A Nine-Switch UPQC with Variable-Band Hysteresis Control
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Distributed generation (DG) is a continuously developing trend for interconnecting renewable sources of energy like solar and wind to the utility grid (UG), in addition to the traditional sources of energy. The intensive integration of these renewable sources, with a large increase in power switching electronics devices at the industrial and domestic level creates significant issues with power quality (PQ). Due to the strict standards imposed on DG systems and consumer demand for higher PQ, flexible AC transmission systems (FACTS) were developed. This thesis is focused on one of these FACTS devices, the nine-switch unified power quality conditioner (UPQC). It is presented as a flexible and effective solution for current and voltage related problems of the DG system by enhancing PQ. Maintaining similar power rating compared to the conventional UPQC, the nine-switch UPQC functions during normal, sag and swell operation while utilizing three less semiconductor switching devices. The equivalent power rating of its semiconductor switching devices, with respect to the conventional counterpart, results in reduced losses and cost of the entire system. A variable-band hysteresis control method that is applied to the shunt and series terminals performs independent control of the nine-switch UPQC currents and voltages. The shunt and series terminal controllers are accurate and fully utilize the nine-switch UPQC dc-link voltage. By avoiding a multi-loop structure and cascaded resonant blocks, controllers provide with fast, robust and stable control, while maintaining switching frequency close to the selected value. As a result, the nine-switch UPQC simultaneously mitigates current and voltage harmonics, provides power factor correction and compensates sag and swell voltage variations. All design procedures are described in details.