COMPUTATIONAL PROBLEMS OF ELECTRICAL ENGINEERING Vol. 7, No. 2, 2017 SIMULATION OF TRANSIENTS FOR DESIGNING MULTIPLE POWER FILTER CIRCUITS Yuriy Varetsky AGH University of Science and Technology, Krakow, Poland jwarecki@agh.edu.pl © Varetsky Yu., 2017 Abstract: The practice of operating multiple single- tuned filters in industrial power supply systems has shown that technological transformer capacitors in switching filters within the systems can cause the damage of filte rs. The most disturbing loads on the power supply systems are powerful AC arc furnaces. Static Var compensators used in the power supply systems contain multiple harmonic filters for harmonic mitigation and reactive power compensation. For examining the impact of the supply system and filter configuration on transient overvoltages and overcurrents in the reactor and capacitor bank of the filter during switching events, an arc furnace power supply system was chosen as an example. The transient analysis has been carried out by simulating transients within Matlab/Simulink software. In the research, the most typical switching events and harmonic impact on the transient overvoltages have been analysed. The paper focuses on the selection of ratings for capacitors and air- core reactors used in multiple single-tuned harmonic filter configurations based on the ANSI/IEEE Standards and the results of transient simulations. The comparison of reactor and capacitor bank ratings of the filter circuits selected on the base of steady state operation and transient requirements has been shown. Key words: Simulation, transients, filter circuit, harmonics, capacitor, reactor, electrical arc furnace, static Var compensator. 1. Introduction High voltage harmonic filters are widely applied in heavy and mining industry where power electronic and electric arcing devices essentially impact the supply system. In most cases, for the power system supplying the powerful industrial loads, the multiple filter circuits (FC) containing a number of single tuned filtering branches must satisfy harmonic mitigation requirements [1–5]. The topology most commonly used in industrial grids is a passive single branch harmonic filter. Usually the most of the filter circuits are designed to limit the harmonic distortion to a specified level and provide the required reactive power output. The component ratings are often specified basing solely on steady state operation and on fundamental harmonic voltages and currents. While selecting capacitors and reactors of the filter circuit, the variations of their capacitance and inductance caused by the effect of environmental conditions and manufacturing tolerances are taken into account. In practice, it is assumed that a chosen filter resonance point h r should be 2–10 % lower than the accurate resonant frequency h of the filter [3]. Although this may be applicable and satisfy stable loads, the discussed method is not adequate for the filter components installed to compensate the time varying powerful loads, where the technological process is accompanied by a lot of transient occurrences. In these applications, the filter circuit failures may occur during normal manufacture switching. Nowadays arc furnaces are very essential in steel production companies for the production of high-quality steel being the most disturbing loads on power systems. Electrical arc furnaces (EAF) are classified as complex loads with nonlinear and time varying load characteristics, which can cause many problems with the power system quality, including voltage dips, harmonic distortion, unbalanced loads and flicker. The practice of operating EAF in industrial power supply systems has shown that equipment installed in the systems is affected by overvoltages and overcurrents during the normal operation of the arc furnace. The changes of electricity consumption during melting process depend mainly on the quality of the stock, accuracy of control circuits and thermal processes as well. Each cycle of the EAF technological process is characterized by the active power changes and the number of switching required, with a trend towards the decrease and stabilization of processes in further cycles. The first period of metal formation is characterized by the highest power consumption, that is, 60–80 % of the total energy consumption of a whole technological cycle. In the following periods there are lower power fluc- tuations due to arc stabilization. In order to ensure the electromagnetic compatibility of the alternating current EAF with power supply systems, a lot of technical solutions exist. The most effective power quality improvement is based on using Static Var Compensators