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.
This report is a summary of a 3-year LDRD project that developed novel methods to detect faults in the electric power grid dramatically faster than today’s protection systems. Accurately detecting and quickly removing electrical faults is imperative for power system resilience and national security to minimize impacts to defense critical infrastructure. The new protection schemes will improve grid stability during disturbances and allow additional integration of renewable energy technologies with low inertia and low fault currents. Signal-based fast tripping schemes were developed that use the physics of the grid and do not rely on communication to reduce cyber risks for safely removing faults.
This quick note outlines what we found after our conversion with you and your team. As suggested, we loaded 1547-2003 source requirements document (SRD) and then went back and loaded 1547-2018 SRD. This did result in implementing the new 1547-2018 settings. This short report focuses on the frequency-watt function and shows a couple of screen shots of the parameter settings via the Mojave HMI interface and plots of the results of the inverter with FW function enabled in both default and most aggressive settings response to frequency events. The first screen shot shows the 1547-2018 selected after selecting 1547-2003.
Understanding of semiconductor breakdown under high electric fields is an important aspect of materials’ properties, particularly for the design of power devices. For decades, a power-law has been used to describe the dependence of material-specific critical electrical field (Ecrit) at which the material breaks down and bandgap (Eg). The relationship is often used to gauge tradeoffs of emerging materials whose properties haven’t yet been determined. Unfortunately, the reported dependencies of Ecrit on Eg cover a surprisingly wide range in the literature. Moreover, Ecrit is a function of material doping. Further, discrepancies arise in Ecrit values owing to differences between punch-through and non-punch-through device structures. We report a new normalization procedure that enables comparison of critical electric field values across materials, doping, and different device types. An extensive examination of numerous references reveals that the dependence Ecrit ∝ Eg1.83 best fits the most reliable and newest data for both direct and indirect semiconductors. Graphical abstract: [Figure not available: see fulltext.].
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.
Interest in the application of DC Microgrids to distribution systems have been spurred by the continued rise of renewable energy resources and the dependence on DC loads. However, in comparison to AC systems, the lack of natural zero crossing in DC Microgrids makes the interruption of fault currents with fuses and circuit breakers more difficult. DC faults can cause severe damage to voltage-source converters within few milliseconds, hence, the need to quickly detect and isolate the fault. In this paper, the potential for five different Machine Learning (ML) classifiers to identify fault type and fault resistance in a DC Microgrid is explored. The ML algorithms are trained using simulated fault data recorded from a 750 VDC Microgrid modeled in PSCAD/EMTDC. The performance of the trained algorithms are tested using real fault data gathered from an operational DC Microgrid located on the Kirtland Air Force Base. Of the five ML algorithms, three could detect the fault and determine the fault type with at least 99% accuracy, and only one could estimate the fault resistance with at least 99% accuracy. By performing a self-learning monitoring and decision making analysis, protection relays equipped with ML algorithms can quickly detect and isolate faults to improve the protection operations on DC Microgrids.
DC microgrids envisioned with high bandwidth communications may well expand their application range by considering autonomous strategies as resiliency contingencies. In most cases, these strategies are based on the droop control method, seeking low voltage regulation and proportional load sharing. Control challenges arise when coordinating the output of multiple DC microgrids composed of several Distributed Energy Resources. This paper proposes an autonomous control strategy for transactional converters when multiple DC microgrids are connected through a common bus. The control seeks to match the external bus voltage with the internal bus voltage balancing power. Three case scenarios are considered: standalone operation of each DC microgrid, excess generation, and generation deficit in one DC microgrid. Results using Sandia National Laboratories Secure Scalable Microgrid Simulink library, and models developed in MATLAB are compared.
The Port of Alaska in Anchorage enables the economic vitality of the Municipality of Anchorage and State of Alaska. It also provides significant support to defense activities across Alaska, especially to the Joint Base Elmendorf-Richardson (JBER) that is immediately adjacent to the Port. For this reason, stakeholders are interested in the resilience of the Ports operations. This report documents a preliminary feasibility analysis for developing an energy system that increases electric supply resilience for the Port and for a specific location inside JBER. The project concept emerged from prior work led by the Municipality of Anchorage and consultation with Port stakeholders. The project consists of a microgrid with PV, storage and diesel generation, capable of supplying electricity to loads at the Port a specific JBER location during utility outages, while also delivering economic value during blue-sky conditions. The study aims to estimate the size, configuration and concept of operations based on existing infrastructure and limited demand data. It also explores potential project benefits and challenges. The report goal is to inform further stakeholder consultation and next steps.
