Study On Backup Protection And Control Strategy For Critical Lines Against Cascading Trips

Power system security is of crucial importance. With the expanding of interconnected power grid, the undesirable trip of backup protection caused by transferring power flow has become one of the contributing causes of cascading failures and blackouts in power systems. From the perspective of strengthening overall system security and avoiding large-scale blackout, novel wide-area backup protection and emergency control strategy for critical lines against cascading failures are studied and proposed in this dissertation.Based on survivability assessment theory and complex network theory, a novel method is proposed to solve the problem of critical line identification. The concept of backbone-grid is defined to describe the key components and important network structures that effectively sustain the survival of system, and the corresponding effectiveness indices for backbone-grid are also presented. To avoid computational burden resulted from enumerating all the possible reconfiguration schemes of network topology, the problem of exploring backbone-grid is formulated as an integer nonlinear programming problem. Furthermore, the method to extract critical lines from various backbone-grids is presented. Without simulating and randomly sampling specific paths of cascading failures, the proposed method can identify critical lines essential to the survival of system, and thus provide cost-effective locations to update the overall performance of backup protection and emergency control system.To prevent undesirable trip of backup protection caused by transferring power flow, two backup protection operation strategies are proposed. To avoid immediate overload trip of backup protection caused by initial branch removal events, the adaptive adjustment strategy of backup protection is proposed. The heuristic method for searching adaptive strategy and the logic diagram for implementing adaptive strategy are both given. The adaptive strategy is obtained when the system is in normal operation state, and the proposed strategy can avoid the undesirable trip of backup protection on basis of reliable clearance of internal short-circuit fault. Furthermore, to eliminate the undesirable trip of backup protection due to the mismatch between the predetermined adaptive adjustment strategy and the unexpected removal of a series of lines, the fast strategy for blocking undesirable trip of backup protection is further given based on the relay operation information from neighboring substations. The blocking strategy is with simple and reliable principle and can be updating in a real-time manner, and the desirable operation of backup protection can be obtained.To determine the constraint branch set for alleviating overload in critical line, two methods for searching power flow transferring path set are proposed. Firstly, a method for searching transmission section is presented. Accoding to the distribution characteristics of voltage phase at the both sides of transmission section, the strategies for clustering buses and exploring generation zones and load zones are presented, and the method for searching transmission sections between adjacent region grids and within region grid is further given. Secondly, as for the situation that overload critical line is outside transmission sections, an on-line method based on graph theory is further given to explore power flow transferring path set. The mathematical model for searching the shortest path and shorter paths between both sides of the overloaded critical line is established, and taking the shortest path lengths obtained in normal operation state and inherent topology characteristics of power flow transferring paths as the heuristic information, the fast method for exploring the mathematical model is proposed, and thus the power flow transferring path set of the overloaded critical line can be obtained. Moreover, based on the concept of bi-connected component in graph theory, the method for partitioning power flow transferring areas is presented, and thus the power flow transferring path set can be explored in single power flow transferring area. As the result, the searching speed is independent of the whole network size.To alleviate the overload both in critical branch and its power flow transferring paths, an emergency control strategy is proposed based on sensitivity method. Taking the control node pair consisting of generation scheduling bus and load shedding bus as a whole, the mutual coordination ability between control nodes to alleviate overload both in critical branch and its power flow transferring paths is evaluated, and the methods for selecting optimum control node pair and its control amount are further given based on the comprehensive sensitivity and osculating value. Furthermore, the fast method for calculating sensitivity coefficient is presented based on the concept of power flow transferring area. The proposed method can alleviate the overload both in critical branch and its power flow transferring paths in large-scale power grid.