Operational Risk And Vulnerability Assessment Of Power System Based On N-k Contingeilcy

With the development of interconnected grid and the rise of smart grid, the structure of the modern grid has become increasingly complicated. In this background, worldwide blackouts occured frequently in recent years, causing huge economic losses to society. As N-k contingency is one of the most critical parts of blackout, it is extremely significant to conduct a comprehensive assessment of its impact on the grid. In view of this, a series of researchs are carried out in this dissertation, focusing on several security issues of N-k contingency risk and vulnerability assessment. The main works are summarized as follows.The uncertainty of the grid operating conditions is not fully considered in N-k contingency assessment via traditional reliability. In order to solve this problem, component reliability models applicable to N-k contingency are established, which include two parts:one is outage model of transmission line as primary equipment. Operational reliability theory is introduced to obtain outage probability of line, taking various outage factors into account. The other one is hidden failure model of protection as secondary equipments. Markov method is introduced to obtain reliability indexes under different proctection configurations, taking logic relationship of proctection action into account. On that basis, the probability model of hidden failure is proposed according to the composition of relay protection system and hidden failure classification. The proposed models lay a theoretical foundation for subsequent system-level analysis of N-k contingency, combining primary and secondary equipment model together.In view of the deficiency in traditional static security assessment method, an operational risk assessment algorithm for transmission system based on N-k contingency is put forward. The search process of N-k contingency is determined in accrodance with the real-time operational status of grid, to ensure that contingency paths seached out are reasonable and effective. Based on the analysis of hidden failure consequences, the risk assessment method and related indexes of N-k contingency for transmission system are further proposed under market conditions by means of operational risk theory, achieving a good combination of security and economy. The example analysis indicates that N-k contingency risk assessment applying operational reliability and risk theory can reflect the state of power system accurately and verifies the practicality of the proposed algorithm.In order to study the impact of large-scale wind power on grid blackout, the risk assessment model for N-k contingency considering wind farms integration is formulated. The reliability model of wind turbines is established firstly based on wind speed prediction. The output variation of wind farm is considered as an influence in simulatiorr of N-k contingency process. From the perspective of overload, low-voltage and load-loss, N-k contingency risk assessment model is established on the basis of Utility Theory and importance factors. Taking IEEE-RTS79system as an example, the impacts of wind power penetration, integration schemes and wind randomness on N-k contingency risk indexes are analyzed in detail. The results will provide scientific reference of planning, operation and blackouts defence for grid integrated with wind farms.With regard to the relativity of operational risk indexes, the forewarning classification algorithm for N-k contingency is proposed. N-k contingency probability levels are classified at first, based on Fuzzy C-Means (FCM) Cluster and correcting method. Then the indexes and membership function sets of failure severity reflecting static security level of grid are defined, and comprehensive severity is classified via fuzzy reasoning and objective weighting of variation coefficient. At last, forewarning levels of N-k contingency risk are evaluated according to fuzzy comprehensive evaluation vector, with combination of probability and severity. The example results indicate that the forewarning model can make effective judgement on the operational state of grid following failure through N-k contingency simulation, and provide beneficial references for blackout forewarning research.For the purpose of illustrating the relationship between blackouts and grid vulnerability, the algorithm of grid vulnerability assessment based on N-k contingency is presented. A risk indexes system for N-k contingency is set up from the perspective of static security. And a composite risk index for N-k contingency is obtained by application of the entropy and Analytic Hierarchy Process (AHP). According to vulnerability functions of risk and structure, a new vulnerability index is proposed for the quantitative analysis of grid vulnerability concering N-k contingency. Taking a district grid in China for example, the vulnerable buses and lines in it are identified. The example analysis demonstrates that, compared with the vulnerability evaluation method considering risk only, the method considering risk as well as structure vulnerability achieves higher precision and its result is more instructive for power grid monitoring.