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.
Sandia National Laboratories sponsored a three-year internally funded Laboratory Directed Research and Development (LDRD) effort to investigate the vulnerabilities and mitigations of a high-altitude electromagnetic pulse (HEMP) on the electric power grid. The research was focused on understanding the vulnerabilities and potential mitigations for components and systems at the high voltage transmission level. Results from the research included a broad array of subtopics, covered in twenty-three reports and papers, and which are highlighted in this executive summary report. These subtopics include high altitude electromagnetic pulse (HEMP) characterization, HEMP coupling analysis, system-wide effects, and mitigating technologies.
Flicker, Jack D.; Hernandez-Alvidrez, Javier; Shirazi, Mariko; Vandermeer, Jeremy; Thomson, William
In order to evaluate the ability of a Grid Bridge System(GBS), or energy storage-backed grid forming inverter, to provide spinning reserve in an islanded microgrid with significant variable generation, we have developed a high-fidelity system model. In a loss-of-wind scenario, the GBS system significantly improves both the frequency nadir as well as the transient overvoltage response of the system. This case was analyzed for a system with one, two, and three diesel generators both with and without the GBS. System stability for the one generator plus GBS case outperformed all generator system, even for the three-generator case. This indicates that spinning reserve can be shifted from the generators to the GBS without sacrificing system stability, allowing the diesel generators to operate at more economical conditions, saving significant fuel costs over the long-term implementation of the GBS.
While the concept of aggregating and controlling renewable distributed energy resources (DERs) to provide grid services is not new, increasing policy support of DER market participation has driven research and development in algorithms to pool DERs for economically viable market participation. Sandia National Laboratories recently undertook a 3 year research programme to create the components of a real-world virtual power plant (VPP) that can simultaneously participate in multiple markets. The authors' research extends current state-of-the-art rolling horizon control through the application of stochastic programming with risk aversion at various time resolutions. Their rolling horizon control consists of day-ahead optimisation to produce an hourly aggregate schedule for the VPP operator and sub-hourly optimisation for the real-time dispatch of each VPP subresource. Both optimisation routines leverage a two-stage stochastic programme with risk aversion and integrate the most up-to-date forecasts to generate probabilistic scenarios in real operating time. Their results demonstrate the benefits to the VPP operator of constructing a stochastic solution regardless of the weather. In more extreme weather, applying risk optimisation strategies can dramatically increase the financial viability of the VPP. The methodologies presented here can be further tailored for optimal control of any VPP asset fleet and its operational requirements.
Inverters using phase-locked loops for control depend on voltages generated by synchronous machines to operate. This might be problematic if much of the conventional generation fleet is displaced by inverters. To solve this problem, grid-forming control for inverters has been proposed as being capable of autonomously regulating grid voltages and frequency. Presently, the performance of bulk power systems with massive penetration of grid-forming inverters has not been thoroughly studied as to elucidate benefits. Hence, this paper presents inverter models with two grid-forming strategies: virtual oscillator control and droop control. The two models are specifically developed to be used in positive-sequence simulation packages and have been implemented in PSLF. The implementations are used to study the performance of bulk power grids incorporating inverters with gridforming capability. Specifically, simulations are conducted on a modified IEEE 39-bus test system and the microWECC test system with varying levels of synchronous and inverter-based generation. The dynamic performance of the tested systems with gridforming inverters during contingency events is better than cases with only synchronous generation.
In order to study the relative degradation between co-located PV modules and microinverters in an ACPV configuration, four 260 Watt PV modules and four 250 W microinverters purchased on the open market have been co-located in a thermal chamber set at a static temperature (69°C). Instantaneous electrical/thermal measurements have been taken on the microinverters with periodic dark IV measurements on the modules. After over 10,000 hours of testing, no failures or observable degradation have been seen in either the module or microinverter. Using average measured field-temperature data with Military Handbook analysis, this indicates an approximate field use of 44 years of operation lifetime for PV modules, and 13 years of operation for microinverters with reliability of 66.87% with a lower one-sided confidence level of 80%.
