0093-9994 (c) 2019 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. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2019.2927669, IEEE Transactions on Industry Applications > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract— In the field of high voltage transmission systems, different degradation phenomena affect the reliability of the employed components. In particular, under DC stress, the space charge accumulation phenomenon is believed the most responsible of the dielectrics lifetime reduction. To measure the accumulated space charges in flat specimens, the PEA method is one of the most used techniques. The working principle of the PEA cell is based on the acoustic waves propagation and detection. As is well known, the acoustic waves propagating in different means are partially transmitted and partially reflected. Therefore, the piezoelectric sensor of the PEA cell is subjected of both direct and reflected waves. The latter, which are also present in the PEA cell output signal, if are not taken into account could be easily confused as a charge accumulation. For this reason, in this work the acoustic waves behavior within the PEA cell has been simulated in order to avoid incorrect interpretation of the PEA cell output charge profiles. In this paper, compared to other works present in literature in which only the surface charges were considered, the space charges accumulated in the insulation bulk are also considered for a complete analysis of the waves behavior during the PEA measures. The results found in the present work are very useful for cables manufacturing industries in which the choice of the dielectric materials is strongly influenced by the space charge accumulation phenomenon. Index Terms—Space charge, PEA method, HVDC, modelling, FDTD method. I. INTRODUCTION N high voltage systems, especially under AC stress, the partial discharges phenomena are considered the main dielectric degradation causes [1-5]. In high voltage direct current (HVDC) systems, instead, the space charge This paragraph of the first footnote will contain the date on which you submitted your paper for review. It will also contain support information, including sponsor and financial support acknowledgment. For example, “This work was supported in part by the U.S. Department of Commerce under Grant BS123456.” The next few paragraphs should contain the authors’ current affiliations, including current address and e-mail. For example, F. A. Author is with the National Institute of Standards and Technology, Boulder, CO 80305 USA (e- mail: author@ boulder.nist.gov). S. B. Author, Jr., was with Rice University, Houston, TX 77005 USA. He is now with the Department of Physics, Colorado State University, Fort Collins, CO 80523 USA (e-mail: author@lamar.colostate.edu). T. C. Author is with the Electrical Engineering Department, University of Colorado, Boulder, CO 80309 USA, on leave from the National Research Institute for Metals, Tsukuba, Japan (e-mail: author@nrim.go.jp). accumulation phenomenon is believed the main factor causing the dielectric lifetime reduction [6-8]. Considering that, several space charge measurement techniques were proposed [9]. By making a comparison among the all techniques, the Pulsed Electro-Acoustic PEA method [10] can be considered the most simply to be used [11]. This measurement method is essentially founded on the acoustic waves originated by charges vibration and on its detection, after propagating within the PEA cell, by means of a piezoelectric sensor. Considering the PEA cell is composed by different components with different materials, the reflection phenomenon occurs [12]. Therefore, based on the PEA cell components and sample features, the PEA cell output profiles could be affected by noise signals due to wave reflections. This, means that the obtained charge distribution is distorted and thus the real charges accumulation distributions could be misinterpreted. In order to analyze and better understand the acoustic wave behavior, in literature several PEA cell models are reported. In [13], Galloy et al elaborated a one-dimensional electroacoustic model based on the Redwood’s equivalent circuit of piezoelectric and non-piezoelectric materials for the acoustic part of a PEA cell. In particular, they found a way to improve the charge distribution resolution by optimizing the values of the acoustic detector parameters. Another PEA cell simulation model has been developed with commercial software COMSOL® [14]. In this work, the effect of the piezoelectric sensor thickness in the PEA cell output signal was evaluated, without considering the attenuation and dispersion of acoustic waves. In [15], Chahal and Reddy, proposed a PSpice model in order to simulate the acoustic waves behavior within the PEA cell. The authors modeled the entire PEA cell by means of cascade connected lossy transmission lines. In this work, the effect of the PEA cell components dimensions in the output profile was analyzed. In the articles above mentioned, as well as in that previous published by the authors [16], only the surface charges deposited in the sample interfaces were taken into account. In the proposed work, instead, the developed model has been implemented in Matlab environment and for a complete analysis of the waves behavior during the PEA measures, the space charges accumulated in the insulation bulk are also considered. Thanks to the use of numerical software, especially in the case of accumulated space charges, the implementation of the pulse generators employed to simulate The Role of Right Interpretation of Space Charge Distribution for Optimized Design of HVDC Cables A. Imburgia, P. Romano, Senior Member, IEEE, G. Ala, Senior Member, IEEE, E. Riva Sanseverino, G. Giglia I