Dynamic Voltage and Current Assignment Strategies of Nine-Switch-Converter-Based DFIG Wind Power System for Low-Voltage Ride-Through (LVRT) Under Symmetrical Grid Voltage Dip

This paper proposes the dynamic voltage and current assignment (DVCA) strategies for doubly fed induction generator (DFIG) wind power system, which is fed by nine-switch converter (NSC). With the proposed DVCA strategies, the voltage and current capacities of switches in NSC are dynamically assigned to the rotor- and grid-side branches, depending on different grid conditions. With dynamic voltage assignment, the rotor-side gets more voltage to suppress the overcurrent when the symmetrical grid voltage dips, and with dynamic current assignment, the grid-side gets more current capacity for reactive current compensation. Based on the proposed DVCA strategies and configuration, NSC-based DFIG system can be globally optimized, so that the rating capacities designed in the normal condition can also be fully used during low-voltage ride-through (LVRT) period, and no extra capacity margins or auxiliary circuits are needed. The proposed DVCA strategies and global optimization are designed, and also compared with those typical B2B-based DFIG systems. The experimental results from a 1.5-kW NSC-based DFIG system verify the validity of the analysis.