【作者】 孔明；

【导师】 汤广福；

【作者基本信息】 中国电力科学研究院 ， 电力电子与电力传动， 2014， 博士

【摘要】 电压源型换流器高压直流输电(Voltage Sourced Converter based HVDC VSC-HVDC)技术是一种新型的直流输电技术,已有的工程运行经验表明,其非常适合孤岛供电、新能源并网和城市配电网增容等领域应用。近年来围绕VSC-HVDC技术的研究取得了一系列的突破,其中最引人注目的是模块化多电平换流器(Modular Multi-level Converter, MMC)在直流输电工程上的成功应用。MMC自身储能单元能量的有效控制是保证系统暂稳态运行性能的基础和关键。然而,随着输送容量和直流电压等级的提升,MMC所需储能单元急剧增加,对控制系统提出了更高的要求。再者,拓扑结构和工作机理的不同使得MMC在某些工况下的暂态特性相比于传统两电平、三电平换流器有较大区别。本文以MMC-HVDC换流器能量平衡控制机理为出发点,对换流器在对称以及桥臂子模块数和电网电压两种非对称工况下的暂稳态特性及其优化控制策略进行了研究。1、研究了计及各桥臂子模块数量差异的MMC-HVDC数学模型。建立了完整的MMC开关周期平均模型,推导了桥臂交流小信号模型；考虑子模块电容电压之和与直流电压的差异,建立了系统对称和两种典型非对称工况下MMC的低频等效模型。2、研究了MMC-HVDC换流器能量平衡控制策略及功率运行区间的优化方法。分析了桥臂分段电容电压平衡机理,提出了两种段间电容电压平衡控制策略；分析了MMC开关频率的影响因素,提出了以满足周期内最值控制要求为目标的段内电容电压优化控制策略,有效降低了器件的等效开关频率；分析了上下桥臂、相间以及总的子模块能量平衡控制机理,讨论了不同站级有功类控制方式下总的子模块电容电压控制的实现方式；提出了基于三倍频调制电压注入和子模块基值调整的MMC-HVDC换流器功率运行区间优化方法。3、研究了基于分桥臂电流控制的子模块故障非对称容错控制策略。分析了故障子模块旁路退出对MMC内部及输出特性特性的影响,讨论了保证系统持续运行的子模块故障数量上限；提出了改进的分桥臂电流控制策略,结合两种子模块电容电压控制目标,改善了MMC的子模块故障非对称容错控制效果。4、研究了基于子模块电容电压和环流预估的非对称电网故障穿越优化控制策略。基于瞬时功率平衡理论,提出了子模块电容电压在线预估实现方案；讨论了非对称电网电压下各序分量对MMC内部及直流侧变量的影响；明确了暂态期间子模块电容电压和桥臂环流等内部变量的预期控制目标；提出了基于子模块电容电压预估调制和桥臂环流预估控制的复合控制策略；结合具体的站级控制策略,仿真分析了复合控制策略对改善系统暂态运行性能的控制效果。5、开展了401电平MMC-HVDC动模系统相关控制策略的试验研究。介绍了401电平物理动模、混合实时仿真平台的系统架构和主要功能,基于两种试验系统分别进行了分段电容电压平衡和非对称电网故障穿越优化控制动模试验,试验结果验证了相关理论分析和所提出的控制策略的正确性。更多还原

【Abstract】 Voltage Sourced Converter based HVDC (VSC-HVDC) systems is a new kind of HVDC technology. It has been indicated that VSC-HVDC is very suitable for island power supply, renewable energy source integration and urban distribution network capacity increase etc. There has been a series of striking breakthroughs in VSC-HVDC technology in recent years, one of which is the successful application of Modular Multi-level Converter (MMC) in an HVDC project.The effective control of the energy stored in MMCs is a basic and key requirement to ensure the system’s well-performance both under the steady and transient states. However, with the enhancement of the power transmission capacity and dc voltage level, the sub-module number in MMC increases dramaticly, which places greater demands on the control system. Furthermore, because of the topology and operating mechanism, the transient characteristics in some conditions are of great differences compared to the traditional two-level or three-level converter. Based on the study of the energy-balance mechanism of MMC, this dissertation mainly focuses on the transient-performance analysis and the optimization control under balanced and the two typical unbalanced conditions.1. Taking into account the variation of sub-module numbers, the mathematical models of MMC-HVDC are studied. Based on the cycle by cycle switch average model of MMC, the ac small signal model of MMC is presented. Considering the difference between the sum of capacitor voltages in each arm and dc voltage, the low-frequency mathematical models of MMC under the unbalanced sub-module and grid voltage conditions are set up.2. The energy-balance control strategy and operation region optimization method are studied. According to the mechanism of arm-segment control scheme, two voltage-balancing control strategy among segments are proposed. The factors influencing the switching frequency of IGBTs in MMC are analyzed, and an optimization capacitor voltage balancing control strategy within each segment is proposed to reduce the switching frequency. The energy balance mechanism among arms and the total energy control for each MMC in a two-end system is introduced. Combing with the total energy control schemes of MMC in different stations, the power operation region optimization method, based on the injection of triple modulation voltage and the adjustment of capacitor voltage, is discussed.3. The asymmetric sub-module fault-tolerant control of MMC based on the arm current-based control scheme is studied. The impact of sub-modules asymmetric faults on the internal and output characteristics of MMC is analyzed and the ceiling of the failure sub-module is determined. An improved arm current-based control strategy is proposed. Based on the two voltage control targets, the performance during and after bypassing the failure sub-module is improved obviously.4. The unbalanced grid fault ride-through optimization control strategy bases on the sub-module capacitor voltage and the circulating current estimation are studied. An online sub-module capacitor voltage estimation scheme is proposed. The effect of each sequence component to the internal and the dc-side characteristics of MMC is discussed. The control targets of sub-module capacitor voltage and circulating current under unbalanced condition is determined. Then, a compound control strategy include the sub-module capacitor voltage predicted based nearest level modulation and circulating current predicted based direct circulating current control is proposed. Combined with specific station-level transient control strategy, the validity of the compound control to improve the transient performance is analyzed.5. The proposed control strategies are tested based on the401-level MMC-HVDC dynamic simulation experiment system. The system architectures and main features of the401-level physical dynamic simulation platform and digital-analog hybrid real-time simulation platform are introduced respectively. The algorithm for arm-segment capacitor voltage balancing and the optimization control strategy for unbalanced grid conditions are tested and verified respectively.更多还原