Research On LLCL-type Output Filters Of Single-phase Grid-connected Inverters

As the interfacing device of distributed generation systems to the public grid, the design of grid-connected inverters directly determines the quality of output current. With the increasing penetration of grid-connected inverters in the grid, grid-side current harmonic are increasing with potential risk of. It can cause harmonic oscillation, and even more, resulting in the collapse of the power system. In order to solve this problem, there are two ways to improve, which are control-based methods of the inverter and optimization of output filters. At present, many researchers mainly focus on the former, namely the improvement of grid-connected inverter performance from the view of optimization control, while the topology optimization of output filters relatively less. However, the topologoies of the filters affects the design of the controller greatly. So the output filters of single-phase grid-connected inverters are researched in this thesis, starting from the traditional LCL-type output filter, and then carring out further analysis and investigation on LLCL-type filters proposed in recent years. The main contrnts are concluded as follows:1. Analyze the structure and basic principle of the LCL-type filter. Analyze the influence of the parameters on filtering effect by using frequency response. Introduce two kinds of passive damping methods, in which case capacitor series and parallel resistances, and state the equivalent method of active damping based on the passive damping that capacitor parallel resistance.2. Research the structure and analyze the filtering performance of the LLCL-type filter, including the basic principle, the influence of parameters on filtering effect, and the restriction of parameter design. And the parameter design is addressed.3. For the resonance problem of the LLCL-type filter system, active and passive damping are analyzed, which are the comparison of filtering performance between eight kinds of passive damping methods and the active damping method based on the feedback of capacitive current. In case of the double feedback of the capacitive current and grid current, the analysis and simulation are taken to verify the equivalence of filtering effect for active and passive damping methods.4. Combining the respective advantages of LCL and LLCL-type filters, a composite LLCL filter is obtained. The filtering effect of the composite LLCL filter is analyzed in theory. Finally, a parameter design method is proposed for the composite LLCL filter from the view of application.5. The experimental platform prototype is set up to study the filtering performance comparison of the LLCL-type filter and the composite LLCL filter and to reveal the improved features of the composite LLCL filter.