Study On LCL Filter In Grid-Tied Inverter

In distributed energy generation system, grid-connected inverter is one of the most importantparts. It converts DC power from distributed energy to AC power to grid. This thesis analyzed thedead-time effect for grid-connected inverter and then proposed an optimized design scheme of LCLfilter for minimal total inductance.Firstly, this paper illustrated the mechanism of dead-time effect, its harmonic spectrumdistribution and it influence to fundamental output voltage. Combining the resonance frequency ofLCL filter, the dead-time effect was discussed in the cases of resonance frequency located in differentfrequency bands. Then based on the basic analysis and the bandwidth of system closed-loop transferfunction, the paper quantitatively analyzed the suppression effect on dead-time and the closerelationship between the closed-loop bandwidth and the suppression effect in the occasions of singleloop controller and dual loop controller. The experiments were carried out on a250-kW grid-tiedinverter to verify the suppression effect on dead-time. Simulation and experimental results agree withthe theoretical analysis pretty well.Combining with the transfer function of LCL filter, this paper introduced the operating principleof LCL filter and discussed the influence of each parameter to the filtering performance. According tothe analysis, an optimized filter design scheme was proposed which warrants a minimal totalinductance with a need filtering performance. With introducing specific constraint conditions, thefilter design procedure converts to searching optimization solutions mathematically. An inverterprototype with several different LCL filters was made to verify the validity of the optimized designscheme.