bluuebunny/crossref_metadata_2025 huggingface dataset:
Reflection of electromagnetic waves by a layer of periodically stratified dielectric medium—a remedy for TV signal interference caused by tall buildings
Authors: Yamasaki Tsuneki , Hinata Takashi , Hosono Toshio
Published: July 2007

This paper deals with numerical analysis of the reflection of electromagnetic waves from a conductive plate with periodic dielectric distribution. Analysis is based on the Fourier series expansion method (FEM) whereby (i) the EM fields in a periodic inhomogeneous medium are Fourier‐expanded and from this the propagation constant and modal function are found by an eigenvalue equation and (ii) the reflection coefficient is obtained from the boundary conditions between the inhomogeneous medium and free space. Previously, when the dielectric constant E(x) was Fourier‐expanded but the expansion coefficients did not converge absolutely, this method would be abandoned and another method would be sought. In this paper, even in this case, we show that the FEM is still effective. If ε(x) is expressed as ε(x) / ε0 = q(x)/r(x) and the expansion coefficient of q(x) and r(x) can be obtained analytically, it can be shown that there is no need for numerical integration. As a result, the applicability of the FEM for systematic numerical analyses of periodic and inhomogeneous media is greatly extended. Actual numerical analysis is applied to the cases of square, cubic and exponential distributions of the dielectric constant toward avoidance of reflections of the periodic, inhomogeneous layers of the medium. Also, this method may be applied to optical grating problems.
Generating functions for sampling theorems
Authors: Kida Takuro
Published: July 2007

A general form of sampling theorem is considered in which the signal is uniquely interpolated by sample values taken at a number of uniformly spaced sample points for a band‐limited time function of the form f(t) such that ∫−∞∞|f(t)|2 dt < +∞ and whose Fourier spectrum contains components within |ω| < ω0. One of the necessary and sufficient conditions establishing such an interpolation function is derived. According to the theorem, this interpolation function can be given in terms of time functions that correspond to the coefficients obtained by Fourier series expansion with respect to a function of angular frequency and time, called the generating function. Next, for a signal of the same form considering a generalized form of the sampling theorem that deals with nonuniform sampling points, the necessary and sufficient condition for the interpolation function that ordinarily establishes this is shown. In this case it is also shown that the concept of the generating function can be extended. Further, by combining the well‐known Silverman‐Toeplitz theorem for summation of series and the sampling theorem as derived above from the outline of the generating function, several sampling theorems can be derived.
Theoretical analysis of diffraction field fluctuations due to position irregularity of diffraction gratings
Authors: Yoshida Junichi , Adachi Saburo
Published: July 2007

The effect on the diffraction pattern attributable to positional irregularity in a linear and equi‐spaced diffraction grating, if correlation exists between the position irregularities with respect to the reference points, is examined. The nature of these positional irregularities is twofold. In the first case, the probability distributions of the positional irregularities centered around a reference point are all identical. In the second case the effect of random variables causing the positional irregularities is accumulated and the subsequent probability distribution of the diffraction grating is strongly affected. As an example of the first case the positional irregularity of a diffraction grating resulting from thermal expansion of a waveguide is considered. One example of the second case is positional irregularity attributable to refractive index fluctuation from an acoustic wave propagating in an optical waveguide to create periodic modulation of the index. Another example of the second case is the orientation error when a grating is placed at equal intervals with respect to a previously placed grating. In the analysis it is assumed that the radiation loss at each grating is so small that the accumulation in the optical waveguide is negligible.
IL‐3 is an important differentiation factor for the development of prostaglandin E2‐producing macrophages between C57BL/6 and BALB/c mice
Authors: Kuroda Etsushi , Noguchi Junko , Doi Takahiro , Uematsu Satoshi , Akira Sizuo , Yamashita Uki
Published: July 2007

