Research On Virtual Synchronizer Control Method In The Two-phase Stationary Coordinate Frame

With the global energy crisis arising,distributed generation has been received more and more attention.Virtual synchronous generator(VSG)technology can improve the voltage and frequency modulation performance and increase inertia damping of distributed generation efficiently which is significant in the stable operation of distributed generation system.As the VSG is the core research object,the influence of VSG parameters changing on system stability and the synchronization problem under non-ideal grid voltage condition are investigated in this paper.The main contents in this article covers: the design of virtual synchronous machine control system in two-phase stationary coordinate frame,the stability analysis of virtual synchronous machine in two-phase stationary coordinate frame and the design of virtual synchronous machine control system without grid voltage sensor.In order to avoid the complexity of coordinate transformation and model decoupling in rotating coordinate system,the virtual synchronous machine system under stationary frame is presented in this paper.In order to make the proposed control method be provided with frequency and voltage modulation ability,the power control loop design is based on mechanical equation of synchronous generator and droop control equation,and PR controller is adopted in the inner current control loop design.The strategy to estimate main parameters of the VSG control system is proposed based on the requirements of distributed generation connect to the power grid system.Stability analysis is the key point of the research in this paper.To analyze the stability of the VSG system in the two-phase stationary coordinate frame,the 10-order state space equation of the VSG system is established firstly.Then the influence of the VSG control parameters and controller parameters on the system stability is analyzed according to the root trajectory of the system.The operations of the system are analyzed when the parameters are taken as the critical stability point and the unstable point.The virtual synchronous machine control method without voltage sensor is proposed in view of unstable operation and grid connection problem existed under ideal drop,flicker grid conditions.Firstly,the virtual flux is built through stator current,DC bus voltage and inverter signal,and then the grid voltage can be simulated by using the relationship between the flux and the voltage,so as to achieve virtual synchronous machine control without sampling the grid voltage.The proposed control strategies and stability analysis is verified on the three-phase photovoltaic experiment platform,whose core of control is DSP and rated capacity is 15 kW.Experiments verify the effectiveness of virtual synchronous machine control under stationary frame,the accuracy of the small signal model and the stability analysis of VSG system in the two-phase stationary coordinate,and the feasibility of virtual synchronous machine control without grid voltage sensor.