Research On Voltage-controlled Grid-connected Strategy For Photovoltaic Inverter Under Unbalanced Grid Condition

In recent years,with fossil fuels increasingly exhausted and env ironmental problem unceasingly highlighted,the percentage of the new energy system in the whole energy structure rised year by year.Solar power is a clean,less-polluting,sustainable and renewable new energy,so it gets attention in the era of widesprea d development of energy.The photovoltaic inverter which has the most important and practical significance to the control studies of inverter is the core part.Especially when the condition of power grid is worse,how to make inverter be safe and efficial has been a hot spot at the present stage.This article is based on the three-phase grid-connected inverters,which makes us carry out in--depth study on account of the voltage control strategy of sagging control under unbalanced three-phase grid.Finally,the purpose of the common coupling point(PCC)voltage of the stable parallel network inverter is achieved,and the system is safe and reliable.Firstly,this artical introduces the development status of the inverter and its future trend.Combined with the characteristics of the existing photovoltaic inverter,the topological structure of a two-stage three-phase photovoltaic parallel converter is proposed.And the parameters of the dc and ac side filters are designed.In combination with the three-phase unbalanced network,the mathematical model under grid-on mode is established.This paper analyzes the effect of unbalanced grid on inverter and puts forward the problem of network control strategy,which based on droop control and the imbalance of grid voltage.In order to improve the performance of the voltage parallel inverter in the electric grid,using the negative sequence extraction scheme based on the second order generalized integrator,the voltage bases frequency component is extracted rapidly and accurately so as to obtain the dc component and the ac component of the inverter power,which provides the basis for the efficient output current compensation.In view of the grid inverter circuit features into virtual impedance,reasonable adjustment of inverter output characteristic,make the inverter presents perceptual output impedance,in order to make sure the power decoupling of system.In order to solve the problem of voltage imbalance,two kinds of unbalanced control strategies under unbalanced power grids are proposed in the two-phase stationary frame mathematical model.Positive sequence power improved( P+-w* ? Q+-U*)drooping control and Positive and negative sequence improved( P+-w* ? Q+-U* ?Q--G*)drooping control.Effectively improve the inverter and the outlet voltage,realize the target of the output power stable and the PCC point three phase voltage balance.In this artical,the three-phase parallel inverter simulation platform is set up in the Matlab/Simulink/Plecs,the correctness of the voltage control strategy based on drooping control under unbalanced power network is verified efficially.Finally,the experimental system of the network inverter based on TMS320F28335 is constructed,and the feasibility of the control strategy proposed in this paper is verified.The experimental results can prove that the droop control strategy improved under the condition of unbalanced power grid,which can achieve purpose of stable PCC point voltage and the output power of inverter,and reduce the influence of unbalanced voltage on the grid running quality of the inverter.