Power-angle Characteristic Optimization And Transient Stability Control Of AC/DC Parallel System

With the development of modern power system and the promotion of "west-to-east power transmission and the national interconnection" strategy,large-scale AC/DC power system is gradually becoming the basic pattern of China’s power grid,which brings increasingly problems on the transient stability of AC/DC power system.Furthermore,the application scale of new HVDC mode based on voltage source converter is constantly expanding,making the dynamic process of the system more complicated.Based on the background above,this paper analyzes the transient operating characteristics and the optimal control of the AC/DC paralleled system after summarizing the basic theory and control model of the traditional and the new HVDC transmission systems,which provides a certain theoretical reference for the operation and modulation of AC/DC system.Firstly,this paper analyzes the influence mechanism of DC line access on transient stability margin of AC/DC system based on the equal area criterion.Taking stability margin as an index,the effect of DC overload capacity,initial power angle,fault clearance time and power distribution coefficienton on the system transient stability is analyzed,the effective region for stability improvement of AC/DC system by DC link is proposed as well.simulation performed in MATLAB/Simulink validates the previous analysis.Secondly,aiming at the particular issues such as angle trajectory incoherency and electromagnetic power time-varying in the multi-machine power system,an analytical method to evaluate the overall transient angle stability is proposed,using the relative motion of complementary cluster’s equivalent power angle.A coherent cluster partitioning strategy is also put up based on CCCOI transformation as well as coherency indicator.The feasibility of the method is verified at the end of this chapter by simulations.Finally,for the transient stability issues of the AC/DC system,the DC power modulation technique is adopted to optimize the system fault characteristics and improve the transient stability.On the basis of the mechanism analysis of DC power modulation improving the system transient stability,the optimization strategies used for two power modulation schemes are put forward.Contraposing the DC large mode modulation,the input signal source is selected by the prony identification,and the parameter optimization strategy of variable step-size hierarchical search based on the coherency index of equivalent power angle is put forward.For the DC power upgrade/backdown modulation,considering that the modulation parameters are directively related to acceleration and deceleration area in the concept of equal area criterion as well as its particular characteristics of multi-variables and nonlinearity,the paper adopts the particle swarm optimization algorithm,taking unbalanced energy as a performance index,to optimize the modulation strategy.The validity of the two kinds of optimization strategies is verified by MATLAB and BPA tr ansient simulation,the optimized modulation strategy can effectively improve the transient angle stability,providing some reference for the DC power modulation strategy in practical engineering.