Peter J. Theisen

The Role of Energy Storage in a Microgrid Concept: Examining the opportunities and promise of microgrids.

A Microgrid is a cluster of distributed generation (DG), renewable sources, and local loads connected to the utility grid. A microgrid provides a solution to manage local generations and loads as a single grid-level entity. It has the potential to maximize overall system efficiency, power quality, and energy surety for critical loads. The Microgrid Exchange Group, an ad hoc group of expert and implementers of microgrid technology, has defined a microgrid as a group of interconnected loads and distributed energy resources within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid. A microgrid can connect and disconnect from the grid to enable it to operate in both grid-connected or island mode.

270-V Dc Hybrid Switch

High-voltage direct current (dc) power is one of the most efficient, reliable, and lightweight methods to generate and distribute energy. This power system is not in common use today on aircraft or other weight- and reliability-sensitive applications because of difficulties in switching high-voltage dc power. This difficulty becomes acute when switching at high altitude because of the very low mobility of an arc in low pressure. Direct application of power semiconductor switching devices is feasible but would result in severe thermal problems for other than the lowest current applications. A novel switching device is described that overcomes these shortcomings. This device is based on a unique combination of power semiconductor and contact structure switching systems. This hybrid switching device exhibits arcless switching for currents below two times the device rating. Above this current level the contact structure limits the current to a level that can be handled by the power semiconductor. The contact structure develops this currentlimiting capability over a range of pressure from atmospheric to 20 torr. At low pressures a high arc mobility is achieved through a novel approach where the arc develops its own environment. The arc plasma decomposes plastic wall materials to produce an environment that is conducive to high arc mobility.

Investigation of arcing effects during contact blow open process

A theoretical model has been developed to simulate the blow open process considering contact constriction force, contact spring force, and plasma pressure. Meanwhile, monochromatic high speed imaging technique has been used to characterize arc behavior, contact vapor distribution and determine contact blow open gaps. Modeling results are in good agreement with test results. The results show that plasma pressure has a significant effect at the initial arcing stage of a blow open process, but it decreases rapidly due to the increase of arc gap and the gas pressure equilibrium around moving contacts. The plasma pressure and the contact spring force (or applied magnetic force) basically determine the arcing time during the blow open process. However, contacts can be held open by plasma pressure till current zero if piston type arc chamber is employed. Higher contact spring force results in shorter arcing time, higher arcing current and more likely leads to contact weld. Due to the fact of arc pressure decreasing rapidly during the blow open process, the blow open contact gap is less than 2.5 mm for all the contactors tested at the blow open current level. The results indicate that larger allowed arc gap does not necessarily reduce contact weld possibility, since other factors such as arc motion and heat conduction through contacts also have strong influence on contact weld.

Hybrid DC switch for solar array fault protection

Photovoltaic (PV) systems have the intrinsic nature of distributed DC power generation that requires additional protection to reduce the fire and shock risks from various PV faults. Due to the current limited nature of PV generation, the short circuit current of a PV panel is typically 1.2 times of its rated current. This poses a unique overcurrent protection challenge, because traditional overcurrent protection, such as fuses and breakers, are designed for cable protection and they provide protection only for high reverse current faults in solar arrays. This paper proposes a solar fault protection system, including a hybrid DC switch based overcurrent protection device and a protection and coordination scheme. An optimized switch timing control is proposed for the hybrid DC switch. The proposed control method utilizes the parallel electromechanical contactors arc voltage to determine the solid state devices switch timings, thus significantly reduces the solid state device rating and size. A protection and coordination scheme of using hybrid DC switches in PV system is also proposed and implemented. The proposed scheme guarantees the hybrid DC switches only need to interrupt forward fault current, so that the hybrid DC switches do not need to be sized according to the high reverse fault current. Experimental results verify that the hybrid DC switch with the proposed protection and coordination scheme can effectively protect the solar array faults.

Characteristics of High-Current Arcs Between Insulating Chute Materials

An experimental study has been performed to measure the fundamental characteristics of parallel rail arcs confined in polyester and glass mica slots. The voltage gradients of stationary arcs within polyester slots are higher than those for arcs within glass mica slots. The opposite characteristic was found for moving arcs. That is, moving arcs in glass mica slots produce the highest voltage gradients. For glass mica chutes the arc velocity is linearly dependent on current and inversely dependent on wall gap. Polyester wall materials result in overall lower arc velocities that increase with current at a rate that slows with increasing current. In addition, tests show that under certain conditions the arc velocity decreases significantly with decreasing slot width. The comparison of these arc characteristics is presented along with a description of the possible mechanisms controlling this behavior.

Design of the Fort Sill microgrid

This paper describes the design and operation of the microgrid installed for the US Department of Defense at Fort Sill, Oklahoma. The microgrid is composed of two natural gas generators, a battery energy storage inverter, renewables, and a static switch. Design considerations for this retrofit application with a high penetration of dynamic loads are given. The integration of the energy storage inverter with a commercial isochronous generator control system is described. A comparison of droop control vs. isochronous control is provided, and the benefits of using an existing isochronous generator control system for microgrid applications are described. System performance is validated by simulation results.