| With the increasingly serious problems of energy and environment,more and more attention has been paid to the distributed generation using renewable resources.Micro-grid is an efficient organization form of distributed generators,which not only avoids the adverse effects on grid brought by the direct integration of DG,but also gives full play to the renewable resources.As the key component of energy exchange between renewable resources and grid,inverter has the advantage of flexible control method and quick response.However,it leads to the lack of inertia and damping in micro-grid.The synchronous generator has the natural friendly advantage to the power grid,if we use the working experience of the traditional power system to make the characteristics of inverter present like synchronous generator,we can improve the stability of the micro-grid and the compatibility of the distributed generator.Virtual synchronous generator technology arises at the historic moment.Firstly,this paper summarizes the concept,structure and research status of micro-grid,and analyzes several kinds of traditional control strategies of micro-grid inverter,and introduces a new control technology: virtual synchronous generator(VSG).Then,the two-order model of synchronous generator is used as the mathematical model of VSG,and the control structure of VSG is analyzed,the realization method of VSG is obtained.The speed regulator and excitation controller of VSG are also designed.Then,The indirect virtual impedance method is used to realize the synchronous reactance of VSG to ensure that the proposed algorithm can simulate synchronous generator accurately,and the appropriate impedance selecting range is given.Then,the seamless switching technology based on VSG control is studied.This paper presents a synchronization control method by adjusting the reference value of VSG frequency to control the q-component of output voltage that orientated to the grid voltage to be zero,which can guarantee the switching process between island mode and grid-connected mode seamless.Finally,the control method of VSG under asymmetric condition is studied,put forward a kind of VSG control method in island mode with unbalanced loads:suppress the negative-sequence component of VSG output voltage to zero to ensure that VSG can output three-phase balanced voltage;propose a kind of VSG control method in grid-connected mode while grid voltage is unbalanced: filter the grid negative-sequence voltage,calculate the positive-sequence current as a reference to make sure that VSGoutput three-phase balanced grid-connected current.The simulation is performed on the Matlab/Simulink platform to verify the accuracy of proposed VSG algorithm,to validate the effectiveness of VSG seamless switching technology,to confirm the reliability of suggested VSG control strategy under asymmetrical operating conditions. |
