Research On Improved VSG Control Method Based On Virtual Energy Storage

With the increasing permeability of distributed energy,the microgrid is gradually developing into a low inertia and undamped network,and the stability of the power grid is severely challenged.By simulating the inertia and damping characteristics of the synchronous generator,the virtual synchronous generator(VSG)technology can alleviate the impact brought by the non-inertia of the traditional grid-connected inverter to the power grid,and can effectively suppress the rapid fluctuation of the power grid frequency.It has gradually become the key technology to improve the acceptance capacity of the power grid to the distributed power supply.Thanks to the intermittency and uncertainty of distributed generation,the bus voltage of the DC side of the VSG will fluctuate,etc.However,the realization of the VSG technology depends on the stability of primary energy and enough energy buffer.At present,in the research on VSG technology,the DC side is usually equivalent to a DC power supply and its energy storage equipment.The default capacity of the energy storage equipment is large enough,and the DC side voltage fluctuation is not fully considered,which is inconsistent with the practical application scenarios.This paper puts forward an improved VSG control method based on virtual storage,the guarantee of VSG and node voltage stability at the same time,using the three port bidirectional DC/DC converter,DC/DC converter and energy storage battery,such as topology by moving distributed power fluctuation has a wide range of non-critical load voltage and power,thus ensuring the stability of DC bus voltage,and focus on the coordinated control of multiple virtual energy storage units on the DC side of the VSG.The specific research content is as follows:Firstly,the structure of DC microgrid system is studied and each unit is analyzed and modeled.It includes photovoltaic power generation model,grid-connected inverter,energy storage and AC/DC load,etc.,and focuses on modeling and determining parameters of the existing control topology of VSG.Simulation is conducted in the on-grid/off-grid multimode environment,and the effect before and after the strategy improvement is compared and analyzed.Secondly,the control strategy of VSG based on single virtual energy storage unit is studied.The energy storage requirements of the grid-connected system of the VSG under different damping states are analyzed.The virtual energy storage unit is composed of DC power spring and load,and the control of three-port DC/DC converter,bidirectional DC/DC converter and the typical working mode of the battery are studied.The virtual energy storage unit is introduced into the VSG grid-connected system,and the comprehensive effect of the system energy storage unit is analyzed.Finally,the proposed improved VSG grid-connected system with the introduction of single virtual energy storage unit is simulated and verified.Finally,the cooperative control of multiple virtual energy storage units on the DC side of the VSG is studied.The upper control structure,including the DC bus voltage consistent control and the battery storage state of charge consistent control,is established by using the consistency theory.The lower control structure,namely the local structure,includes the phase shift control,decoupling control and droop control.The simulation results verify the effectiveness of the proposed cooperative control strategy.