【作者】 王璐；

【导师】 徐建源；

【作者基本信息】 沈阳工业大学 ， 电气工程， 2019， 硕士

【摘要】 随着电力电子技术的发展,具有控制自由度高、可向无源系统供电、占地面积小等优势的柔性直流输电系统在远距离大容量输电工程领域中脱颖而出。基于模块化多电平换流器(Modular Multilevel Converter,MMC)的高压直流输电(High Voltage Direct Current,HVDC)技术更是凭借着输电容量大、谐波含量少、开关损耗低、应用领域范围广等优点,在电网中得到了广泛应用。在MMC-HVDC系统中,换流站内部和不对称故障的保护策略直接关乎输电系统的可靠性和安全性,因此重点对MMC-HVDC故障分析和保护策略进行研究具有重要的工程应用价值和现实意义,具体内容如下:(1)从MMC拓扑结构入手,结合子模块的工作状态和MMC每相的上下桥臂变量之间的关系,在静止坐标系下建立出MMC的三相数学模型。为了MMC-HVDC控制系统的设计,通过Park变换得到同步旋转坐标系下的数学模型。(2)在完成系统数学模型建立工作后,对MMC-HVDC系统主电路参数相关设计进行了分析工作,研究出了桥臂子模块数的取值方法、分析子模块电容的相关影响因素,推理出子模块电容的取值公式,以及桥臂电抗器参数的制约因素和不同运行状态的选取方法。然后对MMC-HVDC系统所采用的矢量控制系统进行设计,完成整流侧和逆变侧矢量控制系统的设计,并在PSCAD/EMTDC电磁暂态仿真平台中完成了仿真验证,验证所推导的系统主电路参数与所设计的矢量控制系统的正确性,为后文分析换流站发生故障状态下系统行为和保护策略的仿真验证提供平台与理论依据。(3)对MMC-HVDC系统换流站交流系统故障和换流器内部故障产生的原因进行分析。首先研究了换流站内部的子模块故障和桥臂电抗器故障,针对故障设计了子模块冗余保护和桥臂过电流保护策略,并验证了方案的正确性;(4)建立了MMC在三相不对称下的低频暂态数学模型以及功率模型,针对MMC出现三相不对称状态设计了相序分解环节,并以正序电流和负序电流为控制量设计了内环电流解耦控制系统,然后通过抑制负序电流设计了抑制负序电流的控制策略,完成了MMC-HVDC系统不对称情况下控制器的设计。最后在PSCAD/EMTDC电磁暂态仿真平台中完成了仿真验证,验证了抑制负序电流控制策略的有效性。更多还原

【Abstract】 With the development of power electronics technology,flexible DC transmission systems stand out in the field of long-distance large-capacity transmission engineering by high control freedom,power supply to passive systems,and small footprint.HVDC technology based on modular multi-level converter has been widely used in power grid because of its large transmission capacity,less harmonic content,low switching loss and wide application range.In the MMC-HVDC system,the protection strategy of the internal and asymmetric faults of the converter station is directly related to the reliability and safety of the transmission system.Therefore,it is of great engineering application value and reality to study the failure analysis and protection strategy of MMC-HVDC.The meaning is as follows:(1)Starting from the MMC topology structure,the working state of the sub-module and the relationship between the variables of the upper and lower bridge arms in each phase are combined.Then the three-phase mathematical model of MMC is established in the static coordinate system.For the design of the MMC-HVDC control system,the MMC mathematical model in the synchronous rotating coordinate system is obtained by Park transformation.(2)After completing the establishment of the system mathematical model,the analysis of the main circuit parameters related design of the MMC-HVDC system is carried out.The method of determining the number of bridge arm sub-modules is studied.The value formula of the submodule capacitance is inferred.The constraints of the parameters of the bridge arm reactor and the selection methods of different operating states are analyzed.Then the vector control system used in the MMC-HVDC system is designed to complete the design of the rectifier side and inverter side vector controllers,and the simulation verification is completed in the PSCAD/EMTDC electromagnetic transient simulation platform to verify the derived system master.The correctness of the circuit parameters and the designed vector control system provides a platform and theoretical basis for the simulation verification of the system behavior and protection strategy in the fault state of the converter station.(3)Analyze the causes of the AC system faults of the converter station of the MMC-HVDC system and the internal faults of the converter.Firstly,the sub-module fault andthe bridge arm reactor fault inside the converter station are studied.The sub-module redundancy protection and the bridge arm over-current protection scheme are designed for the fault,and the correctness of the scheme is verified.(4)The low-frequency transient mathematical model and power model of MMC under three-phase asymmetry are established.The phase sequence decomposition link is designed for the three-phase asymmetric state of MMC,and the positive sequence current and negative sequence current are used as the control quantity.The inner loop current decoupling control system is designed,and then the control strategy of suppressing the negative sequence current is designed by suppressing the negative sequence current.The controller design of the MMC-HVDC system is completed.Finally,the simulation verification is completed in the PSCAD/EMTDC electromagnetic transient simulation platform,and the effectiveness of suppressing the negative sequence current control strategy is verified.更多还原