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.

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