We have previously reported that peritoneal and splenic macrophages from Th2‐dominant BALB/c mice produced higher amounts of prostaglandin (PG) E2 than cells from C57BL/6 mice. In this study, we investigated how macrophages from BALB/c mice acquire the ability of enhanced PGE2 production, using bone marrow‐derived macrophages differentiated by M‐CSF, GM‐CSF or IL‐3. There is no strain difference in PGE2 production by GM‐CSF‐ and M‐CSF‐differentiated macrophages; however, IL‐3‐differentiated macrophages from BALB/c mice produced higher amounts of PGE2 and lower amounts of type I cytokines than cells from C57BL/6 mice. IL‐3‐differentiated macrophages from BALB/c mice expressed larger amounts of mRNA of membrane‐bound (microsomal) PGE synthase‐1 (mPGES‐1). The amounts of PGE2 produced by macrophages were significantly reduced in mPGES‐1‐deficient mice, and these mice displayed enhanced Th1 responses after Propionibacterium acnes treatment compared with wild‐type mice. Microarray analysis revealed 63 genes that are differentially expressed more than fivefold in macrophages between C57BL/6 and BALB/c mice. These results indicate that mPGES‐1‐mediated PGE2 produced by macrophages regulates immune responses, and IL‐3 is an important factor for the differentiation of macrophages that produce higher amounts of PGE2 through mPGES‐1 activity in BALB/c mice.
Antigen co‐encapsulated with adjuvants efficiently drive protective T cell immunity
Authors: Heit Antje , Schmitz Frank , Haas Tobias , Busch Dirk H. , Wagner Hermann
Published: July 2007

Compared to "live" vaccines, the immunogenicity of "subunit" vaccines based on recombinant antigen (Ag) is poor, presumably because exogenous Ag fails to effectively access the endosomal Ag‐processing pathways of Ag‐presenting cells (APC). To overcome this limitation, we exploited biodegradable poly(lactic‐co‐glycolic) microspheres (MP) co‐entrapping Ag and Toll‐like receptor (TLR) 9 or 7 ligands as an endosomal delivery device. In vitro, microspheres were rapidly phagocytosed by APC and translocated into phago‐endosomal compartments, followed by degradation of the Ag and concurrent activation of endosomal TLR. As a consequence, full maturation of and cytokine secretion by APC as well as Ag‐cross‐presentation ensued. In vivo, "loaded" microspheres triggered clonal expansion of primary and secondary Ag‐specific CD4 and CD8 T cells. The efficacy of CD8 T cell cross‐priming was comparable to that of live vectors. The potency of T cell vaccination was demonstrated by protective and therapeutic interventions using infection‐ and tumor‐model systems. These preclinical "subunit" vaccination data thus recommend MP as a generally applicable and powerful endosomal delivery device of exogenous Ag plus TLR‐based adjuvants to vaccinate for protective and therapeutic CD4 and CD8 T cell immunity.
General and Efficient One‐Pot Synthesis of Tertiary Phosphane–Borane Complexes Containing Different Alkyl Groups and In Situ Facile Recycling of the Phosphorus Donor Reagent
Authors: Baccolini Graziano , Boga Carla , Mazzacurati Marzia
Published: July 2007

Tertiary symmetric and asymmetric phosphane–borane complexes containing different alkyl groups, including bulky groups, can be obtained by the addition of different Grignard reagents to benzothiadiphosphole 1 followed by direct complexation with BH3·THF. Treatment of the resulting mixture with PCl3 led to the reformation of the starting reagent 1, which was recovered from the reaction mixture by simple crystallization. The phosphane complexes were purified by chromatography of the mother liquor. This procedure is highly atom‐economic and environmentally friendly. All the reaction intermediates were identified.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)
A New Selena‐Aza‐Payne‐Type Rearrangement of Aziridinylmethyl Tosylates Mediated by Tetraselenotungstate
Authors: Sureshkumar Devarajulu , Koutha Srinivasamurthy , Chandrasekaran Srinivasan
Published: July 2007

