Publications

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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.

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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.

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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.

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