Research On Protection And Emergency Control To Prevent Overload Cascading Trips

Along with the growing of the grid scale and complexity of power system structure, large-scale blackout caused by overload poses grave threats to the security and stability of system operation and the reliability of power supply. When the line is overload, there is a need to distinguish overload from fault, in order to avoid that misoperation further dampen the power system and cascading trips result in large-scale blackout. After identifying the overload and ensuring no protection misoperation, choosing appropriate generator tripping and load shedding control node and its operating quantity is needed to avoid large-scale blackout from the source and guarantee the security and stability of system operation. In allusion to the problems mentioned above, this paper has preceded mainly the several aspects as follows:An adaptive identification method of overload based on the composite vector plane is proposed in this paper. First of all, according to the measured phasor under different operating conditions of geometrical distribution characteristics, the virtual voltage drop equation is established, so the calculation formula about the adaptive setting coefficient can be derived and the overloading recognition criterion of adaptive distance protection section Ⅲ can be obtained. Then, combined the overloading recognition criterion and the traditional distance protection action characteristic of section Ⅲ,the overload and symmetrical fault are able to be identified. The methodology involves local information procession instead of communication, which makes it fast, easy to implement, overcomes the problem results from dead zone by simple locking protection after overload identification, and system protection can be realized in the condition of overload. Simulation result of IEEE39node10machine system verifies the correctness and effectiveness of the method.An adaptive distance protection on the basis of the virtual voltage drop equation is proposed in this paper. Same as the first method, according to the geometrical distribution characteristics in different operational states, this method uses the virtual voltage drop equation to solve the adaptive setting coefficient and build criterion of the adaptive distance protection. Based on the method, specifically focuses on the problem that transition resistance interfere distance protection, the author confirms the offset angle of transition resistance voltage and acquire transition resistance compensation coefficient with the help of measurement of negative sequence and zero sequence component, which significantly improve the adaptive distance protection’s ability to resist transition resistance. PSCAD 500kV two machine system verifies the effectiveness of the method and its excellent ability to resist transition resistance.A strategy of emergency control for line overload on the basis of the source and load synergy coefficient is proposed in this paper, which could refresh the source and load branch synergy coefficient and the source and load distribution synergy coefficient according to the real-time operation after overload, and ensure generator tripping and load shedding control node set..On the foundation of the situation of line overload and control node set, the method makes use of PSO with double fitness to optimize searching control strategy, taking minimizing the financial loss of system as the goal. Based on all of these, for the reliability of power supply, the method chooses a part of generator that without generator tripping control to increase power supply in the condition of not overloading other line, drives part of sheded load, recovers the power supply, and improve economical efficiency of the holistic strategy. Simulation result of IEEE39node10machine system verifies the strategy effectively reduces load of overload circuit, and has a very high economic coefficient in the context of ensuring safe and stable system operation.