Design of a discrete-time linear control scheme for a modular UPQC

A discrete-time linear control scheme for a multilevel three-phase Unified Power Quality Conditioner (UPQC) based on Single-Phase Power Cells is presented. The multi-variable, non-linear, and coupled features of these topologies make the control strategy design a difficult task. Controlling this kind of systems with mono-variable linear controllers - as proposed in this work - presents significant advantages compared with other approaches as simplicity in the design steps due to the large amount of tools developed for this kind of schemes. Particularly, a classic design method based on the Root Locus approach is used to choose the controllers parameters in order to achieve a given dynamical behavior. The proposed control strategy in this work is intended to be implemented on a DSP based system and thus its design is carried out on the discrete-time and -frequency domain. Also, due to the inherent asymmetries among the power cells in a modular topology, a dedicated individual control strategy is included to ensure a symmetrical distribution of the power among the power cells. This feature allows the semiconductor devices of each corresponding module to operate under the same voltage and current ratings. Results showing transient conditions are shown to probe the feasibility of the control scheme.