Tetraselenotungstate 1 reacts with simple (N‐tosylaziridinyl)methyl tosylate derivatives to give allylamine derivatives as the only products by an unprecedented selena‐aza‐Payne‐type rearrangement. When the methodology is extended to disubstituted (N‐tosylaziridinyl)methyl tosylates, regio‐ and stereospecific ring‐opening of the aziridines occurs to afford allylamine derivatives as the major products and cyclic five‐membered diselenides as the minor products in good yields under mild reaction conditions without using any Lewis acid or base. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)
Twenty first century work issues: A look back
Authors: Hershel‐Chow Gloria
Published: July 2007

Employment Relations Today is pleased to present the keynote address, presented in Chicago at the 2099 Conference of the International Workplace Society, by Gloria Hershel‐Chow, president of The Globe at Work. The Globe at Work is the world's foremost membership association for human resources executives. I t has 843 organizational members and 2,412 individual corporate representatives.
Interactions and co‐ordination of multiple‐function FACTS controllers
Authors: Wang H. F.
Published: July 2007

This paper presents two study cases of interaction analysis of multiple‐function FACTS (flexible AC transmission systems) controllers: ‐ the contradiction between the voltage and the damping control of a SVC (static VAr compensator); ‐ the interaction between the voltage control of the DC link capacitor and the damping control of a UPFC (unified power‐flow controller). In the paper, an algorithm is proposed for the co‐ordinated design of these two multiple‐function FACTS controllers. Examples are presented to confirm the analytical conclusions obtained and to demonstrate the effectiveness of the algorithm of the co‐ordination proposed.
High power factor ac/dc/ac converter with h‐bridge cascade five‐level pwm inverter
Authors: Lin B.‐R.
Published: July 2007

This paper presents an AC/DC/AC converter which consists of a three‐phase switch‐mode rectifier (SMR) and a three‐phase multilevel inverter with separate isolated DC power supplies. Based on a hysteresis current control scheme in the stationary reference frame, the three‐phase SMR is controlled to supply sinusoidal currents with high power factor and low current distortion. The separate DC bus voltages are provided by regulated switch‐mode DC/DC converters. The series connection of H‐bridge cells is adopted to provide multilevel phase voltage. The control scheme of the three‐phase multilevel inverter is based on a look‐up table with sine‐triangular pulse‐width‐modulation (PWM) method. The voltage unbalance problem between the separate isolated DC bus voltages is improved by using the proposed control scheme. The proposed control algorithm of the AC/DC/AC converter is verified by simulation and experimental results.
New control system for an ac‐excited synchronous machine to achieve a wide field weakening range and a high overload capability
Authors: Haag J. , Rees S. , Gutemann M.
Published: July 2007

In the conventional AC‐excited synchronous machine operated with mains excitation [1,2], the commutation conditions in the machine‐side thyristor converter do not allow the drive to be notably overloaded. In addition operation with field weakening and full four‐quadrant operation are not possible due to the principle. If on the other hand the AC‐excited synchronous machine is excited by a small insulated gate bipolar transistor inverter (IGBT), then the commutation conditions in the machine‐side thyristor converter can be improved with the control system presented here. Therefore a considerable overload capability and a field weakening range of 1:2 and more can be realized with the AC‐excited synchronous machine. Areas of application previously excluded are thus opened up to the AC‐excited synchronous machine.
Comparison of electromagnetic field for two different lightning pulse current models
Authors: Andreotti A. , De Martinis U. , Verolino L.
Published: July 2007

In this paper the electromagnetic field produced by a lightning current analytically described by the so called double exponential model is compared with the field produced by the same current (same peak, rise and decay time) analytically described by the model proposed by Heidler. The exponential model has been widely used in literature for its simplicity and its relatively good accuracy. The Heidler model is more complex, but removes the main problem shown by the double exponential: the non‐zero derivative at the beginning of the lightning pulse in contrast with the physical phenomenon. In the lightning electromagnetic pulse (LEMP) simulations both models are now used. In the paper we aim to make a comparison between the two. In particular we show that the two models are fairly equivalent in the frequency range up to 2 MHz, namely the range of interest of typical lightning phenomena. In addition, the double exponential model is shown to be conservative for higher frequencies.
An advanced methodology for steady‐state security assessment of power systems
Authors: Popovic D. P. , Dobrijevic D. M. , Mijuskovic N. A. , Vlaisavljevic D. J.
Published: July 2007

