【作者】 林磊；

【导师】 邹云屏；

【作者基本信息】 华中科技大学 ， 电气工程， 2007， 博士

【摘要】 工业和经济的发展给人们带来了一系列的新问题:无功和谐波对电网的污染日益严重,环境日益恶化,能源也越来越匮乏。在这个背景下,随着多电平技术的发展和日趋成熟,使用双三电平变换器对大容量异步电机传动系统进行控制成为近年来研究的热点。它具有高功率因数,能量可双向流动,节能效果明显,控制性能优良,对电网污染小等优点,在中高压大容量交流传动领域有着广阔的应用前景。本文紧紧围绕双三电平变换器供电的异步电机直接转矩控制系统展开研究,采用先分别研究三电平PWM整流器和三电平异步电机直接转矩控制系统,再将其作为整体进行综合分析考虑的方法。论文的主要工作和取得的研究成果如下:(1)本文阐述了三电平变换器的空间矢量脉宽调制(SVPWM)方法的相关问题,确定了论文使用的减小开关损耗、限制dv/dt的SVPWM方法。引入电流调制的概念,通过旋转坐标变换的方法,在同步旋转dq坐标系下对SVPWM调制方式下的三电平变换器中点平衡问题进行了理论量化分析。(2)本文在阐述三电平PWM整流器直接电流控制系统设计方法的基础上,提出了三电平PWM整流器的中点电位平衡Bang-Bang控制方法。该中点电位平衡控制方法通过对三相桥臂电流及中点电位方向的检测,舍去SVPWM调制序列对中点电位平衡不利的小矢量开关状态,从而实现对中点电位平衡的控制。同时,该方法不会增大系统的开关损耗和桥臂输出电压的dv/dt。仿真分析和实验结果都证明了其可行性。(3)本文建立了三电平异步电机调速系统的统一数学模型。通过该数学模型,首次分析了各种类型的中点电位不平衡对电机控制性能的影响,得出的结论能有效的指导三电平异步电机调速系统的中点电位平衡控制方法设计。本文先从两电平入手,阐述了直接转矩控制的基本原理及相应关键技术。(4)本文提出了新型固定合成矢量的异步电机直接转矩控制方法,并提出了相应的中点电位平衡Bang-Bang控制方法。论文分析了直接使用三电平自身电压矢量进行直接转矩控制的缺点。论述了基于三电平固定方向合成矢量的直接转矩控制方法,通过理论和仿真分析说明该方法缺乏对中点电位平衡的实时控制。针对此问题,本文对矢量合成方法进行改进,从而得到了具有中点平衡功能的新型固定合成矢量方法。在桥臂输出电流方向及中点不平衡方向检测的基础上,对合成调制序列使用Bang- Bang控制方法,实现了对中点电位平衡的实时控制。通过仿真和实验说明了基于新型固定合成矢量的异步电机直接转矩控制方法的可行性和实用性。(5)本文首次提出了适用于三电平异步电机直接转矩控制系统的“24矢量”法及性能更优良的“24矢量+变调制比”法,并提出了与之相关的5级磁链滞环比较器、11级转矩滞环比较器及开关表的设计方法,有效的减小了定子磁链和转矩的脉动。通过对比的仿真和实验说明了以上两种方法对磁链、转矩控制效果的改进。论文还通过起动实验、负载实验和调速比实验对“24矢量+变调制比”法进行了进一步的验证,获得了令人满意的实验结果。(6)本文首次对双三电平变换器供电的异步电机直接转矩控制系统进行了分析研究。通过理想化的假设,深入分析了双三电平变换器中点平衡问题,通过对比得出双三电平变换器能获得比三电平变换器更理想的中点电位平衡控制效果的结论。本文还建立了双三电平变换器供电的异步电机调速系统仿真模型,以此对论文提出的控制方法进行了仿真验证。结果说明了本文控制方案的可行性。更多还原

