Integrated Vulnerability Assessment Of Power Systems Considering Cascade Failures

Large-scale power system has brought about huge economic benefits, but at the same time, its potential risk is more threatening than before. Huge blackout failures in power system are usually caused by not single fault but cascade faults. Traditional stability analysis can’t guarantee the safety and effectiveness of power grid operating. With huge blackout failures frequently happening, vulnerability of power system has aroused wide attention of scholars both at home and abroad, and vulnerability assessment aiming at the key points of network has become a research hotspot in recent years.Based on an extensive collection of overseas and domestic thesis, this paper gives a descriptive definition to vulnerability of power system. Based on deep analysis of overseas and domestic research status, in this paper, vulnerability of power system is classified into structural vulnerability, state vulnerability and integrated vulnerability in a proper way. Forms of cascade failures are introduced, and under the complex network theory, the fault chain model is adopted to describe the cascade failures. Results show that it conforms to the physical characteristics and actual process of power system blackouts to analyze the cascade failures with the fault chain model.In the aspect of structural vulnerability assessment, the paper introduces the concepts of graph, tree, minimum spanning tree, etc in graph theory, and improves the Kruskal algorithm for minimum spanning tree to apply it to analyze the structure of power network, leading to the "minimum power flow tree", thus a large amount of complicated network structure can be quantitatively analyzed under the same benchmark. On this basis, the overall structural vulnerability of power system is accessed from the aspects of active power regional balance and transmission distance. In the aspect of state vulnerability assessment, the voltage margin and changing rate of buses and transmission power of lines are both considered. Continuous Power Flow with local parameters is used to draw the PV curve of buses, with which we can analyze the static voltage stability and critical voltage. Based on the method of normalization, the indexes of bus voltage and line transmission power limit are constructed respectively, the combination of which to comprehensively reflect the state vulnerability of the system. Based on the risk assessment theory, considering the probability of cascade failures, the integrated vulnerability of power system under cascade failures is assessed with the combination of the structural and state indexes. And on the basis, vulnerability of the lines is assessed to determine the key lines of the system.IEEE-24RTS system as an example, conditions of the system in normal operation, N-1fault, N-2fault are all analyzed, and the key lines in the system are determined, which proves the correctness and efficiency of the proposed method.