Design and implementation of three-phase four-wire active filters

The widespread use of power electronics in all types of industrial, commercial and even residential electrical equipment is causing the deterioration of the quality of the electric supply through distortion of the supply voltage. This has led to the development of more stringent requirements regarding harmonic current generation, as are found in standards such IEEE-519. Passive filters have been used to limit the flow of harmonic currents in distribution systems. However, these tend to be bulky and the design is complex, particularly if the number of harmonic components to be reduced increases. With improvements in power electronic devices and conversion techniques, an innovative concept, the active power filter, has recently been proposed and investigated. Voltage source structures in particular present excellent performance characteristics in three phase configurations. However, in a number of installations, four-wire systems are used. Furthermore, if the three phase load is unbalanced, neutral currents flow and the three phase three-wire active filter cannot adequately eliminate harmonics. This thesis deals with active filters for the general case of three phase four-wire systems. The proposed filter uses three independent current controllers acting on half-bridge inverters. The PWM pattern is generated using a constant frequency ramp comparison technique. A detailed analytical study is presented to design the power circuit in order to ensure operation in single phase and unbalanced load conditions. The quality of the supply current, the magnitude and components of the neutral current and the filter bandwidth are investigated. The theoretical considerations are verified by simulation and by experiments on a industrial prototype. Results demonstrate that the active filter is a viable and a potentially economical solution to mitigating harmonics in distribution power systems