Diplomityö

Tampereen yliopisto

Materiaalitekniikan DI-ohjelma Syyskuu 2021

Lasikuitukomposiittiputkia käytetään voimajohtojen komposiittieristimien ytiminä sekä sähkö- linjojen huoltotöissä käytettävien työkalujen varsissa. Komposiittieristimien tehtävänä on eristää jännitteisiä johtimia sekä kannatella rakenteiden painoa. Sovelluksissa käytettävät putket koostuvat lasikuiduista ja hartsista ja ne voivat olla onttoja tai vaahtotäytteisiä. Putket valmistetaan pultruusiolla, joka on tyypillinen komposiittiprofiilien valmistusmenetelmä. Työssä tutkitaan lasikuitukomposiittiputkien sähköneristävyyteen vaikuttavia tekijöitä. Työn tavoite on selvittää miten erilaiset materiaalit ja rakenteet vaikuttavat putkien sähköneristävyyteen ja mekaanisiin ominaisuuksiin.

Kirjallisuustutkimuksessa kartoitettiin nykyisiä eristinmateriaaleja ja niiden ominaisuuksia. Li- säksi materiaalivalinnoista tehtiin kysely lasi- ja hartsitoimittajille. Lasikuitujen osalta tärkeimmät havainnot olivat booripitoisuuden vaikutus lujuuteen ja haurasmurtumien syntyyn, lasikuidussa olevien seedien eli ilmakuplien vaikutus vuotovirtoihin sekä lasikuidun sizingin eli pinnoiteaineen yhteensopivuus hartsin kanssa. Hartsien kohdalla tärkeimmäksi osoittautui niiden kosteudenimu- kyky, joka vaikuttaa niin vuotovirtoihin kuin haurasmurtumien syntyyn.

Kokeellisessa osiossa tarkastellaan komposiittiputkien rakennetta mikroskooppitutkimuksen ja tunkeumanestetestin perusteella. Mekaanisia ominaisuuksia tutkitaan taivutus- ja puristustes- teillä ja sähköneristävyysominaisuuksia dielektrisillä testeillä. Työssä tutkitut materiaalit koostuvat erilaisten lasikuitujen ja hartsien yhdistelmistä. Lasikuiduilla on vaihtelevat booripitoisuudet ja pin- noitteet. Käytettävät hartsit ovat polyesteri-, vinyyliesteri-, ja epoksipohjaisia. Tutkittavia putkia valmistettiin kolmella erilaisella rakenteella.

Testituloksista selvisi, että lasikuidun pinnoitteella on suuri vaikutus rakenteelliseen yhtenäi- syyteen ja mekaanisiin ominaisuuksiin, mutta sähköneristävyyteen se ei suoranaisesti vaikuta.

Booripitoisen lasin havaittiin olevan boorivapaata huonompi niin mekaanisilta kuin sähköneristä- vyysominaisuuksiltaan. Hartseista epoksi pärjäsi parhaiten sekä sähköisesti että mekaanisesti ja polyesteri osoittautui huonoiten sähköeristimiin soveltuvaksi sen alhaisen kosteudenkestävyyden takia. Komposiittirakenteen kerrosmäärällä ei havaittu vaikutusta sähköneristävyyteen, mutta pie- nempi kerrosmäärä heikensi mekaanisia ominaisuuksia. Rakenteeseen lisätty lasihuopa puoles- taan heikensi sähköneristävyysominaisuuksia kosteusaltistuksen jälkeen, mutta sen vaikutus mekaanisiin ominaisuuksiin vaatisi lisätutkimuksia.

Avainsanat: sähköneristävyys, dielektriset ominaisuudet, komposiittieristin, komposiitti, lasikuitu, lasikuitupinnoite, polymeeri

Tämän julkaisun alkuperäisyys on tarkastettu Turnitin OriginalityCheck –ohjelmalla.

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PREFACE

This master’s thesis was done for Exel Composites Oyj, which is a Finnish company that designs and manufactures composite profiles and tubes. I would like to thank Exel Com- posites and Kimmo Puoskari for giving me this interesting opportunity.

I would like to thank my supervisors and all the others who guided and helped me through the process. I would like to give special thanks to Sami Pirinen from Exel Composites. I am very grateful for the professional guidance and advice I received.

I am grateful to my family and friends for their support during my studies.

Tampere, 15 September 2021

Niina Palmu

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LIST OF SYMBOLS AND ABBREVIATIONS

BF brittle fracture

BP benzoyl peroxide

DGEBA diglycidyl ether of bisphenol A ECR electric corrosion resistant

EPR ethylene-propylene rubber

EP epoxy

EVA ethylene-vinyl-acetate FRP fiber reinforced polymer

FTIR fourier transform infrared spectroscopy GRP glass reinforced polymer

HV high voltage

MEKP methyl ethyl ketone peroxide

NCI non-ceramic insulator

SCC stress corrosion cracking

SIR silicone rubber

TGMDA tetraglycidyl methylene dianiline

UPE unsaturated polyester

VE vinyl ester

g gram

I current

kV kilovolt

mm millimetre

N newton

R resistance

T time

Tg glass transition temperature

Δt time difference

V voltage

wt% weight percent

µm micrometre

φ phase angle

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1. INTRODUCTION

A dielectric material, also known as an electrical insulator, is a material that does not allow the electric current flow through it. Insulating materials are essential for safe operation of electrical applications. Electric power is transmitted along power lines and ma- jority of long-distance transmission is done by the overhead lines. [1] For more and more demanding power consumption, voltage levels in overhead lines can nowadays range up to 765 kV. An increasing voltage levels and transmission distances place also higher demands on the insulators. [2] High voltage (HV) insulators are used to insulate the live conductors and support the structures of power transmission lines. Insulating tools used in the maintenance of live apparatus are another application where the tubes discussed in this work are used.

Ceramic materials like porcelain and glass are materials traditionally used in high voltage insulators, but nowadays they are often substituted by polymer-based composite materials. Composite insulators have better insulating properties, are lighter weighted, less brittle and less expensive than ceramic ones. Other important factor is their good performance in contaminated and wet environments. [1] The core of the composite insulator can be hollow or foam-filled tube or solid rod. Its material consists of reinforcing glass fibers and polymer resin. Tubes are produced by pultrusion, which is continuous process for produce profiles with constant cross-section.