To determine risk of an electric shock to firefighter personnel due to contact with live parts of a damaged PV system, simulated PV arrays were constructed with multiple 'modules' connected to a central inverter. The results of this analysis demonstrate that ungrounded arrays are significantly safer than grounded arrays for reasonable module isolation resistances. Ungrounded arrays provide current hazards to personnel up to three orders of magnitude smaller than for a grounded array counterpart. While the size of the array does not affect the current hazard in grounded arrays for body resistances above 100,Ω, in ungrounded arrays, increased array size yields increased current hazards- considering that the overall fault current level is still significantly smaller than for grounded arrays. In both grounded and ungrounded arrays, the current hazard has a direct correlation to array voltage. Since the level of fault current in a grounded array can be significant, this work shows that the non- linearity of the array IV curve must be taken into account for body resistances below 600 Ω and array voltages above 1000V for accurate fault current determination. Although module and array isolation resistance is not a factor that modulates fault current in a grounded array, this resistance, Riso, has a significant effect on current hazard to the firefighter for ungrounded arrays.
This work investigates the use of hybrid switched capacitor converter (HSCC) topologies with wide bandgap devices to achieve high efficiency DC-DC power conversion with high gain, high voltage outputs. This class of converter may be useful for several applications that include a medium voltage bus, such as solar PV, electric aircraft, or even all-electric ship architectures. Three converter prototypes are considered and evaluated in hardware, including a basic (unipolar) HSCC and two bipolar HSCC variants. The converter operation is discussed, and the bipolar prototypes are demonstrated to achieve high-gain, high-voltage output. Finally, the latest bipolar switched capacitor prototype is demonstrated to boost 480 V to 10 kV (Gain > 20) with 97.9% efficiency at 4.96 kW output power.
Gallium Nitride (GaN) semiconductors have extremely low switching loss, high breakdown voltage, and high junction temperature rating. These characteristics enable improved device performance and thus improved switch mode power converter designs. This paper evaluates the Pareto-optimal performance improvements for a DC generation system with predicted GaN loss characteristics and a rigorous multi-objective optimization based design paradigm. The optimization results show that the application of GaN can achieve a 6.4% mass savings relative to Silicon Carbide (SiC) and 40% mass savings relative to Silicon (Si) at the same loss level for a 10 kW application.
In this work, a novel DC-DC converter topology, an adaptation of the Hybrid Switched Capacitor Circuit (HSCC), is considered for use in high-gain, high voltage applications that also require high efficiency and superior power density. In particular, a bipolar HSCC design is described, and a candidate control methodology is set forth and developed analytically. The converter performance is demonstrated to be consistent with analysis. In addition, the converter is demonstrated to step 460V up to 8.63 kV (gain of 19) at 3.63 kW and nearly 97.0% efficiency.
In order to determine how material characteristics percolate up to system-level improvements in power dissipation for different material systems and device types, we have developed an optimization tool for power diodes. This tool minimizes power dissipation in a diode for a given system operational regime (reverse voltage, forward current density, frequency, duty cycle, and temperature) for a variety of device types and materials. We have carried out diode optimizations for a wide range of system operating points to determine the regimes for which certain power diode materials/devices are favored. In this work, we present results comparing state-of-the-art Si and SiC merged PiN Schottky (MPS) diodes to vertical GaN (v-GaN) PiN diodes and as-yet undeveloped v-GaN Schottky barrier diodes (SBDs). The results of this work show that for all conditions tested, SiC MPS and v-GaN PiN diodes are preferred over Si MPS diodes. v-GaN PiN diodes are preferred over SiC MPS diodes for high-voltage / moderate-frequency operation with the limits of the v-GaN PiN preferred regime, increasing with increasing forward current density. If a v-GaN SBD diode were available, it would be preferred over all other devices at low to moderate voltages, for all frequencies from 100 Hz to 1 MHz.
