Gapless metal oxide surge arresters(MOSA)have been available in the market for many years since they were first introduced in the 1970u27s. The aim of this study is to investigate some reliable diagnostic techniques to assess the condition of a metal oxide surge arrester when subjected to severe lightning strikes in the field. A number of non-destructive and destructive diagnostic techniques for Metal Oxide Surge Arrester (MOSA) are discussed in this paper. The non-destructive techniques include the standard 1 mA reference voltage, lightning impulse discharge residual voltage and a number of modern diagnostics based on polarization methods: Return voltage and polarization/depolarization current measurements. In order to observe, analyze and correctly explain the degradation phenomena, a number of destructive techniques based on microstructure observation are also conducted. The techniques include optical microscopy, scanning electron microscopy, X-Ray diffraction and energy dispersive spectrometry. The single and multi-pulse currents of 8r20 s wave shape were used to artificially degrade the MOSA. The before and after diagnostic results of the non-destructive and destructive techniques are presented and interpreted to understand the aging mechanism in MOSA. The importance of modern non-destructive electrical diagnostics based on polarization methods is validated by test results and is highlighted in detail in this paper. Finally the correlation of the results of different diagnostic techniques with each other and with the results of standard techniques is discussed.
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