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For the two-stage single-phase converter, including the power factor correction (PFC) ac-dc converter and dc-ac inverter, a second harmonic current compensator (SHCC) can be introduced to handle the second harmonic current (SHC) for removing the undesired electrolytic capacitor. Consequently, the two-stage single-phase converter becomes a multi-converter system. In this paper, the ac-dc stage or dc-ac stage in the system is categorized into two types in terms of the control objectives, i.e., the bus-voltage-controlled converter (BVCC) and the bus-current controlled converter (BCCC). The SHCC is a BCCC since its bus-port current is directly controlled, and the bus-port impedance of the SHCC is derived, which is affected by the ac-dc stage or dc-ac stage since the SHC reference is extracted from them. The system stability is investigated with the impedance-based stability criterion, and it is found that the system stability is different when the ac-dc stage or dc-ac stage is operated as a BVCC or a BCCC. Specifically, only when the ac-dc stage is operated as a BVCC, the system is unstable; while for other cases, the system is certainly stable. For the applications where the ac-dc stage is operated as a BVCC, a virtual resistor is introduced and realized by the control scheme of the SHCC to ensure the system stability. Finally, a two-stage single-phase PFC ac-dc converter is fabricated and tested to verify the theoretical analysis.

Renewable Energy Systems have been in the spotlight of the academic and industrial research for more than two decades, thanks to the development of several fields related to the Electrical Engineering. More recently, with the increasing complexity of the individual renewable energy systems and the interconnection to the grid, the scientific panorama has been witnessing to a convergence of different topics, which span across several IEEE-IES thematic areas: power electronics, electrical machines, smart grids, energy storage, transportation electrification and aerospace. After a brief overview of the renewable energy technologies, this work deals with how the convergence of multiple technologies developed to provide marginal support to the grid has evolved into the foundation of the future utility grid and expanded to transportation sector. It will be shown how the design of a renewable energy system cannot prescind anymore from the electrical grid and from the ancillary services that are requested. Example of convergence are given for a smart transformer application and for a transportation application.

The power grid in a typical micro distribution system is non-ideal, presenting itself as a voltage source with significant impedance. Thus, grid-connected converters interact with each other via the non-ideal grid. In this study, we consider the practical scenario of voltage-source converters connected to a three-phase voltage source with significant impedance. We show that stability can be compromised in the interacting converters. Specifically, the stable operating regions in selected parameter space may be reduced when grid-connected converters interact under certain conditions. In this paper, we develop bifurcation boundaries in the parameter space with respect to Hopf-type instability. A small-signal model in the dq-frame is adopted to analyze the system using an impedance-based approach. Moreover, results are presented in design-oriented forms so as to facilitate the identification of variation trends of the parameter ranges that guarantee stable operation.