Scaling the controllable network transformer (CNT) to utility-level voltages with direct AC/AC power electronic building blocks (PEBBs)

Increases in load, increases in penetration of renewable energy, and limited investment in transmission infrastructure are fostering the need for a smarter, more dynamically controllable grid. Flexible ac transmission systems (FACTs) devices such as the UPFC and BTB are difficult to implement at transmission-level voltages due to complexity and cost. A direct-ac/ac-converter-based controllable network transformer (CNT), because of its fractional converter rating and lack of dc storage requirement, is a simpler and less expensive alternative for applications involving dynamic power flow control. Scaling of the CNT, however, is still a challenge. This paper proposes an approach for scaling the CNT to utility-level voltages and currents utilizing multilevel topologies as well as direct-ac/ac power electronic building blocks (AC-PEBBs). A comparison of various PEBB-based CNT topologies is presented via simulation and experimental results for a 13 kV, 1 MVA application.