Impedance Modeling And Grid Connection Stability Analysis Of Virtual Synchronous Generator

At present,most grid-connected inverters based on renewable energy are current-controlled inverters,which cannot provide beneficial support for the power grid.Therefore,with the rapid growth of distributed generation in the power system,the difficulty of regulating frequency and voltage increases,which brings severe challenges to the control and operation of power system.Virtual synchronous generator(VSG)is a kind of grid friendly power electronic equipment.It simulates the control characteristic of synchronous generator,not only have good function of regulating frequency and voltage,but also introduce the virtual inertia,which improves the transient stability of the system effectively.In addition,we can attach grid voltage feedforward control to VSG to eliminate the influence of the power grid voltage background harmonic on grid current.However,since that the impedance of transmission line and a large number of reactive compensation devices in the grid cannot be ignored,the stability of the interactive system composed of VSG and complex power grid still needs further study.Therefore,this paper studies the grid-connection characteristics of VSG based on the impedance analysis method.Firstly,the complete positive and negative sequence impedance model of VSG is derived in detail,and the admittance of VSG is equivalent to the closed-loop transfer function.According to the pole distribution of the closed-loop transfer function,control parameters are designed,and the influence of parameter variations on the impedance is analyzed.And then using the harmonic linearization method deduce VSG output impedance model under voltage feedforward control,points out that the introduction of the current inner loop and the feedforward control has obvious inhibitory effect on mirror frequency current.Based on the impedance model,we compare the VSG impedance characteristics under different control,and explain the dominant control unit of the characteristics of VSG impedance at different frequencies combining the characteristics of different control loop.Then,based on the impedance analysis method,the interaction between VSG and the weak power grid component system under these two kinds of control is studied,and the improvement effect of feedforward control on grid-connected stability is illustrated.Finally,the resonance of the interactive system to which the compensation device is connected in series is analyzed,showing that the parameter adjustment can restrain the oscillation of the system.In this paper,a simulation model and a hardware experiment platform are built to verify the correctness of the impedance model,and the stability of the interactive system composed of VSG and complex power grid is verified by RT-LAB platform.