Flexible Control Structure of a Smart Transformer for Universal Operation

The smart transformer (ST) is considered as one of the key technologies for modernizing distribution grids and facilitating large-scale integration of distributed generation and battery energy storage systems into them. The new grid scenarios demand that the ST will not be merely a substitute for the conventional transformer operating as a power transmitter, but that it plays a key role in providing both ac and dc connectivity and advanced grid support, and in modifying the functionality of its AC-side converters. Under different control objectives and varying grid configurations, an approach based on a different control structure for each objective presents critical aspects such as smooth transition and flexible reconfiguration of network. To overcome these aspects, in this work, a three-level hierarchical control approach based on the virtual-synchronous generator (VSG) and the stored energy model of the DC links capacitors provides a unified and flexible control structure that allows responding to different control objectives without modifying the control structure and parameters. The control has been validated under different grid scenarios and control objectives.