Dual Control Strategy Based Three-Phase Four-Wire Distribution Systems in Unified Power Quality Conditioner for Power Factor Improvement

Abstract

This paper presents the study, analysis and practical implementation of a versatile unified power quality conditioner (UPQC), which can be connected in both three-phase three-wire or three-phase four-wire distribution systems for performing the series-parallel power-line conditioning. Thus, even when only a three-phase three-wire power system is available at a plant site, the UPQC is able to carry out power-line compensation for installed loads that require a neutral conductor to operate. Different from the control strategies used in the most of UPQC applications in which the controlled quantities are nonsinusoidal, this UPQC employs a dual compensation strategy, such that the controlled quantities are always sinusoidal. Thereby, the series converter is controlled to act as a sinusoidal current source, whereas the parallel converter operates as a sinusoidal voltage source. Thus, because the controlled quantities are sinusoidal, it is possible to reduce the complexity of the algorithms used to calculate the compensation references. Therefore, since the voltage and current controllers are implemented into the synchronous reference frame, their control references are continuous, decreasing the steady-state errors when traditional proportional-integral controllers are employed. Static and dynamic performances, as well as the effectiveness of the dual UPQC are evaluated by means of experimental results..