Interpretation of WB-DFR Results of OIP Transformer Bushings

WB-DFR is an effective diagnostic method for evaluating the moisture content, oil conductivity, and health of the insulation in oil-impregnated transformer (OIP) bushings. DFR involves measuring response of a dielectric material over a range of frequencies. By varying the frequency of the applied field, information about the dielectric properties of the material can be obtained, including its permittivity, dielectric loss, and conductivity.

Concept of DFR

• The dielectric losses of insulating material vary with frequency and are attributed to conduction and polarization within the material. It can be understood from Figure 1.

  
  

  

Figure 1. (a) Parallel Circuit (conduction/charge flow) (b) Series Circuit (polarization/charge orientation

• Tan Delta for parallel circuit can be expressed as

and whereas for series circuit, it is

• The DFR demonstrates underlying polarization of diploes (or charges) and conduction mechanisms in OIP bushings, which is a mix of oil and solid insulation dielectric responses. The results are significantly affected by oil conductivity and moisture content in paper insulation.

Where ε* is a complex relative permittivity, ε′ is a real part of relative complex permittivity responsible for polarization, and ε′′ is an imaginary part of relative complex permittivity associated with conduction and dielectric loss.

• The DFR measures the dielectric response of insulation across a wide frequency range and models the frequency response alongside the measured data. Using an algorithm, it fits the collected data to a reference material model, aligning it with known dielectric properties. From the resulting curve, the instrument calculates key parameters such as moisture content in solid insulation, conductivity of oil and Tan Delta at 20 °C.

• The frequency-dependent dielectric losses in a transformer bushing are governed by multiple physical phenomena, including geometry effects (capacitance grading structure, layers of kraft paper insulation in OIP bushing, oil-paper interfaces in OIP bushing, oil conductivity, and moisture influence in cellulose-based insulation. Table 1 illustrates frequency-dependent loss mechanisms in bushings.

Table 1. Frequency-dependent loss mechanisms in bushings.

Frequency Range	OIP Bushing Dominant Factor
0.01 Hz to ~ 0.1 Hz	Geometry effects
~ 0.1 Hz to ~ 100 Hz	Oil conductivity
~ 100 Hz to ~1000 Hz	Moisture in paper, aging

Interpretation of DFR Results

• The dielectric characteristics of the materials and their polarization mechanisms determine the behavior of Tan Delta in oil-impregnated paper bushings over frequencies.

• At low frequencies (0.001 Hz to ~100 Hz), polarization mechanisms (interfacial polarization, dipole orientation, and ionic conduction) have enough time to respond to the applied electric field. At these frequencies, conductive losses (leakage currents) and polarization losses play a significant role, leading to an increase in Tan Delta.

• At high frequencies, the polarization mechanisms cannot respond to the rapid changes in the electric field, resulting in reduced energy dissipation and a lower Tan Delta. Furthermore, moisture and ionic impurities in oil and paper considerably increase the Tan Delta. These mechanisms dominate at lower frequencies because ions have more time to migrate and align with the field.

• At higher frequencies above 100 Hz and as they approach 1 kHz, electrons in the materials (oil and paper) start to respond to the alternating electric field. This response results in energy dissipation, leading to a subtle rise in Tan Delta.

Evaluation of DFR Results

The assessment limits of moisture and conductivity are mentioned in Table 2.

Table 2. Assessment limits of moisture and conductivity in OIP bushing.

	Assessment limits
Moisture (%)	< 0.5% Acceptable	> 0.5% to < 1.0% Investigate	> 1.0% Not acceptable
Oil Conductivity at 25 °C , pS/m	< 0.37 pS/m Acceptable	> 0.37 pS/m Not acceptable	-----

• CIGRE-445 gives indicative limits of Tan Delta in fresh and old bushings for frequencies of 15 Hz, 50/60 Hz, and 400 Hz as shown in Table 3.

Table 3. Indicative limit values of Tan Delta for condenser bushings at 20°C.

Frequency Hz	Tan Delta of OIP Bushing
	New	Service aged
15 Hz	< 0.5%	< 0.7%
50/60 Hz	< 0.4%	< 0.5%
400 Hz	< 0.5%	< 0.7%

In a technical paper, experts have proposed a limit for Tan Delta at 1Hz (corrected to 20°C) >= 0.2% to <= 0.4%.

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