【Abstract】 With the development of industry and economy, a series of new problem emerges, such as reactive power and harmonic pollutions on electric grid, deteriorated environment, and energy crises. Under this background, together with the development and increasingly mature of multilevel technology, employing dual three-level converters control large capacity induction motor drives system has become a hot topic in recent years. The system has several merits. For example, high power factors, bidirectional power flow ability, remarkable energy-saving effect, excellent control performance, trivial pollution on electric grid, and so on. It has extensive application foreground on medium and/or high voltage high-power AC drive domain.The dissertation mainly focuses on the induction motor direct torque control (DTC) system fed by dual three-level converters. The paper firstly discusses the control system of three-level PWM rectifier and three-level induction motor DTC system separately, and then analyzes the whole system. The studies carried out in the paper and corresponding results are as follow.(1) In this paper, the method of how to attain space voltage vector by space vector pulse width modulation (SVPWM) strategy in three-level converters is discussed in detail. And a SVPWM method that can reduce switching loss and limit dv/dt is introduced, too. And the paper introduces the concept of current modulation. By using coordinate transformation, the neutral-point balancing problem of three-level converters with SVPWM modulation strategy is quantificationally analyzed in dq two-phase rotating reference frame.(2) The direct current control method of three-level rectifiers is detailedly analyzed in this paper. And the neutral-point balancing Bang-Bang control scheme of three-level PWM rectifiers is proposed. According to the direction information of the three-phase leg current and neutral-point potential, the neutral-point balancing control scheme abandons the switching states of short vector in SVPWM sequence, which has disadvantage effect on neutral-point balance. The scheme achieves preferable neutral-point balance results.Simultaneously, the switching loss and dv/dt of output voltage will not increase. Simulation analysis and experimental results proves the feasibility of proposed method. (3) A uniform mathematic model of three-level induction motor drive system inαβtwo-phase stationary reference frame is set up in this paper. And then, the influence of different kinds of neutral-point unbalance on motor performance is analyzed for the first time. The conclusions are able to instruct the neutral-point balancing method design of three-level induction motor drive system. This paper also takes two-level as an example, introduces the principle of DTC system and the corresponding essential technology.(4) This paper proposes a novel fixed synthesizing vectors method. Furthermore, the real-time neutral-point balancing Bang-Bang control method for the novel fixed synthesizing vectors is proposed, too. The shortcomings of directly using three-level’s own voltage vector for DTC system are discussed. Also, a DTC method, which bases on three-level fixed direction synthesizing vectors, is introduced. Theoretical and simulation analysis shows that, the method lacks real-time control ability of neutral-point balancing. To solve this problem, this paper modifies the vector synthesizing technique, finds the DTC algorithm of three-level induction motor drive system with real-time neutral-point balancing Bang-Bang control ability. The neutral-point balancing control method also only requires the direction information of leg output current and neutral-point unbalance. Simulation and experimental results illuminates the feasibility and practicality of the novel fixed synthesizing vectors three-level induction motor DTC method.(5) To reduce the stator flux linkage and torque ripple of induction motor, the paper modifies the novel fixed synthesizing vectors. Proposes“24 vectors”method and“24 vectors + alter modulation index”method for the first time. Both of them are suitable for three-level induction motor DTC system. By comparison, the“24 vectors + alter modulation index”method has better performance. Combined with these methods, the paper also proposes the design method of relevant 5-level flux linkage hysteresis comparator, 11-level flux hysteresis comparator, and switching table. The contrastive simulation and experimental results showed that, the above two methods can effectively reduce the flux linkage and torque ripple. Moreover, a series of experiments, such as start-up experiment, load experiment and variable-speed-rate experiment, are carried out to validate the characteristic of“24 vectors + alter modulation index”method. The experimental results are all satisfied. (6) The induction motor direct torque control system fed by dual three-level converters has been studied in this paper for the first time. By ideal assumption, this paper in-deep analyzes the neutral-point balancing problem of dual three-level inverter. By comparison, the paper obtains the conclusion, that dual three-level converters can achieve better neutral-point balancing results than three-level converters. The simulation model of dual three-level converters feed induction motor drive system is established in the paper. And the simulation verifications are also carried out. The simulation results show the feasibility of the control scheme proposed in this paper.更多还原