【作者】 李鹏；

【导师】 余贻鑫；

【作者基本信息】 天津大学 ， 电力系统及其自动化， 2004， 博士

【摘要】 市场条件下重组的电力系统的运行点比以往更接近稳定极限,这促使电力工业界努力获得对于系统稳定性的更深刻的理解并开发出更加有效的稳定控制方法。本文以非线性动力学为背景,探讨了注入功率空间上电力系统潮流可行域边界的几何性质,揭示了负荷模型对小扰动电压稳定域边界分岔性质的影响,深入分析了电压振荡阻尼和互联系统低频振荡阻尼的性质与机理,同时提出了获取潮流可行域边界的可信而有效的新算法——混合法,指出了“两步法”在小扰动电压稳定域边界确定中的重要性,发展出了新的基于Prony分析的多机系统阻尼转矩系数算法。本文首先结合优化技术与“预测-校正”思想提出了一种在二维空间计算可视化潮流可行域(Power Flow Feasibility Region, PFFR)的新算法——混合法。在将PFFR解耦为“负荷注入子空间的”以及“发电注入子空间的”的基础上,混合法被拓展到了高维空间,能够计算不同子域的“最近”或“最远”边界点。特别重要的是,通过对混合法计算结果的分析,本文发现“发电注入子空间”上的PFFR的边界在电力系统实际可能的运行范围内可以利用超平面近似。然后,本文研究了计及动态负荷模型下的潮流可行域与小扰动电压稳定域边界的关系,发现感应电动机的存在使得系统在Fold分岔点之前就出现由于电动机滞转引起的SNB点,而这些SNB点不一定会造成系统出现电压崩溃。通过对CPF、分岔方法以及两步法的比较,本文指出对大系统而言,更精确的电压稳定极限判定中需要采用“两步法”(先由潮流解得平衡点,再做特征值分析),且当系统中拥有大量感应电动机负荷时,在“两步法”之后通过时域仿真确定所发现的SNB的性质是非常必要的。为了揭示电压失稳的单调模式与振荡模式的起因,本文在单机系统上推导了新的计及感应电动机负荷的P-H模型以及电压振荡阻尼系数γ,从理论上证明了:与纯静态负荷不同,超过一定比例的感应电动机负荷的存在可使小扰动电压稳定域的边界上只出现SNB点而不出现Hopf分岔点; 慢速励磁和大容量并联电容补偿相互作用可产生负阻尼。这些结论也在实际大系统的仿真中得到验证。为了明确与互联系统低频振荡相关阻尼的性质与数值确定方法,本文论述了阻尼转矩系数矩阵的来源与含义,分析了现有的基于时间序列数据拟合的阻尼转矩系数算法的局限性,进而针对该局限性提出了新的基于Prony分析的电磁转矩系数算法。同时,本文还以全国联网系统为背景,提出了大区电网弱互联削弱系统阻尼、引发低频振荡的新机理,澄清了人们对机电暂态仿真中所用同步电机模型的疑虑。更多还原

【Abstract】 Currently, the electric power utility industry in many countries is undergoing a tremendous change. Restructured power systems are expected to be operated at a greater variety of operating points and closer to their operating limits. Concerns for power system stability are prompting utilities to better understand stability problems so as to devise effective, efficient and economic solutions to the stability problems. In this dissertation an attempt is made to give deeper insights into topological characteristic of boundaries of Power Flow Feasibility Region (PFFR) and effects of load modeling on boundaries of Small Disturbance Voltage Stability Region (SDVSR), as well as the inherent meaning behind voltage oscillation damping coefficients and electromechanical oscillation damping coefficients. We propose a new boundary tracing method for PFFR and recommend two-step method for identifying SDVSR’s boundaries more precisely. A novel online identification algorithm of damping coefficients of multimachine systems is also developed. All researches in this dissertation are conducted in the background of nonlinear dynamics. First, we decouple PFFR into two sub-regions: “Region of Load Injection Space” and “Region of Generator Injection Space”. Then a new ‘Hybrid’ method for tracing boundary of PFFR is developed by adopting the basic idea of “predictor-corrector” and combining it with optimization technique. The hybrid method is also modified to account for concave boundary situation. In the subspace of load injection, we extend the hybrid method to calculate the closest boundary point related to current operating point in high-dimension injection space. In the subspace of generation injection, a new algorithm based on the hybrid method thoughts is presented to obtain the farthest boundary point under L2 and L1 norm definition respectively. It is noted that the topologies of boundaries in these two subspaces are different, especially the boundaries of PFFR in generation injection space can be approximated by hyper-planes within possible operation scope. Then the relationships between PFFR’s and SDVSR’s boundaries are analyzed with consideration of dynamic load models. We have observed that Saddle Node Bifurcation (SNB) points induced by stalling of motors always occur well before Fold bifurcation point and SNB does not coincide with Fold in general sense. At the same time, we point out that it is not all SNB caused by asynchronous motors stalling would lead to voltage instability and the system behavior at these SNBs depends on the system configuration and the system operating condition. After comparing CPF method, bifurcation method and two-step method, we show that two-step method is the most appropriate method for identifying precise voltage stability limit, and time domain simulations sometimes are also necessary when induction motors occupy major portion of power system’s load. In order to reveal mechanisms of monotonic voltage instability mode and oscillatory voltage instability mode, a novel Phillips – Heffron (P-H) model based on Single Generator Single Motor system is proposed, and it is used to derive a voltage oscillation damping factor γ. Then we prove that induction motors in load buses of power systems can eliminate Hopf bifurcation points in boundary of SDVSR, and coexistence of slow-acting excitation systems and large shunt capacitor compensation will induce negative damping for voltage oscillation. All these conclusions are validated through simulations of a realistic power system. Since damping analysis is one of basic techniques in low-frequency oscillation studies, we give a series of theoretic explanation of damping torque coefficients. As the current identification algorithm of damping torque coefficient often fails when dealing with multimode oscillations, we propose a new effective algorithm based on Prony analysis and time domain data. To better understand what causes low-frequency oscillations in the interconnection of state power grid of China, we analytically compare damping torques of strong and weak connected systems. It has been shown that the relatively large impedance of tie-lines in weakly interconnected systems can greatly reduce the damping between these systems, and reinforcement of the connection between two bulk power grids is an effective way to mitigate low-frequency oscillations. At the same time, we clarify the doubt about the models of synchronous generators used in dynamic stability studies.更多还原