Vulnerability Assessment And Stability Control Of Power Grid Based On The Fault Chains

With the development of the economy and the stresses from resources and entironment, the modern power grid is characterized as eltra high voltage, long-distance transmission, large capacity generators and cross-regional interconnections.Therefore, the researches on the evolvement, vulnerability and preventive measures of blackouts have been received much attention. Many works have been done and some cascading outage models have been proposed, such as high-level probabilistic model, complex network model and state-steady model. On the basis of the models, the vulnerability has been analyzed, however, the analytical results were imprecise to the practical system because some models were ignored the power flow and stability of system and the uncertainties in the operational conditions. And, in the existing works, preventive measures were presented to partial prediction and control, while the researches of the control methods against the cascading failures were very few. Additionally, the application of vulnerability assessment results in cascading failure controls has not been solved so far. In this paper, the vulnerability assessment based on the fault chain model and the control methods against cascading failures are studied. The main works are as follows:The model of cascading failures is the foundation of vulnerability assessment and control. According to the typical mode of cascading failures, i.e. power flow transfer, sequent overload of lines and damage of transimission segments, the online determination method of the fault chain model is proposed in this paper. The predictive index of fault chain links is the core of the method. A fault chain can be obtained through calculating the index until the system is unstable. The method depending on the dynamic communication of lines and taking the power flow and transient stability into account is accommodated to the system model and computed online efficiently.The fault chain model as the typical sequence of cascading failure can be also regarded as complex domino effect. The occurrence of a fault chain is associated with that of each fault chain link, which is a simple man-mechine system. The occurrence of a fault chain link is influenced by some uncertainties such as human factor, but the human factor quantitative analysis is not enough for power system. Therefore, based on the human reliability analysis, the human security quantitative model is proposed in this paper using the fuzzy clone selection algorithm. The results of human error probability can be applied to fault chain assessment. The fault chains are comparable from the aspects of vulnerability and cascading failures. The comparability is expressed as the probability and the consequence of the fault chain. Therefore, the risk theory is introduced and the risk importance of fault chains is proposed. The risk importance of fault chains integrates the probabilities and consequences of occurrences of fault chain links, which are connected with many uncertain factors during the cascading outages. And then, based on the fault chain risk importances, the vulnerability assessment method of power grid is derived from the importance measures of reliability research. The method can be calculated in an efficient way hardly influenced by the grid size. The validity of the method can be demontrated by the simulations, and the relationship between vulnerability of power grid and stability is also provided in the simulations.On the basis of the vulnerability assessment conclusions, the preventive control and emergency control thoeries and their executive scheme are proposed to address the slow-phase and the speedy-phase of cascading failures. During the course of the slow motion of cascading outages, the prevention of the overload, instability and intentional attack of vulnerable lines is the main work. In this paper, the control method of the vulnerable lines using transient potential energy is proposed against the instability of the most vulnerable line. During the speedy motion of cascading outages, the system is very dangerous so that eliminating the imbalance power is the primary problem. Taking the uncertain factor caused by generator relay and the control cost into account, the real-time initiation approach of power balance emergency control based on fuzzy risk analysis and control stategy using the particle swarm optimization algorithm are proposed in this paper. Finally, the validity and prospect of the methods are testified by the simulations.