What is the dielectric constant of insulating oil?

The dielectric constant (relative permittivity) of insulating oil typically ranges between 2.2 and 2.5. The specific value depends on the type of oil, its composition, temperature, frequency, and purity level.

Here are more detailed points:

Common Mineral Insulating Oil (e.g., Transformer Oil): This is the most widely used type. Under standard power frequency (50/60 Hz) and room temperature (20-25°C) conditions, its dielectric constant is approximately 2.2 to 2.4.

Synthetic Insulating Oils: Such as silicone oils, ester-based oils (synthetic esters, natural esters). Their dielectric constant may be slightly higher:

Silicone Oils: Typically around 2.5 to 2.8.

Ester-based Oils: Generally also in the range of 2.5 to 3.5. Natural esters (vegetable-based) may approach the upper limit or even be slightly higher (sometimes reaching 3.1 - 3.3).

Influencing Factors:

Temperature: The dielectric constant usually slightly decreases with increasing temperature.

Frequency: At power frequency (50/60 Hz) up to mid-range frequencies, the dielectric constant changes little. However, at very high frequencies (RF, microwave), it decreases.

Purity: Contaminants like moisture, impurities, and oxidation products can significantly affect (usually increase) the oil's dielectric constant. Therefore, the dielectric constant of oil in actual operation differs from that of new, pure oil. Measuring the dielectric constant (along with dielectric dissipation factor/loss tangent) is an important method (standard tests like ASTM D924, IEC 60247) for assessing the aging or contamination level of oil insulation.

Comparison:

Air dielectric constant ≈ 1.0

Paper/pressboard dielectric constant ≈ 3.5 - 4.5

Ceramic insulators ≈ 6 - 10

Water dielectric constant ≈ 80 (very high)

Summary:

For the most common mineral insulating oils, a typical, representative dielectric constant value is ≈ 2.3 (under standard conditions).

For synthetic oils, the range is slightly wider, but values generally around 2.5 - 3.0 are common.

Important Note:

This value is a range. The precise dielectric constant for a specific brand, batch, or oil under particular operating conditions should be based on the manufacturer's technical datasheet or actual results measured using standard test methods (e.g., ASTM D924). This is especially crucial for electrical design or insulation coordination calculations.

The dielectric constant itself is not the sole or most critical indicator of insulating oil performance. The dielectric dissipation factor / loss tangent is a better reflection of the oil's insulation efficiency under AC electric fields and its aging/contamination state.

Therefore, if you need a specific value for estimation or general understanding, 2.2 - 2.5 is a reasonable range for mineral insulating oil, and 2.3 is a commonly used representative value. For synthetic oils or specific applications, more precise data should be consulted.