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.
Deep level defects in wide bandgap semiconductors, whose response times are in the range of power converter switching times, can have a significant effect on converter efficiency. Here, we use deep level transient spectroscopy (DLTS) to evaluate such defect levels in the n-drift layer of vertical gallium nitride (v-GaN) power diodes with VBD ~ 1500 V. DLTS reveals three energy levels that are at ~0.6 eV (highest density), ~0.27 eV (lowest density), and ~45 meV (a dopant level) from the conduction band. Dopant extraction from capacitance–voltage measurement tests (C–V) at multiple temperatures enables trap density evaluation, and the ~0.6 eV trap has a density of 1.2 × 1015 cm-3. The 0.6 eV energy level and its density are similar to a defect that is known to cause current collapse in GaN based surface conducting devices (like high electron mobility transistors). Analysis of reverse bias currents over temperature in the v-GaN diodes indicates a predominant role of the same defect in determining reverse leakage current at high temperatures, reducing switching efficiency.
This article proposes a method of predicting the influence of random load behavior on the dynamics of dc microgrids and distribution systems. This is accomplished by combining stochastic load models and deterministic microgrid models. Together, these elements constitute a stochastic hybrid system. The resulting model enables straightforward calculation of dynamic state moments, which are used to assess the probability of desirable operating conditions. Specific consideration is given to systems based on the dual active bridge (DAB) topology. Bounds are derived for the probability of zero voltage switching (ZVS) in DAB converters. A simple example is presented to demonstrate how these bounds may be used to improve ZVS performance as an optimization problem. Predictions of state moment dynamics and ZVS probability assessments are verified through comparisons to Monte Carlo simulations.
Energy storage technologies are positioned to play a substantial role in power delivery systems. They have the potential to serve as an effective new resource to maintain reliability and allow for increased penetration of renewable energy. However, because of their relative infancy, there is a lack of knowledge about how these resources truly operate over time. A data analysis can help ascertain the operational and performance characteristics of these emerging technologies. Rigorous testing and a data analysis are important for all stakeholders to ensure a safe, reliable system that performs predictably on a macro level. Standardizing testing and analysis approaches to verify the performance of energy storage devices, equipment, and systems when integrating them into the grid will improve the understanding and benefit of energy storage over time from technical and economic vantage points. Demonstrating the life-cycle value and capabilities of energy storage systems begins with the data that the provider supplies for the analysis. After a review of energy storage data received from several providers, some of these data have clearly shown to be inconsistent and incomplete, raising the question of their efficacy for a robust analysis. This report reviews and proposes general guidelines, such as sampling rates and data points, that providers must supply for a robust data analysis to take place. Consistent guidelines are the basis of a proper protocol and ensuing standards to (1) reduce the time that it takes for data to reach those who are providing the analysis; (2) allow them to better understand the energy storage installations; and (3) enable them to provide a high-quality analysis of the installations. The report is intended to serve as a starting point for what data points should be provided when monitoring. Readers are encouraged to use the guidance in the report to develop specifications for new systems, as well as enhance current efforts to ensure optimal storage performance. As battery technologies continue to advance and the industry expands, the report will be updated to remain current.
This paper presents a new high gain, multilevel, bidirectional DC-DC converter for interfacing battery energy storage systems (BESS) with the distribution grid. The proposed topology employs a current-fed structure on the low-voltage (LV) BESS side to obtain high voltage gain during battery-to-grid mode of operation without requiring a large turns ratio isolation transformer. The high-voltage (HV) side of the converter is a voltage-doubler network comprising two half-bridge circuits with an intermediary bidirectional switch that re-configures the two bridges in series connection to enhance the boost ratio. A seamless commutation of the transformer leakage inductor current is ensured by the phase-shift modulation of HV side devices. The modulating duty cycle of the intermediary bidirectional devices generates a multilevel voltage of twice the switching frequency at the grid-side dc link, which significantly reduces the filter size. The presented modulation strategy ensures zero current switching (ZCS) of the LV devices and zero voltage switching (ZVS) of the HV devices to achieve a high power conversion efficiency. Design and operation of the proposed converter is explained with modal analysis, and further verified by detailed simulation results.
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.
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.
This paper presents an isolated bidirectional dc/dc converter for battery energy storage applications. Two main features of the proposed circuit topology are high voltage-conversion ratio and reduced battery current ripple. The primary side circuit is a quasi-switched-capacitor circuit with reduced voltage stress on switching devices and a 3:1 voltage step down ratio, which reduces the turns ratio of the transformer to 6:1:1. The secondary side circuit has an interleaved operation by utilizing the split magnetizing inductance of the transformer, which not only helps to increase the step down ratio but also reduces the battery current ripple. Similar to the dual-active-bridge circuit, the phase shift control is implemented to regulate the operation power of the circuit. A 1-kW, 300-kHz, 380-420 V/20-33 V GaN-based circuit prototype is currently under fabrication. The preliminary test results are presented.
In this paper, Triple Active Bridge Converter (TABC) based Cell Level/Sub-Modular level power processing Battery Energy Storage System (BESS) is proposed to provide better efficiency, reliability and safety benefits. One TABC is connected to the two cells/two sub-modules of the battery pack, and all outputs of the TABC's are connected in series or parallel to achieve the required voltage or current. The main features of the proposed configuration are: to avoid the unnecessary charging/discharging phenomena, providing the isolation, and to improve the optimal power flow of each cell of the battery. Also, secondary voltage compensation and droop based decentralized control scheme is proposed to improve the power-sharing of each TABC and voltage regulation at the battery output under the effect of cable resistance considered. The proposed configuration and control strategy is validated under different loading conditions by MATLAB®, SIMULINK®.
The role of power electronics in the utility grid is continually expanding. As converter design processes mature and new advanced materials become available, the pace of industry adoption is poised to accelerate. Looking forward, we can envision a future in which power electronics are as integral to grid functionality as the transformer is today. The Enabling Advanced Power Electronics Technologies for the Next Generation Electric Utility Grid Workshop was organized by Sandia National Laboratories and held in Albuquerque, New Mexico, July 17 - 18, 2018 . The workshop helped attendees to gain a broader understanding of power electronics R&D needs—from materials to systems—for the next generation electric utility grid. This report summarizes discussions and presentations from the workshop and identifies opportunities for future efforts.