The main objective of the thesis is to study the factors influencing the electrical insulation of glass fiber reinforced composite tubes used in high voltage applications. The aim is to study and compare how different materials used in the products differ from each other and what effects they have on electrical insulation and mechanical properties. The effect of moisture-induced aging of the materials is also considered. The main materials focus on this study are different types of glass fibers with varying chemical composition and especially boron content, and with different sizings used to coat the surface of the fibers.

Of the different polymer types, the focus is on polyester, vinyl ester and epoxy. Another variable to be examined is the structure of the tubes. Tubes can be produced with different lay-ups which can influence the properties of the product.

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The theoretical background of the study looks into design and manufacture of insulators, insulator materials and structures and typical operation environment and failures. Com- parison of different materials is made based on important properties of the insulators.

The experimental section includes a survey of material selection conducted for material suppliers. In addition, in the experimental part of this work is studied how different materials and structures withstand electrical and mechanical stresses. The materials and methods used are described in detail after the theory part. They are followed by the presentation and discussion of the results. Conclusions are presented at the end of the work.

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2. HIGH VOLTAGE INSULATORS

Insulators are important components in a high voltage network. They are used to both support the structures and insulate live conductors. Main functions of insulators are bearing the weight of the conductors and insulation of the conductors, so they should have high mechanical durability and electrical resistivity. Other important properties of insulators are absence of internal holes and impurities, resistance to temperature changes, low thermal expansion of coefficient, low insulation coefficient and resistance to moisture penetration and contamination. [1]

Other application where insulating rods and tubes are used is insulating tools for live working. Metallic tools are mounted on the end of the tubes or rods. Tools are used for example to make contact with the live parts, support conductors during working and support load safely and reliably. Addition to good electrical insulation properties and mechanical strength, insulating tools should have a light weight, be easy to operate and wear and tear resistant. [3]

2.1 Ceramic and non-ceramic insulators

High voltage insulators can be divided into two categories: ceramic and non-ceramic.

Non-ceramic insulators (NCI) can also be referred as polymeric or composite insulators.

First ceramic insulators were introduced in the late 1800s as telegraph network components. Ceramic insulators are made from porcelain or toughened glass. Both materials have good resistance to surface arcing due to their inert surfaces. They are also ex- tremely strong in compression. Most of the failures of glass and porcelain insulators can be traced back to material, production, or application of the units. The manufacturing processes of ceramic insulators are complex, but well-made they are highly reliable. [2]

Advantages of porcelain insulators are their low cost, mechanical durability, and relatively easy production process. Compared to glass insulators they have slightly higher durability against bending forces. Disadvantages of porcelain insulators are their brittle- ness, sensitivity to cracking and high weight. Compared to porcelain insulators, glass insulators have higher mechanical compressive strength, and they are more resistant to breaking. The electrical resistance of glass is also higher than porcelain. Due to low thermal expansion of coefficient, glass insulators withstand temperature variations without big deformations. However, temperature variations can lead to break the insulator.

Glass insulators also break easily by impurities in composition and by strong impacts.

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Other disadvantages of glass insulators are that they absorb contaminations easier, and the moisture easily distilled in the surface. [1]

2.2 Composite insulators

Composite insulators were first introduced in the 1960s. The advantages of composite insulators are their light weight, which is said to be 90 percent compared to ceramic insulators. [2] Composite insulators are hydrophobic, erosion-proof and have better insulation properties. They perform better in contaminated environments and are less susceptible to weather changes. [1] Composite insulators have greater resistance to physical damage. They are not brittle, so they do not fail immediately on impact. That is an advantage especially in cases of vandalism, they resist gunshot damages better than ceramic insulators, which break or shatter. [2]

2.2.1 Design of composite insulators

Composite insulators consist of glass reinforced polymer (GRP) core, rubber housing with weathersheds, and metal end-fittings. Design is presented in Figure 1. The GRP core is either rod or tube. It is generally produced by pultrusion, and its purpose is to support weight of the conductor. [2] Hollow-core insulators allows different high-voltage apparatus to be placed inside the insulator and protect them from the environmental stresses [4].

Figure 1. Typical design of composite insulator [5].

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The core contains electrical glass fibers as reinforcements embedded in resin matrix, which is typically epoxy, vinyl ester or polyester [2,5]. The winding angle is usually about +/- 55° to tolerate bending and pressure. An internal protective polyester layer can be added to improve the stability to arc radiation and resistance to acids. [6]

The GRP core is surrounded by a rubber housing which protects the core from external environment. Weathersheds of the housing also raise the leakage distance between the live and ground end of the insulator. Typical housing materials are silicon rubbers (SIR), ethylene-propylene rubbers (EPR) and elastomers based on ethylene vinyl acetate (EVA). [5] Housing is usually hydrophobic, which mean that the surface naturally repel water, forming droplets. That prevents the occurrence of leakage currents and dry band arcing. [2]

Metal end-fittings are mounted to both ends of the core. One end is energized and attached to the HV line, and the other cold end is connected to the tower. [7] End-fittings give the insulator mechanical strength and secure that moisture cannot reach the GRP rod trough the interface where the end-fitting is joined to the insulator. Fittings are nowadays attached typically by crimping because it has proven to give best strength and sealing performance. [2] Typical metallic materials used for end-fittings are aluminium, iron and steel [4].

Most of the insulators are based on the design described above, but they can vary in diameter and length of the GRP core, materials and chemical composition of the GRP core, rubber housing materials and attaching methods, and the type and attaching methods of the fittings. There might also be other less important differences detected in the design of insulators produced by different manufacturers. [8]

2.2.2 Manufacture of composite insulators

Cores of the composite insulator are typically produced by pultrusion. Pultrusion is a continuous manufacturing technique for fiber-reinforced constant cross-sectional profiles, like rods and tubes. The process is usually highly automated. Pultrusion process begins with the fiber pulling. Fibers, usually in form of glass rovings, are pulled from fiber creels to open resin bath. [9,10] Resin is typically liquid thermosetting resin. The resin impregnated reinforcements are guided to the desired shape and drawn into a heated steel die where the material hardens under the influence of temperature. Heat initiates an exothermic reaction in the resin matrix to create crosslinked polymer. The hot profile drawn out of the die can be cooled by air or water while the profile is still pulled nonstop

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by the pulling unit. The product is cut to preferred length at the end of the line with an automatic cutoff saw and removed from the line either automatically or manually. [10,11]

Schematic of pultrusion process is presented in Figure 2.

Figure 2. Schematic diagram of pultrusion process.

