Study On Optimization Of Parallel Operation Characteristics Of Grid-connected Inverters Based On Virtual Synchronous Generator

As renewable energy gradually replaces traditional fossil energy,the penetration rate of new energy power generation in the power system will continue to increase.Due to the intermittency,volatility and randomness of new energy power generation,the large-scale and high-proportion access of inverters will bring challenges to the security and stability of the power system.At present,the grid-connected inverters based on virtual synchronous generator have received extensive attention and research due to its characteristics of inertia,damping,frequency modulation and voltage modulation.At the same time,the grid connected inverters of new energy power generation are connected to the power grid in parallel,which brings problems such as circulating current and uneven power distribution.Therefore,in order to solve these problems,this thesis studies the optimization strategy of parallel operation characteristics of grid-connected inverters based on virtual synchronous generator.Firstly,based on the research of virtual synchronous generator,this thesis designs a parallel system scheme of grid-connected inverters based on synchronverter according to the requirements of inverters’ grid-connected and parallel operation.Based on the parallel system of two grid-connected inverters,the characteristics of circulating current and power distribution are studied.Secondly,aiming at the uneven power distribution caused by the inconsistent output impedance of the inverters during the steady period,a precise power distribution method based on the dynamic compensation of the power reference is studied.On the basis of analyzing the influence of the LCL filter on the output power,the dynamic compensation principle of the power reference is studied,and the relevant control parameters are set to make the output power of a single grid-connected inverter reach the power reference.The matching method of key parameters of virtual synchronous generator such as active power reference,reactive power reference,active power droop coefficient and reactive power droop coefficient are studied to realize the accurate distribution of output power of parallel grid-connected inverters according to the preset proportion.The moment of inertia matching method of virtual synchronous generator is studied to make the power regulation time synchronization.The simulation results show that the method studied in this thesis can effectively improve the accuracy and synchronicity of power distribution.Thirdly,aiming at the problem of active power overshoot and oscillation during the transient period,an active power oscillation suppression method based on the dynamic adjustment of the moment of inertia of virtual synchronous generator is studied.On the basis of analyzing the relationship among the moment of inertia,active power oscillation and transient energy,the dynamic adjustment control method of moment of inertia of virtual synchronous generator is studied,and the main contents are as follows: 1)the method of determining the time point of moment of inertia adjustment based on enabling function,it reduces the interference caused by system and power mutation on the change of moment of inertia.2)The method of virtual rotor motion state judgment based on the power change rate,it reduces the influence of the reference frequency point offset on the virtual rotor motion state judgment.3)The method of sectional value of moment of inertia,it selects the corresponding moment of inertia according to the different motion states of the virtual rotor,which can support the inertia of the system and suppress the power oscillation at the same time.The simulation results show that the method studied in this thesis can improve the stability and dynamic characteristics of the parallel system.Finally,a low-power experimental platform of parallel system of grid-connected inverters controlled by virtual synchronous generator is built,and experiments related to the optimization of parallel operation characteristics are completed.The experimental results show that the grid-connected inverters based on virtual synchronous generator studied in this thesis can output power according to the power reference set by the system in the stand-alone working mode,and can accurately distribute power according to the set proportion and suppress active power oscillation in parallel working mode.