Generic closed loop controller for power regulation in dual active bridge DC-DC converter with current stress minimization

This paper presents a comprehensive and generalized analysis of the bidirectional dual active bridge (DAB) DC/DC converter using triple phase shift (TPS) control to enable closed loop power regulation while minimizing current stress. The key new achievements are: a generic analysis in terms of possible conversion ratios/converter voltage gains (i.e. Buck/Boost/Unity), per unit based equations regardless of DAB ratings, and a new simple closed loop controller implementable in real time to meet desired power transfer regulation at minimum current stress. Per unit based analytical expressions are derived for converter AC RMS current as well as power transferred. An offline particle swarm optimization (PSO) method is used to obtain an extensive set of TPS ratios for minimizing the RMS current in the entire bidirectional power range of - 1 to 1 per unit. The extensive set of results achieved from PSO presents a generic data pool which is carefully analyzed to derive simple useful relations. Such relations enabled a generic closed loop controller design that can be implemented in real time avoiding the extensive computational capacity that iterative optimization techniques require. A detailed Simulink DAB switching model is used to validate precision of the proposed closed loop controller under various operating conditions. An experimental prototype also substantiates the results achieved

Fiscal Regimes In and Outside the MENA Region

The 1990s ushered the world not only into a democracy wave, following the collapse of the former Soviet Union, but also a wave of fiscal rules, where the number of countries adopting this fiscal regime steadily rose from only 10 in 1990 to reach 97 in 2009. Countries that depend on hydrocarbons tend to suffer from fiscal policies that are highly susceptible to energy price shocks. This provides incentives for implementing fiscal stabilization instruments in the form of “fiscal rules”. However, the resource-rich but largely democracy-deficit MENA region has been a fiscal rules-free region. Against this backdrop, this paper asks two fundamental questions: why has MENA chose not to adopt fiscal rules? And what role, if any, resources dependence and political institutions might have played in this outcome? We find that lack of democracy and weak systems of political checks and balances that characterize MENA countries appear to have outweighed the positive impacts of oil resources so that fiscal instability persists despite ample oil revenues. The nascent Arab 'democracy spring' might tip the scale in favor of the adoption of fiscal rules by emerging democratic governments in the region. However, stronger systems of political checks and balances are also needed and, unfortunately, not necessarily a certain outcome. A move toward inflation targeting regimes, as proposed for Tunisia and Egypt, might also provide additional impetus for adoption of fiscal rules as the evidence of Chile and other inflation targeters suggests.Fiscal regimes, fiscal stabilization, discrete-choice panel-data models

Enhanced rare region effects in the contact process with long-range correlated disorder

We investigate the nonequilibrium phase transition in the disordered contact process in the presence of long-range spatial disorder correlations. These correlations greatly increase the probability for finding rare regions that are locally in the active phase while the bulk system is still in the inactive phase. Specifically, if the correlations decay as a power of the distance, the rare region probability is a stretched exponential of the rare region size rather than a simple exponential as is the case for uncorrelated disorder. As a result, the Griffiths singularities are enhanced and take a non-power-law form. The critical point itself is of infinite-randomness type but with critical exponent values that differ from the uncorrelated case. We report large-scale Monte-Carlo simulations that verify and illustrate our theory. We also discuss generalizations to higher dimensions and applications to other systems such as the random transverse-field Ising model, itinerant magnets and the superconductor-metal transition.Comment: 11 pages, 8 eps figures include

Physical Multi-Layer Phantoms for Intra-Body Communications

This paper presents approaches to creating tissue mimicking materials that can be used as phantoms for evaluating the performance of Body Area Networks (BAN). The main goal of the paper is to describe a methodology to create a repeatable experimental BAN platform that can be customized depending on the BAN scenario under test. Comparisons between different material compositions and percentages are shown, along with the resulting electrical properties of each mixture over the frequency range of interest for intra-body communications; 100 KHz to 100 MHz. Test results on a composite multi-layer sample are presented confirming the efficacy of the proposed methodology. To date, this is the first paper that provides guidance on how to decide on concentration levels of ingredients, depending on the exact frequency range of operation, and the desired matched electrical characteristics (conductivity vs. permittivity), to create multi-layer phantoms for intra-body communication applications

Modular multilevel converter with modified half-bridge submodule and arm filter for dc transmission systems with DC fault blocking capability

Although a modular multilevel converter (MMC) is universally accepted as a suitable converter topology for the high voltage dc transmission systems, its dc fault ride performance requires substantial improvement in order to be used in critical infrastructures such as transnational multi-terminal dc (MTDC) networks. Therefore, this paper proposes a modified submodule circuit for modular multilevel converter that offers an improved dc fault ride through performance with reduced semiconductor losses and enhanced control flexibility compared to that achievable with full-bridge submodules. The use of the proposed submodules allows MMC to retain its modularity; with semiconductor loss similar to that of the mixed submodules MMC, but higher than that of the half-bridge submodules. Besides dc fault blocking, the proposed submodule offers the possibility of controlling ac current in-feed during pole-to-pole dc short circuit fault, and this makes such submodule increasingly attractive and useful for continued operation of MTDC networks during dc faults. The aforesaid attributes are validated using simulations performed in MATLAB/SIMULINK, and substantiated experimentally using the proposed submodule topology on a 4-level small-scale MMC prototype

Laboratory Modelling of the Various Components of Backward Erosion Piping Behavior Due to Converging Flow With Different Constricted Outlets

Backward Erosion Piping (BEP) is an internal erosion mechanism which occurs at the down stream of dams or levees. Two conditions needed for this failure to occur: 1) head of water in the upstream of the dam or the levee to drive the seepage forces, and 2) crack in the roof or the blanket layer of the cohesionless material in the downstream of the dam or a levee. Numerous researches were performed in the past in an attempt to predict the critical gradient for BEP to initiate. Some researches are too conservative and other researches are under predicting the critical gradient. In general, the physical models were used in these researches are either one or two dimensional modelling. Due to complexity of this phenomenon and the three dimensional aspect of it as the water flowing from all directions toward the exit. Therefore, this problem needed to be simulated in a three dimensional model. The objectives of the research are to predict the critical gradient in the sand boil throat due the suspended material which lifted in the crack. Also, calculate the local critical horizontal gradient at channels tips. A circular physical model was designed by the army corps of engineering and modified at Utah State University. This model consists of two chambers, the outer chamber is where the water applied. The inner chamber is where the soil is placed. Mariotte tube to connected to the outer chamber and used to control the differential head applied to the soil sample. Four risers with different diameters are used to simulate the sand boil throat and placed individually at the center of the model. Three different soils representing a range of grain size with different hydraulic conductivities are in the research and some one soil used for two hydraulic regimes applied to the sample. Eight differential pressure transducers are installed around the exit and in the bottom of the riser. Results of the differential transducers are used to understand the backward erosion process and to measure the head loss in the riser due to the suspension of eroded soil in the riser. Three dimensional finite element software is used to mimic the erosion occurs for each test and calibrated using the results of the experimental data. Results were used to calculate the local horizontal critical gradients at different locations from the pipe tips