Utilization of a series resonant DC link for a DC motor drive

A high-frequency series resonant DC link converter is utilized as a DC motor drive. This system generates a resonant current in a series link, and switching is done at zero current instants, reducing switching losses to a minimum value. A pulse density modulation (PDM) strategy, utilizing a current rectangular loop and an external motor speed feedback loop, controls the resonant converter. A sinusoidal input fundamental current and nearly unity input power factor can be observed in different load conditions. The overall characteristics of the system, including such variables as maximum power, input current, start up, and transient responses, are presented by digital simulation and were verified on an actual prototype system     

Energy requirement for distributed energy resources with battery energy storage for voltage support in three-phase distribution lines

Integration of distributed energy resources (DER) into distribution systems is a new concept for improving system capacity and stability, feeder voltage, and supply quality and reliability. This paper has addressed voltage support in distribution systems by energy injection from a battery storage distributed energy system. An operation strategy for an inverter interface battery energy storage DER has been developed for maximum improvement in feeder voltage with minimum energy injection from the DER. A control strategy has been proposed for inverter based battery storage DER to regulate network voltage effectively, through operating the DER to generate real (P) and reactive (Q) power with Q priority. The implementation of the inverter interface DER with battery energy storage will save fuel cost of DER but be of much higher capital cost than using a rotary generator. The proposed technique has been evaluated by simulation on a three-phase distribution system with time varying loads. Test results indicate that DER operating with Q priority offers the best solution for maximum voltage improvement. The results also confirm that DER injecting P and Q at the ratio of maximum voltage sensitivity of line presents better solution for power loss reduction than the solution offered by the DER operating with Q priority.     

Adaptive repetitive control to track variable periodic signals with fixed sampling rate

Recent research has shown that the repetitive control is very efficient in tracking periodic signals, where it is required that an integer number of samples in each period. However, in some industrial applications where the signal period varies but other requirements force a fixed sample rate, the number of samples per period may be a non-integer. To address this problem, this paper presents a new adaptive repetitive control, which deals with the non-integer samples per period due to the fixed sampling rate. The proposed adaptive repetitive control consists of two portions, the repetitive controller and nominal controller, where the former uses a fictitious sampler operating at a variable sample rate maintained at multiple times of the signal frequency, while the latter uses a fixed sampling rate. Interpolations are utilized to generate the fictitious samples required for the repetitive learning. The nearly perfect tracking was achieved for non-integer samples per period, when a simple linear interpolation is used. The error due to the interpolation is quantified, which is negligible to the residual tracking error. The comparison of the proposed and the existing schemes shows the significant improvement on the tracking performance. The experimental results on the control of a servomotor demonstrate the effectiveness of the proposed schemes.     

Multiple converter performance and active filtering

The application of active power filtering to power systems in limited by the low switching rate of available high power inverter switches. In this paper, parallel and series connection of multiple voltage source inverter bridges are examined to increase their effective switching rate. The analysis of the inverter voltage spectrum for double edge modulation shows the modulation desired for the separate bridges in open loop. Intuitively “i” bridges in series or parallel increases the effective switch rate by a factor “i”. However the modulation process for each power converter maintains its pattern for its switching period T which gives a roll off at high frequencies and reduces effective bandwidth. The experimental system of two 10 kVA voltage source inverters demonstrated how the closed loop control strategies for the active filter can be applied for multiple bridges implementing periodic optimized error sawtooth feedback control. The double bridge closed loop system showed that the lowest switch frequency terms were double that of the separate bridges. The higher frequency switching lines that should have been cancelled in theory were still discernible due to the finite precision of the edge timing     

VIRULENCE FACTORS ASSOCIATED WITH PATHOGENICITY OF AEROMONAS ISOLATES

Aeromonas species have been associated with diarrhea, and disseminating infections. Diarrhea is either "cholera-like" or "dysentery-like". Both types of diarrhea are normally mild. However, either type can be severe enough to cause hospitalization. The sources of Aeromonas in these infections are foods and water. Two virulence factors associated with Aeromonas diarrhea are a relatively heat-stable cholera-like enterotoxin and a heat-labile cytotoxic enterotoxin. Researchers who observed either of these enterotoxins rarely saw evidence of the other. These differences are probably due to the rare occurrence of the cholera-like toxin and to differences in purification methods. Human feeding studies showed that enterotoxin production is not sufficient for causation of diarrhea. The Aeromonas must also be able to colonize the intestine or invade the intestinal lining. Virulence factors associated with colonization or invasion have not been identified.     

