海上全直流风场高功率密度电力变换及电压源控制理论

The concept of the DC grid based offshore wind farms is proposed in this proposal. In this wind farms, MMC is adopted for flexible DC power collection and transmission of large-scale offshore wind power from the generator side to the grid side, in order to overcome the issues of reactive power and voltage, power loss coursed by a long-distance AC power collecting cable and the impact on grid when large-scale offshore wind power integration. For three power conversions of wind generator, voltage step-up offshore, and inversion onshore in this power electronic network, novel topology, control and modulation method are researched considering offshore special requirement to improve power conversion efficiency, power density and operational stability for MMC under the condition of large step-up ratio rectification, bidirectional conversion on medium frequency and integration of large fluctuated power through inverters. And then, the theoretical framework for MMC efficient and stable operation and control under different frequency and turns ratio is established. So as to solve the basic theoretical problems for efficient and stable operation of the power electronic network of DC grid based wind farms. Next, the mechanism of machine-grid electrical oscillation caused by the converter operation and the instability mechanism caused by PLL will be revealed systematically, and the voltage source control theory of DC based wind farms with three-level power conversion is proposed. It is useful for improve the stability of large-scale offshore wind farm integration and its grid friendly. Above all, the research achievements will lay the theory foundation for the networking, operation and control of DC grid based wind farms.

本课题提出从发电机出口开始即采用基于MMC的柔性直流汇集和传输大型海上风场电力及并网的全直流风场概念，以克服远距离大型海上风场采用交流汇集电力和并网时由电缆引起的无功电压、损耗及电网冲击问题。在这样的电力电子电网中，对应于风电变流、海上升压和岸上逆变三个变换环节，分别针对MMC在大变比工况下的整流、中频工况下的双向运行和常规工频工况下巨量波动功率逆变并网的问题，结合海上风场的特有需求，研究提高转换效率、功率密度和稳定运行的新型拓扑结构和控制与调制方法，建立MMC在不同频率和不同变比工况下的高效、稳定运行与控制理论体系，解决全直流风场电力传输变换的基础问题；系统地剖析由逆变器控制引起的机-网电气振荡和PLL引起的失稳机理，研究含三级电力变换的全直流风场“电压源”并网控制理论与方法，解决大型海上风场稳定并网问题，大幅提升风电并网的友好性。本课题的研究为全直流风电场的组建和运行控制提供理论依据。

本课题提出从发电机出口即采用MMC汇集、传输和并网的全直流海上风电并网概念，以克服远距离大型海上风场采用交流汇集电力和并网时由电缆引起的无功电压、损耗及电网冲击问题。在这样的电力电子互联系统中，在风电变流、海上升压和岸上逆变三个变换环节，课题分别针对MMC在大变比工况下的整流、中频工况下的双向运行和常规工频工况下巨量波动功率逆变并网的问题，研究了提高转换效率、功率密度的拓扑结构和增强系统并网稳定性的控制与调制策略。针对直流输出型风电机组，提出了一种新的适用于高变比工况下运行的BMMC拓扑结构，并建立了BMMC的稳定运行与控制方法。针对直流升压站用大功率DC/DC，提出了隔离型MMC-DC/DC变换器的软开关运行及能量平衡控制策略，探讨了非隔离MMC-DC/DC直流电力变压器拓扑的演变规律，提出了三种新型拓扑结构及其控制与调制策略。针对全直流风场与交流主网的交互问题，系统地剖析了由逆变器控制引起的机-网电气振荡和PLL引起的失稳机理，研究了含三级电力变换的全直流风场“电压源”并网控制理论与方法，为大型海上风场稳定并网及对电网频率支撑提供了一种有效手段。本课题的研究为全直流风电场的组建和运行控制提供了理论依据，形成了成套的设计指导方案。同时，研究成果在直流风电机组、直流配电网及高比例可再生能源稳定友好并网等领域均得到了广泛的应用，并具有良好的发展前景。