Alternative Liquid Dielectrics for High Voltage Transformer Insulation Systems. Группа авторов
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СКАЧАТЬ in Figure 2.6a and b, which is related to the DDF as given in (2.2),

Schematic illustration of the comparison of ACBDV values of different oil samples.

       2.4.1.2.2 Series Representation

      The application of an AC sinusoidal voltage source to the lossy dielectric in series combination with resistance Rs and capacitance Cs gives two components of voltage upper V Subscript upper R Sub Subscript normal s and upper V Subscript upper C Sub Subscript normal s across the resistor and capacitor, respectively, as seen in Figure 2.6c. The power factor angle is represented by φ in Figure 2.6d. The cotangent of the power factor angle is given by the equation:

      (2.4)cotangent phi equals StartFraction cosine phi Over sine phi EndFraction equals StartFraction cosine left-parenthesis pi slash 2 minus delta right-parenthesis Over sine phi left-parenthesis pi slash 2 minus delta right-parenthesis EndFraction equals cotangent left-parenthesis pi slash 2 minus delta right-parenthesis equals tangent delta

      So, the loss tangent may be written as:

      (2.5)tangent delta equals StartFraction upper V Subscript upper R Sub Subscript normal s Subscript Baseline Over upper V Subscript upper C Sub Subscript normal s Subscript Baseline EndFraction equals StartStartFraction upper I upper R Subscript normal s Baseline OverOver StartFraction upper I Over omega upper C Subscript normal s Baseline EndFraction EndEndFraction equals omega upper R Subscript normal s Baseline upper C Subscript normal s

      The power loss is given by:

      (2.6)upper P equals upper I squared upper R Subscript normal s Baseline equals left-parenthesis omega upper C Subscript normal s Baseline upper V Subscript upper C Sub Subscript normal s Subscript Baseline right-parenthesis squared upper R Subscript normal s

      (2.7)equals left-parenthesis omega upper C Subscript normal s Baseline upper V Subscript upper C Sub Subscript normal s Subscript Baseline right-parenthesis squared StartFraction tangent delta Over omega upper C Subscript normal s Baseline EndFraction

      (2.8)equals upper V Subscript upper C Sub Subscript normal s Baseline squared omega upper C Subscript normal s Baseline tangent delta

      From both representations, the loss is expressed in terms of the capacitance (C) and the loss angle (tan δ ). Therefore, the factor Ctan δ is an important parameter to ascertain the quality of an insulating medium.

      For a nonideal dielectric, the δ angle is greater than 0°. This happens because of the existence of some contaminants and some conducting channel is formed in the dielectric. An increase in the value of tan δ indicates that the oil is contaminated because of oxidation and aging and ingress of excess moisture. Generally, it is not possible to have an ideal dielectric in nature. Due to some kind of impurity, a dielectric always has a loss angle less than 90°. The temperature significantly increases when the power dissipation is higher. The lower the DDF value, the better the integrity of the oil. Higher DDF values indicate the presence of contaminations in the oil. Formation of sludge and water leads to decay in dielectric properties in due course of time and increases the DDF value. Better dielectric properties can be achieved when minimum moisture is contained in the insulating oil. Many studies have shown that with aging, the dielectric properties degrade and this results in the increase of the DDF [55]. Oxidation reaction in the oil produces a variety of by‐products, in which some of them further form acids, sludge, and suspended particles [56]. DDF is a measure of the polarity of the oil molecules, and so NEOs cannot be directly equated with MOs, because the nature of the polarity in the triglyceride molecules of NEOs is different than MOs.

      2.4.1.3 Dielectric Constant

      Dielectric constant of insulating oil is a significant parameter of the dielectric spectroscopy analysis. The degradation of these properties makes the oil inappropriate to be used as an insulating medium. It is observed from many studies that the dielectric constant of NEOs is greater than that of the MO. This is because of the triglycerides present in the NEOs, which makes the ester oils strong polar dielectric.

      2.4.2 Chemical Properties

      2.4.2.1 Water Content