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    A method to predict degree of polymerization value of oil-paper insulation using interfacial charge
    (IEEE, 2029-12-20) Singh, Vijeta; Mishra, Deepak; Baral, Arijit; Chakravorti, Sivaji
    Analysis based on Degree of Polymerization (DP) value is assumed to provide reliable regarding insulation condition. However, due to the invasive nature of testing, DP value based-analysis is not feasible in practical cases as it requires a paper sample from the in-service unit. Here, an attempt is made to develop a methodology to predict DP value based on non-invasive testing of oil-paper insulation. In this paper, it is found that DP value maintains a well-known relation with interfacial charge (estimated using Polarization Depolarization Current data) as both these parameters are related to the cellulosic condition. The proposed method successfully applied on various laboratory prepared samples containing different moisture content.
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    Effect of measurement temperature on interfacial charge freed from deep traps located at the interface of oil-paper insulation
    (IEEE, 2018-12-07) Dey, Debangshu; Sarkar, A.; Pal, Sayantan; Kumar, A.; Mishra, Deepak; Baral, Arijit; Haque, Nasirul; Chakravorti, Sivaji
    Accumulation of interfacial charge creates local field distortion during insulation response measurement. Furthermore, such localized field enhancement affects the interaction between polar compounds present within oil-paper insulation and in turn affects its aging process. By getting sufficient trap energy (normally by thermal oscillation) these trapped charges dislocate from their locations and contribute in depolarization current. These interfacial charges include charges dislocated from shallow and deep sites at interfacial region. The charge residing at deep locations takes more time to dislodge themselves compared to charge residing at shallow sites. As dipole present in cellulose has large relaxation time, there might be some relation between deep charge and paper insulation sensitive parameters. In this work, effects of temperature on deep traps are analyzed. Results reported in this paper shows that magnitude of charge freed from deep locations maintains a correlation with measurement temperature and paper conductivity.
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    Effect of interfacial charge on parameters considered for insulation diagnosis of power transformer
    (IEEE, 2017-12-02) Mishra, Deepak; Baral, Arijit; Pradhan, Arpan Kumar; Haque, Nasirul; Chakravorti, Sivaji
    Out of various analysis methods available Polarization Depolarization Current (PDC) measurement is extensively used for monitoring of oil-paper insulation health. In such case, Classical Debye Model (CDM) is used for analysis of recorded PDC data. CDM has limited capacity to simulate the effect of interfacial charge. Due to this interfacial charge, nonlinearity arises in the system which adversely affects the insulation. Conductivity difference between oil and paper is the main reason behind this interfacial charge which creates nonlinearity in system. In the present work, the effect of interfacial charge is analyzed on some of the performance parameters used for diagnosis of insulation like peak value of return voltage spectrum and paper conductivity.
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    Effect of charge accumulated at oil-paper interface on zero of transfer function formulated using classical debye model parameters
    (IEEE, 2017-11) Mishra, Deepak; Pradhan, Arpan Kumar; Baral, Arijit; Haque, Nasirul; Chakravorti, Sivaji
    PDC measurement and analysis is one of widely used tool for reliable diagnosis of power transformer insulation. In different reported methods of analysis it is considered that polarization current is composed of the current due to dipole movement and conduction current. Similarly the depolarization current is assumed to be composed of relaxation of dipoles. However when the dc conduction effect is removed from polarization current it is found the resulting current is not similar to the measured depolarization current. This deviation in both the currents show the presence of nonlinearity in the system. This nonlinearity arises due to migration of trapped charges that reside at the interfacial region of oil-paper insulation. The present paper shows the effect of such free charge on some important performance parameters like paper moisture and zero of Transfer Function of Classical Debye Model.
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    Assessment of interfacial charge accumulation in oil-paper interface in transformer insulation from polarization-depolarization current measurements
    (IEEE, 2017-06-03) Mishra, Deepak; Haque, Nasirul; Baral, Arijit; Chakravorti, Sivaji
    Accumulation of interfacial space charge in oil-paper interface is a critical issue in insulation diagnostics of transformers. This interfacial charge mainly accumulates due to the conductivity difference of oil and paper. Accumulation of interfacial charge leads to localized field enhancement, which further leads to partial discharges and acceleration in the aging of insulation. Therefore, from the point of view of transformer insulation diagnostics, assessment of interfacial charge is very important. However, it is not easy to estimate interfacial space charge behavior from the transformer diagnostics methods currently in use. In case of Polarization-Depolarization Current (PDC) measurement, a well known method for transformer condition monitoring, the effect of interfacial charge is reflected in the non-linearity of current response during polarization and de-polarization. During de-polarization process, a part of the interfacial charge accumulated during polarization period is absorbed by the electrodes producing a current, which is difficult to separate using conventional linear dielectric theory. In this paper, an attempt has been made to separate this current component from de-polarization current through considering charge de-trapping mechanism. Terming this current component as de-trapping current, its relationship with other parameters of transformer insulation is discussed. The developed methodology has been applied on several practical transformers. It was observed that the time constant of de-trapping current is related to the paper conductivity, oil conductivity, dissipation factor and age of the insulation.