Structural modularity is critical to solid-state transformer (SST) and solid-state power substation (SSPS) concepts, but operational aspects related to this modularity are not yet fully understood. Previous studies and demonstrations of modular power conversion systems assume identical module compositions, but dependence on module uniformity undercuts the value of the modular framework. In this project, a hierarchical control approach was developed for modular SSTs which achieves system-level objectives while ensuring equitable power sharing between nonuniform building block modules. This enables module replacements and upgrades which leverage circuit and device technology advancements to improve system-level performance. The functionality of the control approach is demonstrated in detailed time-domain simulations. Results of this project provide context and strategic direction for future LDRD projects focusing on technologies supporting the SST crosscut outcome of the resilient energy systems mission campaign.
With evolving landscape of DC power transmission and distribution, a reliable and fast protection against faults is critical, especially for medium- and high-voltage applications. Thus, solid-state circuit breakers (SSCB), consisting of cascaded silicon carbide (SiC) junction field-effect transistors (JFET), utilize the intrinsic normally-ON characteristic along with their low ON-resistance. This approach provides an efficient and robust protection solution from detrimental short-circuit events. However, for applications that require high-voltage blocking capability, a proper number of JFETs need be connected in series to achieve the desired blocking voltage rating. Ensuring equal voltage balancing across the JFETs during the switching transitions as well as the blocking stage is critical and hence, this paper presents a novel passive balancing network for series connected JFETs for DC SSCB applications. The dynamic voltage balancing network to synchronize both the turn ON and OFF intervals is described analytically. Moreover, the static voltage balancing network is implemented to establish equal sharing of the total blocking voltage across the series connection of JFETs. The proposed dynamic and steady-state balancing networks are validated by SPICE simulation and experimental results.
The Fly-buck converter provides a simple, lowcost solution to generating multiple isolated or non-isolated output voltage rails using secondary windings of the coupled inductor. Under certain operating conditions, the high side (HS) and low side (LS) switches can realize zero-voltage-switching (ZVS) at turn-ON, reducing switching losses and electromagnetic interference (EMI). However, several factors play a critical role in determining the soft-switching criteria. Therefore, this paper presents an in-depth analysis and evaluation of the ZVS operation through developing equivalent circuit models of the fly-buck converter along with a parametric study of the design variables, including leakage inductance, output current ratio, and dead time.
In spite of several advantages of SiC JFETs over enhancement mode SiC MOSFETs, the intrinsic normally-ON characteristic of the JFETs can be undesirable for many industrial power conversion applications due to the negative turn-OFF voltage requirement. This prevents normally-ON JFETs from being widely accepted in industry. However, a cascode configuration, which uses a low voltage (LV) Si MOSFET can be used to enable a normally-OFF behavior, making this approach an attractive solution to utilize the benefits of SiC JFETs. For medium-, and high-voltage applications that require larger blocking voltage than the rating of each JFET, additional devices can be connected in series to increase the overall blocking voltage capability, creating a super-cascode configuration. This paper provides a review of several super-cascode topology variations and presents a comprehensive comparative study, evaluating similarities and differences in operating principles, equivalent circuits, and design considerations and limitations.
Optimized designs were achieved using a genetic algorithm to evaluate multi-objective trade space, including Mean-Time-Between-Failure (MTBF) and volumetric power density. This work provides a foundational platform that can be used to optimize additional power converters, such as an inverter for the EV traction drive system as well as trade-offs in thermal management due to the use of different device substrate materials.