With the next generation of semiconductor materials in development, significant strides in the Size, Weight, and Power (SWaP) characteristics of power conversion systems are presently underway. In particular, much of the improvements in system-level efficiencies and power densities due to wide-bandgap (WBG) and ultra-wide-bandgap (UWBG) device incorporation are realized through higher voltage, higher frequency, and higher temperature operation. Concomitantly, there is a demand for ever smaller device footprints with high voltage, high power handling ability while maintaining ultra-low inductive/capacitive parasitics for high frequency operation. For our work, we are developing small size vertical gallium nitride (GaN) and aluminum gallium nitride (AlGaN) power diodes and transistors with breakdown and hold-off voltages as high as 15kV. The small size and high power densities of these devices create stringent requirements on both the size (balanced between larger sizing for increased voltage hold-off with smaller sizing for reduced parasitics) and heat dissipation capabilities of the associated packaging. To accommodate these requirements and to be able to characterize these novel device designs, we have developed specialized packages as well as test hardware and capabilities. This work describes the requirements of these new devices, the development of the high voltage, high power packages, and the high-voltage, high-Temperature test capabilities needed to characterize and use the completed components. In the course of this work, we have settled on a multi-step methodology for assessing the performance of these new power devices, which we also present.
Flicker, Jack D.; Tamizhmani, Govindasamy; Moorthy, Mathan K.; Thiagarajan, Ramanathan; Ayyanar, Raja
This work has applied a suite of long-term reliability ATs (accelerated tests) to a variety of MLPE devices (module level power electronics such as microinverters and optimizers) from five different manufacturers. This data set is one of first (only [3] is reported for reliability testing in the literature) as well as the largest experimental set in public literature, both in sample size (5 manufacturers including both DC/DC and DC/AC units and 20 units for each test) as well as number of experiments (6 different experimental test conditions) for MLPE devices. The accelerated stress tests include thermal cycling test per IEC 61215 profile, and damp heat test per IEC 61215 profile and they were performed under powered and unpowered conditions. Included in these experiments are the first independent long-term experimental data regarding damp heat as well as the longest term (>9 month) testing of MLPE units reported in literature for thermal cycling. Additionally, this work is the first to show in situ power measurements as well as periodic efficiency measurements over length of experimental tests, demonstrating whether certain tests result in long-term degradation or immediate catastrophic failures. The result of this testing demonstrates the long-term durability and reliability of MLPE units to several accelerated environmental stressors.
The effects of paralleling low-current vertical Gallium Nitride (v-GaN) diodes in a custom power module are reported. Four paralleled v-GaN diodes were demonstrated to operate in a buck converter at 1.3 Apeak (792 mArms) at 240 V and 15 kHz switching frequency. Additionally, high-fidelity SPICE simulations demonstrate the effects of device parameter variation on power sharing in a power module. The device parameters studied were found to have a sub-linear relationship with power sharing, indicating a relaxed need to bin parts for paralleling. This result is very encouraging for power electronics based on low-current v-GaN and demonstrates its potential for use in high-power systems.
The switching characteristics of vertical Gallium Nitride (v-GaN) diodes grown on GaN substrates are reported. v-GaN diodes were tested in a Double-Pulse Test Circuit (DPTC) and compared to test results for SiC Schottky Barrier Diodes (SBDs) and Si PiN diodes. The reported switching characteristics show that GaN diodes, like SiC SBDs, exhibit nearly negligible reverse recovery current compared to traditional Si PiN diodes. The reverse recovery for the v-GaN PiN diodes is limited by parasitics in the DPTC, precluding extraction of a meaningful recovery time. These results are very encouraging for power electronics based on v-GaN and demonstrate the potential for very fast, low-loss switching for these devices.
Electrical performance and characterization of deep levels in vertical GaN P-i-N diodes grown on low threading dislocation density (∼104 - 106cm-2) bulk GaN substrates are investigated. The lightly doped n drift region of these devices is observed to be highly compensated by several prominent deep levels detected using deep level optical spectroscopy at Ec-2.13, 2.92, and 3.2 eV. A combination of steady-state photocapacitance and lighted capacitance-voltage profiling indicates the concentrations of these deep levels to be Nt = 3 × 1012, 2 × 1015, and 5 × 1014cm-3, respectively. The Ec-2.92 eV level is observed to be the primary compensating defect in as-grown n-type metal-organic chemical vapor deposition GaN, indicating this level acts as a limiting factor for achieving controllably low doping. The device blocking voltage should increase if compensating defects reduce the free carrier concentration of the n drift region. Understanding the incorporation of as-grown and native defects in thick n-GaN is essential for enabling large VBD in the next-generation wide-bandgap power semiconductor devices. Thus, controlling the as-grown defects induced by epitaxial growth conditions is critical to achieve blocking voltage capability above 5 kV.
