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- Control of single-phase electrolytic capacitor-less isolated converter for DC low voltage residential networksPublication . Santos, Nelson; Silva, José Fernando; Soares, VascoIn recent years, there has been a desire to improve electricity generation and consumption, to reach sustainability. Technological solutions today allow a rational use of electricity with good overall performance. Traditionally, from production to distribution, electrical energy is AC-supported for compatibility reasons and easy voltage level transformation. However, nowadays most electric loads need DC power to work properly. A single high-efficiency central AC-DC power converter may be advantageous in eliminating several less efficient AC-DC embedded converters, distributed all over a residential area. This paper presents a new single-phase AC-DC converter using one active bridge (most isolated topologies are based on the dual active bridge concept) and a high-frequency isolation transformer with low-value non-electrolytic capacitors, together with its control system design. The converter can be introduced into future low-voltage DC microgrids for residential buildings, as an alternative to several embedded AC-DC converters. Non-linear control techniques (sliding mode control and the Lyapunov direct method) are employed to guarantee stability in the output DC low voltage with near unity power factor compensation in the AC grid. The designed converter and controllers were simulated using Matlab/Simulink and tested in a lab experimental prototype using digital signal processing (DSP) to evaluate system performance.
- High-frequency transformer isolated AC-DC converter for resilient low voltage DC residential gridsPublication . Santos, Nelson; Silva, J. Fernando; Soares, VascoIn a global and growing society, it is necessary to rethink strategies in order to minimize the environmental impact resulting from the progressive increase of energy consumption and the misuse of energy resources. Energy power system converters must be considered as a global way to spare most of the wasted energy. Today, in the power distribution infrastructure, including modern residential buildings, most equipment have power supplies with imbedded AC-DC power converters which may have overall losses as high as 25% regarding the equipment output. Therefore, common DC buses for residential applications are being studied to increase equipment efficiency. This paper presents and designs an AC-DC isolated converter that uses a full-bridge matrix topology with high-frequency isolation transformer and non-electrolytic capacitors to integrate into the future residential buildings DC bus, presenting a reliable alternative to AC power. Non-linear control techniques (sliding mode control and backstepping control) are employed to guarantee stability and disturbance robustness to the output DC low voltage, while enforcing sinusoidal input AC current and power factor correction. Control strategies are described and simulation results are presented and discussed.
- High frequency transformers for solid-state transformer applicationsPublication . Santos, Nuno; Chaves, Miguel; Gamboa, Paulo; Cordeiro, Armando; Santos, Nelson; Pinto, Sónia FerreiraThis paper focuses on the study of the high frequency transformer incorporated in solid- state transformers, specifically on the development of the steps that enable the design of an optimized high frequency transformer and its equivalent model based on the desired characteristics. The impact of operating a transformer at high frequency and the respective solutions that allow this impact to be reduced are analyzed, alongside the numerous advantages that the utilization of these transformers has over traditional 50/60 Hz transformers. Furthermore, the power scheme of the solid-state transformer is outlined, focusing on the power converters, which are immediately before and after the high frequency transformer (HFT). We also investigate a control technique that allows for correct operation and the existence of power bidirectionality. In a novel approach, this paper demonstrates the systematic steps for designing an HFT according to the desired specifications of each given project, helping students and engineers achieve their objectives in power-electronic applications. Moreover, this paper aims at increasing the knowledge of this area of power electronics and facilitating the development of new topologies with high power density, which are very important to the integration of renewable power sources and other applications. Finally, a simulation is presented to validate a high frequency transformer and its control technique.