Three critical technology areas in pultrusion process are composition of the product, material forming and process temperature controlling. The choice of reinforcing fibers has a significant effect on processability and properties of the end product. The type and amount of reinforcement affect to the electrical insulation and corrosion resistance of the composite. The choice of the resin is important because it determines mechanical properties, operating temperatures and electrical insulation, corrosion resistance and flame properties of the product. These properties can be modified by chemical additives and fillers. Material forming happens after the reinforcing fibers have been resin impregnated.

Fibers are positioned unidirectionally or multidirectionally using different guides, which can be very complex. Understanding the forming system requires experience. Control- ling the temperature is important especially with thermosets. If temperature in the die is too low, the material will not cure completely, and the properties of the product are not optimal. On the contrary, too high temperature can cause thermal stress cracking during the cooling of the profile. That result to poor electrical insulation and corrosion resistant properties. [10,11]

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3. MATERIALS AND STRUCTURE OF COMPO- SITE INSULATOR

Pultruded rods and tubes consist of glass fiber reinforcements embedded in resin matrix.

Main tasks of the fibers are act as the main insulation component and bear the mechanical load [1]. The purpose of the reinforcements in composite materials is to give strength and stiffness. Glass fibers have high tensile and impact strength and good resistance to chemicals. Main functions of the matrix are binding the reinforcements together, main- taining the fibers in the correct orientation and spacing, and protect them from environment and abrasion. Matrix also defines the maximum operating temperature, resistance to water and moisture, and oxidative and thermal stability. [9]

3.1 Glass fiber Reinforcements

Glass is an amorphous material that consist of a silica (SiO2) backbone and several oxides. The main ingredients of glass fibers are silica sand, limestone, boron oxide and small quantities of other ingredients. Glass fiber production process consists of batching, melting, fiberization, coating and drying/packaging. Batching mean weighing and mixing raw materials. Composition defines properties of the fibers. Mixture is melted in a high temperature furnace. In fiberization molten glass is extruded into fibers through bushings or spinneret containing several holes. Extruded streams of molten glass are drawn into filaments and the filaments are cooled with water. Fibers are coated immediately after manufacturing to prevent surface scratches during spooling and weaving. After mechanical treatments coating can be removed and replaced with sizing that improves the fiber- matrix bonding. Sized filaments are gathered without twisting into a bundle which forms a strand. Strand is wound into a package which can be further processed into roving, yarn or chopped fiber. [9]

Reinforcing fibers used in composite insulators produced by pultrusion are typically in the form of continuous single-end rovings. Single-end rovings are produced by pulling glass fibers directly from the bushings and winding them into a roving package. Multi- end rovings consists of multiple untwisted filaments or strands bonded together with sizing. Multi-end rovings can have poor strand integrity, which is why they may not process efficiently in pultrusion. [12] Also, chopped fiberglass mats, veils and nonwovens can be incorporated. They can be used to provide smooth finished surface and improve corrosion resistance. [9,12]

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3.1.1 Fiber type

Composite insulators can be manufactured with different glass fiber types. There are several types of glass fibers with different compositions that affect to their chemical and physical properties [11]. Glass fiber types are named according to their specific properties [12]. They are presented in Table 1.

Table 1. Glass fiber types (adapted from [12]).

Glass fibers can be divided in two categories, low-cost general-purpose fibers and pre- mium special-purpose fibers. Over 90% of glass fibers are general-purpose E-glass.

They offer strength at low cost. E-glass contains 4 to 6 wt% of boron oxide. Also, a boron- free E-glass variant is on the market nowadays. It is known by the trademark Advantex.

It is environmentally friendlier option because it does not emit boron. It has also seven times higher resistance to acids than incumbent boron containing E-glass. [12]

ECR-glass refers to corrosion resistant E-glass. ECR-glass is special-purpose glass fiber with high corrosion resistance. It has improved long-term acid resistance and short-term alkali resistance [12]. ECR-glass is boron free, but it does not necessarily meet the requirements and fall into the ASTM standard classification of E-glass [13]. ECR-glass was developed from E-glass by replacing boron oxide for titanium oxide. The addition of titanium and zinc oxides increases the Si-content and Si/O ratio, which have showed con- siderable improvement in resistance to acids and corrosion. [14] The raised amount of silica as a network former and addition of titanium oxide also increases the strength of the fibers [15]. Compositions of E-glass, boron free E-glass and ECR-glass are presented in Table 2.

Letter designation Property

E, electrical Low electrical conductivity

S, strength High strength

C, chemical High chemical resistance

M, modulus High stiffness

A, alkali High alkali or soda lime glass

D, dielectric Low dielectric constant

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Table 2. Compositions of different glass types (adapted from [12]).

Other special-purpose glass fiber types in the market are S- and R-glass. Their boron- and alkali-free compositions gives them high strength. At room temperature their strength is 10-15% higher than E-glass and they have the advantage of withstanding higher temperatures compared to E-glass. D-glass have low dielectric constant, which is achieved due to its very high, 20-26%, boron oxide levels. Pure silica or quartz fibers have increased silica content and they can be used in ultra-high temperatures. [12] Typical properties of different glass fibers are compared in Table 3.

Table 3. Properties of glass fibers (adapted from [12]).

Property

Glass fiber type E Boron-

free E ECR D S Silica/

quartz Density

[g/cm3] 2.54–2.55 2.62 2.66–2.68 2.16 2.48–2.49 2.15

Tensile strength [MPa]

3100–

3800

3100–

3800

3100–

3800

2410 4380–

4590

3400 Young’s

modulus [GPa]

76–78 80–81 80–81 - 88–91 69

Elongation at

break [%] 4.5–

4.9 4.06 4.5–

4.9 - 5.4–

5.8 5

Coefficient of thermal expansion [10^-6/K]

4.9-6.0 6.0 5.9 3.1 2.9 0.54

Dielectric constant at 1 Mhz

5.86–6.6 7.00 - 3.56–3.62 4.53–4.6 3.78

Dielectric

strength [kv/cm] 103 102 - - 130 -

Volume resistivity [log10(Ω cm)]

22.7-28.6 28.1 - - - -

E-glass Boron free

E-glass ECR-glass

Silica (SiO2) 52-56 59-60 58,2

Boron oxide (B2O3) 4-6 - -

Aluminum oxide (Al2O3) 12-15 12,1-13,2 11,6

Calcium oxide (CaO) 21-23 22,1-22,5 21,7

Magnesium oxide (MgO) 0,4-4 3,1-3,4 2

Zinc oxide (ZnO) - - 2,9

Titanium dioxide (TiO2) 0,2-0,5 0,5-1,5 2,5

Sodium oxide (Na2O) 0-1 0,6-0,9 1,0

Potassium oxide (K2O) - 0,2 0,2

Iron oxide (Fe2O3) 0,2-0,4 0,2 0,1

Iron (Fe2) 0,2-0,7 0,1 -

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3.1.2 Seed count

In addition to glass fiber type, the seed count is another factor that significantly affects the insulating properties. Seeds are small gaseous inclusions trapped in the glass fibers.