The basic objective of this paper is to present the relevant aspects of an advanced methodology for steady‐state security assessment of power systems. This methodology consists of the following relevant parts: procedure for forming the unified external network equivalents, with adaptive buffer system selection, procedure for fast contingency selection and simple method for the fast and sufficiently accurate assessment of dynamic variation of power system frequency during the time of its primary control. For the potentially critical contingency, the continuation of iterative procedure is performed. Full contingency analysis is based on specially developed method, which enables successive solution of the load‐flow problem for a set of characteristic post‐dynamic quasi‐stationary states. The characteristics of the developed methodology are tested on the example of the existing electric power interconnection in the Balkans.
An approach to the non‐active power concept in terms of the poynting‐park vector
Authors: Ferrero A. , Leva S. , Morando A. P.
Published: July 2007

This paper reconsiders the approach to the electric systems in terms of Maxwell equations, and reconsiders in particular the energy transfer involved in an electric system in terms of the flux of the Poynting vector. This approach is extended to the three‐phase systems, and the Park transformation is reconsidered by applying it to the Poynting vector. In this way a correct physical meaning can be assigned to the non‐active components of the Park instantaneous power by tracing them back to the components of the Poynting vector.
On techniques for the localisation of multiple distortion sources in three‐phase networks: Time domain verification
Authors: Rens A. P. J. , Swart P. H.
Published: July 2007

Frequency domain network modelling techniques have shown that the localisation of steady‐state distortion sources by monitoring the direction of active power in the harmonic components can not be accomplished through continuous single‐point measurements in the presence of multiple distortion sources. Because frequency domain models of line commutated AC/DC converters could so easily be susceptible to error, the same analysis in the time domain will enhance the credibility of the findings. A time‐domain approach has therefore been adopted by generating the equivalent power system data through repeated ATP (alternative transients programme) simulations and interpreting the resulting trends by means of a Mathcad package. The results obtained confirm the previous findings.
Measurements for the characterization of quasi‐periodic waveforms
Authors: Peretto L. , Sasdelli R.
Published: July 2007

Quasi‐periodic signals feature discrete‐spectrum waveforms with lines located at frequencies expressed by non‐rational numbers. In this paper, a new digital method for the spectral analysis of quasi‐periodic bi‐tone signals is proposed. The results of some simulations showing its good performance are reported and discussed.
Frequency coupling matrices for multi‐stage conversion system analysis
Authors: Carbone R. , Lo Schiavo A. , Marino P. , Testa A.
Published: July 2007

A general approach for harmonic and interharmonic analysis of multi‐stage conversion systems is formalized. The approach is based on frequency coupling matrices (FCMs) relating voltage and current harmonics and interharmonics. Each component of the conversion system is classified as linear time—invariant or non‐linear time—invariant or non‐linear time variant and a systematic procedure to build up its FCM is presented. Then, according to the specific topology of the conversion system, single‐component FCMs are opportunely combined to deduce a whole‐system FCM. This matrix allows to estimate the absorbed current due to voltage interharmonics, to analyze the admittance of the conversion system and to investigate the current amplitude dependence on the supply voltage phase; such possibilities are shown with reference to a test system.
Simulation and digital implementation of a simple technique of active filter for elimination of switching ripples/distortion in the supply current
Authors: Singh B. N. , Chandra A. , Rastgoufard P.
Published: July 2007

