IEEE TRANSACTIONS ON MAGNETICS, VOL. 54, NO. 3, MARCH 2018 7204904 Finite-Element Analysis for Surface Discharge Due to Interfacial Polarization at the Oil-Nanocomposite Interface Jin-Hyun Choi 1 , Su-Hun Kim 1 , Kyunghoon Jang 2 , Masayuki Hikita 2 , and Se-Hee Lee 1 1 Department of Electrical Engineering, Kyungpook National University, Daegu 41566, South Korea 2 Department of Electrical and Electronic Engineering,Kyushu Institute of Technology, Kitakyushu 804-8550, Japan The propagation of surface discharge due to interfacial polarization was numerically analyzed at the oil-nanocomposite interface using fully coupled finite-element analysis incorporating the relative permittivity from experiments. To improve the insulation ability, a new nanodielectric insulating material has been proposed in which a pressboard is coated with epoxy resin mixed with silica nanoparticles; this nanocomposite material can enhance the breakdown voltage in power systems with a certain level of silica nanoparticles. To specify the electric breakdown performance of this nanocomposite material, we measured the bulk relative permittivity of epoxy resin containing different percentages of silica nanoparticles on the pressboard. Surface discharge, or creepage discharge, tends to propagate along the solid–liquid interface and then leads to flashover. The mechanism of surface discharge, therefore, is a critical issue for understanding the dielectric breakdown strength in solid–liquid interface problems. To quantitatively analyze and explain the characteristics of surface discharge, here, the fully coupled finite-element analysis technique has been applied and tested with various relative permittivity values of nanocomposite materials. This phenomenon has been simulated using the fully coupled governing equations using Poisson’s equation for electric field and charge continuity equations, including surface charge accumulation for charge transport. After verification of our numerical setup in a conventional oil-pressboard system, a needle-bar electrode system was proposed and applied to the analysis of surface discharge propagation for the new nanocomposite materials with bulk dielectric permittivity. The propagation speed at the oil-nanocomposite interface was compared with different percentages of nanosilica. Finally, the physical mechanism of surface discharge due to the interfacial polarization was analyzed with the space, bounded, and surface charge densities at the oil-nanocomposite interface based on the numerical results. Index Terms— Charge transport, oil-nanocomposite interface, permittivity difference, surface charge density. I. I NTRODUCTION E LECTRIC discharge phenomena are very complex and involve many influencing factors, such as purity of the insulation, physicochemical components, and thermal conduc- tivity. Electric insulation is regarded as a critical component in electric power apparatus. In the case of the power transformer, most failures of insulating systems are closely related to the breakdown of solid insulators [1]–[2]. Cellulose materials, such as pressboard, have excellent insulating capability in min- eral insulating oil, are low in cost, and have reasonably good performance. The surface discharge, or creepage discharge, on pressboard insulation is considered as one of the failure modes for oil-immersed transformers. This surface discharge tends to propagate along the liquid–solid interface, leading to flashover [3]. Until now, studies on streamer propagation and surface discharge phenomena in insulating liquids have been con- ducted primarily with experimental methods. Recently, numer- ical methods have been developed for analyzing this surface discharge characteristic with the multiphysics analysis tech- nique [4]. Hence, to improve the electric breakdown strength, a new nanodielectric insulating material has been proposed in which a pressboard is coated with epoxy resin mixed with silica nanoparticles. This nanocomposite material can Manuscript received June 27, 2017; revised August 22, 2017; accepted September 6, 2017. Date of publication October 4, 2017; date of current version February 21, 2018. Corresponding author: S.-H. Lee (e-mail: shlees@knu.ac.kr). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2017.2751066 enhance the breakdown voltage with a certain quantity of silica nanoparticles. To analyze the electric breakdown performance, therefore, we adopted the multiphysics analysis technique for this oil-nanocomposite system. This numerical technique has been implemented with the fully coupled finite-element method, including Poisson’s equa- tion for the electric field and charge transport equations for positive ions, negative ions, and electrons. After verification of our numerical setup in a conventional oil-pressboard system, the needle-bar electrode system was proposed and applied to analyzing the surface discharge propagation for the new nanocomposite materials with bulk dielectric permittivity. The propagation speed at the oil-nanocomposite interface was compared with different percentages of nanosilica. Finally, the physical mechanism of surface discharge due to the interfacial polarization was analyzed with the space, bounded, and surface charge densities at the oil-nanocomposite interface based on the numerical results. II. RELATIVE PERMITTIVITY FOR NANOCOMPOSITE MATERIAL With different weight percentages (wt%) of nanosilica, the dielectric strength was tested with the partial discharge initia- tion voltage with a needle-plate electrode system, as shown in Fig. 1. From this breakdown test, a certain amount of nanosilica showed the best result for the dielectric breakdown strength. To understand this mechanism, first, the relative permittivity of nanocomposite material was measured with the different wt% of nanosilica. After preparation of nanocom- posite materials, we immersed them in the transformer oil for 3 and 24 h, and the dielectric permittivity was plotted in Fig. 2. 0018-9464 © 2017 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.