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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Dielectric constant of insulating oil should be
2
2
8
5
2
Quick Summary: The dielectric constant (or relative permittivity, $\epsilon_r$) of transformer insulating oil typically ranges between 2.1 and 2.5. A value of 2.2 is the standard industry benchmark used for mineral-based insulating oils in power equipment.
The dielectric constant (or relative permittivity, ϵr) of transformer insulating oil typically ranges between 2.1 and 2.5. A value of 2.2 is the standard industry benchmark used for mineral-based insulating oils in power equipment.
ϵr=ϵ0ϵ — Relative permittivity where ϵ is absolute permittivity and ϵ0 is free space permittivity.
The insulating oil acts as both an electrical insulator and a cooling medium. A dielectric constant near 2.2 ensures that the electric stress within the transformer insulation system is distributed uniformly between the oil and the solid insulation (like pressboard), which usually has a higher dielectric constant.
Mineral insulating oil is preferred due to its high dielectric strength and chemical stability.
A lower dielectric constant value helps in reducing local electric field concentrations.
The constant remains relatively stable across standard power frequency ranges (50-60 Hz).
Moisture contamination significantly increases the dielectric constant, leading to oil degradation.
Excellent heat transfer properties (convection cooling).
High electrical resistivity preventing leakage current.
Flammability concerns in mineral oils.
Susceptible to oxidation and moisture absorption.
Power transformers
Distribution transformers
Circuit breakers
Capacitor impregnants
The dielectric constant is a dimensionless quantity that measures the ability of a material to store electrical energy in an electric field.
Values lower than 2.0 or higher than 2.5 indicate impurities, oxidation products, or presence of water in the insulating oil.
A is correct — The standard dielectric constant for mineral-based insulating oil used in power transformers is approximately 2.2.
Always remember that while dielectric strength (breakdown voltage) is the most critical parameter, the dielectric constant must be consistent to prevent uneven voltage grading in multi-layer insulation.