【作者】 张建坡；

【导师】 赵成勇；

【作者基本信息】 华北电力大学 ， 电力系统及其自动化， 2015， 博士

【摘要】 相比于传统两电平或三电平电压源型换流器,模块化多电平换流器(Modular Multilevel Converter, MMC)采用模块化拓扑设计,避免了换流器桥臂功率开关器件直接串联,解决了桥臂器件动态均压问题。同时,通过调整子模块串联个数可以实现电压及功率等级灵活变化,并且可以扩展到任意电平输出,使得输出电压电流谐波含量更低。而基于模块化多电平换流器型高压直流输电技术在新能源接入、异步电网互联、无源电网供电、城市配网等诸多领域有着广阔的应用前景,因此吸引了学术界和工业界越来越多的关注。MMC-HVDC控制策略是保证直流输电系统安全、可靠运行的关键。虽然两电平电压源换流器型直流输电(Voltage Source Converter Based HVDC, VSC-HVDC)控制策略可以移植到MMC-HVDC当中,但是由于MMC拓扑、工作机理有所区别,对MMC及其构成直流输电系统进行研究,仍具有重要的实际工程价值。本文采用理论分析和仿真验证相结合研究手段,基于改进子模块拓扑类型,重点研究MMC-HVDC拓扑及其数学模型、环流、稳态和故障控制等相关问题。(1) MMC-HVDC运行机理及其稳态数学模型为了研究MMC-HVDC系统控制策略,首先在分析其两种拓扑特点(HVDC-Light和HVDC-Plus)及其工作原理的基础上,建立了电网电压平衡条件下MMC-HVDC在两种不同坐标系下的数学模型。其次基于MMC的工作机理,分别介绍了适用于MMC的两种调制策略：最近电平逼近(Nearest Level Modulation, NLM)和载波移相正弦脉宽调制策略(Carrier Phase Shifted Sinusoidal Pulse Width Modulation, CPS-SPWM)并进行仿真分析。最后针对这两种不同的调制策略分别设计子模块电容电压均衡改进策略。(2)MMC环流及其抑制策略研究MMC由于本身结构特点和工作机理带来了桥臂环流问题。环流存在使得MMC桥臂电流发生畸变,从而提高了器件额定电流容量,增加了系统成本,同时也造成了不必要的运行损耗。在桥臂电流电压方程基础上,首先利用谐波分析法从不同方面分析了桥臂环流形成原因,进而得出交流电流、桥臂电流、环流和直流电流之间的数量关系。然后通过对方程进一步推导整理,得出了桥臂电流中环流成分的二倍频特性。为实现环流的有效抑制,分别根据环流形成原因及其特性,提出基于电容电压均衡的水平环流抑制策略和基于比例谐振控制器的垂直环流抑制策略,从而在不改变桥臂电感前提下实现环流抑制目标。(3)MMC子模块拓扑研究在直流侧发生故障时,MMC传统半桥型子模块拓扑存在不能够自行阻断直流侧故障电流问题。本文首先总结了现有子模块拓扑特点及其改进类型拓扑。其次通过分析MMC直流侧故障时桥臂电流与模块电容的充放电关系,对传统半桥型子模块拓扑进行改进,设计了串联双子模块拓扑及其改进类型,从而在不影响控制策略和调制及电容电压均衡策略前提下实现了直流侧故障电流阻断功能。同时通过研究闭锁时不同充电路径下交流电压与桥臂等效直流电压关系,定义了反映换流器故障抑制能力的故障抑制系数,并根据系统启动过程中不控整流阶段电容电压的不同,设计了自励启动方法。最后设计了直流侧故障时不同的控制策略,搭建了仿真模型,对直流侧故障特性以及相应控制策略进行分析验证。(4)交流系统故障时MMC-HVDC电流内环控制器及控制策略研究MMC-HVDC通常采用双闭环矢量控制策略,在电网电压不平衡情况下,该策略存在需要调整控制参数多、动态响应较慢问题。为了解决上述问题,首先在两相同步旋转和静止αβ坐标系下,分析了MMC-HVDC电网电压不平衡下数学模型特点,并对其功率分量关系进行相应推导,计算出控制指令,设计低压限流器。然后针对同步旋转坐标下存在直流和交流电流分量的特点,引入基于比例积分和谐振控制器的复合电流矢量控制,实现交直混合电流控制。此外为了实现交流信号无静差控制,在αβ坐标系下引入比例谐振控制器实现正负序电流统一控制。该策略无需繁琐的旋转坐标变换,从而不存在受电路参数影响的耦合项,进而方便了控制器的设计。同时分析了电网电压不平衡下的环流特性并设计了基于比例谐振和复合电流控制器的环流抑制器,最后在PSCAD/EMTDC中搭建的仿真模型基础上,验证了控制策略的有效性。更多还原

