Relationship Between Two Direct Power Control Methods for PWM Rectifiers Under Unbalanced Network

Active power control plays an important role in a pulse width modulation (PWM) rectifier under both balanced and unbalanced network conditions. The prior methods to achieve active power oscillation cancellation (APOC) under unbalanced grid voltages are usually implemented by forcing the grid current to track appropriate current references. They require fine tuning work of a current controller and positive-/negative-sequence extraction of grid voltage/currents and/or converter voltage, which complicates the design of the controller. This paper presents two APOC methods in the frame of direct power control (DPC) for the PWM rectifier and reveals their inherent relationship. In the first method, an appropriate power compensation is added to the original power references without modifying the internal control structure. In the second method, a novel definition of reactive power is employed to replace conventional reactive power, which achieves the aim of APOC automatically. Both methods can be easily integrated with existing DPC schemes with a slight modification. The sequence extraction required in prior methods is eliminated. The inherent relationship between these two methods is investigated, and it is found that they are completely equivalent. The two APOC methods are comparatively studied and implemented in the basic table-based DPC. Simulation and experimental results confirm the theoretical study and the effectiveness of the two methods.