基于模块化多电平换流器的直流输电系统研究

发布时间：2019-04-22 20:09

【摘要】：柔性直流输电技术在解决异步电网互联、远距离孤立负荷供电等问题上有着传统直流输电技术不可比拟的优势。然而基于两电平、三电平的柔性直流输电系统存在电压等级低、损耗较大、滤波器体积庞大等不足，在一定程度上限制了其在直流输电工程中的进一步应用。基于模块化多电平换流器的直流输电系统采用模块化设计，输出电平数高且可灵活调节，以适应不同电压等级和功率等级的要求，省去了大功率交流滤波器，降低了子模块的开关频率，适用于大功率直流输电场合。本文在介绍了模块化多电平换流器工作原理的基础上，，阐述了模块化多电平换流器的调制策略；提出了一种改进的基于排序法的电容电压均压策略，该方法可以有效地实现电容均压，降低器件的开关频率；通过分析换流器内部环流产生机理，设计了负序二倍频环流抑制控制器，仿真证实了所设计的控制器可以有效地抑制换流器的相间环流分量。本文在两相旋转坐标系下建立了基于模块化多电平换流器的直流输电系统的数学模型；分析设计直流输电系统的双闭环控制策略，与阀组级控制策略相结合，实现了对模块化多电平换流器及直流系统的有效控制；通过对两端有源和向无源系统供电的直流系统的电压、电流、有功、无功、子模块各电量的仿真分析，直观地证明了直流系统的优质特性。考虑了负序电流分量对直流输电系统及换流器的影响，本文建立了三相电压故障工况下直流输电系统的数学模型；考虑了负序分量对传统锁相环的影响，设计了故障状态下的相位检测法；以抑制交流侧负序电流为目标，提出与负序电压补偿控制相结合的双闭环控制方法；通过对换流器的子模块电容电压波动、内部环流变化的仿真分析，验证了双闭环控制方法、子模块电容电压均衡策略、环流抑制策略在三相不平衡故障条件时的有效性。
[Abstract]:Flexible HVDC technology has unparalleled advantages over traditional HVDC technology in solving the problems of asynchronous grid interconnection and long-distance isolated load power supply. However, the flexible HVDC system based on two-level and three-level has some disadvantages, such as low voltage grade, large loss and huge volume of filter, which limits its further application in HVDC engineering to a certain extent. The DC transmission system based on modular multilevel converter adopts modular design with high output level and flexible adjustment to meet the requirements of different voltage levels and power levels, thus eliminating the need of high-power AC filters. The switching frequency of the sub-module is reduced, which is suitable for high-power HVDC transmission. On the basis of introducing the working principle of modular multilevel converter, the modulation strategy of modular multilevel converter is described in this paper. An improved capacitance voltage equalization strategy based on sorting method is proposed. This method can effectively achieve capacitance voltage equalization and reduce the switching frequency of devices. By analyzing the internal circulation mechanism of the converter, a negative sequence double-frequency loop suppression controller is designed. The simulation results show that the designed controller can effectively suppress the phase-to-phase circulation component of the converter. In this paper, the mathematical model of HVDC transmission system based on modular multilevel converter is established in two-phase rotating coordinate system. The double closed loop control strategy of HVDC transmission system is analyzed and designed. Combined with the valve group control strategy, the effective control of modular multilevel converter and DC system is realized. By simulating and analyzing the voltage, current, active power, reactive power and electric quantity of each sub-module of the DC system, the high-quality characteristics of the DC system are proved intuitively. Considering the influence of negative sequence current component on HVDC system and converter, the mathematical model of HVDC system under three-phase voltage fault condition is established in this paper. Considering the influence of the negative sequence component on the traditional phase locked loop, the phase detection method under the fault condition is designed, and a double closed loop control method combining with negative sequence voltage compensation control is proposed in order to restrain the negative sequence current on the AC side. Through the simulation analysis of the capacitance voltage fluctuation and internal circulation variation of the sub-module of the converter, the validity of the double closed loop control method, the capacitor voltage equalization strategy of the sub-module and the loop suppression strategy under the three-phase unbalanced fault condition is verified.
【学位授予单位】：重庆大学
【学位级别】：硕士
【学位授予年份】：2014
【分类号】：TM721.1;TM46

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