Schematic picture to illustrate the seeds is presented in Figure 3. Seeds are caused due to formation of gas from the raw ingredients in the glass and by the melting conditions in the furnace. Seeds in the molten glass can range from 10-400 microns in diameter and they can create hollows within the filaments from 1-20 microns in diameter. Larger diameter seeds cause longer hollows. [13] Seeds have remarkable effect on electrical properties of composites because they are the sites for partial electrical discharges and re- duce the resistance to electrical energy [13,16]. High seed count was a problem especially in the original boron-free glass fibers, which made them unsuitable for electrical insulators. Addition of TiO2 results in lower seed count in the boron-free glasses.

Figure 3. Schematic cross-sectional view of glass fiber reinforced composite with seeds in the fibers.

3.1.3 Sizing

Sizing is a surface coating applied to fibers during their manufacture. Fiber sizing significantly influences to fiber and composite processability, performance and interphase adhesion. Although the sizing has so important role, the technology involved is confidential and not much information on the formulations used is revealed by glass fiber producers.

[17]

Sizing is dilute water-based emulsion or dispersion which can contain even ten or more components. Primary components are coupling and film former agents. Coupling agents are almost always organofunctional silane compounds and they are said to be the most important chemicals used in glass fiber and composite industry. They are used to improve the adhesion and bonding of the fiber to the resin and improve the interphase strength. [17] Organofunctional silane compounds create hydrophobic surface by dis- placing adsorbed water on the glass surface. They improve the wetting of the fibers and

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form a strong interfacial bond between the resin and fiber. [18] The silanol group (-SiOH) of the coupling agent reacts with the silanols on the surface of the glass fiber, and other resin-reactive end group connects the resin directly to the fiber [19].

Film former agents are water based polymeric materials that are selected to be well- matched to the resin, because the compatibility can influence to reinforcing properties of the composite. Frequently used film formers are polyesters, polyvinyl acetates, epoxies, polyurethanes and polyolefins. Purpose of the film formers is to protect the fibers during manufacturing process preventing the loss of chemicals during winding and other mechanical treatments. Sizing is applied before gathering the glass filaments to the strand, because the filaments are also very abrasive to each other. In addition, the film formers keep the filaments together in a strand. [17,18]

Sizing can also include lubricant agents, antistatic agents, surfactants, antioxidizing agents, plastifying agents and biocides. Lubricating agents are used to prevent abrasion while glass fibers travel through metal or ceramic guides, bars and pipes. Surfactants lower the surface tension and improve the wetting of surface of the fiber. Antistatic agents are used to prevent glass fibers natural charging by static electricity, because charged fibers can cause problems during processing and manufacturing. Antioxidizing agents are used to prevent the oxidation of organic compounds of the sizing. Oxidation of these chemicals can cause discoloration and degradation of film formers and lubricants. Plas- tifying agents are used to alter properties of the film formers. Plastifying agents are added to film former polymers to make them softer and more flexible and processable. Biocides are used to prevent and stop growth of microbials, which can damage the size. [18]

3.2 Matrices

Polymeric matrices can be divided into thermoplastics and thermosets. Thermosets crosslink together and form irreversible chemical bond when heated during the curing process. Once the polymer is crosslinked, it cannot be remelted and reshaped. Thermo- plastics instead do not crosslink with heat, and they can be remelted and reshaped without affecting materials physical properties. [9] Matrix materials used in composite insulators are typically thermosets. Their strong molecular bonds provide additional strength and chemical resistance to material, and because they cannot be remelted, they have higher temperature resistance than thermoplastics. Most of the current insulators are based on polyester, vinyl ester or epoxy resins. Their typical properties are compared in Table 4 and discussed more in following chapters.

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Table 4. Typical properties of polyester, vinyl ester and epoxy resins (Adapted from [20,21]).

3.2.1 Polyester

The advantage of polyester is low price combined with good properties and processing versatility. Compared to epoxies, polyesters have lower operating temperature and lower mechanical and environmental resistance. Polyester formulation consists of unsaturated polyester (UPE), styrene curing agent and peroxide initiator. [9]

Chemical structure of unsaturated polyester is presented in Figure 4. UPE consists of oligomeric linear molecules with reactive double bonds. Carbon-carbon double bonds remaining along the molecular backbone make UPE susceptible to hydrolytic degradation. [22] UPE is formed by the reaction of dicarbonylic acids with glycols and the properties of the resin can be controlled by the type of these molecular building blocks [14].

Two most common UPE resin types are orthophthalic and isophthalic. Orthophthalic resins are general purpose low-cost resins. They are based on phthalic anhydride.

Isophthalic resins are based on isophthalic acid and maleic anhydride. Isophthalic acid creates high-molecular-weight resins having good mechanical properties and chemical and thermal resistance. Nonpolar glycols improve hydrolysis resistance. [23]

Figure 4. Chemical structure of unsaturated polyester [22].

UPE is cured by free-radical addition copolymerization. It is dissolved to reactive diluent which is commonly styrene. [19] Other diluent options are vinyl monomers or methyl methacrylate [23]. Styrene improves the processability by reducing the viscosity and reacts with polyester chain to form crosslinked structure [9]. Copolymerization with unsaturated styrene forms three-dimensionally crosslinked polymer. Copolymerization is initi- ated by free radical generated by the decomposition of peroxides. [22] Methyl ethyl ketone peroxide (MEKP) is most common for cold curing and benzoyl peroxide (BP) for hot curing [19].

