International Journal of Engineering and Technical Research (IJETR) ISSN: 2321-0869, Volume-3, Issue-6, June 2015 72 www.erpublication.org Abstract— At the time of transformer energisation, a high current will be drawn by the transformer. The mentioned current is called transient inrush current and it may rise to ten times the nominal full load current of transformer during operation. Energisation transients can produce mechanical stress to the transformer, cause protection system malfunction and it often affects the power system quality and may disrupt the operation of sensitive electrical loads such as computers and medical equipment connected to the system. Reduction and the way to control of energisation transient currents have become important concerns to the power industry for engineers. A technique has been proposed to mitigate inrush current in three phase transformer, by a process called pre- fluxing. After setting the initial fluxes of transformer it is energized by conventional controlled switching. In this paper, a system of power transformer of specified rating is simulated in MATLAB/simulink and results were obtained. Index Terms— Transformer, Harmonics, Inrush Current, Prefluxing, MATLAB/Simulink I. INTRODUCTION Inrush Current is a form of over-current that occurs during energisation of a transformer and is a large transient current which is caused by part cycle saturation of the magnetic core of the transformer. For power transformers, the magnitude of the first peak of inrush current is initially several times the rated load current but slowly decreases by the effect of oscillation damping due to winding and magnetizing resistances of the transformer as well as the impedance of the system it is connected to until it finally reaches the normal exciting current value. This process typically takes several minutes. As a result, inrush current could be mistaken for a short circuit current and the transformer is erroneously taken out of service by the over - current or the differential relays [1]. The transformer design and station installation parameters affect the magnitude of the inrush current significantly. Therefore, it is important to have an accurate calculated value of the magnitude and other parameters of inrush current in order to design the relaying to properly differentiate between inrush and short circuit incidents. Also, a proper calculation of the minimum % ratio of 2nd harmonic content of inrush current is an especially important parameter for this differentiation [2]. This paper contains basic principle, fundamental theory and mitigation of inrush current in transformer. The effects of inrush current are described in brief The MATLAB Simulink is used for the simulation. Girraj Singh, Scholar, M.tech, Jagannath University, Jaipur Dr. Mukesh Kr. Gupta, Professor, Deptt. of Electrical Engg, JNIT Jaipur II. NATURE OF INRUSH CURRENT The saturation of the magnetic core of a transformer is the main cause of an inrush current transient. The saturation of the core is due to an abrupt change in the system voltage which may be caused by switching transients, out-of-phase synchronization of a generator, external faults and faults restoration. The energization of a transformer yield to the most severe case of inrush current and the flux in the core can reach a maximum theoretical value of two to three times the rated value of peak flux. There is no direct evidence that the energization of a transformer can cause an immediate failure due to high inrush currents. However, insulation failures in power transformers which are frequently energized under no load condition support the suspicion that inrush currents have a hazardous effect. A more typical problem caused by the energization of transformers is due to harmonics interaction with other system components that develops into over-voltages and resonant phenomena. The study of the energization of a transformer installed in an industrial facility carried out in highlights problems due to harmonics, over-voltages and resonances. In the authors show how the harmonic distortions caused by the switching of lightly loaded or unloaded transformers may be amplified during a power system restoration process, creating high harmonic over-voltages [3] [4]. In the energization of large size transformers in EHV substations with long transmission lines is discovered to cause significant temporary disturbances when harmonic resonances are reached. In particular, when there are transformers already connected to the bus, the disturbances caused by the energization of one more transformer have greater duration and intensity. In it is discussed how transformer inrush current can excite resonance frequencies in inter-connected offshore power systems [5] [6]. Factor Affecting Inrush Current Starting phase angle of voltage Residual flux in core Core material Supply/Source impedance Loading on secondary winding Size of transformer [7] [8] Reduction of inrush current using Prefluxing The innovation behind the prefluxing inrush current reduction strategy lies in the prefluxing device itself. The prefluxing Reduction of Harmonics and Inrush Current of Power transformer using Prefluxing Technique Girraj Singh, Dr. Mukesh Kr. Gupta