Minimum-Loss Control Strategy for a Dual-VSI DFIG DC System

This article addresses the minimum-loss control of the dual voltage-source inverter (VSI) and doubly fed induction generator (DFIG) system connected to a dc link. The minimum-loss operating conditions for field-oriented control based on the airgap flux are obtained analytically using Lagrange multipliers and validated with numerical optimization. As the main contribution of this article, the analysis accounts for core and VSI losses, providing the optimal stator frequency law and rotor/stator d-axis current split ratio, and an implicit expression for the optimal flux trajectory formulated as equality between suitable d-axis and q-axis loss functions. In the proposed implementation, this implicit condition is enforced by using a proportional-integral controller and avoiding look-up tables. Furthermore, the stator and rotor VSI controls are implemented in two independent digital signal processors with no communication, which may ease the use of off-the-shelf VSI units. The optimal conditions and control strategy are fully validated by simulations and experiments on a prototype. The main scope of application is wind-energy dc-grid technology.