DC-Link Voltage Control Strategies for Three-Phase Back-to-Back Active Power Conditioners

The objective of this dissertation is to propose a three-phase back-to-back active power conditioner (APC) with dc-link voltage control strategies for micro-grid applications. The demanded active and reactive power of the APC via bi-directional power flow control can help to regulate the frequency and voltage of the micro-grid to achieve high stability. On the other hand, the dc-link capacitor is an essential component of the back-to-back APC for power flow balancing. In order to provide the ability to improve the power quality and stability of the micro-grid as well as to reduce the dc-link capacitance, four dc-link voltage control methods are developed: a) optimal ac line current regulation (OCR) strategy, b) adaptive dc-link voltage regulation (ADVR) strategy, c) dynamic optimal ac line current regulation (DOCR) strategy and d) flexible DC-link voltage slope (FDVS). Under steady state, the proposed OCR strategy is able to minimize the change of the input current variation as well as to achieve the dc-link regulation in one 60Hz cycle. When an abrupt or continuous power change occurs, the dc-link voltage of the APC will be changed dramatically and the voltage protection could easily be triggered. Therefore, the novel ADVR and DOCR strategy are proposed in order to prevent the false alarm as well as to reduce the required dc-link capacitance. On the other hand, the proposed FDVS can further mitigate the DC-link voltage variation during the large power variation transient. Mathematical equations for proposed control strategies are derived thoroughly. Furthermore, procedures to determine the DC-link capacitance with the proposed strategies are developed. Finally, experimental results obtained from a 5 kVA back-to-back APC verify the feasibility and the performance of the proposed line current regulation strategies.