Resin Density

(g/cm³) Tensile strength (MPa)

Tensile modulus (GPa)

Flexural strength (MPa)

Flexural modulus (GPa)

Cure shrink- age (%)

Glass transi- tion (°C) Poly-

ester 1.1–1.43 34.5–

103.5 2.1–3.45 80–110 3–5.5 5–12 70–120 Vinyl

ester

1.12–

1.32

73–81 3.0–3.5 120–150 2.7–4 5.4–10.3 102–105 Epoxy 1.2–1.3 55–130 2.75–4.1 90–145 2.5–10 1–5 100–270

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3.2.2 Vinyl ester

Vinyl ester (VE) is used when additional performance in corrosion resistance and ele- vated-temperature mechanical properties are needed [23]. Vinyl esters are very similar to polyesters, but their crosslinking densities are lower, which make them tougher. They are more resistant to water and moisture because of fever ester groups. [9]

Vinyl esters are hybrid from polyester and epoxy resin. They are formed by the esterification of an epoxy resin with an unsaturated acid, like methacrylic acid or acrylic acid.

Properties of the resin can be controlled by the monomer selection. Methacrylic acid and epoxy backbone structure based on diglycidyl ether of bisphenol-A (DGEBA) is most common for composite applications. Like UPE resins, VE resin are cured by copolymerization with styrene diluent using peroxides as initiators. [22] Chemical structure of vinyl ester is presented in Figure 5.

Figure 5. Chemical structure of diglycidyl ether of bisphenol-A based vinyl ester resin [22].

The difference of VE and UPE resins are that ester bonds exist only terminal methacrylate groups [14]. VE resins typically have terminal unsaturation and terminal double bonds crosslink with styrene. Less double bonds than UPE makes cross-linking density lower, which make the resin tougher and increases the impact strength and tensile elongation properties. VE have also improved chemical and hydrolytic stability compared to UPE, because the number of ester linkages that can be chemically attacked is minimized by locating only at the end of the molecular chain. [22]

3.2.3 Epoxy

Epoxy (EP) resins are used when high level physical properties are needed. Epoxy resins tend to be more resistant to moisture and other environmental impacts than polyester resins. Epoxy resins have higher flexural strength and interlaminar shear strength compared to vinyl ester and polyester. Their electrical properties and corrosion resistance are excellent. [24] Advantages of epoxy resins are that they allow cross-linking to happen without elimination of condensation products like water and they have lower curing

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shrinkage than other thermosets. They are tough, have high adhesion to many sub- strates and good alkali resistance. Disadvantages are need for heat, slower cure rate and special expertise needed for formulation systems. [19]

The base resin, curing agents and modifiers are three basic elements of epoxy resin formulation. Resin properties can be altered by selecting suitable epoxies, curing agents and additives. Epoxy group is presented in Figure 6. It consists of one oxygen atom and two carbon atoms forming three-membered epoxide ring. It can be attached to large number of molecular bases forming various classes of uncured epoxy resins from low- viscosity liquids to high-melting solids. Three main categories of epoxies used in composite applications are phenolic glycidyl ethers, aromatic glycidyl amines and cycloaliphat- ics. [24]

Figure 6. Chemical structure of epoxy group [24].

Phenolic glycidyl ethers are formed by the condensation reaction between epichlorohydrin and phenol group. Different types of resins are formed based on the structure of phenol-containing molecule and the number of phenol groups. Most used phenolic glycidyl ether is diglycidyl ether of bisphenol-A (DGEBA). It has also multiple variations with higher molecular weights. Other types are diglycidyl ether of bisphenol F, which has lower viscosity and better chemical resistance, and phenol and cresol novolacs, which are derived from phenol and formaldehyde. [25] They have higher reactivity, greater cross-linking density, higher temperature and chemical resistance, high cured glass transition temperature (Tg) and excellent temperature performance compared to DGEBA- based epoxy resins. The second base resin type is aromatic glycidyl amines, which are formed by reacting epichlorohydrin with an amine. Tetraglycidyl methylene dianiline (TGMDA) is most important of that class. It has excellent mechanical and high-temperature properties and high Tg. Cycloaliphatic base resins are the third base resin type. They contain epoxy group that is internal to the ring structure. They have lower viscosity, greater cross-linking intensity, very high mechanical and thermal properties, and superior outdoor weathering resistance. [24] Chemical structures of different type epoxy resins are presented in Figure 7.

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Figure 7. Chemical structures of a) DGEBA, b) TGMDA and c) cycloaliphatic epoxy resin. Adapted from [25,26].

Epoxy resins react with large number of chemicals called hardeners, curatives or curing agents [24]. Curing component contains active hydrogen atoms which react with epoxide groups in a polyaddition reaction. Reaction is not a radical catalytic chain reaction like in case of UPEs, but resin molecules react slowly with curing agent molecules in approximately the same quantities. [14] Most commonly used curatives are amines, amine derivatives and anhydrides.

Amines will cross-link epoxy resin by a catalytic or addition reaction. Most often used are aliphatic amines, which can be used as such, or adducted to alter reactivity, volatility, toxicity and stoichiometry [25]. Primary and secondary aliphatic amines react by addition polymerization where one nitrogen-hydrogen group reacts with one epoxy group [19].

Catalytic reaction of tertiary amines is result from unshared electron pair on the nitrogen.

Tertiary amines do not cause an additional reaction but works as a catalyst and cause epoxide group to self-polymerize. Resins that are cured by homopolymerization have higher Tg and better chemical resistance but are more brittle than resins cured by addition reaction. Cycloaliphatic amines have at least one amino group attached directly to a sat- urated ring. They can be cured quickly in cold, damp conditions and they have higher Tg

and toughness than aliphatic amines. Aromatic amines have nitrogen-hydroxyl group directly bound to an aromatic ring. They need high cure temperature to get high Tg and

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chemical resistance. [24] Rigid benzene ring structure gives higher heat deflection temperature than aliphatic amines [19]. They usually provide the best properties of amine- cured epoxies [25].

Second class of epoxy curing agents are acid anhydrides, which have various physical forms. Phthalic anhydride is the most used. Anhydrides are particularly suitable for electrical insulation applications. Formulations are known from low viscosity and exotherm and long pot life. Cure mechanism is complex and includes competing esterification and etherification reactions. As cured resins they withstand higher temperatures than aliphatic amines, have excellent electrical properties and low curing shrinkage. Disad- vantage is moisture sensitiveness. Atmospheric moisture or water released by cure reactions can cause anhydride hydrolysis to acid. [25]

3.2.4 Additives and Fillers

The purpose of the additives in resin systems is to provide specific performance. Initia- tors are required to cure the thermosetting resins. There is variety of initiators with different levels of activation temperatures. Pultrusion formulations usually use multiple-initiator system to fast processing speed. Polyester and vinyl ester systems use peroxide initiators while epoxy systems use catalyst. Mold releases are used to prevent the curing material from bonding to die wall. Other additives like UV-radiation inhibitor, pigments for coloration and low-profile agents for surface smoothness may also be used.

