【作者】 李进；

【导师】 荆龙；

【作者基本信息】 北京交通大学 ， 电气工程（专业学位）， 2018， 硕士

【摘要】 远距离直流输电可以有效的解决地区之间能源不平衡的问题,模块化多电平变流器(Modular Multilevel Converter,MMC)目前已成为直流输电的主流拓扑之一,该拓扑采用子模块级联结构,在高电压、大容量领域仍具备明显优势。为了提高其直流侧故障清除能力,本文主要研究了半桥子模块(Half bridge sub-module,HBSM)和箝位双子模块(Clamp double sub-module,CDSM)混合而成的混合型MMC,进行以下几个方面的研究:1)对混合型MMC的拓扑结构和工作原理进行阐述。首先介绍了 HBSM和CDSM的拓扑结构和工作原理,分别建立开关模型,并以此为依据分析了混合型MMC的拓扑结构和工作原理,根据混合型MMC具备直流侧故障清除的特点,对子模块冗余配置进行设计,然后建立稳态下的混合型MMC的数学模型,并指出系统正常运行时,1个CDSM可以等效为2个HBSM,适用于半桥MMC(HB-MMC)的功率控制器或者电压控制器、调制策略、均压策略和环流抑制策略等系统控制策略仍然适用于混合型MMC。2)分析混合型MMC从直流侧短路故障发生时刻到故障清除完成时的运行特性。首先分析短路故障时的运行特性,忽略交流侧电流注入,仅对三相六桥臂建立数学模型,利用能量等效将短路故障后的MMC等效为一个RLC电路,建立故障电流方程,求解出直流侧故障电流,根据故障电流表达式分析了电气参数对故障电流的影响;然后分析了混合型MMC清除故障电流的原理,并对混合型MMC中HBSM和CDSM的数量关系进行计算,建立了 MMC闭锁后的等效电路和数学方程,并利用龙格-库塔法计算出桥臂电流、子模块电压、交流电流和直流电流等电气量,求出故障清除完成后子模块的电容电压,为子模块电容耐压设计做出参考;利用仿真与实验验证了上述分析的正确性。3)分析混合型MMC正常运行前启动和短路故障后的重合闸。针对系统正常启动前预充电过程中2种不同类型子模块预充电特性不一致的问题,设计了多段式预充电方式,解决了子模块的自取能问题,能够将各个子模块电容电压充至额定值;短路故障后的重合闸需要降低直流侧二次过流水平和桥臂电流冲击,提出一种旁路部分CDSM的方式来降低系统二次过流水平,同时针对故障后子模块电容电压差异导致的桥臂电流冲击问题,引入冲击电流抑制策略,设定桥臂电流阈值,将桥臂电流限制在上、下阈值之间,平稳地将子模块电容电压恢复至额定值。更多还原

【Abstract】 Long distance HVDC transmission can effectively solve the problem of energy imbalance.Modular Multilevel Converter(MMC)has become the first choice topology of HVDC.The topology adopts sub-module cascade structure,which has obvious advantages in high voltage and large capacity fields,in order to improve its short-circuit fault clearing capacity.In this paper,we mainly study the hybrid MMC consisted of Half bridge sub-module(HBSM)and Clamp double sub-module(CDSM).The following aspects are studied in this paper:1)The topology and working principle of hybrid MMC are discussed.Firstly,the topology and working principle of HBSM and CDSM are introduced,and the switch model are set up respectively.On the basis of this,the topology and working principle of the hybrid MMC are analyzed.according to the characteristics of fault clearance,the sub-module redundant configuration of the hybrid MMC is established.Then,the mathematical model of the hybrid MMC is set up and conclusion that one CDSM can be equivalent to two HBSMs during normal operation,and the HB-MMC power controller、the modulation strategy.the voltage balance strategy for capacitor and the circulation suppression strategy are still applicable to the hybrid MMC.2)Analyzing the operation characteristics of hybrid MMC from the short-circuit fault to fault clearance completion.Firstly,ignoring the AC current injection,the operation characteristics of short-circuit fault are analyzed.The mathematical model of three-phase bridge arm is established and The MMC is equivalent to a RLC circuit by the energy equivalent method.The fault current equation is established and the DC side current is solved.Then the influence of electrical parameters and the system control strategy on fault current are analyzed according to the expression of the fault current.then,analyzing the principle that the hybrid MMC clear fault current and The proportion of the HBSM and CDSM in the hybrid MMC is designed.Also the MMC equivalent circuit and mathematical equation after blocking are established.Using the Runge-Kutta method,the bridge arm current、the sub-module voltage、the AC current and the DC current are calculated.The voltage of the sub-module after the completion of the fault clearance is also calculated,and the reference for withstanding voltage design of the sub-module is made.The correctness of analysis all above is verified by simulation and experiment.3)Analyzing the pre-charging process of the hybrid MMC before normal starting and reclosing process after short-circuit fault.In view of the inconsistency of the pre-charging characteristics of the 2 kinds of modules in the system,a multi-stage pre-charging mathod is designed to solve the problem of the self-power of the sub-modules,and the voltage of each module can be charged to the rated value.then,reclosing after short-circuit fault mainly reduces secondary over-current and current shock.A strategy of bypassing partial CDSM is proposed to reduce the secondary over-current level of the hybrid MMC.In view of the current shock problem caused by the difference of capacity voltage of HBSM and CDSM,a current shock suppression strategy that the arm current threshold is set up and arm current can be limited between upper and lower threshold is introduced,thus each sub-module voltage can be recovered to the rated value placidly.更多还原