Study On Optimal Design Scheme Of LCL Type Grid-Connected Filter

With the increasing power demand for social development, the relationship between power and development is becoming more and more important. People pay more and more attention to new energy for its special advantages, photovoltaic(PV) becomes a hot topic at the same time. As an important technology of photovoltaic grid-connected power generation, filtering technology has a great influence on the quality of grid-connected current. The main works of this thesis are listed as follows:(1) Research on the performance and the mathematical model of LCL filterCompared and analyzed the filtering performances of L, LC and LCL types of grid-connected filters, the result showed that LCL type filter had a better performance for suppressing high frequency harmonic. Therefore, LCL filter was selected as the research topic. However, a split-capacitor method was proposed in view of the shortcomings of the resonant peak of the LCL type filter, and the control strategy of the inverter was suppressed by improving the control strategy.The mathematical models in three-phase static coordinate system and two-phase rotating coordinate system could be obtained by establishing the mathematical models of grid-connected inverter based on LCL type filter. The analysis results indicated that in the mathematical models of LCL filter, designs of the controller structure and parameters of single-phase was consistent with three-phase circuits.(2) Research on control strategy of inverter based on LCL filterIn the part of control strategies analysis, it showed the necessity of the control strategy used in the system since the presence of massive harmonics in the PV grid-connected power generation system and iner harmonics in filter, and explained that why harmonic suppression is the main evaluation factor of a control strategy. The other work is about the passive damping control strategy of grid-connected inverter, comparison and analysis between split-capacitor based on the LCL filter and traditional passive damping proved that the split-capacitor method has a low damping loss; and for the control strategy of the active damping, we did same analysis as those of passive one, it demonstrated the split-capacitor model can pick and choose a larger proportional gain and obtain a better filtering effect. Based on the above mentioned analysis, the technique that cooperative control strategy based on split-capacitor model of LCL filter and detailed design of the new scheme controller were proposed.(3) Research on the values of the filter based on cooperative control strategyThe influence of inductances and capacitances on filtering effect was concluded by analyzing the properties of the LCL filter. At the same time, the selection region of every inductance and capacitance value was found out by analyzing their total values’selection requirements. Then the proportion relations of the two selected inductances or two capacitances were analyzed, and the reference selection method was summarized to ensure that the cooperative control strategy can obtain the optimal filtering effect.(4) Research on the filtering results with the different control strategiesThe simulation model of the three-phase and the single-phase grid-connected inverter based on the new control strategy has been set up in MATLAB/Simulink software platform. The simulation results indicated the following two conclusions:one was that the cooperative control of grid-connected inverter based on LCL filter split-capacitor method could suppress the harmonic content effectively, which proved the validity of the designed control strategy; second, the cooperative control strategy showed a better damping effect on comparison with the active damping control strategy as well as less power loss for the passive damping control strategy through the comparison analysis. The superiority of the proposed control strategy was also verified.