An integrated non-isolated DC-DC converter for applications that demand wide conversion range is proposed in this work. Using a technique known as graft scheme, it is possible to design a single-switch SEPIC-buck converter, where both the input current and the current supplied to the output stage composed of the filter capacitor and the load are non-pulsating when operation occurs in continuous conduction mode (CCM). Therefore, minimised electromagnetic interference (EMI) levels result as a consequence. Besides, the converter requires the use of a single active switch connected to the source reference node, without the need for isolated control circuitry, while it is possible to achieve wider conversion ranges than that provided by the classical buck converter. The qualitative and quantitative analysis of the converter is presented, from which it is possible to design and analyse it. Experimental results are discussed in detail to validate the theoretical considerations.
The doubly fed induction generator (DFIG) and back-to-back converter are very sensitive to power quality disturbances in grid-connected wind energy conversion systems (WECSs). Special attention has been given to protect the system from voltage sags, considering the introduction of several low-voltage ride-through (LVRT) techniques in the literature. However, only few works have really analyzed the behavior of power semiconductors during such phenomena in terms of the thermal stresses, whereas the existing studies are focused on balanced voltage sags only. In this context, this work presents a thermal profile analysis of power semiconductors in the grid-side converter (GSC) and rotor-side converter (RSC) considering a DFIG-based WECS submitted to symmetrical and asymmetrical voltage sags. The system is modeled using PLECS software and results on a 2.0 MW system are presented and thoroughly discussed. The results show that it is possible to meet the ride-through requirements during both balanced and unbalanced sags in terms of acceptable thermal stresses on the semiconductors as long as the back-to-back converter and its respective control system are properly designed.
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