As a new means of power generation, distributed generation (DG) is experiencing a rapid development. The DG systems based on renewable energy source or micro-sources such as fuel cells, photovoltaic (PV) cells, wind turbines, and micro-turbines can reduce greenhouse gas emissions, improve power system efficiency and reliability. All the power generated by each DG in the micro-grid must use an electric inverter to interface with the power system. Basically, the microgrid inverter has two operation modes: grid-connected mode and standalone mode. For standalone operation mode of microgrid inverter, a multi-loop controller is proposed in this work. The voltage differential feedback inner loop is embedded in the outer voltage loop. Also an output voltage decoupling and current decoupling are implemented by only using the output voltage feedback. The proposed control scheme possesses very fast dynamic response at load step change and can also achieve good steady state performance at both linear and nonlinear loads. For grid-connected operation mode, it is demonstrated that the possible grid-impedance variations have a significant influence on the system stability when conventional proportional-integrator (PI) controller is used. To deal with this stability problem, an H controller with the explicit robustness in terms of grid-impedance variations is proposed to incorporate the desired tracking performance and the stability margin. For the transition from grid-connected operation mode to standalone operation mode, this thesis proposes the voltage based and current control based algorithm which can force the current to decrease to zero at a short time thus provide seamless transfers between the two operating modes, avoiding the temporarily uncontrolled output voltage. Finally, the thesis analyzes the transient characteristics of the voltage and proposes an intelligent islanding detection and load shedding scheme which can detect the grid outage fast and do the load shedding accurately to avoid the distortion in the transition. All the proposed controllers and control algorithms in this work are extensively tested and verified through experiments.
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