0885-8977 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TPWRD.2019.2892739, IEEE Transactions on Power Delivery 1 Abstract--Activation energy is popularly used for 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 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 are related to physical, chemical reactions that happens at oil-paper interface. The present paper proposes a methodology which uses de- trapped charge, dislodged from deep and shallow traps, to assess insulation condition and for 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. Index Terms—Space charge, Dielectric measurement, Power transformer, Dielectric materials, Conductivity, Moisture measurement I. INTRODUCTION IELECTRIC spectroscopy based non-invasive insulation diagnosis techniquesare popular among utility providers. As far as insulation condition is concerned, available literatures [1-2] show that both Time Domain Spectroscopy (TDS) and Frequency Domain Spectroscopy (FDS) based methods provide same information. Measurement of FDS data is time consuming especially in the low frequency region.As a result, PDC analysis based non-invasive diagnosis approach has gained popularity in recent times [1-3]. Typical analysis procedure of PDC data involves identification and thereafter interpretation of insulation D. Mishra is with the Electrical Engineering Department, Indian Institute of Technology (Indian School of Mines), Dhanbad, India (e-mail: deepak2010urs@gmail.com). S. Dutta is with the Electrical Engineering Department, Indian Institute of Technology (Indian School of Mines), Dhanbad, India (e-mail: dutta.saurabh2@gmail.com). A. Baral is with the Electrical Engineering Department, Indian Institute of Technology (Indian School of Mines), Dhanbad, India (e-mail: arijit@iitism.ac.in ). N. Haque is with the Electrical Engineering Department, Jadavpur University, India (e-mail: nasirul07@gmail.com). S. Chakravorti is currently the Director, National Institute of Technology, Calicut, India, on lien from Jadavpur University, Kolkata, India (e-mail: s_chakrav@yahoo.com). model parameters. Among available R-C network-based insulation models, Classical Debye Model (CDM) is perhaps the most widely used circuit [3]. It is reported that measurement temperature significantly affects the measured dielectric spectroscopy data [4-5]. PDC data recorded from the same unit at different time instants might be influenced by two distinctly different measurement temperatures. Analysis of such measurement data separately lead to inaccurate assessment of aging that might have happened between the two measurements. Research findings show that CDM parameters, obtained using insulation response (measured at two different temperatures), are related by Activation Energy E a [6-8]. This implies that PDC data recorded at two different measurement temperatures can be successfully compared if the activation energy of the system is known. Activation energy is defined as the average rate of all the reaction (physical and chemical) that involvessolidinsulation. Satisfactory information regarding remaining life of transformerinsulationcan also be obtained using E a [8-10]. It is reported by researchers that value of E a generally lies within a range of 85kJ/mol to 120kJ/mol [9]. Normally its value is considered equal to 111 kJ/mol for real-life transformers [7], [9].Literature shows that E a is not constant for a given unit but depends on paper moisture (%pm)[8]. Its dependency on moisture content indicates that assuming E a to be equal to 111kJ/mol is not always accurate. Unfortunately, very few literatures exist that tries to evaluate value of E a using data obtained through non-invasive testing. One such technique is reported by two authors of the present paper [9]. In [8-9], E a is estimated using an iterative technique that requires PDC and RVM data measured at a specific temperature. Both PDC and RVM data measurement are time-consuming processes. Furthermore, it might be practically difficult to ensure same measurement temperature for both PDC and RVM [11]. The above discussion suggests that it will be beneficial to the utilities if E a identification and insulation diagnosis can be done using a single type of non-invasive measurement, preferably PDC. Apart from measurement temperature, PDC data and hence, its interpretation also gets affected by de-trapped charge [12-13]. It is reported [3] that polarization current is the combination of dipole current (generated due to dipole orientation) and conduction current. Similarly, depolarization current is expected to have current only due to the relaxation of dipoles. If the conduction current is calculated accurately and subtracted from polarization current then the resultant current should represent only dipole Use of Interfacial Charge for Diagnosis and Activation Energy Prediction of Oil-paper Insulation used in Power Transformer D. Mishra, Student Member, IEEE, S. Dutta, Student Member, IEEE, A. Baral, Senior Member, IEEE, N. Haque, Member, IEEE, S. Chakravorti, Senior Member, IEEE D