This paper presents a transient analysis of a unified power flow controller (UPFC), and design of capacitance of the DC-link capacitor based on that analysis. Active power flowing out of the series device in transient states is theoretically discussed to derive what amount of electric energy the DC link capacitor absorbs or releases through the series device. As a result, it is clarified that the active power flowing out of the series device is stored in the line inductance as magnetic energy during transient states. Design of capacitance of the DC-link capacitor is also presented in this paper, based on the theoretical analysis. Experimental results obtained from a 10 kVA laboratory setup are shown to verify the analytical results.
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