1 Abstract-- in this paper, voltage distribution and maximum electric field intensity in power transformer windings, using various electric shields are studied. The study covers disk type and layer type windings. According to the results, disk-type windings with inter-turn shields have the best voltage distribution under lightning impulse conditions. Keywords - transformer winding, lightning overvoltages, high voltage shields, voltage distribution, potential gradient I. INTRODUCTION xcessive voltage of network over its nominal rate is called overvoltage, which may appear in transient or temporary form in electrical network. One the most important types of transient overvoltages are caused by lightning. It is not distributed uniformly in transformer winding and produces significant voltage gradient at the beginning of the winding, which can result in electric breakdown of transformer insulation. Standard lightning waveform is determined by rise time T 1 , the time which waveform reaches to 90% of its peak value and back time T 2 which waveform passes the peak value and is reduced to half of the peak. Thus, lightning waveform is introduced by 2 1 / T T . In lightning waves, T 2 in more than 54% of the cases is greater than 40 μs, and T1 in more than 70% of the cases is more than 1.2 μs. therefore, standard lightning voltage waveform is defined as s s μ μ 50 / 2 . 1 which is used in voltage impulse tests. Frequency of lightning impulses can reach hundreds of thousand kHz. Thus, substantial capacitance of winding cannot be ignored despite normal 50Hz situations. According to equivalent circuit of Fig. 1, parallel and series capacitances are considered between windings and grounded components (i.e. core, tank, etc) and between disks, layers and turns, respectively. Just after the application of a step voltage to the winding, voltage distribution is completely controlled by capacitance values. Charging current of the first capacitances is more than that of the second ones due to the current paths. Therefore, voltage drop on different capacitances is different. Thus the voltage distribution is not uniform at all. Complete capacitance network is very complicated. Differential equation of voltage location in each point of winding with this capacitive model is as follows: 0 v Į v/dx d 2 2 2 2 = - A (1) If winding end is grounded, the answer is: () ( ) Į sinh Įx sinh U x v A = (2) Where, U is the amplitude of the applied voltage at the winding terminal, v(x) is the voltage at any point of the winding, l is the winding length and α is the winding capacitance index, which is defined as: S P C C = α (3) Where, Cp and Cs are the whole winding parallel and series capacitance, respectively. According to the above equations, greater series capacitance results in more uniform impulse voltage distribution and then lower stress on winding insulation. Greater α results in more uniform field and less insulation damage in insulation system [1-5]. Increasing the series capacitance of the winding highly uniforms this distribution. Normally transformer manufacturers reach to this aim by interleaving the winding turns. This method is costly and time consuming. In [6], the electrostatic shielding, used in disc windings to increase the windings series capacitance, and for linearization of its impulse voltage distribution is introduced and its impact on the modification of impulse voltage distribution is compared with the disc winding without shielding. Figure 1- equivalent circuit of a single layer winding. Also in [7], an electrostatic shielding is introduced in the disk winding to increase the winding equivalent series capacitance, and consequently linearize the impulse voltage distribution. Its impact on impulse voltage distribution is evaluated by Comparative Study of the Effect of Various Shields on Lightning Electric Field in Power Transformer Windings M. R. Meshkatodini, A. Shahmohammadi, M. Majidi and M. Karami, Electrical Engineering Faculty, Power and Water University of Technology, Tehran, Iran, E Paper accepted for presentation at the 2011 IEEE Trondheim PowerTech 978-1-4244-8417-1/11/$26.00 ©2011