Control of Smart Transformer-fed Distribution Grid

At present, the distribution grid is undergoing significant changes due to the utilization of distributed energy resources and new loads, like electric vehicles. The increasing penetration of these resources has caused issues in the management of the electric grid. To cope with the emerging grid issues, the Smart Transformer (ST) is a promising solution, which can benefit the grid management and increase the hosting capacity of distributed energy resources, due to its control and communication functionalities and dynamic. The voltage control is one of the most important topics for the ST application in the distribution grid. The ST LV converter enables direct control of the LV grid voltage waveforms in terms of amplitude and frequency. This control, interacting with the connected appliances, determines the performance and stability of the LV ST-fed grid. Though ST can well address grid issues, challenges from the point of view of control are encountered: control interactions with distributed resources, grid resonances, stable operation with grid synchronization, and high power quality during provision of grid services. To address these problems, the knowledge of power system, power electronics and control is combined in this work. For the control interaction, an optimal voltage control as well as design consideration is proposed for ST with stability concern. For the resonance issue, filter-based active damping methods are proposed and implemented in ST to alleviate the issue, and a grid estimation method is used to online estimate the resonant frequency and stability margin. For synchronization issue, a more accurate morel of grid converter with synchronization effect is proposed. Moreover, a virtual impedance-based stabilization method by means of ST is proposed to address the instability caused by synchronization. For the power quality issue, a frequency adaptive control strategy is implemented in ST to maintain good power quality.