ELECTRICAL BREAKDOWN IN LIQUID DIELECTRICS

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Electrical Breakdown in Liquid Dielectrics

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Topic Outline

• Liquids as Insulators

• Classification of Liquid Dielectrics

• Characteristics of Liquid Dielectrics

• Pure Liquids and Commercial Liquids

• Conduction and Breakdown in Pure Liquids

• Conduction and Breakdown in Commercial Liquids

• Effect of Moisture Content on Breakdown Strength of Liquid Dielectrics

• Testing of Insulating Oils (Fluids): Transformer Fluids

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Liquids as Insulators

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Liquid as Insulators

• Liquid dielectrics offer higher dielectric strength than gases due to density (approx. 103 times denser).

• Possess higher dielectric strength, potentially reaching 10^7 V/m.

• Fill the entire insulated volume and dissipate heat through convection.

• Oils are significantly more efficient than air or nitrogen for heat transfer in transformers.

• Actual dielectric strength in practice is around 100 kV/cm, far less than theoretical max.

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Applications of Liquid Dielectrics

• Used as impregnants in high voltage cables and capacitors.

• Acts as heat transfer agents in transformers and arc quenching media in circuit breakers.

• Common liquid dielectrics:

  • Petroleum oils (Transformer oil)

  • Synthetic hydrocarbons

  • Halogenated hydrocarbons for specialized applications.

  • Silicone oils and fluorinated hydrocarbons for high-temperature applications.

  • Toxic isomers like PCBs are largely discontinued due to health concerns.

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Properties of Liquid Dielectrics

• Mixtures of hydrocarbons, generally weakly polarized.

• Must be free of moisture, oxidation products, and contaminants for effective electrical insulation.

• Water presence significantly reduces electrical strength, even 0.01% water can drop strength to 20% of dry oil's value.

• Presence of fibrous impurities exacerbates dielectric strength reduction.

• Table 3.1 presents dielectric properties of common liquid dielectrics.

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Table 3.1: Dielectric Properties of Some Liquid Dielectrics

• Breakdown strength, relative permittivity, Tan S, resistivity, specific gravity, viscosity, acid value, refractive index, saponification, thermal expansion, and maximum permissible water content for various oils.

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Classification of Liquid Dielectrics

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Transformer Oil (Mineral Oil)

• Most conventional liquid dielectric in power apparatus, a mixture of hydrocarbons:

  • Paraffins, iso-paraffins, naphthalenes, aromatics.

• Subjected to high temperatures (~950°C), leading to aging processes:

  • Formation of acids, resins, and sludge.

• Corrosive effects on solid insulations, reduced heat transfer capacity due to sludge deposits.

• Specifications for testing are in IS 1866 and IEC 296 standards.

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Synthetic Hydrocarbons

• Polyolefins are preferred dielectrics for power cables.

• Composed of poly-butylene and alkylaromatic hydrocarbons.

• Their properties resemble those of mineral oils.

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Chlorinated Hydrocarbons

• Produced from chlorination of benzene and diphenyl into compounds known as askarels (PCBs).

• Known for high fire points and excellent electrical properties but banned due to health risks.

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Silicone Oils

• Alternative to PCBs, though expensive with thermal stability at high temperatures.

• Resistance to chemicals and oxidation, usable at higher temperatures than mineral oils.

• Used in transformers as an acceptable substitute for PCBs despite lower flammability.

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Esters

• Natural esters (like castor oil) and synthetic esters (organic and phosphate) used as capacitor impregnants.

• High boiling points and low flammability make these suitable for hazardous applications.

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Latest Developments

• New oils like high-temperature hydrocarbon oils, tetrachloroethylene, and perfluoropolyether.

• HTH oils provide good electrical insulation with higher boiling points.

• Tetrachloroethylene is nonflammable with excellent heat transfer but mixed with mineral oil.

• Perfluoropolyether (Galden HT40) is nonflammable with a boiling point over 400°C, low vapor pressure for efficient heat transfer.

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Properties of High-Temperature Hydrocarbon (HTH) Oils and Tetrachloroethylene

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Characteristics of Liquid Dielectrics

• Essential properties:

  • Good dielectric properties

  • Excellent heat transfer characteristics

  • Chemical stability under operating conditions

• Subcategories:

  • (a) Electrical properties

  • (b) Heat transfer characteristics

  • (c) Chemical stability

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Electrical Properties

• Essential for determining dielectric performance:

  • Capacitance per unit volume or relative permittivity

  • Resistivity

  • Loss tangent (tan δ)

  • Ability to withstand high electric stresses

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Electrical Properties: Detailed

• Permittivities of petroleum oils range from 2.0 to 2.6; silicone oils can reach higher.

• The permittivity changes with frequency for polar liquids like water (e.g., 78 at 50 Hz to 5.0 at 1 MHz).

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Resistivity in Insulating Liquids

• Insulating liquids for high-voltage applications must exhibit resistivities above 10^16 Ω·m.

• Most pure liquids meet this criterion.

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Loss Tangent and Electric Stresses

• The power factor indicates power loss under applied ac voltage.

