Stability Analysis Of Virtual Synchronous Generator Based On Impedance

In recent years,new energy sources such as photovoltaics and wind power have developed rapidly,and the penetration rate of new energy power generation in the power system is increasing.However,the volatility and intermittent nature of photovoltaics and wind power,as well as the lack of inertia and damping characteristics of traditional new energy power generation,have a negative impact on the stable operation of the power grid.In this context,virtual synchronous generator(VSG)technology came into being.The virtual synchronization technology introduces inertia and damping in the control system,which makes the inverter have the inertia and damping characteristics of the synchronous generator,which is beneficial to the stable operation of the power grid.However,in the island mode,the virtual synchronous generator provides voltage and frequency support for the grid,and its stable operation is related to the stability of the whole system.Therefore,it is of great significance to study the stability of virtual synchronous generators.The specific work of this paper is as follows:Firstly,the control technology of VSG is studied in detail.The two modes of the virtual synchronous generator are introduced separately.For the grid-connected mode,active-frequency control,reactive-voltage control,and off-grid switching control are introduced.For the island model,the unbalanced load capacity is analyzed,and the control strategy based on positive and negative sequence is proposed.Secondly,the impedance modeling of the VSG is established.Firstly,the main circuit model of the virtual synchronous generator is introduced.Then the small signal model of the control system is established according to the specific control method.Finally,considering the influence of the active-frequency control loop,the control system and the main circuit system are combined to obtain the impedance model of the system.The basic principle of three-phase impedance measurement method and sinusoidal modulation impedance measurement method are introduced,and the accuracy of the built model is verified by impedance measurement.Then there is the stability analysis of the virtual synchronous generator.The first three impedance-based stability criteria are introduced,and the three criteria are analyzed and compared.Through analysis and comparison,the generalized Nyquist stability criterion is adopted.Then,for the virtual synchronous generator,the influence of system parameters and line impedance on the stability of the system is analyzed.It is analyzed that the line impedance is too large and the system is unstable.The inertia coefficient and damping coefficient of the control parameters have little influence on the stability of the system.Finally,a hardware-in-the-loop test platform including RT-LAB platform and TMS320F28335 digital controller was built.Based on the platform,the modeling of virtual synchronous generator system and the design of VSG control algorithm are completed.The stability analysis and verification of VSG unbalanced load control and virtual synchronous generator are completed.The results of Matlab and hardware-in-the-loop test platform verify the accuracy of the unbalanced load control algorithm and stability analysis proposed in this paper.