Analysis And Control Strategy Research On Transient Stability Of AC/DC System

With the continuous expansion of power grid and the development requirements of responding to the optimization and rational allocation of energy structure,the current large-scale AC/DC interconnection form of transmission has gradually become the basic trend of power grid planning and construction.As the capacity of DC transmission system continues to increase,its impact on the transient stability of AC system becomes more and more significant,making its participation in transient process of AC/DC coordination and transient stability control gradually increase the demand.In this thesis,based on a detailed analysis of the mechanism of the impact of VSCHVDC on multi-machine systems,a VSC-HVDC active power control strategy to improve transient stability is proposed.From the equivalence of rotor equations of motion reflecting the transient stability of multi-machine systems,with the help of the grounded admittance model of VSC-HVDC rectifier station and the contraction of system nodal admittance matrix,the relational equation of the power characteristics and the topological location information of VSC-HVDC rectifier station on the influence of the system equivalent electromagnetic power is deduced.An accurate mathematical model reflecting the quantitative relationship between the active power control of VSCHVDC rectifier station and the transient stability of system is established.For nonlinear relationship between the additional control amount of active power at rectifier station and the equivalent power angle,using the sliding mode variable structure control method,the switching surface is established by using the equivalent power angle change speed and acceleration as state variables to give an additional control strategy for active power at rectifier station that can accelerate the system towards stability.The simulation results show that the proposed strategy can ameliorate the change of equivalent power angle and speed up the recovery of system stability.As the limited overload capacity of VSC-HVDC will affect its ability to improve transient stability,considering that the synchronous machine mechanical power is also an important factor affecting the change of equivalent power angle,in this thesis,a coordinated control strategy of VSC-HVDC active power and valve opening to improve transient stability is proposed.Based on the derived equivalence rotor equations of motion for multi-machine AC/DC systems,a mathematical model for comprehensive analysis of VSC-HVDC rectifier stations and valve opening affecting the system transient stability is established by converting equivalence mechanical power affecting equivalence power angle change into an additional control quantity of valve opening.And according to the sliding mode variable structure control method to determine the rectifier station active power and valve opening additional control amount coordinated with the requirement of state variable change in the switching surface function.Based on sliding mode variable structure control method,the additional control quantities of active power and vavle opening at rectifier station are determined that are coordinated according to the changes of state variables in the switching surface function.During the transient process,two additional control quantities are calculated in real time based on the current moment of operation,which are interrelated and influence each other,and quickly stabilize system by changing the equivalent electromagnetic power and mechanical power.Simulation analysis shows that the combined effect of active power at rectifier station and valve opening coordination control makes up for the limited overload capacity of VSC-HVDC and improves the system transient stability.