Study Of Searching Key Power Flow Transfer Section And Emergency Control Of Preventing Cascading Failure

The safe and stable operation of power grid is of great significance to ensure the orderly operation of social production and life.With the continuous expansion of the power grid,its complexity is also getting higher and higher,which means that the stability,security and reliability of the power grid are facing great challenges.By analyzing the large-scale blackouts around the world in recent years,it can be found that most of the causes of these accidents is: after the failure of the local line of the power grid,the line protection device action makes the fault line be removed,and then the large-scale transfer of the power flow occurs,resulting in the overload of the normal line and the misoperation of the overload protection device,which causes the cascading trip response.Therefore,in order to improve the operation monitoring efficiency of the power grid and reduce the probability of large-scale blackouts,this paper focuses on the search method of related key transmission sections after the fault line is disconnected and the emergency control method of overload line power flow.Firstly,it is difficult to search and determine the key transmission sections timely and comprehensively after the fault line is disconnected in the large power grid.A key transmission section search method based on improved fuzzy C-means clustering algorithm is proposed.Firstly,the particle swarm optimization algorithm is used to improve the clustering center in the fuzzy C-means clustering algorithm to overcome the sensitivity of the clustering algorithm to the initial clustering center,and the initial transmission section is obtained based on the improved clustering algorithm.Secondly,the Floyd algorithm is used to search the shortest circuit and the sub-short circuit containing the disconnected line,and the lines that may be missed are added to the initial transmission section to form the candidate transmission section.Finally,the proposed composite factor criterion is used to select candidate transmission sections,and then the key transmission sections are determined.This method can effectively improve the accuracy and practicability of the key transmission section search results.Secondly,for the overload lines in key transmission sections,an emergency power flow control strategy for overload lines based on comprehensive sensitivity is proposed.This method first calculates the weighted sensitivity of the generator node to the overload section to represent the sensitivity of the generator node to the overall power flow change of the overload section,and classifies the generator node types according to the positive and negative values of the weighted sensitivity,so as to obtain the set of operation buses with added and reduced output.Secondly,the node contribution rate of the generator node to the overload section is calculated to represent the overall overload capacity of the motor node to eliminate the overload section,and combined with the weighted sensitivity to form a comprehensive sensitivity.Then,the buses with the maximum comprehensive sensitivity in the set of added and reduced output operation buses are selected as the adjustment buses,and the node adjustment is calculated according to the principle of adjustment constraint.Different from the existing literature,this method determines the adjustment order of buses by rolling update,realizes the multiple participation of single and multiple buses,gives full play to the ability of each node to eliminate overload,and reduces the number of buses participating in the adjustment to a certain extent.At the same time,the comprehensive sensitivity of the method can fully reflect the ability of the node to eliminate the overall overload of the section,and the selected node can eliminate the overload more efficiently.Finally,in view of the lack of consideration of the influence factors such as the change of load size,the climbing speed of the unit and the maximum adjustable number of the unit in the sensitivity emergency control strategy,an emergency control method of overload line power flow based on multiple constraints is proposed.The objective function of the method is to minimize the auxiliary variables related to the line overload.The mathematical model is established with the load variation constraint,the maximum output constraint of the generator,the climbing speed constraint of the generator,the power balance constraint of the system,the safety constraint of the normal line and the maximum number of adjustable units as constraints.The commercial solver is used to solve the model,and then the final overload line power flow optimization control scheme is obtained.Considering multiple constraints,this method conforms to the actual engineering situation,so the obtained scheme is more practical.