The grid faces a number of challenges related to large-scale integration of intermittent distributed generation (DG) such as photovoltaics (PV). Power quality challenges include voltage regulation issues, flicker, and frequency volatility. Operational challenges include the need for extension of the command-and-control infrastructure to millions of devices anticipated on the low-voltage (service) side of the distribution network. This paper presents an advanced grid-tied inverter controls concept designed to address such challenges. This controls concept is based on reproducing favorable characteristics of traditional generators that result in load-following tendencies, and is accordingly dubbed Generator Emulation Controls (GEC). Traditional generators are analyzed with specific focus on such favorable characteristics as inertial dynamics and controlled impedance. Details of GEC are then presented, and its implementation is outlined based on the evolution of conventional grid-tied inverter controls. This is followed by an examination of the system impact of GEC-operated devices. GEC allows DG inverters to perform voltage regulation support, reactive power compensation, and fault ride-through. GEC also allows DG inverters to form scalable inverter-based microgrids, capable of operating in grid-tied mode or separating and supporting an islanded load. Simulation results are presented to examine the impact on voltage regulation and power losses across a distribution feeder. Two experimental test beds are used to demonstrate voltage regulation support, transient suppression, and microgridding capabilities.
Numerical solution of the time-dependent Schrödinger equation for resonant-tunneling diodes has been impeded by the difficulty in handling open-system boundary conditions. This paper presents a boundary condition method to simulate the interaction with ideal particle reservoirs at the device boundaries. A switching transient is calculated where the device is switched from the peak current state to the valley current state. In addition, this method was used to develop a small-signal analysis of resonant-tunneling diodes. Results for the small-signal equivalent circuit of a particular device versus frequency are presented.
We present a numerical study of the I-X mixing in GaAs/AlAs/GaAs quantum well structures. A P-X mixing model proposed by Liu [Appl. Phys. Lett. 51, 1019 (1987)] is extended to include the effects of self-consistency and nonzero transverse momentum. In the present model, the coupled Schrodinger equations for I' and X electron envelope wave functions are solved self-consistently with Poisson's equation to calculate the electron transmission probability and wave functions, which lead to the current-voltage (I-V) characteristics of single barrier and double barrier resonant tunneling diode structures. The quantum transmitting boundary method is employed in the model for numerical solution of the coupled Schriidinger equations, which proves to be very stable and efficient, even for large (> 2000 A) structures. The features of I-X mixing, such as the resonance/antiresonance in the transmission probability and the virtual bound states, are clearly demonstrated. Additional physical features are observed in the transmission probability and the wave functions under applied bias conditions. Our work shows that inclusion of transverse momentum, variable effective mass, and the self-consistent potential is important in the realistic modeling of 1-V characteristics for structures exhibiting I-X coupling.
The properties of a metal-oxide-metal (M-O-M) tunneling detector are presented and the parameters influencing its operation are discussed. The theory of operation and experimental results for small as well as large signals are presented. The polarity reversal at large-signal levels is predicted theoretically and observed experimentally.
A self-consistent quantum mechanical simulation is used to study the effect of spacer layer thickness on such resonant tunneling diode properties as the peak current and peak-to-valley current ratio. It is found that with a low cathode doping the peak current is insensitive to the commonly used spacer layer thickness. However, for higher cathode doping the peak current decreases with increasing spacer layer thickness. This phenomenon is explained on the basis of the junction potential between the heavily doped cathode contact region and the undoped double-barrier region. Thus, for device applications where a high current density is desired the cathode spacer layer should be designed as thin as possible.
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