Journal Articles

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Author: search.filters.author.Mishra, DeepakSubject: search.filters.subject.Current measurement

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Now showing 1 - 4 of 4
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    Influence of temperature on interfacial charge of power transformer insulation
    (The Institution of Engineering and Technology, 2019-06-21) Dutta, Saurabh; Mishra, Deepak; Haque, Nasirul; Pradhan, Arpan Kumar; Baral, Arijit; Chakravorti, Sivaji
    One of the popular methods for insulation diagnosis is measurement and analysis of polarisation–depolarisation current (PDC). During normal operation, charges get confined at the interface of oil–paper insulation. A part of these accumulated charges get absorbed in depolarisation current and contribute to the overall PDC data. The process through which charges are released from their confinement is known as charge de-trapping, which is highly influenced by thermal energy content of the insulation and hence by measurement temperature. In the current work, an effort has been made to investigate the effect of measurement temperature on de-trapped charge. Two samples are prepared and analysed in the laboratory at different temperatures for this purpose. This is followed by analysis of data collected from several real-life power transformers. Related analysis presented here suggests that measurement temperature of the system plays an important role in determining the amount of de-trapped interfacial charge during PDC measurement.
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    Compensating the effect of residual dipole energy on dielectric response for effective diagnosis of power transformer insulation
    (The Institution of Engineering and Technology, 2017-11-28) Mishra, Deepak; Baral, Arijit; Chakravorti, Sivaji
    Analysis of relaxation current is a widely accepted method for diagnosis of power transformer insulation. The accuracy of such diagnostic tool is dependent on insulation model parameters which are formulated using relaxation current. This implies that the accuracy and hence the reliability of existing insulation diagnosis methods indirectly depends on the accuracy of the recorded polarisation depolarisation current. Sometimes during field measurement relaxation current measurement equipment fails to record proper current, even after application of dc charging voltage. As per utilities, this primarily happens due to improper/loose connections (this cannot be avoided entirely due to the involvement of human factors) and such situation is usually followed by checking and rectifying improper connection. The analysis presented in this study shows that the polarisation current recorded immediately after rectifying the correction is inaccurate and leads to the erroneous diagnosis. Furthermore, it is observed that in these cases, the measured and calculated (using insulation model) values of performance parameters like dissipation factor, polarisation index, and paper-moisture differ by a large extent. This work is aimed at removing the effect of this residual dipole energy introduced during the improper connection phase.
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    Effect of charge accumulated at oil–paper interface on parameters considered for power transformer insulation diagnosis
    (The Institution of Engineering and Technology, 2018-01-02) Mishra, Deepak; Haque, Nasirul; Baral, Arijit; Chakravorti, Sivaji
    Polarisation and depolarisation current (PDC) measurement and analysis is one of the popular tools for effective diagnosis of power transformer insulation. Normally, it is assumed that polarisation current is the combination of the current due to dipole movement and conduction current. Similarly, the depolarisation current is only due to the relaxation of dipoles. However, it is found that after eliminating the effect of dc conduction from polarisation current the resulting current is not similar to that of measured depolarisation current. This shows some non-linearity is present in the system. This non-linearity occurs due to movement of trapped charge that resides in the interfacial region of oil–paper insulation. This study shows the effect of de-trapping charge on various performance parameters that are used for insulation diagnosis like paper moisture and dielectric dissipation factor (tanδ).
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    Estimation of paper conductivity from short duration polarisation–depolarisation current for diagnosis of power transformer
    (The Institution of Engineering and Technology, 2019-07-12) Mishra, Deepak; Haque, Nasirul; Baral, Arijit; Chakravorti, Sivaji
    The value of paper conductivity provides quantitative evaluation of transformer insulation health. However, proper identification of paper conductivity requires complete profile of polarisation–depolarisation current (PDC). PDC measurement being a time-consuming offline process generally takes several hours to complete. Furthermore, magnitude of PDC becomes very low at larger value of time, which makes it sensitive to changes in environmental conditions and field noise. Hence, accuracy of paper conductivity identification can be ensured by conducting multiple measurements. This in-turn prolongs shutdown time of equipment and become less advantageous to utilities. Here, a method is proposed which is capable of estimating paper conductivity using PDC data recorded for only 800 s. The proposed technique is tested on data collected from several real-life in-service transformers. In order to illustrate the accuracy of the proposed technique, paper conductivities (calculated from short duration PDC) were compared with those computed using PDC measured for 10,000 s.