International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056 Volume: 02 Issue: 01 |April-2015 www.irjet.net p-ISSN: 2395-0072 © 2015, IRJET.NET- All Rights Reserved Page 132 Sensitivity Analysis of Maximum Overvoltage on Cables with Considering Forward and Backward Waves Hamed Touhidi 1 ,Mehdi Shafiee 2 , Behrooz Vahidi 3 , Seyed Hossein Hosseinian 3 1 Islamic Azad University, Bam Branch, Department of Electrical Engineering, Bam, IRAN 2 PhD Student, Department of Electrical Engineering, Ferdowsi University of Mashhad, Mashhad, IRAN 3 Professor, Department of Electrical Engineering, Amirkabir University of Technology, Tehran, IRAN ----------------------------------------------------------------------***--------------------------------------------------------------------- Abstract - lightning is known to be one of the primary sources of most surges in high keraunic areas. It is well- known fact that surge overvoltage is a significant contribution in cable failures. The other source of surge voltage is due to switching and it is pronounce on extra high voltage power transmission systems. The effect of both lightning and switching surges is weakening the cable insulation. The progressive weakening of such insulation will lead to cable deterioration and eventually its failure. Each surge impulse on the cable will contribute with other factors towards cable insulation strength deterioration and ultimately cable can fail by an overvoltage level below the cable basic impulse level (BIL). The maximum lightning overvoltage for a given cable depends on a large number of parameters. This paper presents the effect of model parameters (e.g., rise time and amplitude of surge, length of cable, resistivity of the core and sheath, tower footing resistance, number of sub conductors in the phase conductor (bundle), effect of surge arrester, length of lead, relative permittivity of the insulator material outside the core, power frequency voltage, stroke location, cable joints, shunt reactors, sheath thickness) on maximum cable voltage. The simulations show that the maximum overvoltage. Keywords: Maximum overvoltage, Sensitivity analysis, Forward and Backward waves. 1. INTRODUCTION power transmission network has developed during the last decades based on the use of over head line and cables [1].The insulation of underground insulated cables is not self restoring and the BIL can experience a decrease as a result of aging. Lightning overvoltage can be a major cause of underground cable failure; therefore it must be taken into account when designing the insulation system of a cable.IEC specifies one, two, or three BIL levels for each system voltage, thus giving the customer some room for adapting the BIL to the actual lightning overvoltage conditions. The manufacturer is simply required to produce cables that satisfy the lightning test voltages. CIGRE WG B1-05 is currently undertaking a study to assess the maximum voltage stress on long cables. This study is motivated by the possibility of reducing the required test voltage for long cables that are protected by arresters, as the cable attenuation may reduce the maximum cable voltage well below the existing B)L’s. Therefore it is necessary to develop a technique which is able to estimate maximum overvoltage in power cables considering effective parameters. The resulting overvoltage at the cable ends has been subject to several investigations, e.g. [2-4] while in reality the maximum voltage may occur inside the cable [5, 6]. This maximum voltage can be found by calculating the voltage at selected positions along the cable (discretized model) [7]. This can be done by subdividing the cable into a number of small sections but error accumulation lead to inaccurate results. References[6,8,9] presents an improved approach where a regular line model is used for obtaining the voltages and currents at the line ends which are next used in the calculation at internal points using an off-line time step. However, in the case of steep-fronted lightning surges, the maximum voltage can only be determined with good accuracy if one of the selected positions is sufficiently close to the position of the maximum voltage. Article [10] is presented an analysis of the maximum voltage inside distribution cables from lightning strokes based on analysis by lattice diagrams. However, the technique does not cover the cable attenuation effects found in long transmission cables and the exact position of maximum voltage. In [11], maximum voltage is calculated by adding the peak value of the voltage wave in the forward and backward direction. The study presented in this paper is aimed at determining effect of model parameters on maximum lightning overvoltage (close back flashover) with the method which is explained in [11]. This goal is achieved by performing sensitivity analysis considering effective parameters on the maximum overvoltage. Simulation results show the importance of this study. 2. COMPUTATIONAL PROCEDURE The maximum voltage was calculated based on the technique presented in [11]. The computation was carried out in three steps: 1) Calculation of the voltage at the exposed end of the cable assuming infinite cable length. 2) Calculation of the maximum incoming voltage wave at the remote end of the cable using the result from previous stage as a known voltage source at the exposed end.