适用于直流串联型风电场的风电变流器及风机间解耦方法

The DC wind farm with series connection which connects several groups of DC wind turbines in series to transmission voltage level, might be a suitable solution for the construction of future offshore wind farm with low-cost, because high-capacity offshore converter station eliminated. After in depth analysis of coupling mechanism and impact factor of wind energy curtailment in DC wind farm with series connection, optimized design and control of MMC based wind power converter of DC wind turbine series connected are studied with regards to high turns ratio, low frequency, fluctuated power and voltage operation. Specific decoupling method are proposed with regards to different time scale. Based on wind power distribution and wind speed correlation of any two points in wind farm, wind turbine layout and its number in series-connected DC wind farm will be investigated in order to make wind power generation difference minimal on time scale of minute above. Based on time scale of minute below, several new solutions of power decoupling methods are proposed, such as increasing rotator speed and adding storage devices. All proposed control methods can solve the problems existing in the single-input-multiple-output system with strong coupling characteristics, in order to reduce to maximal extent or totally avoid the wind curtailment.

直流串联型风电场将场内直流风机串联升压至传输电压水平，舍弃海上升压变电站，尤其适宜于未来低成本的海上风电场建设。在深入分析直流串联型风电场耦合机理和弃风影响因子后，针对串联直流风机变流器高变比、低频、波动功率和电压的运行条件与设计需求，研究基于MMC的风电变流器的优化设计与控制，并根据不同时间尺度针对性地提出直流串联型风电场解耦方法。在对风场内部风功率分布及任意两点间风速相关性研究的基础上，研究直流串联型风场风机的布局方式及串联机组数量，提出优化设计方法，使分钟级以上的时间尺度上串联机组间出力的差异性最小。在分钟级以下的时间尺度上，研究利用转子变速运行储存和释放能量、增设储能单元等手段弱化机组间功率耦合的控制方法，解决直流串联型风电场作为多随机输入-单输出、机组间强耦合系统所存在的固有问题，实现最大程度减少风机弃风或避免弃风。

着重分析了直流串联型风电场的稳态特性、动态特性和串联型直流风机的外特性边界。通过对串联直流风机控制特性的描述，提出带风机出口电压限幅的串联风机控制策略。为了实现风电变流器的轻量化，提出一种新型的分叉结构模块化多电平变换器，能够将电压等级较低的交流系统连接至电压等级较高的直流系统，并具有较低的损耗与较小的电容使用量。当串联风机间存在风电功率差异时，在分钟级以上时间尺度下，可通过风电机组的合理布局和内网联接方式的选择使得串联风机间的风电功率尽量平均分布，在分钟级以下时间尺度上，可通过串联风机的转子变速控制和增设储能单元来弱化串联风机间的耦合，减小弃风以提高直流串联型风电场的风能利用率。当串联风机簇中有风机切除时，可通过风场直流内网的设计来允许一定数量串联风机的切除，同时可以通过岸上换流站对直流传输电压的控制，来降低风机切除时对其他正常运行的串联风机的耦合影响，以提高直流串联型风电场的容错能力。