Analysis and design of active power filter topologies to cancel neutral current harmonics in low voltage electric power distribution systems

Abstract

Recent surveys of 208/120V three phase four wire electric systems, buildings and industrial plants with computers and other nonlinear loads, show excessive currents in the neutral. Under normal operating conditions with the loads reasonably balanced, the current in the neutral is expected to be small, if load currents have an undistorted sinusoidal waveshape. However, with the growing use of nonlinear loads, load currents are nonsinusoidal and rich in harmonics. The zero sequence harmonic components in the load current (i.e. 3rd, 9th, etc.) are co-phasal and do not cancel each other in the neutral. This results in excessive neutral currents which are fundamentally third harmonic and their presence is tied to wiring failures, elevating of neutral potentials, transformer overheating, etc. Survey results of 146 computer sites across the country indicate that 22.6 percent of the sites had neutral currents in excess of 100 percent of the phase current. In response to these concerns, this research work focuses on investigating active power filter topologies particularly suitable to cancel neutral current harmonics in low voltage electric power distribution systems. In this thesis a new active power filter scheme is proposed to cancel neutral current harmonics. The proposed approach employs a star/delta transformer along with a two switch PWM controlled active filter. The closed loop control of the active power filter guarantees cancellation of neutral current harmonics under varying load conditions. The proposed system drastically improves the overall system performance and virtually eliminates transformer overheating due to harmonics. Design and implementation of the closed loop control system is presented. Key design issues involving the power circuit of the proposed active filter is elaborated. Experimental results from a 3.6 kVA laboratory prototype, validates the design approach and confirms the suitability of the proposed scheme.