Research On Power System Vulnerability Assessment Methods

In recent years, several large area blackouts occurred in succession worldwide, whichencourages people to know that the traditional power system security analysis methodbased on the power system stability analysis is already not able to effectively ensure safeand stable operation of power system. With the development of the electric power systemsecurity risk analysis method, people began to take the concept of vulnerability into thepower systems, to find out the weak point of power system and take correspondingpreventive measures to improve the system operating state to prevent large area blackoutaccident, also to provide valuable reference data for the safe and stable operation of powergrids and early warning. Therefore, researching how to accurately analyze the currentvulnerability and the potential danger of the grid, and to further prevent cascading failureshas important significance.Firstly，this paper summarizes the present situation and main problems of domesticand foreign electric power system vulnerability assessment, and then, combined powersystem structure vulnerability analysis with risk theory, researches in power systemvulnerability assessment methods and assessment indicators.This paper studies power system structural vulnerability analysis methods based oncomplex network theory, comparative analyzes the three models of power networktopology vulnerability analysis: un-weighted betweenness, weighted betweenness andelectrical betweenness. According to the actual operation characteristic of electric powersystem, using the model of power network topology analysis based on electricalbetweenness in vulunerability to obtain more practical and accurate results. The electricbetweenness definition and calculation process are discussed, and it takes IEEE-14systemas an example, calculates and sorts the electrical betweenness of branches and nodes in thesystem, and it verifies the validity of electrical betweenness in system structurevulnerability assessment to recognize vulnerable branches and nodes.By combining the power system structural vulnerability analysis method based onelectric betweenness with the theory of risk, this paper proposes a system vulnerability assessment method. It gives the definition of branch vulnerability risk indicator, whichequals to the multiplication of branch cut probability scale value and branch vulnerability,in which, branch vulnerability indicates the severity of branch vulnerability，and iscombined by three indicators: the electrical betweenness of the branch, the loss of load ofthe real system when the branch is cut off and the number of overload branches caused bythe cutting off of this branch. Branch cut probability scale value indicates the probability ofthe branch’s being cut off, to obtain the scale value, we have to consider various reasons ofthe branch’s being cut off, so the fault tree analysis method is used to construct the threelevels of evaluation indicators of the branch cut probability scale value. And the standardsof grading these evaluation indicators are given. Based on the branch vulnerability risk, it’sdefined that the node vulnerability risk equals to the sum of the vulnerability risk of all thebranches that are directly connected with the node, and it’s defined that the transmissionsection vulnerability risk equals to the sum of the vulnerability risk of all the branches thatcompose the transmission section.Branch cut probability scale value is required by the weighted summation of bottomevaluation indicators step by step. AHP and Delphi method is proposed to constructjudgment matrix and to calculate the weight of each evaluation indicator. The advantagesand disadvantages of the nine scale method and the three scale method in constructingjudgment matrix are discussed，and this paper advises to use the nine scale method whenthe number of indicators to be compared is not more than three，otherwise，to use the threescale method, and examples of the weight calculation process are given.Finally, the research results are summarized, and the future research work isprospected.