Research On Control Methods Of Grid Connected Inverter Based On LCL Filter In Distrubuted Energy System

Distributed generations develop rapidly in recently years with substantial benefits,including flexible electricity production and high energy utilization.As the interface of grid and distributed generations,grid-connected inverter's performance is important to guarantee the power quality.Grid is often weak because of a large number of distributed generations tying to it,putting forward higher requirements on grid-connected inverters.This paper takes an LCL filtered transformerless grid-connected inverter as the research object.And researchs on paramerters optimize design and resonace damping of LCL filter,stability analyze of grid-connected inverter in weak grid,grid current's DC component suppressing.A mathematic model of the LCL filter was built based on LCL type single-phase grid inverter topology.System stability caused by parasitic resistance,LCL fiter's parameters change and grid resistance was analyzed.LCL filter parameters limitation and data range was confirmed,LCL filter parameters was designed based on minimum conductors' value and minimum change of resonace frequency caused by grid resistance.A 3.3kW single phase grid-connected inverter was set up as the experimental platform at last.Control parameters of the method based on capacity current and grid current were designd.Stability of the system after the control method adding propotional feedforward or all feedforward of grid voltage was analyzed respectively.The role of grid all feedforward functions to the grid background harmonics were explained.Differenctial elements of feedforward function were configured by low-pass fitler to suppress the introducing noise.When gird resistance ranges,the adaptation of time constant in low-pass filter was discussed.The wave shaking problem was researched,which was caused by digital signal process time delay,filter time delay and PI controller's intergrate time delay.The solving methods were proposed such as using proper filter structure,designing proper filter paramerters and using proper sample and control interrupt to design more effective control methods.The active damping method based on double current sampling needs to add the current sensor used in sampling capacitive current.However,LCL filtered inverter's resonance can be damped by cascading notch filter in forward channel of current controller.When grid is weak,the variation of impedance can cause the change of resonance frequency which leading to the failure of notch filter.In order to solve this problem,a method was proposed to correct the notch frequency of notch filter by detecting the system resonance frequency on line.It was difficult to digitalize and calculate.So a biquad filter instead of notch filter as the correct object was proposed.It was used to correct notch frequency and the proportion factor of PI controller.Simulation and experiments validated the proposed method can improve the weak grid adaption of LCL-filtered inverters efficiently.DC componenet in transfoermerless grid-connnected inverter needs to be suppressed.Based on the mathematic model,generating mechanism of grid current's DC component was discussed from inconsistent action of power switchs,error of measuring elements,error caused by digital control respectively.Relationship between DC bus ripple voltage and grid current was discussed.Quantitative description of DC component was got by integrating the different section of DC bus voltage in one work cycle.This quantitative value was used to correct the reference current on line.Simulation and experiment results verify that the method can effectively suppress the DC component without any extra peripheral circuits.Through the above research,the grid inverter's performance was further increased,and its core competitiveness in the market was improved.This thesis adapts the demand to our country to develop the new energy and improve the core competitiveness,and has certain theoretical foundation and practical application value.