Control of grid-tied inverters for nano-grids

In recent times, with the development of renewables, the concept of micro-grid emerged, representing a novel bottom up power distribution organization. The micro-grid can integrate the nearby distributed, and mostly renewable, energy sources, the storage devices and the loads into the grid, with increased efficiency, flexibility and reliability, showing significant economical and environmental benefits. The micro-grid concept can be further scaled down to the range of a single house or small building, and differentiated by a new terminology, nano-grid. The proposal of the nano-grid concept is aimed at simplifying the application scenarios, so that a hierarchical bottom-up power distribution network can be established, where the nano-grid plays the lowest-end role. It can not only operate autonomously, feeding the typical household appliances from the available renewable sources; thanks to the modular smart grid architecture, it can also be conveniently interconnected to other similar units, operating in parallel and harmoniously energizing a larger region in a city, a small-island or a village. In addition, the nano-grid also has the possibility of self-integrating into the utility grid, exchanging power with the mains when needed, thanks to a specifically designed grid interface converter. Referring to the latter, a variety of requirements are defined by applicable standards, in terms of load power quality, grid support functionalities, abnormal condition ride-through and protection means. The realization of the above functionalities is heavily dependent on the control of the grid interfacing inverter hosted within the nano-grid, about which numerous solutions have been proposed in the existing literature. However, few of them can realize all the functionalities simultaneously in a single controller. The target of this dissertation is therefore proposing, analyzing and testing a high-quality, multi-functional control scheme for grid-tied inverters. This goal is reached in three steps: i) a deep literature review, ii) the identification, study and realization of the multi-functional inverter controller, and iii) the implementation of further, higher level functionalities, like the grid-supporting and parallel operation capabilities. Accordingly, the study is initiated from step i), with an overview of existing control strategies and key functionalities of grid-tied inverters. The comprehensive review of a research topic is, in any case, very advantageous to define the state of the art solutions and to evaluate the margins for improvement in the existing technology. In this research case, it allowed to understand that a triple-loop controller structure is the most suitable to achieve high-performance control of the nano-grid electrical system and the most promising as to the capability of implementing multiple interface and protection functionalities jointly. In the second step ii), a large-bandwidth triple-loop controller is proposed, whose implementation is the first contribution of this dissertation. The peculiarity of the proposed controller is the large-bandwidth control of the injected grid current, which brings in many beneficial features. Leveraging on this controller organization, multiple functionalities are later implemented by means of a superimposed flexible mode-transition manager and an auto-tuner, altogether forming a high quality, multi-functional control scheme for grid-tied inverters. This represents the second contribution delivered by this dissertation. Finally, in step iii), the extended scenario of multiple parallel-connected grid-tied inverters is discussed, targeting the realization of distributed grid-supporting functionalities in grid-tied mode and the automatic balanced power sharing in parallel-islanded mode. The final implemented control scheme provides a feasible solution for the forthcoming smart nano-grids and represents the third contribution of this research activity.