强励双馈风电系统及其在严重电网故障下的优化控制研究

During the grid faults, the wind generation system should not only be able to ride through the grid faults, but also support the grid voltage by injecting reactive power to the grid. However, it is usually very difficult to satisfy the grid codes by simply improving the control strategies during the severe grid faults. Although the back to back converters in the Doubly-Fed Induction Generation (DFIG) system can be effectively protected from the grid faults by adding additional hardware in the rotor-side or the stator-side of the DFIG, it limits the dynamic-reactive-support ability of the DFIG and even aggravates the grid faults on the other hand. .By referring to the forced excitation idea of the synchronous generator, a new research idea is proposed in this proposal which can enhance the robustness of the DFIG system against the grid faults by building the forced excitation system of the DFIG rather than adding additional hardware devices. Firstly, the minimum DC-link voltage for ensuring the controllability of the rotor current should be calculated using minimum principle. Secondly, the forced-excitation converter for the DFIG system will be investigated based on the recently proposed Y-source concept. The topology optimization and the modulation strategies of the forced-excitation converter system will be researched. Further, the control scheme is studied for the proposed DFIG system to meet the lowest requirement of the grid codes. Its limitation is studied. And a novel optimal control strategy of reactive power support based the improvement of the PCC voltage is proposed. The influence of the reactive power support of DFIG on the damping ratio will be analyzed based on properly modelling the whole system. The methods to increase the damping ratio to suppress the oscillation will be investigated too. The ability of LVRT and dynamic support of DFIG under severe grid faults will be improved significantly. The project will promote the development of the wind energy generation.

电网故障时，需要风力发电系统实现故障穿越且提供动态无功支撑。严重电网故障下仅靠控制方法的改进难以满足并网导则的要求。在双馈发电机（DFIG）的转子或定子侧增加装置能够保护励磁变换器，但不利于发电机对电网的动态支撑，甚至使电网故障进一步恶化。.借鉴同步发电机强励的思想，提出与其增加其它装置，不如构建强励系统来增强DFIG对电网故障的适应和调节能力的研究思路。首先，利用极小值原理确定转子电流可控所需的最低直流母线电压。其次，提出基于Y-Source类直流母线的强励变换器方案，研究其拓扑的优化、控制和调制策略。第三，实现满足并网导则最低要求的无功支撑控制，分析其局限性，提出基于并网点电压改善的无功支撑优化控制策略，以实现对电网的有效支撑。建模分析DFIG的无功支撑对系统阻尼的影响并研究增强系统阻尼减少振荡的方法。有望显著提高DFIG在严重电网故障下的穿越和无功支撑能力，推动风力发电事业的发展。

当电网发生故障时，需要风力发电系统不仅不能脱网还应向电网提供动态无功支撑。双馈风电机组是我国已安装的变速恒频风电机组主流，但它需要通过不断地增加额外装置—— Crowbar、STATCOM来应对电网对风电机组越来越高的要求，这将导致系统成本增加、控制复杂、不可靠性增加、效率降低。本项目拟通过构建双馈风电机组的坚强励磁系统，一步到位解决双馈风力发电机的故障穿越和无功支撑问题。. 传统的双PWM励磁变换器的瓶颈是直流母线电压低于暂态转子感应电动势。将直流母线电压提高数倍，不存在技术实现的障碍，但会带来稳态损耗增加、性能下降的负面影响。构建坚强的励磁系统，不是简单地提高电压，扩大容量。因此，双馈强励变换器拓扑结构的研究是本项目研究的重点。首先，深入研究了阻抗源型逆变器（如Z Source、Y Source）应用于双馈强励拓扑的可行性。这类变换器能够实现暂态提升直流链电压，但电压的提升受限于电容电压，控制电容电压的升高需要一段时间，不能满足需要。其次，在此基础上提出了基于变结构改进qZ-Source的双馈强励变换器拓扑，通过理论、仿真和实验证明了其应对严重电网故障的有效性。但该方案存在着需要较高电压等级开关管的问题。第三，项目组经过研究，提出了新的能够用现有主流功率器件实现的拓扑，已通过仿真验证了其有效性，目前已初步制作了实验室样机，该方案有望应用于实际。. 拓扑的选择、功率器件的选择，取决于暂态转子反电势。因此，推导出最严重电网故障下的转子感应电势的最大值，是整个项目的关键之一。项目组经深入研究，解决了这一问题，仿真和实验验证了理论分析的结果。. 在进行课题研的过程中，还取得了其它几点收获：提出了基于MSOGI、DSOGI的感应电机磁链观测新方法，实现了对输入信号直流偏移、谐波的抑制，提高了磁链观测的稳态和动态性能；提出了电感磁芯损耗的精确计算方法；提出了基于MSOGI的电容部分电流提取方法，实现了电网背景谐波的抑制。