Statistical Analysis of Breakdown Voltages in Virgin and Aged LDPE Using Johnson S B and Weibull Distribution NURUL A. Bani a * , ZULKURNAIN Abdul-Malek b and HUSSEIN Ahmad c Institut Voltan dan Arus Tinggi, Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia. a nurulaini.kl@utm.my, b zulkurnain@utm.my, c hussein@utm.my Keywords: Electroluminescence, Weibull distribution, Johnson SB distribution, Insulation material. Abstract. Polymeric material such as low density polyethylene (LDPE) has been used for decades as insulating material. Any polymeric material will experience degradation after prolonged application of high electrical stresses. Deeper understanding of the long term electrical degradation of the insulating material is necessary to predict the life of high voltage cable. Electroluminescence method (EL) is used to detect the breakdown voltages of thin film LDPE. This method utilizes a Peltier cooled electron multiplying charge coupled device (EMCCD) camera to detect the breakdown of the sample. Statistical distribution of the AC breakdown voltages of 100μm virgin and aged LDPE has been analysed. Comparison for the best fitted distribution was made for Weibull distribution and Johnson SB distribution using Anderson-Darling (A2) goodness-of-fit and Kolmogorov-Smirnov (D) goodness-of-fit (GOF). Johnson SB is rarely used in high voltage engineering application. The probability density function (PDF) and the cumulative density function (CDF) for both distributions are defined in this article. The statistical parameters used are estimated based on Maximum Likelihood Estimation (MLE) for both distributions. Based on the statistical analysis, it is observed that Johnson SB provide better fitting than Weibull distribution with lower fitting error and that 3-parameter Weibull is much better fitting than 2-parameter Weibull distribution for most cases. It is also found that the median breakdown voltage of LDPE samples decreases with increasing aging temperature. Introduction An electrical system depends highly on the performance and reliability of its insulation system in order to carry out its designated purposes for a very long period of time. A thorough investigation on the dielectric properties of the polymer is necessary so that its utilization can be optimized. It has been known that ageing affects the morphology of a material and consequently changes its properties. Ageing affects the breakdown and dielectric strength [1] and also the breakdown voltage value [2]. A comprehensive review of fault detection and diagnostic techniques in power system distribution network have been conducted in [3]. One alternative but reliable method known as electroluminescence (EL) method is used to investigate the electrical properties of polymeric material. In this method, light is emitted when polymer atoms are excited to new excitation state as a result of dielectric material being subjected to high electric field. The emission of light can be observed through electron multiplying charge coupled device (EMCCD) camera. Statistical techniques have been extensively applied to dielectric failure data [4] and can be used to determine the voltages that the insulating material most likely to breakdown at from the analysis of the breakdown voltage distribution [5]. Weibull distribution has been widely used as a tool to analyse the electrical performance of high voltage engineering device and insulation performance of electrical equipment [6] for its reliability to perform in predefined and extreme values conditions. However, several other statistical distributions with lower fitting error have been opted