International Journal of Applied Power Engineering (IJAPE) Vol. 14, No. 2, June 2025, pp. 373~381 ISSN: 2252-8792, DOI: 10.11591/ijape.v14.i2.pp373-381  373 Journal homepage: http://ijape.iaescore.com/ Effective assessment of power transformer insulation using time-varying model without temperature constraints Sachin Yashawant Sayais 1 , Chandra Madhab Banerjee 2 1 Department of Electrical Engineering, Medi-caps University, Indore, India 2 Department of R&D, Electrical Engineering, Philips, Pune, India Article Info ABSTRACT Article history: Received Mar 26, 2023 Revised Nov 12, 2024 Accepted Nov 28, 2024 This paper proposes a method for insulation diagnosis using the time- varying model. The parameters of the stated model are unique and can be recognized by the polarization current. Several methodologies have been reported for insulation diagnosis using various insulation models. However, moisture information without considering the effect of measurement temperature is bound to provide inaccurate result. Temperature significantly affects the dielectric response of materials. As the temperature rises, various important changes take place. Increased temperatures can boost molecular mobility, resulting in higher polarization, which in turn affects the material's dielectric constant and loss. Additionally, higher temperatures typically raise conductivity due to improved charge carrier mobility, further influencing the dielectric response. Hence, the effect of measurement temperature on insulation diagnosis is discussed in this paper so that responses recorded at different temperatures can be effectively compared. The proposed methodology for determining insulation state is tested using data from real- life power transformers. Keywords: Aging Insulation condition Insulation model Oil-paper insulation Power transformer Temperature This is an open access article under the CC BY-SA license. Corresponding Author: Sachin Yashawant Sayais Department of Electrical Engineering, Medi-caps University AB Road Pigdamber, Rau, Indore, Madhya Pradesh 453331, India Email: sachinsayais@gmail.com 1. INTRODUCTION Insulation models have been crucial in advancing various non-invasive diagnostic techniques [1]-[3]. Recent developments show that RC-based insulation models are essential for analyzing temperature effect [4]-[6], non-uniform aging [7], and the influence of copper-sulphide [8]. Simple models like the conventional Debye model (CDM) need modifications before they can represent in-situ transformer insulation affected by practical conditions [9], [10]. Hence, a few modifications were proposed which resulted in the formulation of the modified Debye model (MDM) and the modified Maxwell model (MMM) [9], [10]. However, these models are essentially modifications of the base structure CDM. CDM assumes that the insulation comprises a finite number of dipole groups [9], whose distinct properties can be approximated by series RC branches for a practical transformer the insulation response at a given time instant is influenced by several dipole groups, whose number and properties vary with the unit's operating age. As the number of dipole groups to be considered during CDM formulations is not fixed, the overall count of branches in a CDM for a specific unit is not fixed [3]. Non-unique branch parameters in CDM (and other modified versions of CDM) affect the accuracy and reliability of all available insulation model-based diagnosis methods. Thus, it is only logical to have an insulation model that analyses insulation response by considering the collective behavior of all dipole groups rather than concentrating on a few distinct dipole groups. Such a model, by definition, will be unique for a given transformer and hence will be better suited for developing reliable