Transient Characteristics of a Unified Power Flow Controller, and Design of the DC Link Capacitor

Abstract

The purpose of this paper is to perform theoretical analysis of active power flowing into, or out of, a series device in a unified power flow controller (UPFC), and to reveal what amount of electric energy the dc link capacitor absorbs or releases through the series device in a transient state. This paper theoretically derives that the electric energy flowing into, or out of, the series device during a transient state is equal to the difference between the energy stored in the line inductance in a steady state before the start of the transient state and that in another steady state after the end of the transient state. This analysis helps us to determine the required capacity of the dc link capacitor, as well as the required response of dc voltage regulation in the shunt device. Experimental results obtained from a 10-kVA laboratory setup agree well with analytical results, as well as computer-simulated results.