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Battery energy storage systems (BESSs) are one of the main countermeasures to promote the accommodation and utilization of large-scale grid-connected renewable energy sources. With the rapid increase in the installed capacity of BESSs, the security problem and economic problem of BESSs are gradually exposed. On the one hand, fire accidents happen on occasion; on the other hand, the operation efficiencies and battery utilizations of BESSs are not high, resulting in considerable economic losses. In this paper, the relationship between the construction scheme of a BESS and the power conversion system (PCS) is analyzed. The structures, control methods, and grid-connected/islanding control strategies of PCSs are categorized, evaluated, and compared in detail. And the design schemes of high capacity BESSs as well as relevant considerations are systematically discussed. The test waveforms of a 10-kV BESS based on a cascaded H-bridge high-voltage straight hanging PCS are shown to prove the feasibility of this advanced transformerless BESS scheme. Finally, the future development directions of high-capacity BESSs and PCSs are prospected.

Terahertz polarization converter has drawn great attention for its properties of wavefront control in recent years. However, the low transmission efficiency of these converters remains a major obstruction to be resolved. A promising solution is the all-dielectric metasurface, based on which, this study proposes a high-efficiency transmissive terahertz linear-to-circular polarization converter. The simulations show the transmission efficiency of the converter is beyond 88% with an ellipticity