Abstract—Most serious failure of power apparatus is due to the insulation breakdown. Partial Discharge (PD) that damages insulation by gradual erosion is major source of insulation failure. In electrical engineering, partial discharge (PD) is a localized dielectric breakdown of a small portion of a solid or fluid electrical insulation system under high voltage stress, which does not bridge the space between two conductors. So, An important tool for improving the reliability of HV insulation systems are partial discharge (PD) measurements. Accurate simulating of PD is more important for insulation study. In this paper, the mechanism of PD has been simulated by using EMTP. Keywords—Partial Discharge , Void , Simulation, Insulation , EMTP. I. INTRODUCTION Artial Discharge (PD) generally begins within voids, cracks, at conductor-dielectric interfaces within a solid insulation system, or in bubbles within liquid dielectrics. Since discharges are limited to only a portion of the insulation, the discharges only partially bridge the distance between electrodes. PD can also occur along the boundary between different insulating materials. Partial discharges within an insulating material are usually initiated within gas- filled voids within the dielectric. Because the dielectric constant of the void is considerably less than the surrounding dielectric, the electric field (and the voltage stress) appearing across the void is significantly higher than across an equivalent distance of dielectric. If the voltage stress across the void is increased above the corona inception voltage (CIV) for the gas within the void, then PD activity will start within the void. Once begun, PD causes progressive deterioration of insulating materials, ultimately leading to electrical breakdown. PD can be prevented through careful design and material selection. In critical high voltage equipment, the integrity of the insulation is confirmed using PD detection equipment during the manufacturing stage as well as periodically through the equipment’s useful life using On- Line Partial Discharge surveys. PD prevention and detection are essential to insure reliable, long-term operation of high voltage equipment. Error in such equipments causes in outgoing and many economical disadvantages. Many errors Mehdi Nafar is with Department of Electrical Engineering, Marvdasht Branch, Islamic Azad University, Marvdasht, Iran, (phone: +98917-301-8583; fax: +98728-331-1172; e-mail: mnafar@miau.ac.ir). that result in power system apparatus outgoing is related to insulation system and partial discharge that destroy the insulation gradually, in the most important fault resource in insulation of power system equipment [1-3]. Location techniques of this phenomenon are very important in maintenance and reparation of equipment. many researches are done in this domain [1-9]. Accurate modeling and simulating PD mechanism is the first stage for all PD localization studies. In most done researches, the simulations produced current Impulses are used. But the mechanism of PD cannot be simulated in these researches. In this paper accurate mechanism of PD is simulated in EMTP software. The response of simulation is like results of laboratory experiments. II. PD MECHANISM PD usually begins within voids, cracks, or inclusions within a solid dielectric, at conductor-dielectric interfaces within solid or liquid dielectrics, or in bubbles within liquid dielectrics. Since discharges are limited to only a portion of the insulation, the discharges only partially bridge the distance between electrodes. PD can also occur along the boundary between different insulating materials. Fig.1. an infected insulation Simulation of Partial Discharge Mechanism using EMTP Mehdi Nafar P International Conference on Innovations in Electrical and Electronics Engineering (ICIEE'2012) Oct. 6-7, 2012 Dubai (UAE) 249