Static Security Control Of AC/DC Hybrid Power System Based On Security Region

The interconnection of large regional power grids and the hybrid connection of AC/DC systems makes the steady-state operating point of the power system get closer and closer to its stable operating limit.At the same time,the coupling mechanism of different types of variables in AC/DC hybrid power system is complex,which brings great challenges to the security and stability control of the AC/DC hybrid power system.In the process of static security control,how to consider and simplify the various security constraints in the optimal control of AC/DC hybrid power system from the overall macro perspective,and then formulate the corresponding control scheme efficiently and quickly are of great significance to ensure the security and stability operation of the system.The theoretical advantages of the Steady-state Security Region(SSR)provide an effective solution to such problems by providing the overall operating state of the system and simplifying the consideration of various constraints in optimization problems by the boundary surface represented by hyperplane.Therefore,based on the theoretical advantages of steady-state security region,how to quickly formulate the corresponding control schemes for AC/DC hybrid power system in different operating states is studied in this paper.The main research contents are as follows:Firstly,the mathematical model of the SSR of the AC/DC hybrid power system is established.For the boundary surface of SSR in high dimensional space,the quasi-orthogonal method is used to search for a certain number of critical operating points on the boundary surface,and then the least square method is used to perform linear fitting after normalizing these critical operating points with different dimensions,and the linear expression of the boundary surface is obtained.At the same time,the judgment standard of the fitting effect of the boundary surface,as well as the calculation formula of the Steady-state Security Distance(SSD)and Steady-state Security Margin(SSM)are given.At last,in order to realize the visualization application of SSR,a description method of the section of the SSR is given,so as to the relative position of the operating point to each boundary can be seen intuitively.Secondly,a preventive control optimization model of AC/DC hybrid power system based on SSR is proposed.On the basis of considering the difference of control cost of different variables,the model takes the lowest control cost as the objective function and the linear expression of SSD as the constraint,which can solve the drawback of the traditional preventive control model with a large amount of calculation and provides the possibility for online application.At the same time,in view of the disadvantages of large-scale AC/DC hybrid power system with many anticipated accidents in the future,the dominant event idea is adopted to screen the anticipated accidents,and only the effective anticipated accidents are retained,which can not only shorten the solution time of the boundary surfaces of SSR,but also simplify the optimization space of prevention control model,thus the solution efficiency can be improved.Thirdly,on the basis of preventive control,considering the probability and severity of different anticipated accidents,a two-layer mathematical optimization model of coordinated control between prevention and correction is established.The model takes the lowest total cost of coordinated control as the objective function,and decomposes the pressure of security control into pre-accident and post-accident.For the outer-layer optimization of the two-layer optimization model,a "constraint relaxation" optimization method is proposed,which removes the anticipated accident with the lowest SSM from the preventive control subset and incorporates it into the corrective control subset,which can effectively avoid the dimension disaster caused by the exhaustive method.For the inner-layer optimization,the SSD is taken as the constraint condition,which effectively reduces the complexity of the model and improves the efficiency of solution.Finally,in order to formulate the corresponding security control scheme more quickly and efficiently,the concept of Decoupling Security Region(DSR)is proposed based on the SSR.Taking the multi-infeed AC/DC hybrid power system with Static Synchronous Compensator(STATCOM)as an application scenario,the influence of STATCOM on the transmission characteristics of the strong DC and weak AC system is analyzed.Then the key variables of the system are extracted,the DSR is described and the evolution characteristics of the DSR are analyzed.At last,based on the online application of the DSR,the security control methods of fast power flow control and decoupling control of key variables are proposed,and the solution time of the control scheme can be reduced to a millisecond level.