【Abstract】 Compared with the traditional two-level or three-level voltage source converter, because modular multilevel converter (Modular Multilevel Converter, MMC) adopts modular design, there are no requirement of converter valves with directly series connected switching devices and problems of dynamic balancing voltage any more. At the same time, through adjusting the number of sub modules, MMC can change the voltage and power levels flexibly, extend to any level of output easily which can reduce the output voltage and current harmonic content. Modular multilevel converter based high voltage direct current (MMC-HVDC)transmission system has broaden the application prospects in the renewable energy integration, power supply for the passive power grid, asynchronous grid interconnection, distribution network and etc. Recently, MMC-HVDC has attracted more and more engineers’and scientists’ attention.MMC-HVDC control strategy plays an important role in ensuring system safe operation. Although the control strategy of two level voltage source converters based HVDC (Voltage Source Converter Based HVDC, VSC-HVDC) can be ported to MMC-HVDC, because their topologies and working mechanism are different with each other, the research of MMC and DC transmission system is still necessary and has important practical engineering significances. In this dissertation, MMC-HVDC topology and mathematical model, sub module topologies, circulating current, steady and control strategies are investigated by means of theoretical analysis and simulation validations.(1) Operation mechanism and steady mathematical model of MMC-HVDCFirstly, two general MMC-HVDC topologies (HVDC-Light and HVDC-Plus) and operation mechanism are analysed to investigate the control strategies. The mathematical models are built in the two different reference frames under balanced grid voltage condition. Secondly, based on the working mechanism of MMC, two modulating strategies are presented:nearest level modulation (NLM) and carrier phase shifting sinusoidal pulse width modulation strategy (CPS-SPWM). Lastly, improved capacitor voltage balancing algorithms are also designed with respected to these two different modulating strategies.(2) MMC circulating current and suppressing strategiesMMC structure and working mechanism have caused circulating current problem. Circulating current not only distorts arm currents but also increases the rated current of power devices, which may increases system cost and unnecessary power loses. Based on the arm current equations, firstly, the circulating current forming reasons are studied from different aspects according to the harmonic analysis theory, then the quantitative relationship between the ac current, arm current, dc link current and circulating current is derived. Secondly, through the deep analysis, the double base frequency characteristic of the circulating current is deduced. In order to reduce or suppress the circulating current, three measures, such as a modified voltage balancing algorithm, a vertical circulating current suppressing controller based on proportional resonant and an arm current filter are developed according to circulating current forming reasons and characteristics without changing the arm inductance.(3) Research on MMC sub module topologiesIn the event of the dc link failures, traditional half bridge sub module topologis of MMC can’t clear the dc link fault current after insulated gate bipolar transistors (IGBTs) are blocked because the forward diodes still conduct as rectifier bridges. In this case, the ac breaker has to be opened to cut off the fault current which may cause some adverse effects. This paper firstly summarizes the existing sub module topologic features and improved type of topologies. Secondly, the relationship between the arm current and capacitor’s charge or discharge states are analyzed after dc link fault, the traditional half bridge sub module topology is improved, series connected double sub modules topology (SDSM) and improved hybrid sub module type (IHSM), which can achieve dc link current blocking capability without affecting the control strategy, modulating and capacitor voltage balancing strategy, are designed. What is more, through investing the different current charge paths, the fault current blocking coefficient is defined as the ratio between arm equal DC voltage and ac voltage magnitude to indicate the blocking effect. The self startup methods are also designed according to the different capacitors rectified stages. Lastly, the different control strategies after dc link fault are designed and the simulation model is built to verify these topologies’and strategies effectiveness and feasibility.(4) Research on MMC-HVDC inner loop current controllers and strategies under unbalanced grid voltage conditionMMC-HVDC usually adopts double closed-loop vector control strategy under unbalanced grid voltage in positive and negative rotating reference frame, which has problems such as too many parameters needed to be adjusted and dynamic response being unsatisfied. To solve these problems, firstly, the mathematical model of MMC-HVDC in this case are analyzed in the two phase synchronous rotating and stationary a P reference frame, power characteristics and the corresponding relations between different power components are derived to calculate the current order for different control strategies. What is more, the corresponding voltage dependent current order limiter (VDCOL) is also designed to solve possible over current problem. Secondly, becaue there are ac and dc sequence currents, a hybrid current vector controller based on proportional and integral is used to control this hybrid current. In order to control the ac signal with no static error, the proportional resonant (PR) controller is introduced to. control positive and negative seence current uniformly in α β reference frame. This control strategy does not need the complex reference transformation, so the disadvantageous effect of coupled voltage which is easily influenced by circuit parameters is eliminated and the control algorithm becomes easy to be realized. At the same time, the circulating current characteristics under unbalanced grid voltage condition are also analyzed; the circulating current suppressor based on PR is designed. At last, the simulation platform of MMC-HVDC is built in the electromagnetic transient simulation software PSCAD/EMTDC to verify this controller’s effectiveness and feasibility.更多还原