Dielectric Dissipation Factor as a Diagnostic Tool for Insulating Oil Degradation in Transformers

Transformers rely on insulating oil for both electrical insulation and thermal management. Over time, the oil degrades due to thermal stress, oxidation and contamination leading to reduced dielectric performance and increased risk of failure. Traditionally oil analysis methods include dissolved gas analysis (DGA), interfacial tension (IFT), and acidity tests. However, the dielectric dissipation factor also known as tan delta or power factor offers direct assess the quality and performance of insulating oils, making it a valuable indicator of insulation health.

What is Dielectric Dissipation Factor (DDF)?

The dielectric dissipation factor quantifies the energy loss in an insulating material when subjected to an alternating electric field. Ideally, the current through a perfect insulator leads the voltage by 90°. However, impurities such as moisture, oxidation by-products, and contaminants introduce a resistive component, causing the current to deviate from this ideal phase angle. The tangent of this deviation angle (δ) is called the dissipation factor.

Measurement Standard

Two primary standards are used globally for measuring DDF:

• ASTM D924 – Standard Test Method for Dissipation Factor and Relative Permittivity of Electrical Insulating Liquids. Test performed at 2 different temperatures (25 oC and 100 oC)

• IEC 60247 – Measurement of relative permittivity, dielectric dissipation factor, and DC resistivity. Test performed at 90 oC only

Factors Affecting DDF in Insulating Oil

Several factors influence the dissipation factor of insulating oils:

1. Temperature: Higher temperatures increase molecular activity, leading to greater dielectric losses. DDF tends to rise with temperature, especially at low frequencies.

2. Moisture Content: Water in oil reduces breakdown voltage and increases DDF. Moisture ingress can occur due to poor sealing or aging of the insulation system.

3. Oil Aging: Oxidation and thermal stress degrade oil quality, increasing its conductivity and DDF. Aging also leads to the formation of acidic compounds and sludge.

4. Contaminants and Conductivity: Presence of conductive particles or chemical degradation products increases the resistive component of the current, thereby raising the DDF.

Significance in Transfomer A high DDF indicates poor insulation quality and increased energy losses. It means that DDF may influenced the power factor or insulation resistance of transformer windings. Regular monitoring of DDF helps in:

• Preventive maintenance

• Quality control during oil acceptance

• Condition assessment of aging transformers