Active current shaping for better utility interface

Abstract

Increased use of nonlinear electrical loads injects harmonic currents to power systems. High levels of power system harmonics create voltage distortion and enlarge power quality problems. Harmonics result in poor power factor, lower efficiency and interference to adjacent communication systems. The harmonic currents flow into the utility supply lines produces extra losses. An active power filter uses a switching inverter to produce harmonic compensating currents. The major objective of this project was to eliminate effects of harmonics and to improve power factor of a typical nonlinear load. Attempts were made to apply active power filters for current shaping of a specific load, contrary to its common applications of applying at the point of common coupling. The National Instruments USB-6008 multifunction data acquisition (OAQ) module was used to acquire data from the sample load viz. the computer power supply. A potential divider was incorporated to the circuit to acquire voltage signal. Current signal was acquired using a series resistor. Filter current was implemented by switching an inductor using four insulated gate bipolar transistors (IGBT) arranged in H bridge configuration. The simulation circuit was implemented using M·ATLAB Simulink software tool. Inductance of the switching inductor, voltage of the step up transformer and the switching frequency of the system were obtained by simulation. Subsequently the above circuit parameters were validated for variable loads using total harmonic distortion as the discerning criterion. It was possible to reduce THO of the current wave of computer power supply from 107% to 12%. Power factor was improved from 0.66 to unity. By increasing the power factor to unity, the current flow can be reduced by approximately 34%. The observations made herein are applicable for harmonic elimination in nonlinear loads in general with necessary modifications.