Research On Complex Power System Vulnerability Assessment Methods

The safe and stable operation of power system is crucial for national economy aswell as people’s livelihood and national security, but the blackouts occurringfrequently in recent years have attracted extensive attention. In order to enhance therobustness of power system, we must modernize the power grid through highlystructured and optimal design because of the capacity and scale of power systemexpanding gradually. As the power system becomes increasing complex, the complexsystem which is always considered as robust one is getting more and more vulnerable,which means robustness creates complexity while complexity creates vulnerability,causing threats on the security of power system that is brought by vulnerability. Muchresearch has been observed that blackouts are often triggered by cascading failures,enlarging by the vulnerabilities of power system. Vulnerability of power grid isextending concept of security contributing to evaluate the security and stability ofpower system from global perspective. Research on vulnerability of power system ismaking up for its security study, but also it worth to be realized deeply as well tobuild stronger foundation for strategic power infrastructure defense system.This paper is focused on the practical requirements of power systemvulnerability assessment, and then in-depth researches have been carried out bymethods of complex power grid vulnerability assessment, including identification ofvulnerabilities of power system and power system vulnerability assessment. The mainstructure and the research content of this paper are summarized as follows:Firstly, the weighted power grid was established based on complex networktheory by choosing impedance of transmission lines as the weighted parameter, andthen the structural vulnerability assessment method for power grid was presented.With regional network of Changsha as example, complex network feature parameterswere applied to vulnerability assessment of power grid. The vulnerabilities andstructural characteristics of Changsha regional network had been analyzed andreviewed. The time-domain simulation results show that vulnerable component outagewould lead to catastrophic failure in power system. What’s more, the structuralvulnerability assessment of Changsha regional network and IEEE118-bus system,which based on static and dynamic analysis method, was also combining with running state of power grid. Simulation results indicated that intentional attack on vulnerableelements could lead to serious problems such as cascading failure.Secondly, To figure out how large-scale blackouts take place, animpedance-based topological model and cascading failure model for weighted powergrid was proposed. Considering differences of both node and edge to define a newnetwork structure entropy which was used to describe the faults evolutionquantitatively and unbalanced distribution edges in power network structure. Thestructure entropy had important influence on the spread of cascading failures. Thestandard structure entropy was proposed by GA(genetic algorithm)in order to easeinfluence of varying size of power grid during cascade. The change of standardstructure entropy during cascading failure was a way to capture “heterogeneity” ofpower grid. Simulation results of IEEE118-bus system and North-China powersystem indicated that intentional removal of some edges according to their standardstructure entropy could decrease heterogeneity of the grid structure and migratecascading failure.Thirdly, studying the complex network characteristics of power grids andfeatures of cascading failure progress was extremely important to anticipate blackouts.The physical laws of power flow through network were also studied. However therewere some typical complex network models which were not incorporate the physicallaws of electricity explicitly. Thus, this paper defined each distance between pairs ofnodes calculated according to superimposed principle based on the informationcontaining character of power grid network. The identification model of power gridwas built on a more comprehensible physical background than the existing models.This new model was based on the electrical coupling connectivity metric which wasdefined according to electrical distance between nodes. Numeric example of IEEE testsystems show the internal heterogeneous structural character of power grids. Then theelectrical coupling connectivity metric was applied to critical nodes identification inpower grids. With IEEE39-bus system and regional power grid of Hunan province asexample, the time domain simulation as well, all simulation results indicated theindex of this paper were more reasonable and effective than the topological degree.The dynamic model which summarized the key features of power grid waspresented by employing influence model to describe the interaction between nodes,while the evolution of the system states was within Markov chain. A vulnerabilityassessment method was presented according to a theory that the probabilit y of power system state changed from normal to collapse equaled to the corresponding element ofleft eigenvector of the irreducible influence matrix associated with its dominanteigenvalue of1based on the binary influence model. Vulnerability was treatedanalytically and numerically for several networks with different topological structureby employing assessment model based on node degree and edge betweenness. Resultsindicated that the scale-free network was more vulnerable than the random andsmall-world network. The more realistic evolvement model was built by consideringthe energy of nodes and equivalent electrical distance between nodes which determinethe influence between corresponding nodes. Numeric examples of the EPRI-36andGuangdong power grid demonstrate the validity and simpleness of the proposedmethod compared with the node betweenness and node importance according tonetwork cohesion degree, therefore it is beneficial to online security analysis from asystem perspective.Finally, this paper proposed a search method for power system cascading outagesbased on dynamic power flow which considered the voltage and frequencydependence of loads and generator regulation characteristics. The voltage stabilityevaluation index of power system and its critical value were derived according to thevariation of the Jakob matrix diagonal elements when the operating point of a powersystem was close to voltage collapse point. The voltage stability evaluation index ofpower system would decline gradually but its critical value was steady at around0.5when the load lever increased. The voltage stability evaluation index was applied toidentify vulnerable lines. With IEEE39-bus system as example, simulation resultswere compared with the results typical index such as transmitted power and weightedline betweenness. The results show that the proposed evaluation index was moreuseful and the time-domain simulation results also verified that the proposed methodwas reasonable and valid.