针对可再生能源分布式并网发电系统高变换效率和高电能质量的要求，本项目在总结无变压器型并网逆变器技术特点的基础上，研究其拓扑结构和调制原理的形成规律和演绎方法，进而提出新型的电路拓扑和调制策略。通过附加开关管的控制解耦，实现共模对地漏电流的消除；通过倍频调制方式，满足并网电能高质量的要求；通过功率双向四象限调制方法，实现并网发电有功和无功的调节。项目还研究非理想条件下开关器件寄生结电容、并网滤波器结构等实际问题对无变压器型并网逆变器工作状态的影响，针对国际标准中严格的并网规范和安规要求，提出相应的优化设计方法，实现理论分析和设计实践的统一。本项目的开展，为太阳能光伏、电动汽车等新兴产业中相关问题的解决提供可参考的理论和技术基础；积极促进国家在并网逆变技术领域的研究和发展，增强国家在该领域知识产权保护的话语权和国际竞争力，加速可再生能源及分布式并网发电技术在我国的应用和推广。

Focused on the requirements of high conversion efficiency and high power quality in distributed generation systems, and based on the overview of the existing studies and technologies in grid-connected inverters, the general laws of the topologies derivation and the modulation principle of transformerless grid-connected inverters are researched and explored in this project. And the novel circuit topology and modulation strategy are proposed by the general law. Furthermore, the common-mode leakage current is eliminated through controlling the additional switches and decoupling the DC and AC sides of the circuit. The double-frequency SPWM strategy is achieved, and the harmonic contents of output currents are reduced and the power quality is improved accordingly. The four-quadrant operation with bidirectional power flow is proposed in order to implement the regulations of active and reactive power. Moreover, the influences of nonideal factors on the operation modes of transformerless grid-connected inverters, such as the switches' parasitical junction capacitance, the grid-connected filter structure, etc., are discussed detailedly. Accordingly, the effective optimization design methods are presented to accord with the grid-connected standards and security specifications of the grid-connected power system. As a result, the project will provide referable theory base and technological solution for the emerging industries including solar energy, electric vehicle, etc.. Furthermore, its implementation will greatly motivate the research and development in grid-connected inverters, strengthen the nation's rhetorical power in the protection of the intellectual property and also its international competitiveness in this field, and accelerate the application and popularization of renewable energies and distributed generation systems.