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.