Regulation of the capacitor voltages in a direct-like cascade AC-AC converter for FACTS controllers

This paper develops a scheme for the regulation of the capacitor voltages of a recently introduced direct-like AC/AC converter which uses the cascade converter as a building block. In this AC-AC converter topology the cascade converter is intended to replace the standard valve, which typically comprises a number of switching devices in series. Previous investigations in the area of FACTS controllers have shown that this new converter approach has inherent limitations when operating as a synchronous voltage source supplied from the line. To overcome this, the scheme injects a small compensation voltage into all converterpsilas valves and controls their currents according to a multiple DC voltage regulator system. Compensation voltage is orthogonal to the line voltage and allows independent control of capacitor voltages. The condition when a quadradure voltage and current components at the fundamental frequency and the situation where low order harmonic voltage and current components, as applied to the case of a UPFC-like power controller, are investigated. Experimental results from a low-power AC-AC converter prototype validate the control scheme and illustrate the ability of this new power converter topology to control, independently, the active and reactive power in the line.