Research On Control Strategies For Three-Phase Grid-Connected Inverter In Distributed Generation

With today’s growing energy crisis, the development and utilization of renewable energy, including solar and wind energy, are becoming a very important issue, in which distributed generation (DG) plays a vital role.As an interface between power grid and renewable energy, grid-connected inverter is an indispensable part of DG and its performance has an important impact on the implementation of DG. In this dissertation, grid-connected inverter used in DG is researched and a new control strategy is proposed to improve its stable and dynamic performance. Also, the analysis of different islanding detection methods is included.First, this dissertation introduces the development of DG and its application in renewable energy in recent years, after which the ongoing research of grid-connected inverter is summarized. Then, the working principle of three-phase half-bridge grid-connected inverter is analyzed and a10kW prototype is designed, including the parameters of LCL-filter.After the hardware design, a continuous model of the system is built, based on which the parameters of multi-resonance controller are selected according to the root locus theory. The effectiveness of the proposed design method is verified in experiments.Next, in order to damp the resonance of the LCL-filter, an improved discrete model of the system is built, based on which the state variables feedback is adopted to configure the closed-loop poles of the system. By placing the poles in their best positions, the dynamic performance of the system can be greatly improved. Additionally, in order to reduce the cost of the whole system, a Luenberger observer is applied.Due to the unsatisfactory performance of the observer, LCCL control strategy is employed. Although this strategy can suppress the resonance of the LCL-filter successfully, its dynamic performance is limited. Combined with state variables feedback, the performance of LCCL control strategy can be improved, and with the employment of state observer, the cost of the system can be kept unchanged. This proposed strategy is verified both in simulation and experiments on the prototype.Last, as required by IEEE standard, islanding detection methods are necessary for DG. In this dissertation, a wide range of islanding detection methods are researched, especially the basic positive ones and frequency shift methods. Their non-detection zones are analyzed theoretically and verified in experiments.