• In transformers, oil losses are comparatively negligible.

• Dielectric strength, crucial for application suitability, depends on multiple factors, including electrode material and liquid properties.

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Heat Transfer Characteristics

• Heat transfer in equipment occurs primarily via convection.

• Factors influencing heat transfer include thermal conductivity (K) and viscosity (v).

• Higher K is ideal for high-temperature equipment; high viscosity may lead to localized overheating.

• Silicone oils may contribute to overheating issues in certain applications.

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Chemical Stability of Liquid Dielectrics

• Thermal and electrical stresses may degrade liquid dielectrics in presence of oxygen, water, and other impurities.

• Degradation leads to solids and gases that can affect heat transfer and electrical properties, necessitating monitoring.

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Pure Liquids vs. Commercial Liquids

• Pure liquids are chemically pure, while commercial liquids consist of complex organic mixtures and unavoidable impurities.

• Using pure liquids simplifies the study of conduction and breakdown mechanics.

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Purification of Liquid Dielectrics

• Main impurities include dust, moisture, dissolved gases, and ionic impurities.

• Purification methods:

  • Filtration, centrifuging, degassing, distillation, and chemical treatments.

• Dust reduces breakdown strength; moisture and gases further impact electrical strength.

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Additional Purification Methods

• Gases like oxygen and carbon dioxide must be controlled via distillation and degassing.

• Ionic impurities lead to poor conductivity; removal methods include utilizing drying agents and other purification techniques.

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Liquid Purification System

• Closed cycle systems are used to maintain liquid quality.

• Key processes include distillation, drying, gas removal, and filtration.

• Ensures the oil is purified for reliable testing in electrical applications.

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Breakdown Tests for Liquid Dielectrics

• Conducted using test cells integrated into purification systems.

• Breakdown measurements are affected by electrode geometry, surface condition, and impurities.

• Pure liquids exhibit high breakdown strengths (1 MV/cm) compared to lower strengths in commercial oils.

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Conduction and Breakdown in Pure Liquids

• Under low electric fields (< 1 kV/cm), the current characteristics link to purification residue; high fields lead to rapid current fluctuations and approach breakdown conditions.

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Breakdown Mechanics in Pure Liquids

• Current-electric field characteristics can show distinct regions leading up to breakdown:

  • Low fields: current from ion dissociation.

  • Intermediate fields: saturation point.

  • High fields: electron multiplication phenomena leading to breakdown.

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Breakdown Mechanism in Pure Liquids

• Breakdown occurs via field emission and mechanisms similar to Townsend’s ionization processes.

• Various factors contribute to breakdown voltage, impacting the strength and conditions under which breakdown occurs.

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Maximum Breakdown Strengths

• Breakdown strength of highly purified liquids and gases varies significantly.

• The presence of electronegative gases can enhance breakdown strength, as can hydrostatic pressure increases.

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Summary of Pure Liquid Breakdown

• Breakdown in pure liquids involves electronic breakdown through primary and secondary ionization processes.

• Breakdown facilitated by electrode surface irregularities or at impurity interfaces leads to electrical discharge.

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Breakdown in Commercial Liquids

• Commercial liquids possess impurities that significantly impact breakdown strength and mechanisms involved during breakdown events.

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Breakdown Mechanisms in Commercial Liquids

• Factor-in impurities on the breakdown process:

  • Suspended particles, bubble mechanisms, and stressed oil volumes all contribute variably to breakdown outcomes based on composition.

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Suspended Particle Theory

• Solid impurities act as points of weakness. Their interaction with the electric field can lead imperfections that foster breakdown.

• Local ripples of stress may promote breakdown in the presence of significant amounts of conductive particles.

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Bubble Theory

• Breakdown strength connected to gas bubbles and their influence when subjected to hydrostatic pressures and electric fields.

• Different factors promote bubble formation that leads to breakdown through elongation or critical voltage levels becoming reached.

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Thermal Mechanism of Breakdown

• Thermal breakdown theory addresses localized heating due to high current densities near cathode surfaces.

• Formation of vapor bubbles and their elongation acts as a precursor to dielectric breakdown, influenced by liquid structure.

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Effect of Moisture Content on Breakdown Strength

• Moisture reduces dielectric strength, the impact varies with temperatures and phase states (globules vs. vapor bubbles).

• Total moisture content should be kept below 50 ppm to maintain effective dielectric properties.

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Stressed Oil Volume Theory

• Breakdown influenced by specific regions within the liquid, wherein the weakest section sets limits on overall breakdown strength.

• Breakdown voltage dependent on liquid composition, stress distribution, and volume.

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Testing of Insulating Oils (Fluids)

• Properties and quality checks for oils (i.e., transformer fluids) to ensure they meet operational standards.

• Key tests include dielectric breakdown, acidity, moisture content, color, and particulates.

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Conclusion Summary

• The complexity of breakdown phenomena prevents a single theory from explaining all observations.

• Advancements in study techniques provide in-depth insight into mechanisms of liquid breakdown, influencing future use of synthetic dielectrics and liquid types.