Research On Grid-connected Stability Of Inverter Based On Virtual Synchronous Generator

In the context of global environmental pollution and energy crisis,renewable energy has increasingly begun to raise people’s attention.Renewable energy sources(RESs),mainly including photovoltaic power generation and wind power generation,are integrated into the grid as distributed sources.With the continuous improvement of the capacity of renewable energy,it has a negative impact on the stability of the traditional power system.The renewable energy generation is mainly integrated into the power grid through power electronic equipment which is lack of inertia and damping,and it leads to the lack of inertia and damping of power system.In this context,the virtual synchronous generator technology based on energy storage has entered the eyes of researchers.Firstly,the principle of virtual synchronous generator is analyzed,and the mathematical model of virtual synchronous generator is established.The ontology and control model of virtual synchronous generator mainly include the following parts: rotor motion equation,stator voltage equation,virtual governor and virtual exciter.After building the basic module of the virtual synchronous generator,the voltage and current double closed-loop control is designed to make the output voltage of the virtual synchronous generator more stable and the response faster,and then a complete grid connected inverter using the virtual synchronous generator algorithm is obtained.Then,a complete virtual synchronous generator model is established.The virtual synchronous generator model is used to verify the islanding operation and grid-connected operation.Under different working conditions,the important parameters of the virtual synchronous generator,such as moment of inertia,damping coefficient and active frequency droop coefficient,are simulated to verify their influence on the performance of the virtual synchronous generator.The results show that the system will obviously oscillate in grid-connected mode.The moment of inertia has a very important impact on the system oscillation.The larger the moment of inertia is,the smaller the frequency overshoot is,the longer the frequency response time and adjustment time are.Finally,the power loop of the virtual synchronous generator is decoupled and the small signal model of the active power loop is established.According to the VSG stability margin,frequency adjustment time,cut-off frequency of the active power loop and grid connection standard,the reasonable values of the moment of inertia J and damping coefficient D are determined to ensure the stability of the system.Due to the introduction of inertia,the virtual synchronous generator is more prone to power oscillation.In this paper,the nonlinear relationship between inertia and angular velocity is analyzed,and adaptive neural network control is applied to transmission for the first time.Based on this concept,an improved virtual synchronous generator(VSG)based on energy storage system is proposed to overcome the characteristics of low inertia and under damping caused by largescale renewable energy integration.Firstly,the radial basis function(RBF)neural network with simple algorithm,strong learning ability and fast learning speed is used to adaptively adjust the virtual inertia.This strategy not only improves the response of the system,but also reduces the frequency overshoot when tracking the steady-state value.Then,based on the fixed damping ratio,the damping coefficient is adjusted adaptively according to the change of inertia to further suppress the power oscillation.