Power Sharing and Power Quality Improvement in Parallel Connected Power Converters in the Microgrid

Power electronics-based Voltage Source Converters (VSCs) have been widely used as an interface between energy source and the grid/load in microgrid system. The VSCs are required not only to provide the basic function that achieves reliable and accurate power sharing in a microgrid, but also to improve the power quality. Specifically, the VSCs need to dampen the high-order LCL filter resonance and to compensate harmonics caused by nonlinear loads. These multi-functional requirements are putting challenges to VSC converter control.

To address above issues, this thesis investigates VSC control methods that can effectively share the load power demand in islanded microgrids as well as actively mitigate the resonances and harmonics in grid-connected mode. A modified droop control method with a minimum number of current sensors is investigated to accurately share the active and reactive power demand among the VSCs connected to the Point of Common Coupling (PCC) bus. The method requires no grid current information or communication links between VSCs, which can significantly reduce the implementation cost. Next, an improved PLL algorithm is presented to achieve fast grid synchronization for converters even under a distorted/unbalanced grid voltage. Then, a virtual RLC damper is proposed to provide active damping for the LCL filter resonance to ensure the system stability. Moreover, a unified selective harmonic compensation strategy using the VSC in the presence of nonlinear local loads in both grid-connected and islanded microgrids is presented. The proposed control methods of VSCs provide power-electronics based solutions to power sharing and power quality improvement in the microgrids, which feature high reliability, adaptability and cost-efficiency.