Active power filter for unbalanced distribution networks using a flying-capacitor multi-level inverter

Widespread growth of the consumer electronics market and the upward trend in harnessing electrical power from renewable sources has increased the number of power electronic converters connected to utility distribution systems. Power electronic converters offer flexibility in voltage and current control as well as improved energy efficiency, but generate harmonic currents which have detrimental effects on the power system. A particular challenge appears in 4-wire networks where zero-sequence harmonics exist; such currents accumulate in the neutral conductor and pose threats of overheating and failure. This work investigates harmonic compensation using shunt active power filters with specific application to unbalanced distribution networks. Owing to its increased availabil- ity of switching states, the flying-capacitor multi-level inverter (FCMI) is explored; how- ever an additional challenge exists in the balancing of capacitor voltages. Furthermore, synthesis of unbalanced inverter output volt ages necessitates a three-dimensional pulse- width-modulation (3D-PWM) strategy. This work proposes a new modulation scheme, specifically suited to 4-limb FCMIs, which combines 3D-PWM and natural flying-capacitor voltage control. To develop an optimal harmonic compensation strategy, it is important to understand the nature of harmonic distortion experienced in typical distribution systems. New data on harmonics generated by common single-phase appliances are presented and a compre- hensive power-quality study is performed on a typical 4-wire network. Findings are used to develop a computer model which emulates realistic harmonic load current waveforms. The final contribution in this work lies in the development of an unbalanced harmonic extraction technique based on synchronous reference frame theory and a new modified deadbeat controller which compensates delays and improves reference current tracking. In conjunction with the proposed 3D-PWM scheme, the complete control strategy pro- vides compensation of positive, negative and zero-sequence harmonic currents in addition to alleviating phase load imbalance. A bespoke FCMI test rig is developed, providing experimental validation of the proposed system.EThOS - Electronic Theses Online ServiceGBUnited Kingdo