Research On Photovoltaic Grid-connected Control Technology Based On Virtual Synchronous Generator

Distributed generator(DG)units have very little or no moment of inertia and damping characteristics compared to conventional large power plants where synchronous generators dominate.As DG penetration levels continue to increase,the effects of low inertia and damping effects on grid stability and dynamic performance are increasing.The solution to improved grid stability is to provide virtual inertia through a virtual synchronous generator(VSG).The parallel connection of the photovoltaic power generation unit to the power grid also brings power decoupling and inverter circulation,which reduces the dynamic response capability of the photovoltaic power generation unit voltage and frequency.Based on the layered control,the active frequency adjustment strategy of photovoltaic grid-connected inverter is proposed to realize the imbalance of output power and improve the dynamic response of the system frequency.By studying the mathematical model of synchronous generator,the paper aims to simplify the VSG structure,and design the first-order lag link with the inertia characteristics of the frequency converter and the rotor.The simplified control structure is more robust in power dynamic adjustment,and at the same time avoid parallel resonance problem.Set the VSG operating impedance to a nonlinear parameter.The virtual negative impedance method based on coordinate transformation controls the precise distribution of reactive power to reduce the circulation between inverters.The power decoupling control is realized by coordinate transformation.Design the environmental parameters of the VSG system and select the key components of the main circuit.Through experiments,the rationality of the control strategy is verified.Analyze the simulated SG quasi-synchronous parallel operation to achieve fast frequency modulation and power adjustment performance of the distributed power supply.The typical second-order VSG control structure is used to realize the parallel process of multi-energy complementary power generation,which improves the stability of multi-energy interconnected power supply.In order to make the photovoltaic unit in the two-machine interconnection system have flexible and rapid frequency modulation and voltage regulation capability,a virtual synchronous generator(VSG)control scheme with minimum adjustment parameter design is proposed.The least-parameter frequency modulator has a first-order inertia characteristic,which can solve the resonance problem when the two-machine interconnection is connected to the grid.The transient operation of the nonlinear reactance simulation system in the VSG impedance model achieves the accuracy and flexibility of the VSG adjustment of the interconnected system during the transient steady state transition process.The simulation results verify the feasibility and effectiveness of the least-parameter VSG method.