Research Publications

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Author: search.filters.author.Haque, NasirulSubject: search.filters.subject.Dielectric measurement

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    Feasibility of using normalized de-trapped charge for diagnosis of power transformer insulation
    (IEEE, 2018-10-07) Mishra, Deepak; Baral, Arijit; Haque, Nasirul; Chakravorti, Sivaji
    In order to use de-trapped interfacial charge as a performance parameter for diagnosis, it is necessary to reduce the influence of insulation geometry. Available literature shows that this can be done by considering dc insulation resistance R 0 as the normalization factor. Such normalization may not be always practically feasible as information regarding R 0 requires measurement of temperature sensitive low amplitude current. In this paper it is shown that geometric capacitance C 0 (instead of R 0 ) can be used as an effective normalization parameter to reduce effect of insulation geometry. Effect of normalizing freed interfacial charge using C 0 and R 0 and its subsequent effect on insulation diagnosis are presented in this paper.
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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.