Hardware-in-Loop-Based Research On Virtual Synchronous Generator Under Connected And Off Grid States

With the energy and environmental problems worsening,distributed generation technology continues to evolve.Although distributed generation technology has many advantages,the relatively uncontrollable intermittent power supply such as photovoltaic power generation has a certain negative impact on power quality.In order to coordinate the relationship between distributed power and grid,battery energy storage system has been widely used.Converter is the the basic power electronic interface unit in the battery power storage system,so the research on the control mechanism of battery energy storage converter has far-reaching theoretical and practical significance.In this paper,the energy storage device and it's connected/off grid control strategy is studied,based on China EPRI project "energy storage system connected/off grid characteristics and detection technology research".Because the energy storage system needs to achieve stable operation under both grid-connected and island operating mode and smooth seamless switching between operating modes,this paper studies the control strategy of virtual synchronous generator based on energy storage converter.First of all,based on the control principle and mathematical model of the voltage-source converter,the paper analyzes the double closed-loop decoupling control strategy and the modeling method of virtual synchronous generator,and gives the control parameter selection method.Secondly,small signal models of energy storage system based on virtual synchronous generator control strategy in grid-connected mode and two converters in parallel operation mode are established.The influence of inertia on system stability is analyzed and the results of the analysis are verified by simulation.According to the power sharing mechanism of multi-VSG parallel systems,when the frequency crosses the limits,secondary frequency modulation is performed on the parallel system of multiple machines.It is proposed that decentralized frequency modulation is susceptible to error so that the parallel converter power can not be accurately averaged.And the decentralized frequency modulation is suitable for single VSG,while centralized secondary frequency modulation is not easily affected by error,so the power distribution is not affected by the secondary frequency modulation dynamic process and is more suitable for multi-VSG in parallel.Thirdly,according to the requirements of seamless switching of operation modes,the pre-synchronization principle are analyzed,and the island switching is divided into planned and unplanned respectively for simulation analysis.Finally,based on the hardware-in-loop experimental platform,the virtual synchronous generator control program and the hardware circuit are designed,and the connected and island control strategy of the energy storage system is experimentally verified.So that the feasibility of the control strategy is further verified by hardware in-loop experiments.