© 2017 Published by “Petru Maior” University Press. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 11 Scientific Bulletin of the „Petru Maior” University of Tîrgu Mureş Vol. 14 (XXXI) no. 2, 2017 ISSN-L 1841-9267 (Print), ISSN 2285-438X (Online), ISSN 2286-3184 (CD-ROM) STUDYING TRANSIENT STATE USING STATE PARAMETERS Katalin ÁGOSTON “Petru Maior” University of Tîrgu-Mureș, Romania Nicolae Iorga Street, no.1, 540088 Tîrgu-Mureș, Romania katalin.agoston@ing.upm.ro Abstract This paper presents the state variables method in the analysis of the transient state. Using this method the behaviour of transformers at on/off switching is studied. The transient state is a short event caused by any changes in the system which is followed by an energy modification stored in the system. The energy is stored in capacitors and coils and any modification needs time and has reactions. These reactions can be high impulses of energy which propagate through the network and influence or even destroy other circuit elements or devices. The transient state of an RLC circuit and of a transformer is studied. Differential equations are used and the nonlinearity of the coil is highlighted. Differential equations and the equivalent circuit are used to deduce the state parameter model for the transformer. Equations and equivalent circuit for a real transformer are deduced with and without load at the output. The behaviour of the transformer was studied while functioning in an open circuit and with different loads at the output. To study the output parameter variations state space model simulation was used. Keywords: transient state model, state parameters, transformer, Simulink model 1. Introduction The transient state is a short event in a system caused by a sudden change of the functioning state. The transient state can be also characterized through a relatively short event of energy modification in a system caused by a sudden change in state. By switching on/off the circuit elements or devices, a short transition state appears between the two steady states. This relative short transition time theoretically is infinite, but practically it is 4-5 times longer than the circuit constant. The transient state appears not only by on/off switching of the circuits, but also by any changing in the circuit. The length of the transient state is depending on the type of the circuit and the state which it is going to reach. The produced impulse and shock propagate through the net and can damage instruments and equipments. The phenomena of the transient state affect the operating way of the circuit elements or devices also. [5] The characteristics of the transient state depend on the charging and discharging of parasite capacitors, and the change of the magnetic field of the induction coil. These changes are relative long and influence the circuit elements and equipments. For example, coupling an electrical motor, the starting current is six times larger than the nominal current, and this shock propagates in the electrical network a long distance. This shock appears at other equipments like overvoltage. Any change and energy modification in the system appears as an oscillation. Mathematically, it can be modeled as a damped harmonic oscillator. In electrical engineering, oscillations are caused by a sudden change of the circuit structure and this change is studied through transient response of the system. The transient response gives the variation of the output signal at a given variation of input. There are many types of equipment which operating mode is followed by short or large transient state. For example there are switching power supply, induction motors with variable speed, equipments which operate in switching mode, or other circuits which produce voltage or current oscillations during the working period. Voltage regulators and filters are used to prevent transients in electricity and to protect instruments and other equipments. Different mechanical or electrical systems are studied applying impulse and/or step input. These are specific inputs used to analyze the behavior of the system. The step input and inverse step input can be like on/off switching. The transient response is not necessarily bound to on/off switching but to any event that affects the state of the system. [3],[4] The graphic form which shows how the new steady state is reached depends on the system structure and on the parasite elements. The transient