Generally used fillers are calcium carbonate, alumina silicate and alumina trihydrate.

Calcium carbonate is the most common one. It is used to replace the raw material to lower cost of the resin. Alumina silicate fillers are used to improve electrical properties and corrosion resistance. Alumina trihydrate fillers are used to provide better flame and smoke resistance properties and electrical-arc and track resistance. [10]

3.3 Structure

Composite insulator cores can be hollow tubes, foam-filled tubes, or solid rods. Cores have uniform circular cross-sections and are straight in lengths. According to the standards IEC 1235 and IEC 60855-1, external diameters of hollow tubes can vary between 10 to 100 mm, solid rods between 10 and 15 mm and foam filled tubes between 32 and 77 mm. [27,28] Also, the wall thickness of the composite tube can vary, but there are no specific limits given them in the standards.

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3.3.1 Lay-up and fiber orientation

Composites are laminated materials which consist of individual layers. A layer of composite material with unidirectional fibers or woven fabric is called ply. A single ply and lay-up where all the plies or layers are stacked in the same direction is called lamina.

Unidirectional lamina is strong and stiff in the 0° direction parallel to the fibers, where the reinforcing fibers carry longitudinal tension and compression loads, but weak in the 90°

direction where the transverse tension load is carried by polymeric matrix, which is much weaker. [9]

A lay-up where plies are stacked in multiple directions is called laminate. The properties of the laminate depend on the orientation of the plies. If the loads are high in one direction, the laminate should have most of its plies oriented parallel to that direction. [29] It is typical to orientate the plies in multiple directions to balance the load-bearing capabil- ity. A laminate that bears equal loads in all four directions has the same number of plies at 0°, +45°, -45° and 90° angles. Such laminate is called a quasi-isotropic. [9] Because of anisotropy, GRP cores and especially tubes damaged easily by compression and crushing [30].

Coordinate system is required in design and analysis of composite structures. The most common coordinate systems are presented in Figure 8. 1-2-3 system is used to identify the lamina directions. 1-axix is parallel to the fibers (0°), the 2-axis perpendicular to the fibers (90°) and 3-axis is normal to the plane of the ply. X-y-z system refers to the overall laminate directions. [29]

Figure 8. Coordinate systems of composite structures [29].

3.3.2 Foam filling

Foam is insulating material that composed of closed cells. Foam filling can be used inside the insulating tube to prevent the ingress and migration of moisture. Foam should be free of voids, separations, cracks, and other defects. Typical foam filling material is polyure- thane. Foam filling should be bonded to the wall of the tube. [28]

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4. OPERATION ENVIRONMENT AND FAILURES

4.1 Operation environment

Materials and structures of the insulators should have a lifetime of 20 to 30 years in challenging environments and extreme temperatures [6]. Insulators are subjected to mechanical, electrical and environmental stresses that can lead to mechanical and electrical failures [5].

The environment in which the insulator operates have a great impact. In contaminated environments, insulators performance can deteriorate. [2] In operation various factors, like static and dynamic loading, electrical discharge, electrical current, UV radiation, humidity, rain and pollution, etc. affects to insulators performance. [31]

4.2 Failures

In most catastrophic cases, the insulator carrying power line breaks mechanically and failure lead to the drop of the energized transmission lines. In addition to the discon- nected line, freely hanging energized conductors cause a serious safety risk to people.

Another undesirable situation is an electrical failure that causes disconnections in the network. [4]

Most of the insulator failures are caused by bad design, material selection or quality control. In this regard, most manufacturers today produce high quality products, and the future failures depend primarily on the aging of materials. Degradation process of composite core is not common, but it may result in mechanical failures. End-fittings may slip out of place because of degradation. In addition, GRP core can break normally due to overload, or a brittle fracture can occur in the core. The lifetime of the insulator usually remains unchanged below the damage limit, where the fiberglass begins to break. How- ever, brittle fracture can be caused also at low loads. [4]

4.2.1 Brittle fracture

The most important failure mode of composite insulators is electro-mechanical brittle fracture (BF) failure, where stress corrosion cracks are formed inside the rod [5]. If the composites are subjected to pure tensile stresses parallel to the fibers without any acids, the fracture surface is broom-like. It consists of multiple fractures, fiber pull-outs and matrix cracking. In the BF process, where in addition to the tensile stress, the composites

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are exposed to acids, the fracture surfaces are planar and run perpendicular to the fibers.

The number of fiber pull-outs and debondings is very limited and fiber and matrix are almost in the same plane. [8] The size of the smooth planar region can reach even 80%

of the cross-section of the rod [7]. Failure occurs typically either just outside the energized fitting or inside the fitting. The morphologies of the fractures vary according to their location. If the failure occurs outside the fitting, it is typically characterized by severe resin decomposition. If the failure is inside the fitting, no significant decomposition of the resin usually occurs. Severe surface contamination is typical and can be so strong that the single fibers on the fracture surface cannot be detected. [8]

In microscopic level, the formation of mirror, mist and hackle zones on the fracture surfaces of single glass fibers is typical [8]. Mirror zone is smooth and circular, and it typically contains the starting point of the fracture of the single fiber. There the crack propagation is relatively slow. Mist zone has matt and rougher surface than mirror zone. It is a transition region between mirror and hackle zone. There the propagation of the crack be- comes rapid. Hackle zone is rough-textured, and it has radical irregular fracture lines running from the mirror zone, indicating rapid crack propagation. Hackle lines run in the direction of crack propagation. [32]

4.2.2 Cause of brittle fracture

The brittle fracture of composite rods has been widely accepted to be caused by stress corrosion cracking (SCC). SCC can occur when the rod is simultaneously exposed to tensile stress and corrosive environment containing free hydrogen ions. [8] At the molecular level, mechanism of SCC is described by ion exchange reaction. Non-siliceous oxides, like the large metal ions of glass fibers (Al, Ca, Fe, Mg) are leached out and replaced by hydrogen ions. It is considered that leaching of calcium and aluminum is the main reason for acid corrosion, because their amount is about 35 wt% of the glass, while all non-siliceous oxides together make up 45 wt%. [33] Leaching weakens the fibers and fractures can form if low tensile stress is applied along the fibers [8]. BF can form at loads that are only 5 to 20% of the maximum load-bearing capacity of the composite rod [7].

