This paper presents a Voltage-Modulated Direct Power Control (VM-DPC) with an additional parallel compensator for the Doubly Fed Induction Generator (DFIG) under unbalanced grid conditions. The proposed method not only guarantees a satisfying steady-state performance but provides also a regulating property of the negative-sequence output currents through designed negative-sequence parallel compensator. It can provide symmetrical stator currents and suppress the ripples in both active and reactive powers under unbalanced grid conditions. The performance of the proposed method are verified by comparing it with three different control strategies in simulations carried out in Matlab/Simulink SimScape Power System. Finally, the effectiveness of the proposed method is evaluated in an experimental prototype, which proves the proposed VM-DPC with the additional compensator has a satisfactory steady-state performance and a fast power transient response under unbalanced grid conditions.
The impedance analysis has been proposed and proved to be an effective way to analyze the grid-connected stability of Voltage Source Converter (VSC) based energy conversion systems. Most of the existing impedance analyzes are discussed based on vector oriented control, while there are few discussions about the impedance analysis of Direct Power Control (DPC). In this paper, an impedance modeling approach of VSC using the DPC is firstly proposed and analyzed, which fills the gap of impedance analysis of the DPC based converters and provides a basis for studying the corresponding grid-integration stability problems. Since there is no need for PLL and Park transformation, the impedance matrix is built directly using the αβ reference frame and features symmetrical characteristics. The proposed impedance matrix can be transformed into a positive-sequence impedance and analyzed by using SISO Nyquist criteria. The stability of a weak-grid connected VSC using DPC is analyzed in this paper. The main factors affecting system stability, including the grid and control parameters, are considered in the analysis. The effectiveness of the proposed method is demonstrated by the simulations carried out in the Matlab/Simulink Simscape Power Systems as well as a hardware-in-loop system.
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