Research On Analysis And Prevention Of Power System Cascading Failures

With the increasing interconnection of regional power grids and the further advancement of electricity markets,the modern power grid is becoming more and more complex.Blackouts involving cascading failures have directly threatened the safe and stable operation of power system.To prevent large-scale power blackouts effectively,it is urgent to carry out in-depth analysis on cascading failures which leads to accidents and formulate suitable control strategies to eliminate potential security risk which will influence system reliability.To achieve the goal of prevention and control of cascading failures,this paper aims at exploring the key factors such as crucial branches and multiple contingencies which might trigger cascading blackouts from the very beginning,according to the current system operation state.Meanwhile,as for cascading overload caused by power flow transferring,the active power correction strategy based on sensitivity is designed to block the propagation of cascading failures in time.For identifying the key part of complex power grid,the branch importance is evaluated from the overall perspectives of structure and running state.In terms of structure,the index of active power transmission betweenness considering transmission margin rate is proposed based on the complex network theory and the characteristics of power grid;In terms of running state,the index of impact based on improved transfer entropy of power flow is proposed by making use of such electrical information as the amount of power transferring,the branch security constraint,the initial power flow,etc.An assessment index of synthetical importance of branches is set up considering the relevance and complementarity between system structure and running state,including indices of active power transmission betweenness and impact.The test on an IEEE 39-bus system demonstrates the method proposed is valid and rational.To solve the issue of large amount of multi-contingency combinations,a new method of N-2 contingencies rapid screening and ranking is proposed.According to line outage distribution factors and real-time power flows,the line overload impact factor(LOIF)is defined.Based on LOIF,all N-2 contingencies to be analyzed are divided into two types for screening simultaneously,and the performance index is used to measure the severity of important anticipated N-2 contingencies selected for generating a list of contingencies.Several simulation tests of IEEE systems show that this method is accurate and reliable,and can reduce the scale of N-2 fault sets quickly and effectively by using the first-order fault calculation results,greatly improving the analysis efficiency.As for the problem of line overload caused by power flow transferring,a load shedding strategy based on the matching sensitivity and improved integrative sensitivity is proposed.This strategy takes the control node pair including generation scheduling bus and load shedding bus as a whole so that the mutual coordination ability between control nodes to eliminate overload can be fully evaluated.The selection range of control nodes is narrowed down according to the division method of generalized power flow transferring zone,and the principle of concentric relaxation is used to screen branches which are prone to overload for constructing the security constraint set,thus reducing the computation amount and improving the emergency control speed.Combined with the matching sensitivity,the effective control node pairs which can eliminate overload are selected.Followed by that,the optimal control unit with the best load shedding effect is determined based on the improved integrative sensitivity.By taking into account the constraints of adjustable nodal power,overload elimination and the redundant power of normal branches,the adjustment power amount is calculated,and rounds of adjustment are utilized to guarantee the control process can gradually approach the direction of overload eliminated thoroughly.The test result of an IEEE 39-bus system shows that the load shedding strategy proposed in this paper is highly feasible and can effectively solve the problem of multi-branch overload correction.