It is generally approved that BF is caused by the simultaneous application of a tensile stress and an acid. However, the source of the acid controversial. Theory of Montesinos et al. [34] claimed that SCC can be caused by hydrogen ions in terms of water and that BF can happen due to water and mechanical stresses, and it occurs more probable with water than acids. [35,36] However, that theory has been questioned by the study where water could not cause SCC without either high electric fields or heavy contamination [36].

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According to Tourreil et al. [37] the source of the acid is either nitric acid formed in service or carboxylic acid generated by hydrolysis.

Based on the nitric acid theory, corona or dry band discharges in presence of moisture and oxygen can produce nitric acid. If the housing is damaged and the core is exposed to the atmosphere in area where electric discharges may occur, nitric acid can be formed.

[37] Nitrogen oxides are formed as a by-product of gas discharges and when they react with water nitric acid is formed [8]. This theory is supported by the observations that most of the BF failures have appeared just outside the live end-fitting and some inside the fitting. Failures have not occurred close the grounded end or in the distance from the fittings. This scenario also explains how the higher number of BF failures is related to higher transmission line voltages. [38] In many cases the housing is damaged, and rod exposed. This favours the theory that acid comes from external sources rather than internal ones. [39,40] The identification of nitrate from the surfaces of failed insulators have confirmed the generation of nitric acid in service. The process has also been simulated under laboratory conditions. [38]

According to carboxylic acid theory, water can under certain conditions, affect all epoxy, vinyl ester and polyester resins and form carboxylic acid because of hydrolysis [37].

Epoxy formulations consist of epoxy and hardener. Epoxy formulations are discussed more in chapter 3.2.3 Epoxy. Most common hardener belong to phthalic anhydride group. Hydrolysis of the hardener transfers them into acids. Acids can be generated before or during the fiber impregnation and picked up by the fibers when they pass through the resin bath in the pultrusion process. In the finished product, clumps of hydrolyzed hardener may be randomly located on or near the surface, or somewhere inside the core.

If water meet acidic clump, it enables the acid to enter the cracks and start the BF process. [39,40] In the other case small amount of unreacted hardener remains in the finished pultruded rod. This can happen if the resin is not completely cured due to too short curing time or a low temperature. [37] If water reach the rod during service life, it will hydrolyze the hardener still present. This newly formed acid can, in combination of tensile stress, lead to BF. Water acts as a carrier for the acid and without water in contact with the rod, the occurring of BF is very unlikely even in the presence of acid clumps. [40]

It has been confirmed by several studies [37,39,40] that hydrolysis of the hardener can turn them to acids. The reaction is described in Figure 9. Type of the hardeners used in the studies is not mentioned but most likely they belong to anhydride family, which is mentioned in the articles as the most common one. Laboratory test proved that clumps of hydrolyzed hardener can lead to BF in the presence of moisture [39]. Field case study

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shows that same process explains field failures. Fourier transform infrared spectroscopy (FTIR) analysis of failed insulator shown that small amount of unreacted hardener was present in the rod and presence of the acid that is obtained by the hydrolysis of hardener.

[40] Field failed insulators, including polyester, vinyl ester and epoxy-based materials, were studied with FTIR analysis to detect presence of nitric and carboxylic acid. Of 18 field failed insulators, 11 failed because of carboxylic acid, 2 because of nitric acid and 5 cases were not clear. Based on the study, it is 5-10 times more likely that BF is caused by an acid that is a by-product of the manufacture of the rod itself, instead of nitric acid occurred in service. [37]

Figure 9. Reaction where hydrolysis of the hardener leads to creation of carboxylic acid [40].

Observations that support the carboxylic acid scenario rather than nitric acid scenario are that BF have occurred even though the housing or the fitting seal that prevent water from entering the core have not been damaged, and BF have occurred far away from energized end fitting where electric field is very low, and expose rods have not fail even they have been years in service exposed to environment and acids. Based on that the source of acid cannot logically came outside the insulator but must be internal. [39,40]

The theory is also criticised. According to Kumosa et al. [38] it is chemically possible only with epoxy/anhydride resin systems and most of the failed insulators have been E- glass/polyester rods and failures of E-glass/epoxy rods is rare. Nor can the model explain why the failures typically occur outside the fitting. Failures should be able to appear at anywhere along the core, but the location typically correlates with electric field concen- tration.

4.2.3 Prevention of brittle fracture

Common things in the models are that BF is caused by SCC and that water ingress initiates the process [35,36]. BF can be prevented by not allowing water or acid inside the insulator or designing the rod with very high resistance to SCC. Even the best pro- tection against moisture fails over time. It would be more efficient to design the composite core to be SCC and BF resistant. [35]

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Among glass fibers, resin and their interface, the glass fibers have been found to be most vulnerable to SCC. [33] Fiber and resin type, resin fracture toughness, surface fiber exposure, interfacial strength and moisture absorption affects BF resistance. [41,42]

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5. IMPORTANT PROPERTIES OF THE INSULA- TORS

There are standards defining requirements for insulating tubes that are intended to be used equipment, devices and tools operating at voltages above 1 kV. IEC 1235 applies to insulating hollow tubes and IEC 60855-1 to foam-filled tubes and solid rods. According to the standards material and design of the tubes should have insulating properties and external surface of the tubes should have hydrophobic properties. Dielectric tests are performed to verify the ability of the tubes to withstand dielectric stress before and after exposure to humidity. In addition to electrical requirements, tubes should have mechanical resistance properties. Different tests define how the tubes should withstand bending, torsion, crushing and fatigue. [27,28]

According to literature in the field [41,42], tubes should also be resistant to SCC and BF, which are the main failure types of composite insulators. Resistance to BF is affected by fiber and resin type, resin fracture toughness, surface fiber exposure, interfacial strength and moisture absorption. Moisture absorption can also affect the insulation properties and mechanical characteristics of composites [41].

5.1 Dielectric properties

Dielectric materials do not contain freely moving charges. When dielectric material is subjected to electric field, every electron and nucleus experience a force, but they are so tightly bonded that they are not free to move, so material do not conduct electric charges. Dielectric properties depend on the chemical composition and structure. In general, the electric conduction of polymers is low, and conduction is often caused by impurities that provide charge carries in the form of electrons or ions.

