2014 International Conference on Lightning Protection (ICLP), Shanghai, China Current Distribution in MOV Element Stressed by 4/10 μs Impulse Current Naoyuki Tsukamoto Otowa Electric Co., Ltd. Hyogo, Japan n-tsukamoto@otowadenki.co.jp Shunsuke Imato Doshisha University Kyoto, Japan duo0316@mail4.doshisha.ac.jp Yoshihiro Baba Doshisha University Kyoto, Japan ybaba@mail.doshisha.ac.jp Masaru Ishii The University of Tokyo Tokyo, Japan ishii@iis.u-tokyo.ac.jp Abstract— This study focuses on current distribution in a metal oxide varistor (MOV) element stressed by 4/10 μs impulse current studied using the finite-difference time-domain (FDTD) method. After 100 kA shots, V 1mA/ mm values , which indicate deterioration of nonlinear characteristics of MOV, in edge parts of MOV elements decreased more than those in center parts. FDTD analysis was conducted in two ways, namely 1-D and 3-D models. The varistor is represented with many nonlinear cubic cells of 3 mm × 3 mm × 3 mm. It turns out from the 3-D FDTD analyses that the skin effect is insignificant for a lightning current having a rise time of 4 s. However, if there is small difference in the nonlinear characteristic of a ZnO element between its periphery and other parts, the distributions of current density are considerably affected, which may result in enhanced degradation of a varistor element in its periphery. Keywords-component; MOV, ZnO, varistor, surge withstand capability, SPD I. INTRODUCTION Lightning protection using surge arresters and surge protective devices (SPD) plays an important role for improving reliability in power delivery. Metal oxide varistors (MOVs) are their important components. In order to satisfy diverse needs for SPDs, various tests using impulse currents to confirm thier performance are difined by standards. The performance is evaluated by the peak current value, the charge value and the number of repetitive shots. It has been reported that the failure modes and degree of degradation depend on the test current waveform (1) . The authors reported on the behavior of varistor voltage, V 1mA /mm, stressed by a single shot of impulse current with various waveforms, and discussed its degradation mechanism (2)(3) . Different current waveforms with the same charge transfer of an impulse bring about different behavior of varistor voltage drop, an indicator of degradation of MOV. For example, at 10/350 μs waveform, varistor voltage drop occurred in the region above 0.2 C/cm 2 in charge transfer and showed -10 % change above 0.35 C/cm 2 . On the other hand, at steep waveforms like 4/10 μs or 8/20 μs, the degradeation move forward rapidly. Varistor voltage drop occurred at 0.03 C/cm 2 and reached -10 % at above 0.1 C/cm 2 of charge transfer. In the present study, impulse current of 4/10 μs is mainly used because it is for preconditioning of operating duty test of surge arresters according to IEC standards. (4) By the investigation using spot electrodes for MOV elements and analysis of impulse current distribution by two ways of FDTD anayisys, the authors try to clarify the mechanisms of degradation of MOV by impulse currents. II. INVESTIGATION ON TESTED MOVS 4/10 μs, impulses were used in this study and the test procedure was based on IEC standards. (4) Two MOV samples having the same chemical composition and size (diameter 41 mm and height 36 mm) were prepared. Then, 2 shots of 100 kA were applied to one sample. The 100 kA current impulse is typical value in the operating duty test for distribution-class surge arresters. In order to investigate distribution of degree of degradation in a MOV element after application of impulse currents, the spot electrode method was employed. MOV elements were horizontally sliced into 3 disks to check the deviation in the height direction. Then, numerous silver electrodes having a 2- mm square area were attached on the upper and lower surfaces by silver paste printing on sliced pieces of MOV elements. After drying, the voltages at DC 1mA are measured for each spots and V 1mA /mm values were obtained for each part. Table 1 shows the result of V 1mA /mm measurements for spot electrodes at sliced MOV elements both with and without application of 4/10 μs impulse currents.