This paper deals with a digital signal processor (DSP) based implementation of a three‐phase shunt active filter (AF) to compensate harmonics and reactive power of nonlinear loads. A three‐phase diode bridge rectifier with R‐L and R‐C loads is taken as the nonlinear load. It is observed that the AF systems suffer from the problems of switching ripples (in case of R‐L nonlinear load) and distortion (in case of R‐C nonlinear load). The switching ripples and distortion in the supply currents contribute considerably to its Total Harmonics Distortion (THD). This investigation proposes a new, cost free and simple approach for the elimination of switching ripples and distortion in the supply current. A pulse width modulation (PWM) current controlled voltage source inverter (CCVSI) is used as the AF. The AF system is tested with two types of PWM switching techniques, namely, carrier‐less (hysteresis) and carrier based techniques. Two types of current control techniques, namely, direct current control and indirect current control are designed to obtain PWM switching signals for the devices of the AF. The capability of the indirect current control technique towards the elimination of switching ripples and distortion is established. Simulated and experimental results for steady state and transient operating conditions of the AF for R‐L as well as R‐C nonlinear loads are given and discussed in detail.
Some remarks concerning the measurement of the ferromagnetic losses under non‐sinusoidal conditions
Authors: Ţugulea Andrei
Published: July 2007

The present standards concerning the measurement of the losses in ferromagnetic media, namely the IEC publications 404‐2 (1978), 404‐;3 (1982) and 404–10 (1988), refer only to the magnetic flux densities between 1 T and 1.85 T, if the magnetic sheets are grain oriented, and 1 T and 1.5 T if the magnetic sheets are nonoriented. The time variation of the magnetic fluxes must be almost sinusoidal at industrial frequencies of 50 Hz or 60 Hz and medium frequencies between 400 Hz and 10,000 Hz. In the above‐mentioned standards there are no references concerning the measurement of losses under non‐sinusoidal conditions. The paper deals with some theoretical aspects concerning the power‐flows under non‐sinusoidal time variation of the electromagnetic fields. The theoretical conclusions are applied to the study of the losses in ferromagnetic media. It is shown that new norms or standards are necessary. Some practical suggestions are given at the end.
A class of analytical functions to study the lightning effects associated with the current front
Authors: Heidler F. , Cvetić J.
Published: July 2007

The lightning currents are measured on tall structures and by artificially initiated lightning. Additionally they are deduced from the remote electric or magnetic field radiated during the so‐called return stroke phase, when the lightning current is flowing to earth. For this purpose return stroke models were developed giving the dependency between the field data and the current along the ionized return stroke channel. The most frequently employed models use the so‐called channel‐base currents, which are commonly expressed by analytical functions to simplify the calculation procedure. A review is given about a class of analytical functions being very convenient in such calculations. The special attention is given to the type of current function most frequently used in lightning research and standardization. With these functions the fine structure in the current front can be studied, which is of basic interest with respect to the lightning protection.
A novel non‐linear TCSC control scheme for a multi‐machine power system
Authors: Zheng S. L. , Lei X. , Li X. Y. , Povh D. , Retzmann D.
Published: July 2007

In this paper, a novel non‐linear thyristor controlled series compensation (TCSC) control scheme for damping power oscillations and improving transient stability of a multi‐machine power system is presented. Based on the analysis of TCSC power modulation in a two‐machine system, a non‐linear control law for the TCSC is established via local measurements. With the help of the feedback linearization technique, the non‐linear model of a multi‐machine power system is transferred to an equivalent linearization model. To this equivalent linear system a well‐developed optimal linear design methodology with integral‐of‐time‐multiplied‐absolute error criterion (ITAE) is adopted to achieve optimal control performance. The effectiveness and robustness of the non‐linear control scheme proposed are demonstrated with a three‐machine system equipped with a TCSC. In comparison with a conventional control scheme, significant improvements in dynamic performance are achieved by the proposed TCSC control strategy.
Reduction of operating frequency for power cables
Authors: Steinbrich K.
Published: July 2007

The article describes the effects of operating frequency reduction on the transmission behaviour of high power cables. Strongly reduced current losses, lowered induced voltages in the sheaths and lower charging currents offer new solutions for the transmission of energy by cables. The following facts can be obtained by reduction of the operating frequency: an increase of the current carrying capacity, an enlargement of the transmission distance, lower thermal stress of the cable and better efficiency of the power transmission. The above mentioned advantaes are typical for direct current (d.c.) transmission systems, but can now also be achieved for usual alternating current (a.c.) cables by reduction of the operating frequency. The positive effects of frequency reduction are represented for four different types of power cables.
Load‐prediction with neural and statistical components in power systems with instationary load patterns
Authors: Wilfert H.‐H. , Voigtländer K.
Published: July 2007

