Transformer Oil Tan Delta

Transformer oil tan delta is a parameter with a wide range of applications for assessing the condition of dielectric fluids. Transformer oils, including highly refined mineral oils, silicone oils, and natural ester-based oils, are used to insulate current-carrying parts of electrical equipment. Their primary functions include providing dielectric properties and facilitating heat dissipation. These oils are widely used in transformers, capacitors, and cable impregnation.

Understanding the Concept of Dielectric Losses

All substances, regardless of their composition, interact with electric fields under certain conditions. Materials that contain fewer than 100 million free positive and negative charges per cubic inch are classified as dielectrics. These materials serve as insulating components in electrical and electronic systems. Their interaction with electric fields is quantitatively evaluated to determine their effectiveness as insulating materials.

When an electric field is applied to a dielectric material, it induces bulk heating within the material. This phenomenon results in what is known as dielectric losses, which represent a specific measure of interaction between the material and the electric field. Dielectric losses occur regardless of whether the charges generating the electric field are positive or negative, or whether their polarity changes over time. Even at low charge densities, the presence of charges within a dielectric material leads to leakage currents that penetrate the material, thereby generating heat.

The relationship between the electric field and the properties of a dielectric material is critical for evaluating its performance as an insulator. If the charges generating the electric field remain stable, the electric field induces a current through the dielectric sample. The magnitude of this current depends on the resistance of the dielectric material. However, it is important to note that, similarly to a metal rod that eventually breaks when bent repeatedly at the same point, dielectric materials are also vulnerable to deterioration caused by alternating electric fields. These fields affect the molecular structure of the dielectric material, potentially leading to breakdown over time.

What is Transformer Oil Tan Delta, and What Does it Indicate?

Transformer oil tan delta is used to measure dielectric losses in transformer oil. Dielectric losses occur when an electric current flows through the dielectric material in the oil as a result of an applied electric field. In order to accurately quantify these losses, it is necessary to assess the behavior of the dielectric material under both direct current (DC) and alternating current (AC) conditions.

In this context, the dielectric behavior is capacitive, meaning that the current is out of phase with the voltage. This phase shift, represented by an angle (φ), indicates the timing difference between the applied voltage and the resulting current flow. The total phase shift includes an additional angle (δ) resulting in a nonzero tangent of the loss angle, known as tan delta (tan δ).

In an ideal dielectric, the phase shift would be 90°, and the loss angle (δ) would be zero, indicating no energy loss. However, in real materials, dielectric losses are inevitable; therefore, the loss angle (δ) is always greater than zero. The tangent of this angle, tan delta (tan δ), is used as a quantitative measure of dielectric losses, helping to assess the efficiency of dielectric materials in applications where electrical insulation and minimal energy dissipation are critical.

The dielectric loss tangent is a critical parameter of insulating materials. The higher the tan delta (tan δ), the greater the dielectric losses, which can lead to thermal breakdown of the insulation. Factors such as moisture, ionization of entrapped gas, and contamination can all increase the dielectric loss tangent. Additionally, the value of tan δ is influenced by the insulation temperature, test voltage, and frequency.

Dielectric losses in transformer oil are directly related to its composition, particularly the presence of impurities such as resins, soaps, and water. Transformer oils typically operate at temperatures ranging from 68°F to 257°F (20°C to 125°C), and under these conditions, the tan delta remains relatively low.

Resins, both neutral and acidic in nature, are a primary source of dielectric losses in transformer oil. These resins can remain in the oil due to incomplete filtration or develop as the oil ages due to oxidation and polymerization reactions. Such resins are poorly soluble in oil and tend to form colloidal particles, which are a major cause of electrophoretic conductivity. For example, the presence of just 0.5% resins in the oil can increase the tan delta by a factor of 20.

Soaps, formed by reactions between metals and organic acids (including those occurring during the oil aging process), can also contribute to dielectric losses. These soaps do not dissociate into ions in hydrocarbon solvents; however, when present in sufficiently high concentrations, they can cause catastrophic increases in dielectric losses.

Depending on the oil grade and operating conditions, soaps may exist either as a true solution or in colloidal form, and in both cases, they affect the tan delta. Notably, when oils containing soaps are heated to 212°F (100°C), both sharp increases and decreases in tan delta may be observed. If the oil is subsequently cooled, the tan delta may continue to decrease, and the initial curve will not be reproducible during subsequent heating. This behavior indicates a steady alteration in the colloidal state of soap.

Water can exist in transformer oil in either dissolved or emulsified forms. The dielectric losses caused by water are determined by the water state rather than the total water content. Water in true solution does not impact the dielectric losses  of oil, but when present as fine emulsions, it can sharply increase these losses. The threshold at which water changes from dissolved to emulsified state depends on the composition of oil. In oils with the same water content, one may dissolve the water, while another may form an emulsion. The latter will considerably increase the tan delta value.

Therefore, nearly all oxygen-containing compounds (such as alcohols, acids, phenols, and resins), metal salts, organic acid salts (soaps), and water can noticeably increase the tan delta once they reach a certain concentration — particularly near the solubility limit, where a secondary phase forms as microemulsions or colloids. Heating the oil can disrupt these formations, decreasing the tan delta.

FLD T Transformer Oil Tan Delta Tester

The FLD T tester is designed to measure the dielectric loss tangent, electrical permittivity and resistivity of transformer oils and other dielectric fluids at a frequency range of 45–65 Hz. It operates according to industry standards such as IEC 60247-2004-02, VDE-0380-2:2005_01, ASTM D924-08, and ASTM D1169-02. Transformer oil tan delta tester is used to assess the quality and performance of transformer oils, ensuring their compliance with the required specifications for safe and efficient operation.

This device is particularly useful for utility companies and maintenance teams that need to monitor the condition of dielectric fluids and ensure that they are within acceptable limits for optimal performance.

The maximum permissible values of oil tan delta (tan δ) for new equipment are defined by the relevant standards or equipment specifications. For equipment in operation, these values are determined by the manufacturer’s documents and the applicable regulatory requirements of the end user.