Contents I. Introduction
In The last two decades, the doubly fed induction generator (DFIG) has been a popular choice for wind energy conversion systems (WECSs) interfaced with the ac mains, due to the significant savings in the power converter—usually rated to 30% of the overall WECS power [1]. With the growing interest towards dc transmission and distribution systems—especially for wind farm interconnection [2] and dc microgrids [3], [4], researchers have started looking at the options and benefits of integrating a DFIG into a dc power network. A simple solution to minimize the number of controlled converters and their power rating is the so-called DFIG-dc system. It adopts only a single, de-rated voltage-source inverter (VSI) on the rotor side and a fully rated diode bridge on the stator, both connected to the same dc-link or dc grid [5]. This scheme effectively implements the cheapest power electronics while allowing high dynamic control and some degree of freedom to optimize losses, but suffers from some severe drawbacks, such as harmonic-related extra losses and torque-ripple. Although the torque-ripple can be mitigated at the control level [6], torque-ripple and harmonic compensation cannot be achieved simultaneously [7], without adding extra hardware such as active filters or multipulse rectifiers. In order to improve the DFIG-dc system, Nian et al. [8] proposed to replace the diode-bridge with a stator-side VSI and operate the DFIG with a slip equal to −1, then realizing a dual-VSI DFIG-dc system where both the VSIs convert half of the total power. The dual-VSI topology for wound-rotor induction machines (WRIM) was explored in the past for high-power motor drive applications in order to achieve a 2-p.u. speed range and using converters rated to 1 p.u. [9]–[13]. This inherent modularity is attractive for multi-MW wind turbines, where a larger speed range can also improve the amount of yearly energy harvested.
