Research On Operational Trends And Critical Links Identification Of Power Systems

The scale of power systems becomes larger and the modern society is more dependent on the power supply excessively, so the damage due to widespread blackout is more severe than before. Evaluating operation states of a power system and taking rapid corrective measures are of importance to avoid blackouts of the power system. By effectively evaluating the system operation states, the system operators can identify the system security level, monitor the key links which may lead to severe accidents, and take effective preventive control measures in time to ensure safe operation of the power system. Therefore, identification of the operation states and the key links of a power system is of great significance for preventing cascading failures and blackouts.The identification approaches of the operational trends and the key links are investigated systematically and thoroughly in this thesis. The main work is summarized as the followings:(1) A quick search approach for transmission sections is presented considering optimization of geographical partition boundary. According to the definition of the transmission sections, based on the geographical partition of a power grid, the simplified partition structure graph of the grid can be obtained by applying the reasonableness test and partitioning optimization to the boundary stations. Automatic quick search of the transmission sections can be completed by using adjacency matrix and path matrix of the simplified graph of the partition structure as well as corresponding logical operations. An importance index of the transmission section is defined based on the transmission capacity and the safety margin of the section. According to the index, key sections of the grid can be sorted and filtered. Application of the proposed method to New England 39-bus standard system and an example of the practical power grid demonstrates the practicality and validity of the proposed method.(2) A concept of power betweenness is proposed and applied to identify the vulnerable links of a power grid. The power betweenness of transmission lines and that of nodes are defined respectively according to network connectivity and the allocation of power transferring, and are sorted to recognize the vulnerable links of a power grid. Taking New England 39-bus standard system as the testing example, the effectiveness of the power betweenness is validated through the cascading attacks such as the breakage or overload, and results show that these elements with high power betweenness are the vulnerable links in a power grid indeed. Furthermore, the stability under the static attack is verified by network efficiency function and power flow entropy respectively. Results show that the higher the power betweenness of electric elements are, the greater the influence on power grid operation is.(3) A concept of grid key line sequence is proposed for prevention of power grid cascading failure. A comprehensive importance index of line sequence is constructed combining structural vulnerability indexes with network performance indicators. The key line sequence can be selected using the multiple attribute decision making method. Furthermore, particle swarm optimization algorithm is employed in this paper to sort the multiple attributes of large-scale line sequences. The New England 39-bus standard system and the Hebei southern power gird are employed as examples to validate the proposed approach of key line sequence recognition. The simulation results show that the proposed method can effectively filter the key line sequences and provide useful guidance for preventing the power grid cascading failures.(4) An identification approach for the moving trends of power system operation states is proposed. It allows the system operators intuitively identify the unsafe conditions and continuously monitor the distance between the real state and unsafe state as well as the direction and speed of the trends. The Fuzzy Analytic Hierarchy Process (FAHP) is employed to evaluate the operation states using the observed variables. The trends of the generated indicator are identified by Qualitative Trend Analysis (QTA), from which dynamic operational trends of the whole system can be easily obtained. Simulation results show that the proposed approach can effectively identify the moving trends of the power system operation states, therefore it can be applied to monitor and assess the security level of power system operation.(5) An online monitoring system of operation states and key links of a practical power gird is developed. Real-time assessment of operation states and automatic identification of the key equipments are online implemented. The system has been put into operation in the Hebei southern power system.