Transient analysis of grounding systems for wind turbines Damir Cavka a, * , Dragan Poljak a , Vicko Doric a , Ranko Goic b a Department of Electronics, University of Split, Rudjera Boskovica 32, 21000 Split, Croatia b Department of Power Engineering, University of Split, Croatia article info Article history: Received 14 June 2011 Accepted 9 November 2011 Available online 20 December 2011 Keywords: Wind turbine Lightning Grounding system Transient analysis Wire antenna theory abstract The paper deals with transient analysis of grounding systems wind turbines. A typical wind turbine grounding system arrangement based on ring electrode is analyzed. Special attention is focused to the influence of additional vertical and horizontal electrodes, respectively. Influence of grounding wire placed in cable trench on the transient behavior is studied in detail. The mathematical formulation in the frequency domain is based on the thin wire antenna theory and the related set of Pocklington integro- differential equations. The corresponding transient response is obtained by means of the Inverse Fourier Transform (IFT). The set of Pocklington integro-differential equations is solved by the Galerkin-Bubnov Indirect Boundary Element Method (GB-IBEM) featuring the use of isoparametric elements. A number of illustrative computational examples are presented in the paper. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Grounding systems, such as buried vertical or horizontal elec- trodes and large grounding grids are important for safety of personnel and for the protection of electrical equipment in indus- trial and power plants. The principal task of such grounding systems is to avoid the values of transient step and touch voltages which determine the health hazard. The secondary purpose of grounding systems is to provide common reference voltage for all connected electrical and electronic systems. Relatively recently, the development and installation of integral lightning protection system for wind turbines (WT) is of particular interest [1e 10]. Namely, wind turbines are often struck by lightning due to their special shape, complex construction and the fact that they are usually placed in isolated locations, mainly at higher alti- tudes. Available relevant statistics indicates that between 4% and 8% of wind power in Europe suffers damages due to lightning strikes each year [6]. This situation is even worse in the southern parts of Europe, like Croatia, due to the increased number of thunder storms and usually relatively low soil conductivities. Despite the fact that the methodology for WT lightning protection has been already proposed in [1], a number of issues concerning transient behavior of grounding system, in the case of lightning strike, are not quite clarified. Namely, the grounding methodology described in IEC 61400-24:2010 [1] is completely subjected to the IEC 62305-3:2006 [11], which handles lightning protection for general structures including houses and buildings. The foundation and grounding system of a WT are generally much smaller compared to the grounding systems of the buildings of the same height. Further- more, the lightning protection level for a WT is much higher than that of a normal building having an equivalent foundation to a turbine. Therefore, low-impedance grounding system is a major prerequisite for an effective protection of WT from lightning strikes. Namely, proper grounding for the protection of the WT should be designed in a way to reach the grounding resistance of preferably less than 10 U (for an isolated WT, without the contri- bution of the residue grounding system of wind farm) [1]. This task is very difficult to fulfill in the case of the high specific resistance of the soil. Hence, setting the optimal mathematical model in terms of accuracy and efficiency is of great importance. This means that smaller grounding systems of a WT may not have sufficient capacity for lightning protection compared to the conventional equipment. Also, the standards for grounding systems [11] are based on the steady state or low frequency analysis. Therefore, all practical aspects of grounding systems design are mostly relied on the steady state analysis. However, such analysis does not account for a transient behavior of grounding system in the case of lightning strike. Importance of the transient analysis lies in the fact that the appearance of high impulse currents leads to an increase of the grounding system potential related to zero ground during the * Corresponding author. E-mail addresses: dcavka@fesb.hr (D. Cavka), dpoljak@fesb.hr (D. Poljak), vdoric@fesb.hr (V. Doric), rgoic@fesb.hr (R. Goic). Contents lists available at SciVerse ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene 0960-1481/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2011.11.042 Renewable Energy 43 (2012) 284e291