A photovoltaic (PV) inverter is a balance-of-systems (i.e., every component except for the module component whose purpose is to control and convert power flow through the PV system). Namely, the inverter transforms the nominal DC power produced by the PV module to AC power, which can be transported through the electrical power grid or used on-site by various power-consuming units (Figure 14.1). As the interface between the DC and AC sides of the system, the inverter must meet rather stringent requirements for both.
We have examined ground faults in PhotoVoltaic (PV) arrays and the efficacy of fuse, current detection (RCD), current sense monitoring/relays (CSM), isolation/insulation (Riso) monitoring, and Ground Fault Detection and Isolation (GFID) using simulations based on a Simulation Program with Integrated Circuit Emphasis SPICE ground fault circuit model, experimental ground faults installed on real arrays, and theoretical equations.
PV faults have caused rooftop fires in the U.S., Europe, and elsewhere in the world. One prominent cause of past electrical fires was the ground fault detection blind spot in fuse-based protection systems uncovered by the Solar America Board for Codes and Standards (SolarABCs) steering committee in 2011. Fortunately, a number of alternatives to ground fault fuses have been identified, but there has been limited adoption and historical use of these technologies in the United States. This paper investigates the efficacy of one of these devices known as isolation monitoring (or isolation resistance monitoring, Riso) in small (∼3kW) and large (∼700 kW) arrays. Unfaulted and faulted PV arrays were monitored with Riso technology and compared to SPICE simulations to recommend appropriate thresholds to the maximize the range of ground faults which could be detected while minimizing unwanted tripping. Based on analytical and computational models, it is impossible to determine a trip threshold that provides fire safety and negates unwanted tripping issues. This paper mathematically demonstrates that appropriate Riso trip thresholds must be determined on an arrayby- array basis with sufficient leeway by system operators to adjust trip threshold settings for their particular usage cases.
In order to identify reliability issues associated with advanced inverter operation and array states (e.g. volt-VAR control, high DC/AC ratios), we have collected system and component-level electro-thermal information in a controlled laboratory environment under both nominal and advanced functionality operating conditions. The results of advanced functionality operation indicated increased thermal and electrical stress on components, which will have a negative effect on inverter reliability as these functionalities are exercised more frequently in the future.
The continued exponential growth of photovoltaic technologies paves a path to a solar-powered world, but requires continued progress toward low-cost, high-reliability, high-performance photovoltaic (PV) systems. High reliability is an essential element in achieving low-cost solar electricity by reducing operation and maintenance (O&M) costs and extending system lifetime and availability, but these attributes are difficult to verify at the time of installation. Utilities, financiers, homeowners, and planners are demanding this information in order to evaluate their financial risk as a prerequisite to large investments. Reliability research and development (R&D) is needed to build market confidence by improving product reliability and by improving predictions of system availability, O&M cost, and lifetime. This project is focused on understanding, predicting, and improving the reliability of PV systems. The two areas being pursued include PV arc-fault and ground fault issues, and inverter reliability.
Ground faults in photovoltaic (PV) systems pose a fire and shock hazard. To mitigate these risks, AC-isolated, DC grounded PV systems in the United States use Ground Fault Protection Devices (GFPDs), e.g., fuses, to de-energize the PV system when there is a ground fault. Recently the effectiveness of these protection devices has come under question because multiple fires have started when ground faults went undetected. In order to understand the limitations of fuse-based ground fault protection in PV systems, analytical and numerical simulations of different ground faults were performed. The numerical simulations were conducted with Simulation Program with Integrated Circuit Emphasis (SPICE) using a circuit model of the PV system which included the modules, wiring, switchgear, grounded or ungrounded components, and the inverter. The derivation of the SPICE model and the results of parametric fault current studies are provided with varying array topologies, fuse sizes, and fault impedances. Closed-form analytical approximations for GFPD currents from faults to the grounded current carrying conductor-known as %E2%80%9Cblind spot%E2%80%9D ground faults-are derived to provide greater understanding of the influence of array impedances on fault currents. The behavior of the array during various ground faults is studied for a range of ground fault fuse sizes to determine if reducing the size of the fuse improves ground fault detection sensitivity. The results of the simulations show that reducing the amperage rating of the protective fuse does increase fault current detection sensitivity without increasing the likelihood of nuisance trips to a degree. Unfortunately, this benefit reaches a limit as fuses become smaller and their internal resistance increases to the point of becoming a major element in the fault current circuit.