PORE SIZE EFFECT IN THE FIREEZE DRYING PROCESS

SUMMARY— Experiment were conducted to determine the influence and/or effect of the structure of the dried material itself on transport rates. Drying rates were measured in the 0.2–3.0 mm Hg pressure range, the usual operating range of commercial freeze driers. Thermal conductivities of dried beef and heat and mass transfer were calculated from drying rate data and the structure of the freeze-dried material was studied by optical means. A simplified relationship between thermal conductivity and pore structure was derived. Results show drying rates were related to basic transport properties, thermal conductivity and pore structure.     

Placement of shunt VAR systems

Static VAR Systems (SVSs) are becoming increasingly important in the damping of power system oscillations and a number of papers discussing the control of SVS for this purpose have been published. In this paper, however, the position of the SVS is discussed and it is shown that in simple cases the SVS should be placed slightly closer to the generator than to the load, and that for long lines several SVSs will give the best control. Further analysis is included to estimate the optimum placement of the SVS for a variety of loads and operating conditions. Computer calculations and model transmission line measurements are presented to support this analysis. The former includes an investigation into the effectiveness of the SVS as a function of its position for several cases. The conclusions are that, although no easily recognisable rule of placement emerges, poor choice of position can drastically reduce the influence of the SVS on transient stability and the calculations presented show that a compromise solution may be reached for any line whose operating conditions can be predicted.     

Stability boundary analysis of the dynamic voltage restorer in weak systems with dynamic loads

In this contribution, a stability analysis for a dynamic voltage restorer (DVR) connected to a weak ac system containing a dynamic load is presented using continuation techniques and bifurcation theory. The system dynamics are explored through the continuation of periodic solutions of the associated dynamic equations. The switching process in the DVR converter is taken into account to trace the stability regions through a suitable mathematical representation of the DVR converter. The stability regions in the Thevenin equivalent plane are computed. In addition, the stability regions in the control gains space, as well as the contour lines for different Floquet multipliers, are computed. Besides, the DVR converter model employed in this contribution avoids the necessity of developing very complicated iterative map approaches as in the conventional bifurcation analysis of converters. The continuation method and the DVR model can take into account dynamics and nonlinear loads and any network topology since the analysis is carried out directly from the state space equations. The bifurcation approach is shown to be both computationally efficient and robust, since it eliminates the need for numerically critical and long‐lasting transient simulations. Copyright © 2011 John Wiley & Sons, Ltd.     

Active power flow control in a distribution system using discontinuous voltage controller

This paper proposes a hybrid discontinuous control methodology for a voltage source converter (VSC), which is used in an uninterrupted power supply (UPS) application. The UPS controls the voltage at the point of common coupling (PCC). An LC filter is connected at the output of the VSC to bypass switching harmonics. With the help of both filter inductor current and filter capacitor voltage control, the voltage across the filter capacitor is controlled. Based on the voltage error, the control is switched between current and voltage control modes. In this scheme, an extra diode state is used that makes the VSC output current discontinuous. This diode state reduces the switching losses. The UPS controls the active power it supplies to a three-phase, four-wire distribution system. This gives a full flexibility to the grid to buy power from the UPS system depending on its cost and load requirement at any given time. The scheme is validated through simulation using PSCAD.     

FREE-RADICAL RETROGRADE-PRECIPITATION POLYMERIZATION: A MATHEMATICAL MODELING STUDY OF POLYMERIZATION OF STYRENE IN DIETHYL ETHER

Polymerizing a monomer above the lower critical solution temperature (LCST) of its polymer-monomer-(non)solvent mixture has demonstrated better control characteristics than conventional free-radical polymerization kinetics. Reaction kinetics of polymerization in a poor solvent are strongly influenced by heat and mass transfer properties, as understood from modeling the transport phenomena in our earlier work. The study has now been extended to model the reaction kinetics in a styrene-diethyl ether system. The model was based on the CCS model for free radical polymerization, with the modification proposed by Achilias-Kiparissides. Computer simulation results agree well with those obtained from experiments carried under similar conditions, with the onset of phase separation as the only adjustable parameter. Drawbacks of the model are lack of analysis for the effect of monomer concentration and the absence of an appropriate radical trapping mechanism.