Control Schemes For Three-level PV Grid-connected Inverter Under Non-ideal Grid Conditions

The PV grid-connected power generation is an impormant utilization mode of the solar energy,and the popularity of it is significant. In order to improve the power quality, nowdays the three-level converters are still the mainstream of PV grid-connected inverters. In this paper, with the control of three-level PV grid-connected inveter under non-ideal grid conditions as the object of research, key technologies and unbalanced control strategies of it were studied deeply.The correct value of the fundamental phase and fundamental frequency positive/negative sequence voltage are essential for the unbalanced control strategies. The paper researched on the ANF based phase locked method, proposed enhanced ANF and moving average-ANF by considering the influence of DC-offset and harmonics to the ANF’s performance. The proposed methods can extract the fundamental phase accurately with the DC-offset and unknowned harmonics. Also researching on dual second order general integrator-frequency locked loop and improving it, fundamental frequency positive/negative sequence voltage vectors are detected rapidly and accurately even with the unknown harmonics. The improved methods are further researched by simulation and experiments, and the results proved the validity of the proposed methods.The reasons of midpoint potential imbalance of three-level PV grid-connected inveter are studied. Through the research of different influence made by different basic voltage vectors, the paper proposed a new neutral-point potential balance algorithm based on the combination of vector displacement and balance factor. In the full consideration of the basic voltage vectors which good for the neutral-point potential balance, and with the efficacious control of balance factor, the new method can control the neutral-point potential balance. The simulation results show the validity of the proposed neutral-point potential balance algorithm.Considering the three-level PV grid-connected inverter will produce negative current in AC side and double frequency fluctuated voltage in DC side when the grid voltage unbalanced, the strategy of suppressing the negative current in AC side and the strategy of suppressing the second harmonic voltage in DC side are proposed separately. By using the proposed stratesies, the three-level PV grid-connected inverter can operate with high performance, and the simulated results prove the validity of the proposed strategies.Finally, the virtual line-flux-linkage was introduced for reducing costs and improving system robustness. Moreover, the virtual line-flux-linkage observation method based on DSOGI-FLL wase proposed, which can extract the fundamental frequency positive/negative sequence of virtual line-flux-linkage accurately under unbalanced grid voltage conditions. With the virtual line-flux-linkage observation technology, the unbalanced control strategies of three-level PV grid-connected inverter without line voltage sensors are obtained. The validity of the proposed strategies was proved by simulation results.