Systems Prevention And Control In Power Cascading Failure Analysis And Its

Recently,many blackouts have occurred all over the world,which caused extreme damage to human society in many aspects.Blackouts are mainly caused by cascading failures,which mainly propagate by the successive removal of system components,and they may also include a series of complex dynamic phenomena,which evolve together and gradually weaken the performance of power systems.To reduce the risk of failures and blackouts,it is important to analyze the propagation of cascading failures and the relevant vulnerabilities.Besides,under the background of large-scale renewable energy integration,the uncertainty of output has brought greater challenges to the operation of power systems.For the analysis of cascading failures,except for the needing of taking into consideration the system failures caused by external accidents,the uncertainty of renewable energy may also lead to endogenous violations and it may further induce cascading failures.Therefore,the uncertainty also makes the analysis of cascading failures more complex.In this dissertation,the analysis,prevention,and control related to cascading failures are studied.The main work and achievements are as follows:(1)For analyzing the propagation and vulnerability to cascading failures,a cascading failure graph model is proposed,which can be used to analyze cascading failures from the perspective of network science and statistics.Based on the cascading failure graph,the weighted degree index is proposed to evaluate the vulnerability of transmission lines,and three indexes are proposed to evaluate the vulnerability of the transmission network.The three indexes measure the impact of cascading failures on the vulnerability of transmission network from the viewpoints of average failure vulnerability,average failure propagation intensity and average failure influence respectively.Using the weighted degree distribution model and the assortive coefficient,the first-order and second-order distribution characteristics of the cascading failure graph are analyzed,and the propagation mechanisms of cascading failures are revealed.Results show that the distribution of line vulnerability in power systems is highly heterogeneous,and it meets the power-law distribution for large-scale systems;In the cascading failure graph,the vulnerable lines present the assortive property,which indicates that the lines with high vulnerability in the system are prone to fail successively,besides,the larger the assortive coefficient,the more concentrated the vulnerable lines in the cascading failure graph.(2)For analyzing cascading failures in renewable energy integrated power systems,considering higher safety criteria,the Random Chemistry algorithm is used to sample initial failures,the cascading failure graph is reconstructed using failure data,and the relationship between the distribution characteristic of line vulnerability and the overall vulnerability of transmission network is analyzed.Combined with the distribution characteristic of the line vulnerability,an index is proposed for comprehensively reflecting the vulnerability of transmission network.Based on that,combined with the operation scenarios of the system and the topological differences of cascading failure graphs,the concept of vulnerability uncertainty and the corresponding index,and the line vulnerability variation index are proposed.The variations of vulnerability to cascading failure under different operation states and different uncertainty levels are analyzed.Results show that when considering the N-2 safety criterion,there are only a few lines that take part in the propagation of cascading failures,and some lines occupy the propagation center of the cascading failure graph;Considering renewable energy integration,the power system vulnerability to cascading failures is sensitive to the changes of operation state and uncertainty level.When the uncertainty level exceeds a certain range,the vulnerability of the system rises drastically.(3)To tackle the endogenous violation problem caused by the uncertainty of forecast error,related methods are proposed from three aspects,that is,an optimization method that is based on the linear programming approximated AC power flow model is proposed to identify endogenous violations,This method can identify the potential overloaded lines and voltage violation nodes quickly and effectively;a random sampling-based cascading failure graph method is proposed to analyze the propagation path of cascading failures caused by endogenous violations,by using this method,the most likely propagation path of cascading failures can be presented directly under uncertain conditions;and a minimum load shedding method is proposed to analyze the system reliability,this method is based on the linear programming approximated AC power flow model,which can simulate the reactive power distribution of the system,and has the advantage of fast solving with its linear character.Results show that the endogenous violation lines present aggregation property in the system,and most of them have a common node;The cascading failure graph can effectively reflect the failure propagation characteristics considering random sampling.Although the failure path in the cascading failure graph has many propagation modes,its main path can be obtained by network reduction;Results also show that the proposed method is more accurate than the method based on DC model,and the system reliability decreases with the increase of uncertainty level.(4)For the prevention and mitigation of cascading failures,a simulation-based decisionmaking method is proposed for transmission network expansion planning.Based on the concept of cascading failure graph,a new graph evolution model is proposed,and the evolution law of line vulnerability is revealed by using the model,it includes the law of vulnerability distribution,the law of line connection tendency and the change law of the ranking of vulnerable lines.Based on that,a cascading failure mitigation planning method is further proposed,this method can effectively reduce the complexity of the searching of vulnerable lines.Results show that the vulnerability of lines in the graph evolution model presents heterogeneous evolution and gradual scale-free distribution,which indicates that the most vulnerable lines can be identified in the early evolution process;The proposed planning method can effectively reduce cascading failures.When considering N-2 accidents,the number of cascading failures and the size of blackouts can be effectively reduced.(5)To control and block cascading failures,the time limitation of conventional topology optimization control method for handling cascading failures is analyzed.Considering the normal operation state of the system before contingency and the emergency reserve of the generator,a modified optimal transmission switching model is proposed to obtain the corrective solution.Considering the available correction time,the tripping time of lines after a contingency is calculated based on the line temperature model,which is used as the proxy for the available correction time.Two algorithms are proposed to verify the practical feasibility of the two different corrective solutions.Considering the available correction time and the actual failure propagation and time characteristics,a comprehensive control algorithm is proposed to block the propagation of cascading failures.Results show that when considering the available correction time,there is a high proportion of infeasible solutions in the solutions obtained from the optimal transmission switching model.The proposed control algorithm can obtain the feasible correction solutions to block the propagation of cascading failures.