A hybrid pattern algorithm is presented combining statistical and neural methods to forecast hourly load of an electrical power supplying system. Compared with ordinary neural techniques which require a large stationary data‐set for the parametrization of the huge number of net‐weights, the algorithm yields to a sufficient prediction even with a small reference data‐set and is especially suited for power utilities with instationary load patterns. In this sense the choice of appropriate model structures and parsimonious parametrization are considered in particular. The presented modular design yields to a high transparency of the entire prediction algorithm. Furthermore a clear assessable performance measure of the prediction accuracy of the four individual steps of the forecasting algorithm is presented.
Technical and economical analysis of 3‐LIMB and 4‐LIMB three phase transformers in YN/YN network
Authors: Elleuch M. , Poloujadoff M.
Published: July 2007

This paper deals with the behaviour of the three phase three limb transformer (with or without tertiary winding) as well as the four limb transformer under unbalanced steady states in a YN/yn network. Technical and economical comparative analysis has been established among the different configurations of the transformers. The opportunity of the replacement of the four limb transformer, currently used in the Tunisian distribution network, by a three limb transformer equipped by a tertiary winding has been discussed. During the analysis, the example of a 20 KVA transformer with a tertiary winding and a fourth limb (which could be removed) has been tested. A satisfactory concurrent is obtained between theoretical and experimental results.
System management of a wind energy converter to control a wind specific power fluctuation
Authors: Sourkounis C. , Kanakis A. , Lemogo‐Kana Ch.
Published: July 2007

The fluctuating production of electric energy converters caused by the dynamic changing wind velocity leads to a system perturbation which can be called flicker. Apart from this system perturbation the quality of the electric power is also reduced because of the tower shadow effect. The periodic appearance of this effect overlaps and intensifies the flicker. Within the scope of this contribution a system management will be introduced, which reduces the tower shadow effect and which damps the short‐term power fluctuation caused by the changing wind velocity. The mode of operation of this system management will be shown on a simulated test model (7.5 kW) of a real wind energy converter (Ventis 100/20). In this simulation, the model of the aerodynamic behaviour of the wind rotor and the mechanical subsystem reproduces the real stationary and dynamic behaviour. The control of power is realised by the system management based on a microcontroller and a three phase A.C. power controller.
A black box identification in frequency domain
Authors: Mercorelli P. , Terwiesch P.
Published: July 2007

Harmonic oscillations as integer multiples of the fundamental frequency in a power system are caused by nonlinear physical effects such as switching or saturation. Modelling and detection of these harmonics are crucial for power system control and protection. The present paper proposes the use of wavelet networks with smooth local trigonometric functions as activation functions. A new algorithm is proposed, together with the use of the Cross Entropy function as a tool for evaluating the model quality. The algorithm consists of recursive dual iterations with biorthogonal smooth local sine and cosine wavelet packets in order to calculate the adjustable parameters related to the activation functions. The algorithm efficiently minimizes the Shannon Entropy function by adoptively choosing the best time‐frequency cells on the wavelet packet tree. During every loop the Cross Entropy function between estimated outputs and target outputs is checked. A procedure by using trigonometric wavelet packets is proposed as an effective tool for disturbance detection, power quality analysis and non‐linear harmonic circuit modelling. Simulations of a converter bridge for traction drives are included to illustrate the effectiveness of the algorithm and the choice of the activation function.
Numerical determination of characteristic temperatures in directly loaded power oil transformer
Authors: Radakovic Z.
Published: July 2007

The paper presents the application results of original non‐linear non‐stationary thermal model of the oil power transformers. The main advantage of the model is precise treatment of non‐linear convection heat transfer influence to the transient thermal process. It is convenient to calculate the model parameters from the short‐circuit heating experiment. The aim of this paper is to investigate the precision of such obtained parameters application to a transformer in normal operation. For that purpose, the precise calculation of power losses distribution is needed and is proposed in the paper. For experimental verification of the model accuracy the measuring method for continuous monitoring of mean winding temperature of directly loaded transformer is developed.
New passive filter design for neutral current cancellation in balanced 3‐phase 4‐wire non‐linear distribution systems
Authors: El‐Saadany E. F. , Salama M. M. A. , Chikhani A. Y.
Published: July 2007

