Publications

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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 three-year research program to create the components of a real-world virtual power plant (VPP) that can simultaneously participate in multiple markets. Our research extends current state-of-the-art rolling horizon control through the application of stochastic programming with risk aversion at various time resolutions. Our rolling horizon control consists of (1) day-ahead optimization to produce an hourly aggregate schedule for the VPP operator and (2) sub-hourly optimization for real-time dispatch of each VPP subresource. Both optimization routines leverage a two-stage stochastic program (SP) with risk aversion, and integrate the most up-to-date forecasts to generate probabilistic scenarios in real operating time. Our results demonstrate the benefits to the VPP operator of constructing a stochastic solution regardless of the weather. In more extreme weather, applying risk optimization strategies can dramatically increase the financial viability of the VPP. As a result, the methodologies presented here can be further tailored for optimal control of any VPP asset fleet and its operational requirements.

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 project is part of a multi-lab consortium that leverages U.S. research expertise and facilities at national labs and universities to significantly advance electric drive power density and reliability, while simultaneously reducing cost. The final objective of the consortium is to develop a 100 kW traction drive system that achieves 33 kW/L, has an operational life of 300,000 miles, and a cost of less than $\$6$/kW. One element of the system is a 100 kW inverter with a power density of 100 kW/L and a cost of $\$2.7$/kW. New materials such as widebandgap semiconductors, soft magnetic materials, and ceramic dielectrics, integrated using multi-objective cooptimization design techniques, will be utilized to achieve these program goals. This project focuses on a subset of the power electronics work within the consortium, specifically the design, fabrication, and evaluation of vertical GaN power devices suitable for automotive applications.

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Sandia National Laboratories performed analysis to develop conservative hazard guidelines regarding firefighters working near photovoltaic (PV) arrays. Assuming implementation of NFPA 70 system shutdown requirements, the analysis focused on DC hazards only. Several different PV variables were considered, including system grounding and DC voltage classes. The hazard scenarios considered the contact conditions, current paths through the body, and PPE. Guidelines for the hazard definitions for men and women were based on the IEC TS 60479-1 guidelines. The importance of PPE was illustrated in the results.

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The photovoltaics industry is in dire need of a cheap, robust, reliable arc fault detector that is sensitive enough to detect arc faults before they can develop into a fire while robust enough to noise to limit unwanted tripping. Management Sciences has developed an arc fault detector that is housed in a standard PV connector, which will disconnect the PV array when it detects the surge current from an arc fault. Sandia National Labs, an industry leader in the detection, characterization, and mitigation of arc faults in PV arrays, will work with Management Sciences to characterize, demonstrate, and develop their arc fault detection/connector technology.

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.

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.

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