Study Of Emergency Control Strategy To Prevent Cascading Trip Caused By Power Flow Transferring

With the development of China's electric power industry,the power exchange between the regional power grid is becoming more and more frequent,mounting the burden of the line.In this context,the power flow transferring caused by fault and other factors furtherly deteriorate the working conditions of lines,thus increase the risk of brackout.To this end,this paper studies the emergency control strategy to prevent cascading trip caused by power flow transferring,so as to ensure the correct action of backup protection that can avoid the occurrence of major power outages and improve the safe operation level of the power system.First of all,based on the feature of power flow transferring for multiple branches removal event,the calculation formula of line power flow is deduced to fast recognise the flow transferring in network.By analyzing the composition of the flow on the normal line and the distribution difference of flow transferring between fault branches,we introduce the equivalent power flow and virtual branch model to simplify the refraction and reflection process.Through this way,we get the fast calculation formula to identify the power flow transferring after lines cut off.And then,according to the composition of the line power,two algorithm to searching power flow transferring serious lines based on the equivalent power flow and the power flow transfer density(FTIL)separately are presented.The former one reestablish a new path searching scheme on the basis of equivalent power flow,which creatively extend the path search to multiple branches removal event.What's more,the strategy that conduct searching process with correction is adopted to avoid missing lines.The latter one offers the use of FTIL to define generalized power flow transferring critical area,that can simplify distribution of power flow transferring for multiple branches broken,and the aboved area can be defined with modified breadth first search(BFS)algorithm,automatically avoiding small FTIL value areas and reducing search range by introducing FTIL into BFS termination condition,which improves the searching efficiency of serious lines.As for the problem of line overload,a load shedding scheme based on virtual control unit and heuristic search is established.This method firstly uses the principle of concentric relaxation to reduce the size of network,filterring devices and elements that is preferential to participate in adjustment.Secondly,the concept of virtual control unit is introduced,which can validly identify the difference of load shedding effects between the overload branches and improve the resolution of the traditional sensitivity algorithm.Then combined with the equivalent generator tripping and the load shedding control strategy,the heuristic search method of the optimal control unit,the calculation method of the optimal control unit adjustment amount and the control processes are put forward.Finally,after rounds of adjustment,the grid gradually towards the optimal direction of the load shedding until the overload eliminated.Noticeably,in each turn,the calculation processes of optimal control points and adjustment values is simple and qucik,and the overload will be feedback at the end,which achieves closed-loop control.Finally,the IEEE standard test system is used as the example to verify and analyze the algorithm presented in this paper.The simulation results show that the proposed algorithm can achieve the anticipated effects under the situation of multiple branches removal.In conclusion,this article provides a more complete emergency control strategy to prevent cascading trip caused by power flow transferring,and it furtherly improves the safety and stability of power grid.