Study On The Control Strategies Of Three-phase Grid-connected Inverter Based On LCL Filter In Microgrid

In recent years, due to the increasingly serious energy crisis and environmental pollution, distributed renewable energy generation and grid technology has received wide attention. Obviously limited by nature conditions, the distributed power supplies have a considerable impact on the power system, moreover, the high stand-alone access cost and control problems both add to the difficulties. However, as a controllable micro system, which could combine distributed sources and loads together with a uniform power control, the microgrid gives the solutions to the problems above. Throughout the microgrid, the majority of the power supply sources are in renewable forms, and all of these supplies are led to the grid by power inverters. Thus, the grid-connected inverters becomes the core component, whose control strategies under various microgrid operated states should be placed in a very important position for study. Started from microgrid, this dissertation studies various control strategies of grid-connected inverters, besides, the simulation studies are performed in PSIM9.0and an experimental platform is established.The structure and basic components are briefly introduced in the beginning, as well, the grid-connected and stand-alone control strategies of different micro supplies are discussed. The control strategies of micro sources are all centered on the output power, which apply PQ control on dispatchable units and MPPT control on nondispatchable units. When the microgrid operated in islanded mode, droop control is applied on the diapatchable units for adjusting the PPC voltage and frequency. Simulation in PSIM9.0verifies the correction of the control strategy.LCL filters are employed in more and more high-power occasions for its distinctive advantages. This dissertation compared L,LC and LCL filters, and then analyzed active damping and passive damping control strategies taking account of LCL filter’s harmonic problems. Based on the simulations in PSIM, it is inferred that the resonant peaks can be cut by the double current closed-loop control strategy. As voltage harmonics are contained in the grid, the currents drained into the grid would appear obvious distortions. An improved grid voltage feed-forward control strategy is recommanded to solve this problem, and then the control strategy is verified effective by the simulations in PSIM.Based on the achievements above, experimental investigations are carried out. Then, the3-phase grid-connected inverter systems basing on LCL filters are designed, and the3kW experimental platform is established while the operating programs are compiled. Open loop experiment, stand-alone/grid-connected operating experiments are all proceeded on the inverter, showing the results that the system could operated in the stand-alone mode as well as the110V2kW grid-connected operation with both excellent steady-state and dynamic performances. These works has accumulated some experience for system design and practical operation, having a significant value in both theoretical and engineering aspects.