【作者】 郝正航；

【导师】 余贻鑫；

【作者基本信息】 天津大学 ， 电力系统及其自动化， 2011， 博士

【摘要】 为了深入认识电网扰动下双馈风力发电机(DFIG)的暂态行为,分析了机电解耦条件和机电解耦导致的结果。然后针对单机(DFIG)无穷大系统,定义了DFIG功角并分析其暂态行为,揭示了双馈电机的机电解耦性所导致的功角暂态行为的物理本质—纯粹的电磁暂态过程,具有快变特点。功角快变特性的外在表现是电网扰动后的有功功率快恢复特性。通过仿真分析和动模试验检验了理论分析的正确性。为了研究风电场对电力系统小扰动稳定性的影响,提出了双馈风电场动态频率特性的概念,通过数学证明,给出了判定双馈风电场对电力系统阻尼影响的性质及大小的判据:相频特性决定阻尼的性质,幅频特性决定阻尼大小。研究发现,DFIG动态频率特性与励磁控制参数密切相关,双闭环PI控制器的比例系数、积分系数对DFIG的动态频率特性有显著影响,进而造成DFIG的阻尼大小甚至阻尼性质因调节器参数变化而发生显著改变。在此基础上,设计了植于风电变流器上的附加阻尼控制器,利用电力系统分析综合程序(PSASP),通过二区域和多区域的算例分析证实,该控制策略具有提高电力系统阻尼的明显效果,而且具有参数易整定、调试方便和易于工程实现的优势。为了研究含风电场的电力系统暂态稳定性,分析了双馈感应发电机功角快变特性对常规同步发电机(SG)的首摆稳定性产生的重要影响。研究发现,DFIG和SG的功角摇摆曲线可能存在两类交点—主动交越点和被动交越点,交越的性质决定了DFIG对SG稳定性影响的性质。大扰动时,SG的首摆期间有主动交越交点,DFIG将降低系统的暂态稳定性;后续摇摆期间有被动交越交点,DFIG起正阻尼作用。在此研究基础上,设计了DFIG的暂态功角控制策略,该控制策略能对风电场附近的常规同步发电机产生显著致稳作用,提高了含风电的混合电力系统整体稳定性。为了揭示双馈风电机组的轴系扭振机理,基于电磁转矩的观点,发现双闭环PI调节参数和前馈补偿参数是影响电磁转矩性质的两个关键因素。由于设计用的互感系数很难保证就是电机的实际参数,故存在或正、或负的误差,误差的正、负性和大小对阻尼转矩的方向和大小有直接影响。上述两个独立因素共组合为4种情形,研究了各种情形下电磁转矩的阻尼性质,揭示了2种将产生电气负阻尼和可导致轴系扭振的情形。为了辨识和监测轴系扭振,还提出自然工况下轴系扭转模式、电磁模式的在线辨识方案,引入子空间法进行系统辨识和振荡模式提取。更多还原

【Abstract】 In order to penetrate to the nature of transient behaviour of doubly-fed induction generator under grid disturbance, the conditions of electromechanical decoupling is concluded and the results of electromechanical decoupling is investigated. Power angle of DFIG is defined and its dynamic behavior is analyzed based on a single machine (DFIG) vs infinite system. It is demonstrated that the transient behavior of the power angle, not depending on rotor position and featuring complete electromagnetism transition, often shows fast-change character. Therefore, while disturbances happen in power grid, the active power of DFIG can be restored quikly to original value. Both digital simulation cases and physical experiments verify the proposed theory viewpoints.In order to investigate the influence of DFIG-based wind farms on small-signal stability of power system, the conception of frequency-power dynamic characteristic for DFIG was presented in this paper. Based on the proposed conception, two propositions are proved to be available for examining the damping of generic component. The phase of frequency-power dynamic characteristic of generic component can tell whether the damping performance is positive or not, and its magnitude can tell how strong the damping is. The research reveals that the regulator parameters of excitation system, such as integrator and proportion coefficient, have strong influence on the frequency dynamic characteristic of DFIG. Hence, the daming contribution of DFIG may extensively vary if the parameters of excitation regulator change. Using theoretic viewpoints above, an additional control strategy is designed for DFIG convertor to improve power system damping. Based on Power System Analysis Software Package (PSASP), 2-area and multi-area power systems cases show that the proposed control strategy can increase power system damping. The proposed strategy has some advantages of simple construction, fast tuning parameters, convenient debugging, and easy realization.For studying the transient stability of power systems with DFIG-based wind farms, the influence of DFIG on the first swing stability of power systems is investigated through illustrating the fast-change characteristic of DFIG power angle. It is revealed that two sorts of power angle curve cross will be produced when DFIG and parallel synchoronous generators are integrated into power grid. In this paper, these sorts of angle curves cross are defined as positive cross and passive cross, which have a significant effect on the stability of synchoronous generators. When a large disturbance happen in power grid, the positive cross will appear in the period of first swing and transient stability of power system will be weakened. But, the passive cross will occur in the succeeding seconds and DFIG will accordingly enhance damping of synchoronous generators. According analysis results above, an excitation control scheme is designed for DFIG convertor to improve transient performance of power angle of DFIG, and the proposed control strategy can also restrict the first swing of the synchoronous generators nearby the wind farm during grid disturbance.To reveal the mechanism of torsional oscillation of DFIG shaft, based on electromagnetic torque conception, it is demonstrated that the parameters of PI regulator and forward feed compensation are key factors that have great impacts on the electromagnetic torque. Because the controller design value of mutual inductance between stator and rotor windings is hardly equal to its real value, the error between design and real value can be positive or negative, which is the other crucial factor relating to the sign and magnitude of damping torque. Four kinds of cases are studied to find out the feature of damping torque, and two of them are confirmed to be the mechanisms of torsional oscillation of negative damping torque. To monitor shaft oscillations, an on-line identifying scheme for mechanical modes and electrical modes is developed, which employs subspace identification algorithm to identify system model and extract the dominating modes under the natural operation conditions.更多还原