借助于传输容量大、有功功率快速可控等优势，LCC自提出以来便得到了良好的应用。同时为了改善其对交流电网依赖程度高、需要消耗大量的无功功率等缺点，具有有功和无功控制灵活、潮流反向容易、易于接入弱系统等优点的MMC也逐渐得到了广泛关注。但是由于MMC采用了全控型器件，其传输容量以及经济效益相比于LCC仍有较大差距，因此为了充分发挥二者的优势，LCC-MMC混合直流输电系统为高压直流输电技术提供了新的可能。由于LCC-MMC目前仍处于研究初期，许多理论与工程实践的基础仍需深入分析，本文即对LCC-MMC混合直流输电系统的控制策略及故障穿越策略进行研究：首先针对LCC-MMC混合直流输电系统主接线方式进行研究，结合目前的技术水平提出了可行性较大的四种混合方式，并分析了各个方式的特性和优缺点。同时为了研究混合直流系统的可行控制策略，详细分析了LCC-HVDC与MMC-HVDC的基本运行特性与控制原理，并分别对LCC-HVDC与MMC- HVDC的阀控以及换流器级控制策略进行了研究。在此基础上，根据所提出的控制策略搭建了各自的仿真模型，对LCC与MMC换流器的稳态与暂态特性进行了必要的分析。然后针对LCC-MMC混合直流输电系统的稳态及暂态控制策略进行研究。选取最具有工程实践意义的LCC-MMC端-端混合直流系统作为研究对象，提出了其可行的启动及稳态控制策略，并进行了仿真验证；同时，基于所建的仿真模型，对LCC-MMC高压直流输电系统的故障特性进行了仿真分析，尤其是针对其直流故障不能自清除的原因进行了深入的理论分析与验证。最后基于对上述原因的分析，提出了基于集成直流断路器的模块化多电平换流器（MMC integrated with DC circuit breaker, IDCB MMC）和双向旁路晶闸管组成的新型混合直流输电系统。在发生直流短路故障时，通过换流器与集成直流断路器在控制上的配合，IDCB-MMC可以将故障电流的能量转移到线路中间的两个能量吸收支路，从而实现以半桥子模块作为基础的具有故障电流隔离能力的MMC。仿真结果显示该系统具有良好的故障自清除能力与应用经济性，且不改变LCC-MMC原有的稳态特性，具有良好的工程应用前景。
Since firstly proposed, LCC-HVDC having the advantages of large transmission capacity and rapid control of active power, was widely applied. Yet disadvantages such as close dependence on the AC grid and large consumption of reactive power turned up, the conquest of which led to MMC, a more flexible control of active and reactive power, an easier reverse of power flow, and an easier access to weak systems. With all the merits, MMC-HVDC adopting the full controlled device lags behind in transmission capacity and economic efficiency to LCC. In this sense, LCC-MMC hybrid transmission system giving full play to their individual advantages provides new possibilities for HVDC transmission technology. In its initial researching stage, LCC-MMC system needs in-depth analysis into its theoretical and engineering practice basis, into which the paper designated to look to explore the control strategy and fault clearance strategy of this system.First, main connection modes of all types of LCC-MMC hybrid transmission system were studied, in which four kinds of most feasible transmission systems in the current technology situation were brought up and carefully analyzed in terms of their own characteristics and pros and cons. Then the operating characteristics and control principles of LCC-HVDC and MMC-HVDC were looked into in detail to establish the control of hybrid transmission system strategy. Next, the paper analyzed the valve control and converter level control strategies of LCC and MMC respectively, on which the distinct simulations were built through different control strategies, and the steady-state condition and transient characteristics of the LCC and MMC were carefully study.Then, the steady-state condition and transient control strategies of LCC-MMC Hybrid transmission system were studied. Based on the analysis of the existing Hybrid transmission system, the PTP LCC-MMC Hybrid system with the most practical engineering model was selected as the research object, and commissioning strategy and steady state operation strategies were proposed and simulated for verification. At the same time, based on the established simulation model, the fault characteristics of LCC-MMC Hybrid transmission system were simulated and analyzed, especially focusing on deep theoretical analysis into the reasons why its DC fault cannot be automatically cleared.The last part summarized the above description and came up with a new LCC-MCC hybrid system based on an MMC topology integrated with DC circuit breaker （IDCB MMC） and double bypass thyristors. Should a DC short-circuit fault occurred, IDCB-MMC can transfer the energy of the fault current to the two energy absorption branches in the middle of the line through the control of the converter and the DC circuit breaker, and half-bridge submodule can be used as fault current capability of the MMC. The simulation proves sound fault self-clearing capability and economic feasibility, absent from altering its original steady-state characteristics in such system, offering a bright prospect in engineering application.
