With increasing penetration of the renewable energy, the grid-tied PWM inverters need to take corresponding responsibilities for the security and stability of future grid, behaving like conventional rotational synchronous generator (RSG). Therefore, recognizing the inherent relationship and intrinsic differences between inverters and RSGs is essential for such target. By modeling the typical electromechanical transient of grid-tied PWM inverters, this paper first proves that PWM inverters and RSGs are similar in physical mechanism and equivalent in mathematical model, and the concept of static synchronous generator (SSG) is thereby developed. Furthermore, the comprehensive comparison between RSG and SSG is carried out in detail, and their inherent relation is built. Based on these findings, the rationality and feasibility of migrating the concepts, tools, and methods of RSG stability analysis to investigate the dynamic behaviors and stability issues of SSG is therefore confirmed. Taking stability issues as an example, the criteria of small signal and transient stability of a typical grid-tied PWM inverter is put forward to demonstrate the significance of the developed SSG model (including synchronizing coefficient, damping coefficient, inertia constant, and power-angle curve), providing clear physical interpretation on the dynamic characteristics and stability issues. The developed SSG model promotes grid-friendly integration of renewable energy to future grid and stimulates interdisciplinary research between power electronics and power system.

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Power electronics solutions based on multiple converter configurations offer cost-effective solutions by integrating a number of components at input or output power stages. This paper proposes a new multi-input isolated three-level converter for renewable and sustainable energy systems adopting high dc link voltage. Multiple dc sources are integrated to the three-level dc/dc converter before the isolation stage, resulting in reduced part-count, determining dc link voltage level and allowing flexibility in transformer design. The proposed architecture eliminates two boost switches which are present in the two-stage counterpart. The input inductors are operated in discontinuous conduction mode; thus, power can be shared between input sources through proper selection of input inductors. A low voltage prototype has been designed to serve as a proof of concept.Index Terms-multi-input converter, three-level converter, renewable energies, dc/dc converter, pulsating current cell.

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