Simulation of Transformerless Multilevel Series APF based on Improved SRF Theory Katabathina Venkateswarlu M. S. Sujatha M.Tech student, Dept. of EEE Professor, Dept. of EEE Sree vidyanikethan engg. College Sree vidyanikethan engg. College, Tirupati, India Abstract— The nonlinear load in the distribution system rapidly increases, affects the source voltage and damage the sensitive equipment connected at the point of common coupling (PCC). The main aim of this paper is to regulate the source voltage against voltage sag and harmonics by employing transformer less multilevel series active filter based on improved version of synchronous reference frame theory with modified PLL. A cascaded multilevel inverter with separate dc sources has been proposed with carrier shifted sinusoidal pulse width modulation switching method is employed to reduce the harmonic distortion in the output of the inverter. The main advantage of this configuration is no need of injection transformer and reduces the filter requirements. The proposed method is validated through MATLAB/SIMULINK software. Key Words: Voltage sag, harmonics, Series active power filter, Cascaded multi-level inverter, Synchronous reference frame theory, Sinusoidal PWM, Modified PLL, Non linear load, MATLAB/SIMULK I. INRODUCTION The power quality becomes very important issue because of rapid growth of nonlinear loads. The recent advancements in power electronics leads to saving of energy, cost, operation and control of equipment but at the same time the generation of harmonics affect the source voltage as well as the sensitive equipment connected at the point of common coupling (PCC). So the source needs to supply the harmonic component along with fundamental component this leads to additional generation cost and the high magnitude of currents damages the windings of motors, transformers and other equipments, etc. The depreciation of voltage from the nominal voltage is called sag or voltage dip typically voltage lies between 0.2 to 0.9 pu. In the recent year the loss of production due to voltage sag is very high and severe stress on the equipment insulation and many problems arises due to insulation failures. Swell is the excess amount voltage typically lies between 1.2 to 1.9pu of the nominal voltage. However these are less dominant compared sag. Now a days the recent advances in power electronics makes the use of nonlinear loads are rapidly increases. This affects the source voltage as well sensitive equipment connected at the point of common coupling. There many other PQ problems like transients, flickers, notches etc. but voltage sag and harmonics are dominant in present power system. There are many solutions are available to protect the voltage at PCC like DVR, D-STATCOM, shunt active filter, series active filter, UPQC etc. The proposed series active power filter is a series compensating device. It acts as a controlled voltage source and can compensate all voltage-related problems, such as voltage harmonics, voltage sag, voltage swell, flicker, etc at the source side of distribution system. The multilevel inverter has been proposed and it has an advantage of low harmonic distortion. The control mechanism for APF is derived from synchronous reference frame theory. The proposed method is implemented in MATLAB/SIMULINK software. II. SERIES ACTIVE POWER FILTER The series APF injects a voltage in series with the supply voltage and reduce harmonic components in voltage waveforms. Therefore series APF can be regarded as harmonic filter. The basic structures of conventional and proposed series APF are shown in fig. 1(a), (b) respectively. Series APF is a voltage source inverter (VSI) with control mechanism of detection of disturbances and generation of required signals. The proposed scheme has an advantage of omission of injection transformers which suffers from magnetic core saturation and other problems and this can be done by the use of cascaded multilevel inverter with separate dc sources. The proposed series APF with fixed dc voltage source makes only real power exchange between the line and the series APF. If we use capacitor as dc source makes reactive exchange between Tirupati, India Vol. 3 Issue 6, June - 2014 International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 www.ijert.org IJERTV3IS060901 International Journal of Engineering Research & Technology (IJERT) 681