Direct Power Control For NPC Three-level Voltage Source PWM Rectifiers Using A Vector Influence Table Method

Three-level rectifiers have a number of advantages such as low harmonic distortionof input currents, low switching voltage stress and low du/dt. Therfore, they havebeen widely used in medium and high power applications such as reactive powercompensation of the power systems, high voltage transmission and distribution, highvoltage frequency control of motor systems, etc. The DPC (direct power control)strategy is based on the evaluation of the active and reactive instantaneous powererror values to choose voltage vectors to control the active and reactive power. In thispaper, the DPC control of NPC (neutral-point-clamped) three-level rectifiers is mainlyanalyzed.To overcome the shortcomings of the conventional DPC strategy such as currentdistortion and voltage ripples of the neutral-point, in this paper, a novel direct powercontrol (DPC) strategy is proposed for NPC three-level rectifiers. In order to controlthe active and reactive power and the neutral point potential, three pre-calculatedvector influence tables are established and the optimized switching state is selected byan objective function. This paper defines the influence functions to analyze therelationships between the voltage vectors of NPC rectifiers and the change rates of theactive power, the reactive power and the neutral point potential. By discretizing andaveraging the influence functions, the influence factors of the active power, thereactive power and the neutral point potential fξ, fμ, fδare obtained, and they haveapproximately linear relationships with the change rates of the active power, thereactive power and the neutral point potential, respectively. Furthermore, threepre-calculated vector influence tables are established based on the influence factorsand by minimizing an objective function the appropriate vector can be selected.The performance of the novel DPC has been tested by simulation and experimenton the platform of PWM rectifier. It can be concluded that compared to theconventional DPC method, remarkable ripple reduction of the active and reactivepower and the neutral point potential can be achieved by the proposed method.