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A European efficiency enhancement strategy for the grid-tied inverters with common dc bus and ac bus in parallel operation is proposed in this paper. Based on the inherent relationship between the dc bus voltage and the grid-feeding power, two dc bus voltage regulators with different output limits are employed in each grid-tied inverter. The operation of the dc bus voltage regulators depends on the grid-feeding power level. Therefore, the grid-tied inverter has priority up to the maximum efficiency operation point, and then, to operate at full load mode with the increase of the grid-feeding power. As a result, the European efficiency of the grid-tied system is improved without any communication link and extra hardware cost. The operation principle of the grid-tied inverter system is analyzed in detail with the implementation of the proposed control, and the design guidelines for key parameters are given and discussed. A prototype with three grid-tied inverters in parallel operation is built and experimental tests have been conducted on it in terms of steady-state performances and dynamic performances. The experimental results show that the operation modes of each grid-tied inverter depend on its sensing gain of the dc bus voltage. The European efficiency of the grid-tied inverter system with the conventional control and with the proposed control are 94.74% and 95.17%, respectively, which means the European efficiency has been enhanced by 0.43%. These results verify the feasibility and effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

Voltage-source-converter (VSC) technologies present a bright opportunity in a variety of fields within the power system industry. New modular multilevel converters (MMCs) are expected to supersede two- and three-level VSC-based technologies for HVDC applications due to their recognized advantages in terms of scalability, performance, and efficiency. The computational burden introduced by detailed modeling of MMC-HVDC systems in electromagnetic-transients (EMT)-type programs complicates the study of transients especially when these systems are integrated into a large network. This paper presents a novel average-value model (AVM) for efficient and accurate representation of a detailed MMC–HVDC system. It also develops a detailed 401-level MMC-HVDC model for validating the AVM and studies the performance of both models when integrated into a large 400-kV transmission system in Europe. The results show that the AVM is significantly more efficient while maintaining its accuracy for the dynamic response of the overall system. [ABSTRACT FROM AUTHOR]