Decoupled Control And Optimized Design Of Grid-connected Inverter With LCL Filter

The voltage source PWM grid-connected converters(GCIs) are widely used in modernindustry application, such as distributed generation systems where sources are mostly re-newable energy, and modern power system. To pursue green power as well as to meet thestringent harmonic standard, the conventional L filter is gradually replaced by the LCL filter,for its better harmonic elimination capability. However, this brings challenges to suppressthe resonance. Among diversity control strategies that focus on active damping for LCL-type GCIs, the systematic state space methods are preferred and discussed in this thesis, fortheir full degrees of freedom in control by which the bandwidth and stability margin can bewell traded off.The principle of state space control for LCL-GCIs is introduced in detail, including thedesign approaches such as the linear quadratic(LQ) method and the direct pole placementmethod, and also two general type state feedback controllers known as integer-based typeand PI-based type. To overcome the difficulty in design of PI-based controller due to theshortage of regular way, the LCL-GCI together with the PI-based state feedback controller ismodeled in discrete controllable form to facilitate the computer aided design. By means ofperformance evaluation in terms of reference tracking, grid rejection and robustness underweak grid, and the required control effort is also considered, a clear guidance for designingthe state feedback controller is proposed.Due to multi-feedback of state variables, the coupling between grid voltage and gridcurrent turns to be complicated, especially in the discrete plane. By means of continuoustransformation, a full grid voltage feedforward (GVFF) decoupling strategy is proposed tomake the feedforward work well with the discrete state feedback controller. The robust-ness analysis of the proposed strategy to the grid impedance is also performed. Moreover, agrid voltage estimated by state variables instead of measured is employed for the full GVFF,which not only retains the important information of grid voltage but also eliminates the influ-ence of accompanied noises. As a result, the accuracy and robustness is well traded off underwith only a moderate computational burden. Hence, a high-quality steady performance isattained. To overcome the defection of weak dynamics performance, an investigation on simplified GVFF strategy is carried out subsequently based on different state feedback con-trollers, leading to a more practicable solution.In order to take advantage of the inherent harmony between filter and controller, a novelintegrity design method that starts with standard PI controller for grid current is proposed.Given expected stability margin, the suitable region of resonance frequency of LCL filter isidentified to accommodate the preset controller; hence no resonance damping is employed.The parameters of filter are further determined to comply with the IEEE harmonic standardsas well as practical constraints. Moreover, a unit proportional feedforward of grid voltageis employed not only to suppress the grid background harmonic but also to boost the gridimpedance compatibility. The shortcoming of comparatively large total filter inductancerequired by the high resonance frequency is overcome subsequently, either by substitutingthe LCL filter with LLCL filter, whose harmonic attenuation is sharper; or by determining apracticable lower LCL-resonance frequency for a specified PI controller. The superiority ofthe proposed design is verified by the super steady dynamic and steady waveform that evenunder weak grid, which is attained in low cost and in easy way.From the whole research works in this thesis, the dynamics as well as the couplingbetween grid voltage and grid current are systematically investigated and analyzed, basedon which, a simple design of high performance LCL-GCI is proposed also, and it pave theway to the wide applications of LCL-GCIs.