Research On The Control Strategy Of Cascading Failure In AC/DC Hybrid Power System

With the application of HVDC transmission technology,China has formed a basic pattern of AC / DC hybrid power systems with high voltage levels and long-distance transmission,which improves the ability to optimize the allocation of resources in a wide range and ensures the power demand of different regional loads.However,at the same time,it also makes the operation of the power grid face numerous tough challenges.Different from the traditional pure AC power grid,the component connection mode of the AC-DC hybrid power grid has changed greatly,and the coupling relationship between the parts is closer.The constraints to be met for normal operation are more complex,which aggravates the risk of cascading failure in the hybrid power grid.Based on the general propagation process of cascading failure in the power grid,considering the action characteristics of line backup protection,the evaluation form of whether the cascading failure is caused after the initial failure of the power grid is given.Then the electrical distance between different operating states is discussed in detail,and the electrical distance that is most likely to cause chain failure is taken as the safety level evaluation index of the system.Aiming at the cascading failure phenomenon of AC power system,a control model for preventing the cascading failure of AC power grid is established by taking the safety level index as the objective function,taking the output of the generator set as the control variable,and combining the feasibility constraints of AC power grid operation.Finally,the particle swarm optimization(PSO)algorithm is used to solve the model,and the simulation verification is carried out in the corresponding system.On this basis,the prevention cascading failure control strategy of AC-DC hybrid power network is analyzed.Firstly,comprehensively evaluate the significance of AC lines from the perspective of network structure and operation status.At the network structure level,the line transmission contribution index is considered to quantify the power transmission capacity of each line;From the perspective of power grid operation status,the hazard index consists of the reciprocal of the product of the overall voltage relative margin and the branch transmission margin rate.Immediately after that,the selected AC circuit is regarded as the initial fault of the AC-DC hybrid power network,and the evolution process of the chain failure of the hybrid system is briefly discussed.Given the initial AC failure scenario of the single DC feed AC hybrid system,the impact of the initial fault on the operation of the hybrid system,is analyzed from the aspects of protection action characteristics and phase transition failure.And the judgment formula for the chain failure of the hybrid transmission system is given.Then,the operation status of the hybrid system is expressed by node injection power.The safety margin index of the hybrid power network is reflected in the electrical distance between the current operating state of the power grid and the critical state that can cause a cascading failure.This index can quantitatively characterize the safety level of the hybrid system.Finally,by optimizing the output of the generator set,aiming at improving the safety margin index of the hybrid system as much as possible,taking into account the constraints of the operation of the hybrid system,the defense control model of the AC / DC hybrid power grid is built,and the improved IEEE14 node system and IEEE39 node system are simulated and verified.The simulation results show that the proposed preventive control strategy can effectively improve the safety level of power grid operation and avoid the occurrence of cascading failures.At the same time,the preventive control model of a single DC-infeed hybrid system can provide a reference for further research on the prevention of cascading failures in multi-DC-infeed hybrid systems.