Research On Power Flow Control Strategy Of AC/DC Hybrid System Based On Steady-state Security Rregion

Large-scale AC/DC hybrid power grids have become a new form of power grid development in China.In large AC/DC hybrid power grids,different types of variables are intertwined,the coupling mechanism is complex,and there may be contradictions between control objectives and operational constraints.It brings great challenges to the operation and control of the system.Especially in the case of serious faults such as emergency block of largecapacity DC transmission,how to realize the potential of coordinated control of multiple DCs and AC power grids is significant to ensure the safe and economic operation of the system.This paper makes use of the theoretical advantages of the steady-state security region to carry out corresponding coordinated control research for different operating phases and control levels of AC/DC hybrid systems.The main research contents are as follows:(1)A steady-state security region mathematical model for an AC/DC hybrid system is established,and the definition and characterization method of the two-dimensional projection of the steady-state security region are given.The current system’s operating constraints can be obtained by the security region boundary screening method.In order to make full use of the potential of coordinated control of DC systems and AC grids,the use of controllable series capacitor commutated converters is proposed to expand the size of the steady-state security region.The basic principle and control system of the controllable capacitor commutated converter are analyzed,and a station-level control strategy based on the commutation voltage time area is proposed.Based on this,the MOV-parallel gap combination protection device is used to shorten the failure recovery time of controlled series capacitor converter and improve the ability of the weak receiving end system to resist commutation failure.(2)Based on the steady-state security region theory of AC/DC hybrid systems,a linear fitting of the boundary surface of the steady-state security region is proposed to improve the calculation efficiency of the steady-state security region method.Based on the fitted steadystate security region boundary,the calculation methods of modified security distance and its sensitivity are given.The method of scaling the steady-state security region is used to realize the quantification of the importance of each dimension variable.A calculation method for the security correction guidance vector is proposed,and the security distance sensitivity of each control variable to the guidance vector and the actual adjustment ability of each control variable are comprehensively considered to determine the priority of its participation in the correction control.The security correction strategy optimization model with the minimum control amount as the optimization goal was established,which can make full use of the potential of coordinated control of multiple DC and AC power grids to restore the security of system’s power flow as soon as possible after DC block.(3)Taking full account of the vulnerability of the AC/DC hybrid system after the emergency control to withstand secondary risks,a steady-state security region mathematical model that takes the N-1 constraints of lines with heavy load into account is established,and based on this,the depicting method of optimization dispatch security sub-region in threedimensional space are given.The security and economics of the optimization dispatch strategy are comprehensively evaluated with the system generation cost,voltage deviation and security margin.By calculating the security distance sensitivity of each control variable to the transmission limit of the crucial section,the sensitive generators and DCs that have a significant effect on the transmission power of the critical section are determined to reduce the difficulty of observation in multivariable systems,and the projections of security sub-region of highsensibility control variable to optimization dispatch target,generation cost,voltage deviation and the crucial section are depicted under the constraints of different security margin,providing richer operation information and strategic guidance for optimization dispatch.(4)A general model for controlling the security region and the optimal adjustment curve in the region with time characteristics is proposed.Based on this,combined with the calculation method of the main circuit parameters of the HVDC transmission,the steady-state relations of the electrical quantities such as the active and reactive power compensation,the firing angle,the extinction angle and the transformer tap were determined,and the DC active power control region was completed.An optimal adjustment curve model with the minimum average voltage fluctuation of the sensitive buses as the optimization goal is proposed,which can guarantee the global stability and security of the AC/DC system in time dimension and limit the voltage fluctuation to the maximum extent.This method provides a feasible solution for the problem that the control target of AC/DC hybrid system will be difficult to achieve due to the uncoordinated time response characteristics of the components.