Research On Safety Evaluation Of Wind Power Integrated Power Systems Based On Complex Network Theory

In China,the expanding development of clean energy,such as wind power,could optimize the structure of energy sources and enhance the sustainable development of both society and economy.A higher penetration of wind power may bring enormous social and economic benefits.However,it also brings some uncertainty issues for power grid safety.In recent years,the world has witnessed several large-scale blackouts triggered by the serious tripping of renewable energy,which has drawn great attention from electric industries across the world.The safety issue among those accidents could attribute to the high penetration of renewable energy in power system.When the large-scale renewable energy has tripped off from the system,the large amount of power shortage leads to cascading failures,thereby leading to the eventual instability of the system.Therefore,it is great importance to study the safety evaluation for wind power integrated power system by utilizing a comprehensive evaluation on both the static and dynamic safety and proposing the improvement measures to prevent large-scale blackouts.Previous studies involving the identification of power grids' crucial nodes generally analyze the impact of the node failure on network integrity and connectivity under cascade failure using the static analysis method and based on network topology.The existing node importance research,only analyzes the importance of nodes in the network by defining the corresponding network indicators with statistical methods.However,the research heavily relies on the availability of the nodes in the network or the fixed connection path between neighbor nodes,with little consideration given to the dynamic characteristics of the power system.For this reason,this dissertation proposes the electrical coupling relation between power network nodes considering the power flow direction among nodes.The dependent utility for two kinds of nodes in a network,i.e.source nodes and terminal nodes,is established and the computing method for electrical coupling coefficient is further improved.Furthermore,this paper utilizes an improved PageRank computing method to calculate electrical coupling coefficient of neighborhood nodes and then analyze dynamic transmission process the node contribution in the network,which puts forward the method of sorting the nodes.The validity of the proposed algorithm is tested in IEEE 39-bus system with wind power.Test results show that it can effectively identify the nodes that have a greater impact on the stability of the system and regard them as vulnerable nodes.The topological structure of the power grid is targeted as research object.By establishing a Kuramoto-like model of the power system,this dissertation analyzes the impact of the topological structure on the ability of power system to maintain synchronization.According to power property,node connection in the system is divided into homogenous node connection and heterogeneous node connection.In addition,the influence of the connection between homogenous nodes and heterogeneous nodes on the synchronization ability of power system is analyzed.The results indicate that removing the tie line between the heterogeneous nodes(generator and load)can improve the synchronization ability.The influence of the two schemes by which wind power is integrated into the power system on the system's synchronization ability(integration through load nodes and integration through synchronous machine nodes)is further analyzed.The results show that the synchronization ability can be improved when the wind power is connected with the load nodes(the heterogeneous nodes).A Kuramoto model suitable for transient stability analysis of wind power integrated systems is established.A sample for the initial operation after a large disturbance occurs is formed.By adopting the probability analysis,the number of initial operation points that can return to the equilibrium point before disturbance is counted.Moreover,these initial operating points are projected in phase space to estimate the size of attraction domain of the system after disturbance.In consideration of the large blackout caused by the trip-off of high penetration wind power,this research strives to analyze the influence of system parameters on the size of attraction region after large disturbances by using the power system dynamics model based on complex network.The results show that by increasing the coupling strength between nodes(the maximum transmission power),the size of the attraction region of the system can be effectively increased and the system's ability to resist large disturbances can be improved.When the coupling strength is a constant,the increase of the generator output has a negative impact on the transient stability of the power system.When the generator output is large enough,the system may be totally unstable.Furthermore,the method of the system attraction region size estimation of wind power integrated system is studied,and the relationship between wind power permeability and transient stability is further analyzed.The results show that the probability of transient instability increases with the increase of wind power penetration.The higher the wind power penetration is,the more noticeable the effect is.Based on the above,this research selects nine evaluation indicators for the impact of wind power on power system safety,so as to the comprehensively evaluate the safety of the wind power integrated system.The nine indicators fall into the two categories of static safety evaluation and dynamic safety evaluation.A single-indicators evaluation method based on yardstick competition is proposed.By constructing the benchmark scenario,the safety evaluation results of the system with different wind power permeability are obtained.Based on grey correlation analysis,the comprehensive evaluation model of system safety is further established.With the variation coefficient method the coefficient of variation method,the index weight is determined to realize the dynamic empowerment of the evaluation index.Finally,the validity of the proposed method is verified by comprehensively evaluating the safety of a 2020-planning provincial power grid in China.The research results provide guidance about the planning and decision-making of wind power integrated systems.