Dielectric properties are described by dielectric constant, which defines how easily a material can be polarized when subjected to an electric field. In polarization positive charges move with the electric field and negative charges against it. The greater tendency to electric polarization, the higher is the value of dielectric constant. Typically, nonpolar polymers with symmetrical structure and covalent bonds are the best insulators.

Most of the polymers have polarized covalent bonds and dipoles orientates to lign up with electrical field, which rise the dielectric constant. Another important property is dielectric strength, which describes the breakdown phenomena. Every material has specific dielectric strength, which tells the maximum value of the electric field that a material can

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withstand without breakdown. In order for insulators to withstand electrical field, they must have high dielectric strength. [43]

Leakage current is the most important parameter to obtain information about the behaviour of an insulating material. Leakage current is superposition of three components:

polarisation current, ionisation current and conduction current. Polarization current is generated as an inherent dielectric response to the applied electric field. Corona and unstable partial discharges on the surface of dielectric material cause the ionisation current. Conductive contamination layer on the insulating surface creates the conduction current. [44] High leakage current indicates poor performance of the insulator. Leakage current can be caused by aging of the insulator, damage of the core or pollution. [45]

Main concern of polymeric insulators during service life is material degradation due to aging. One of the most important factors reducing dielectric properties of the insulator is pollution. Pollution layer decreases the resistance of the surface and can lead to the flow of leakage current. [46]

The aging of the materials can be analysed by examining the level of the leakage current and phase angle. The phase angle between the voltage and the leakage current changes as the material ages. In the beginning the leakage current passes through the insulator material and not along its surface. Behaviour is capacitive and the leakage current lead the voltage by 90 degrees. The resistivity of the surface starts to decrease when it be- comes contaminated or wet. That result in an increase of the resistive leakage current that flows through the surface and a decrease in the phase angle. Aging behaviour is noticeable in phase angle and surface resistance even for early stages when there is hardly any discharge activity and leakage currents are low. [47]

5.2 Resistance to leakage current and moisture

Seed count of the glass fiber can affect dielectric properties, especially in the presence of moisture. Any absorption of water will increase dielectric constant of composite and humidity increase the leakage currents. [48] In addition, moisture absorption can affect to mechanical properties of the materials and initiate BF process [41]. Materials that absorb high amounts of water, absorbs also more acidic solutions [49]. That can contribute to the formation of brittle fracture. BF do not occur if there is no acid involved but moisture can lead to electrical failures. [50] Due to the negative effects on electrical and mechanical properties of the insulators, moisture absorption resistance should be good.

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In the literature [41,48-50] has been found that composites with high seed ECR-fiber absorb less moisture than composites with low seed ECR-glass and E-glass. Difference is explained by the different diameter of the fibers. The 20 mm diameter of high seed ECR-glass fibers was larger, while the diameters of E-glass and low seed ECR-glass were 14 mm in the studies. Fibers with larger diameters form less fiber/resin interfaces which impacts to the moisture absorption process. [50] They prevent the moisture absorption as the diffusion into the material slows when the path lengths around the fibers increases [48]. It must be considered whether the diameters of the glass fibers in the studies were of different sizes by a chance, since all types of glass fibers are generally produced with several different diameter options.

However, composites with high seed ECR-glass fibers have worse electrical insulation properties than composites with E-glass fibers [16]. High seed count can affect to dielectric properties in the presence of moisture, but no correlation between the moisture absorption and leakage currents has been found, so higher leakage currents cannot be explained by higher moisture absorption. [42,48] Instead, the seed count has significant effect on leakage current. It has been studied that high seed ECR-glass fibers have hun- dred times higher leakage current than low seed ECR-glass fibers for the same amount of absorbed moisture. [41,42] Leakage currents in low seed ECR-glass fibers are like in their E-glass counterparts [41]. Absorbed moisture changes the electrical field in the material. The moisture content within the materials is higher on the outer edges and decreases towards the center and so does the dielectric constant. When material with different dielectric constant is in the electric field, the induced charge increases on voids, which are in the medium where the dielectric constant is lower. [48]

The polymer resin type affects the moisture absorption of the composite more than the glass fiber type [48]. Based on the literature of the field [41,42,48-50] polyester-based composites have the worst moisture absorption resistance with high moisture diffusion rate and high maximum moisture absorption. Because of their low resistance to moisture uptake, they cannot be recommended on insulator applications. [50] Both modified polyester and vinyl ester-based composites diffusion is single phase Fickian diffusion. They saturate quickly, but vinyl ester take moisture less and more slowly. [42,50] Epoxy-based materials take more moisture than vinyl ester based, but epoxy have tendency to not fully equilibrate at first phase. [41,49] They keep absorbing moisture at second slower rate and moisture content gradually increased with time. This is called non-Fickian diffusion. Full saturation might take years to reach. Various phases in moisture absorption leading to a final equilibrium can mean unstable behaviour when exposed to moisture.

Factors influencing the electrical insulation of glass fiber composite tubes

Niina Palmu

FACTORS INFLUENCING THE ELECTRI- CAL INSULATION OF GLASS FIBRE

COMPOSITE TUBES

Master’s thesis Faculty of Engineering and Natural Sciences Examiner: Associate Professor Mikko Kanerva Examiner: M.Sc. Nazanin Pournoori

September 2021

ABSTRACT

TIIVISTELMÄ

PREFACE

CONTENTS

LIST OF SYMBOLS AND ABBREVIATIONS

1. INTRODUCTION

2. HIGH VOLTAGE INSULATORS

2.1 Ceramic and non-ceramic insulators

2.2 Composite insulators

2.2.1 Design of composite insulators

2.2.2 Manufacture of composite insulators

3. MATERIALS AND STRUCTURE OF COMPO- SITE INSULATOR

3.1 Glass fiber Reinforcements

3.1.1 Fiber type

3.1.2 Seed count

3.1.3 Sizing

3.2 Matrices

3.2.1 Polyester

3.2.2 Vinyl ester

3.2.3 Epoxy

3.2.4 Additives and Fillers

3.3 Structure

3.3.1 Lay-up and fiber orientation

3.3.2 Foam filling

4. OPERATION ENVIRONMENT AND FAILURES

4.1 Operation environment

4.2 Failures

4.2.1 Brittle fracture

4.2.2 Cause of brittle fracture

4.2.3 Prevention of brittle fracture

5. IMPORTANT PROPERTIES OF THE INSULA- TORS

5.1 Dielectric properties

5.2 Resistance to leakage current and moisture