| Currently, electric power networks dispersed geographically in China are being interconnected with the emphasis on transferring large amount of power from the west region to the east and keeping a generation/load balance between the south region and the north. With the huge interconnected power network, complex dynamic phenomena appear in operation and damages by voltage instability accidents can be significantly great to the whole society. Voltage instability presents a common feature of their sudden and concealment. During the development of accidents, they are difficult to be detected and control measures cannot be taken timely and effectively. Once voltage instability happens, power systems are difficult to survive. The existing voltage stability assessment methods still cannot satisfy the need of on-line analysis. Therefore, it is necessary to develop new methods for on-line voltage stability assessment through advanced technology of measurement, communication and computation according to the practical operation of power systems. It is of great significance to prevent voltage instability of power systems and improve the reliability of transmission networks. In this dissertation, considering the impacts of generator reactive power reserves, load characteristics and On-Load Tap Changers (OLTCs) on voltage stability, a fast voltage stability assessment method based on power transmission paths is studied in detail. The framework of on-line voltage stability assessment for large scale power systems are designed to use synchronous information provided by the Wide Area Measurement System (WAMS). By the method of power transmission path analysis, the voltage stability of the practical Shandong power system in China is assessed and the schemes to improve voltage stability by Static Var Compensators (SVCs) are compared. Based on the improved power-flow tracking algorithm, a new method to determine key power sources is studied. The main contributions of the dissertation are as follows.Based on power transmission paths, a fast assessment method for static voltage stability is proposed. Although power transmission paths are very complex in practical power systems, the voltage stability degree of power systems can be represented by the voltage stability of the power transmission path which is the most prone to voltage instability due to the local nature of voltage instability problems. Starting from the voltage stability of a simple system, a voltage stability index is proposed based on the definition of power transmission paths and system equivalence. To reduce the calculation in complex power systems, the local voltage stability index is used to determine weak buses. Based on electrical distance, participant buses and key power sources on the reactive and active power transmission paths are decided. The weak power transmission paths are searched and the weakest one is computed through the equivalent system. The weakest path voltage stability index incorporating the key generators reactive power reserve is used as a tool for voltage stability assessment. By the method, the weak areas and the key factors contributing to voltage instability can be detected. Load characteristics have a great impact on voltage stability. Through the improvement of both the local voltage stability index and the equivalence method, load characteristics are taken into account in the power transmission paths analysis and a new voltage stability index is proposed. Simulation results of the New England test system demonstrate that the proposed method is of the following features. Differing with the Thevenin equivalence oriented the whole system, the calculation process of the method only require the information of weak power transmission paths. Reactive power reserve of key generators has an important influence on the voltage stability. So the voltage stability of the weakest power transmission path may decline or/and the weakest one may shift when some generators reach their limits. Combining the voltage stability index and the reactive power reserve of key generators in the process of voltage stability assessment, the results will be satisfactory. When the compositive load model is adopted, the voltage stability limit will not coincide with the power transfer limit. By the voltage stability index considering load characteristics, the voltage stability of power systems can be assessed correctly.Regarding to the voltage instability problems caused by OLTC during the long-term dynamics process, effects of OLTC on the voltage stability are analyzed in the simple power system. The analysis of the simple system demonstrates the following results. Considering the excitation reactance of induction motors, three different changing trends of power transfer limit are found in the normal regulating range of OLTC. They are monotonic increase, monotonic decrease and decrease after increase, respectively. With the regulation of ratio, two critical points, that is, the critical point of regulating voltage effect and voltage stability changing will shift. Under different trends of power transfer limit, by analyzing the relative position of the two critical points, the voltage instability mechanism caused by reverse action is explained and the reason of different effects on voltage stability caused by reverse action is pointed out. According to the different trends of power transfer limit, different principles of emergency control should be taken. The effective control depends on whether the power injected to induction motors after ratio regulating can meet the need of the mechanical load. For constant power loads, the successive regulating effect of OLTC will be more complicated if different trends of power transfer limit are considered. There is not only the successive obverse or reverse action of OLTC, but also the complex mix actions when the ratio is raised stepwise. By the equivalent method of power transmission paths, the research conclusion of the simple system are expanded to the complex system and the power transfer limit of complex systems can be predicted. By choosing the key power transmission path, the effect of OLTC on -voltage stability in complex systems can be analyzed fast.According to the need of on-line voltage stability monitoring, three main parts of power transmission paths analysis are simplified through the information provided by WAMS and the framework for on-line voltage stability assessment is developed. Based on power transmission paths analysis, the voltage stability of Shandong power system is analyzed. Weak buses are decided by the local voltage stability index. Key path voltage stability indices of power transfer from the west to the east and local reactive balancing are computed with load increasing. The reactive power reserve indicator is also calculated. Simulation results demonstrate that weak buses in Shandong power system are primarily located in the east areas, such as Qingdao and Yanwei. In these areas, load increases rapidly, but the generation is of shortage. Therefore, large power needs to be transferred from the west areas and the ability of voltage support from the local generators is weak. Under contingency conditions, operators should control the reactive power flow of key power transmission paths reasonably and maintain the ability of voltage support from the local generators. They are the key factors to ensure the voltage stability of Shandong power system. Computation results of the software Voltage Security Assessment Tool (VAST) prove the feasibility of the power transmission paths analysis. The schemes to improve voltage stability of Shandong power system by SVCs are compared. The results show that the voltage stability limit can be improved by SVCs under different modes and the effect in Laoshan substation is the best.Based on the improved power flow tracking algorithm, a new method to determine key power sources is proposed. In the process of power transmission path analysis, it is important to determine the key power sources. The accuracy of the calculation depends on whether a reasonable choice of the parameter values. If the parameter values are too big, the number of weak power transmission path will increase greatly and too much calculation follows. Conversely, if the parameter values are too small, the weakest power transmission path may be missed and the calculation error is large. In the application process, suitable parameter values may be achieved only through lots of simulations combined with much practical experience. In the improved power flow tracking algorithm, distribution factor matrix is decoupled by the reactive power flow equivalence model and line power flow decomposition model. Therefore, the calculation speed is accelerated. Through power flow tracking results before and after the perturbation, the variation of generators' shares on load increasing is calculated. It can be used as a tool to determine key power sources. The proposed method avoids the problem of parameter values selecting in the original method and the accuracy is improved.
|
PDF Full Text Request