Alternative Liquid Dielectrics for High Voltage Transformer Insulation Systems. Группа авторов

Figure 2.5 Comparison of ACBDV values of different oil samples.

Figure 2.6 (a) Parallel representation of the insulation. (b) Phasor diagram for parallel circuit. (c) Series representation of the insulation. (d) Phasor diagram for series circuit.

(2.2)

The power loss P can be represented in Eq. (2.3),

(2.3)

       2.4.1.2.2 Series Representation

      The application of an AC sinusoidal voltage source to the lossy dielectric in series combination with resistance R_s and capacitance C_s gives two components of voltage and 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)

      So, the loss tangent may be written as:

      (2.5)

      The power loss is given by:

      (2.6)

      (2.7)

      (2.8)

      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

The water content in any dielectric liquid affects the performance of the apparatus containing it. The water holding limit of VO is much higher than that of MO, which is an added advantage in case of NEOs. It also absorbs the moisture present in solid insulation as compared with MO, thus keeping the paper dry and slows down its degradation process in the long run. Natural ester fluids undergo hydrolysis at higher temperatures, consuming the available

Скачать книгу