200 zyxwvutsr 1 Annual Report Conference on Electrical Insulation and Dielectric Phenomena zyxw Optimisation of a Numerical Model for Analysis of Partial Discharge Phenomena in a Flat Cavity. zyxw R. Candela, R. Schifani, P. Romano Dipartimento di Ingegneria Elettrica, Universitci di Palermo. Palermo, ITALIA Abstract The failure probability of the insulation in zyxwvuts HV components can be studied through Partial Discharge (PD) measurements to detect defects. In this paper a numerical model, previously developed by the authors for the simulation of PD activity in a spherical void embedded in epoxy resin, is implemented to a flat cavity under divergent electric field. This situation simulates very well the real working conditions encountered in the insulation of electrical machines. The model is developed taking into account all the geometric and electrical parameters, indeed it is carried out a fine examination on physical parameters playing a fundamental role on the PD phenomenon. In particular, a more accurate formulation of the work function and a Finite Elements Analysis (FEA) of electric field inside the cavity has been applied. Indeed a new approach is investigated to reproduce the uncertainty of the discharge phenomenon. A probability fimction distribution, obtained by Weibull analysis, gains the best results. zyxwvutsr In order to obtain the best fitting zyxwvut of the model output to the experimental PD data, a strong procedure based on an heuristic algorithm has been performed. This algorithm detects the minimum of a quality function that evaluates the distance between the PD patterns of the numerical model and of the experimental data. After a description of the numerical model adopted, a comparison between the experimental and the simulated data are presented together with comments and remarks. Introduction The PD model is based on certain physical principles already studied in [2,3,4,5]. These principles have been described and presented in detail in [l]. From the research carried out by the Authors it appeared that there are some parameters having a greater influence than others on the model's performance. The aim of this paper is to identify the optimal parameters to reproduce the PD phenomenon. The PD model working mechanism is described in detail in literature [2,3,4,5]. As a result, this paper focuses on the optimization of these parameters not only in a uniform electric field, but also in a divergent one. Finally, the experimental results are reported and commented. The PD model The PD phenomenon can be reproduced using a numerical model, whose development has been widely described in literature and in this paper, the influence of some interesting parameters of physics PD mechanism, has been analyzed using a stochastic numerical model [ 11 and an evolutionary optimization technique has been used for their exact assessment. In particular, this research focuses on streamer discharges as they are pulse-shaped and are associated with higher charge pulses inside a test cell, called CIGREII. This specimen is characterized by a divergent electric field and by an air gap that should simulate a flat cavity (Figure 1). HV Electrode r" Epoxy resin Specimen A zyxwv 7 25 c 50 Figure 1 - The CIGRE I1 test cell 0-7803-7053-8/1/$10.00 2001 IEEE 689