Journal Articles

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Author: search.filters.author.Haque, Nasirul

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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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    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.
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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.
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    Use of interfacial charge for diagnosis and activation energy prediction of oil-paper insulation used in power transformer
    (IEEE, 2019-01-13) Mishra, Deepak; Dutta, Saurabh; Baral, Arijit; Haque, Nasirul; Chakravorti, Sivaji
    Activation energy is popularly used for the estimation of remaining life of transformer insulation. It is defined as the average rate of all reactions that happen with cellulose. Existing literature shows that the activation energy of oil-paper insulation can be obtained from polarization depolarization current (PDC) and return voltage measurement (RVM) data that are measured at a specific temperature. It is practically difficult to ensure the same measurement temperature for both PDC and RVM data. On the other hand, PDC data and its analysis get influenced by de-trapping current. This de-trapping current is generated by ionic charge carriers that get freed from trap sites during PDC measurement process. Formation of these trap sites is related to physical, chemical reactions that happen at oil-paper interface. This paper proposes a methodology which uses de-trapped charge, dislodged from deep and shallow traps, to assess insulation condition and for the prediction of activation energy. Thus, eliminating the need of RVM data. The proposed method is tested using data collected from various real-life in-service transformers.
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    Condition assessment of power transformer insulation using short-duration time-domain dielectric spectroscopy measurement data
    (IEEE, 2019-10-14) Mishra, Deepak; Baral, Arijit; Haque, Nasirul; Chakravorti
    Utilities prefer noninvasive methods for assessing the condition of power transformer insulation. Analysis of polarization-depolarization current (PDC) is one such popular method. One such analysis involves the estimation of trapped charge released from the interfacial region of oil-paper insulation. The literature shows that such charges can be reliably used for the diagnosis of transformer insulation. However, such analysis requires a complete profile of PDC. PDC measurement (an offline technique) takes a large amount of time (several hours) to complete. The magnitude of PDC data for a larger value of time is also sensitive to changes in environmental conditions and field noise as its magnitude is low. Hence, a reliable estimation of detrapped charge may require numerous PDC measurements. This situation is not convenient for utilities as it prolongs shut down time. In this article, a method has been proposed which is capable of estimating detrapping charge using PDC data measured for a short span of time. The proposed method is tested on data collected from several real-life in-service transformers.