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This paper investigates the switching patterns of direct power control (DPC) strategies for three-phase grid-connected voltage-sourced dc-ac converters. The instantaneous variation rates of active and reactive powers by applying each converter voltage vector in 12 different sectors are deduced. In terms of hysteresis look-up-table DPC, based on the power variation rates, modified switching tables are presented to alleviate the active power pulsations, and an asymmetrical hysteresis controller is proposed for active power control to eliminate its steady-state errors. As to predictive DPC, according to the power variation rates, the cause is identified to that the values of the predicted duration times for the auxiliary active voltage vector become negative. Consequently, to solve this problem, two new alternative vector sequences are proposed and compared. A simple compensation method is further added to the basic control scheme of predictive DPC to deal with grid voltage unbalances. Experimental results on a 1.5-kVA grid-connected dc-ac converter are presented to validate the feasibility of the improved switching patterns and the unbalanced compensation method on the DPC strategies for dc-ac converters.
Date of Publication: Dec. 2011