Wide Area Backup Protection And Control Strategy To Prevent Cascading Trips

With the rapid growth of load demand and the execution of strategy we called"power transmitting from west to east, mutual complementary between north and south and national power grid interconnection", a super large-scale nationwide power network is shaping in our country. Furthermore, with the deregulation of power industry, power flow exchanges occur more frequently among those interconnected power grids. Besides, the load of transmission lines is increasing. Once some key lines with heavy load condition are tripped after fault, cascading trips, even catastrophic blackout are possibly triggered for the domino offect of flow transferring and irrational activation of backup protection. This would result in enormous loss of society and economy. By introducing the wide area information into backup protection design, novel wide area backup protection and control system to prevent cascading trips is proposed in this dissertation.Based on the impact of network topology and parameters on flow transferring between branches in the network, the mathematical description and definition of flow transferring relativity factor (FTRF) are presented, which can reflect the flow transferring relationships between branches in power network. The matrix calculation formulas of FTRF are separately deduced and proposed under the condition of phase-to-ground admittance in consideration or not. The characteristics of the FTRF are summarized as well. By analyzing refraction and reflection process of flow transferring, the calculation formula of FTRF for multi-branches chain removal event is deduced. Moreover, the calculation formula is proved according to the definition of FTRF. Therefore a sound FTRF theory frame is founded.Based on FTRF theory, as the transient period after the condition of single branch removal or multi-branches removal event is concerned, the transient flow transferring identification algorithms are presented. Moreover, a wide area backup protection design scheme is also proposed separately. Based on the relationship of node injection currents and branch currents, network correlation coefficient matrix is introduced; and a wide area measurement system (WAMS) based flow transferring identification algorithm with limited measurement points to distinguish flow transferring in transient period is presented. The principle of measurement point selection is proposed based on analyzing the characteristic of network correlation coefficient.In order to mitigate the overload caused by flow transferring, two kinds of controlstrategies are presented in this dissertation. One is fast generator tripping and load shedding control strategy based on network correlation coefficient; the other is generator and load adjustment control strategy based on optimization planning theory. The former adopts an equal-quantum generator tripping and load shedding control scheme. According to the characteristic of network correlation coefficient, the principle of control nodes selection of load shedding and generator tripping and the calculation method of control quantum are proposed. The algorithm of the former control strategy is simple and fast. At the same time, since the equal-quantum control scheme would not result in overload of other healthy branches, the validity of the control strategy is guarantied. The latter control strategy, which aims at minimizing the loss of load, applies optimization planning theory into generator and load adjustment. A novel simplified approach and solving strategy of the optimization model is proposed to speed the searching process. Both two kinds of control strategies integrate the on-line measurements of WAMS, and then can guaranty minimum control cost and eradicate cascading trips simultaneously.