A Ćuk-based modular DC-DC converter for medium-voltage direct-current (MVDC) grid applications

A reliable modular DC-DC converter that is well-suited to the MVDC grid applications is presented in this dissertation. This converter benefits from zero current turn-off and soft turn-on switching. Each power module is designed and controlled such that it employs two small film capacitors for transferring the power from the input to the output. This feature eliminates the need for electrolytic capacitors that have high failure rates. Furthermore, each power module is configured as an isolated converter, using an integrated high frequency transformer. Each power module in the proposed modular converter sees only one semiconductor on the conduction path to transfer energy from the input to the link capacitors or from the link capacitors to the output, which results in relatively low conduction losses. Another advantage of the proposed converter is the possibility of having bidirectional flow of power. The zero current turn-off feature in the proposed converter allows using any type of active switches, even Silicon Controlled Rectifiers (SCRs), which are naturally commuted switches. SCRs have numerous advantages over the other power switches, including higher voltage ratings and current ratings, lower power losses, lower cost, and higher reliability, which are all of particular importance in the MVDC grid application. The main limitation of SCRs is that they have natural commutation and cannot be turned off with gate signals. The approach employed in this dissertation enables the use of SCRs in the proposed modular DC-DC converters. The input and output terminals of the power modules can be connected in series or in parallel to form modular converters that facilitate sharing voltage or current in high power applications. In this dissertation, an input-parallel output-series (IPOS) modular configuration is considered to increase the voltage blocking capability at the output and handle high currents at the input of the converter. The dissertation studies the principles of the operation of this converter and presents its design procedure. Also, a modified topology for reducing the number of inputs switches is proposed. Multi-port configurations that allow having several independent input sources in this modular DC-DC converter are presented in this dissertation. In addition, a brief comparison between the proposed topology and a dual active bridge (DAB)-based modular converter, which is the most common DC-DC converter topology used in MVDC grid, is carried out. In addition, the performance of the proposed modular converter is evaluated through simulations and experiments--Author's abstract