Different types of non‐linear loads are expected to proliferate into the distribution system, causing the harmonic distortion levels on these systems to increase. Third harmonic and all other triplen harmonic currents have little diversity among different loads and add in the neutral. The neutral current in a low voltage three‐phase four‐wire distribution system is expected to increase resulting in significant problems. The factors that affect the neutral current magnitude as well as the phase currents distortion are investigated. A new technique, namely, reactance one‐port compensator is presented in order to cancel the neutral current and improve the overall system distortion levels. The attenuation and diversity effects are considered during performing this study. The analysis uses the electromagnetic transient program (EMTP) to model the loads as well as the overall system. The proposed filter drastically improves the system performance and substantially reduces the neutral current.
An economic parallel processing technology for faster than real‐time transient stability simulation
Authors: Beaudin S. , Marceau R. J. , Bois G. , Savaria Y. , Kandil N.
Published: July 2007

The need to increase power flows is forcing electric power companies to consider the rapid determination of transfer and security limits in energy management centers. At the present time, transient stability transfer limits are generally calculated off‐line due to the limitations of numerical computers. This paper introduces an innovative approach for simulating transient stability based on the use of a new type of chip technology designed for real‐time image processing. Power system equations in matrix form are solved by means of rapid matrix operation techniques on the PULSE (parallel ultra large scale engine) technology, resulting in outstanding low cost performance. This paper confirms the efficiency of this approach on the 9 bus Western Systems Coordinating Council (WSCC) test power system. Simulation results based on a preliminary version of the PULSE technology clearly indicate that economic, faster than real‐time transient stability simulation is feasible for large networks.
Fast analytical short‐circuit current calculation of rectifier‐fed auxiliary subsystems
Authors: Kunz M. , Herold G.
Published: July 2007

The course of time of a three‐phase rectifier system which its alternating valve participation can be interpreted as a continuous sequence of alternating switching states. To allow a more convenient calculation, the substitutional circuit with the converter is transformed into state‐space coordinates. Hereby each operational mode of the rectifier can be represented by two linear independent space‐phasor component networks. In the state‐space, an analytical solution for this boundary value system can be carried out. After a retransformation back into the time domain, its time functions can be derived. In contrast to other calculation methods, no assumptions or simplifications have to be made like ideal smooth DC currents. Furthermore, all states of operation of the rectifier bridge can be easily calculated, which cover DC side idle‐running to DC short‐circuit.
MPP control of a photovoltaic energy system
Authors: Hua C. , Lin J. , Tzou H.
Published: July 2007

Photovoltaic (PV) power converters are used to convert the electrical energy from the PV modules to a stable and reliable power source. The object of this paper is to analyse and design a DC/DC converter with current regulation for PV energy systems. An on‐line tracking method with a discrete time control and a Pl compensator is used to track the maximum power points (MPPs) of the PV modules. With the temperature‐loop and insolation‐loop control, the proposed MPPT control features accurate MPPT and fast transient response. The large‐ and small‐signal model and transfer function for the system are developed by using the principle of energy conservation. The Tl320C25 digital signal processor (DSP) was used to implement the proposed on‐line MPPT control. Simulations and experimental results show excellent performance.
A new synthetic loading of induction machines based on phase modulation
Authors: Soltani J. , Szabados B.
Published: July 2007

Synthetic loading of induction machines are used to test machines for temperature rise and total losses without the need for attaching mechanical load to the shaft. A new phase modulation technique of the voltage is proposed in lieu of the traditional two‐frequency method. A simple bang‐bang DC switch is used to modulate the excitation current of an induction generator. System modeling was performed using the traditional two‐axis method, and has proven that the method is feasible. Practical test results are shown to validate the simulated performance. It is shown that the only constraint of this synthetic loading is to maintain the RMS values of the current and voltage at rated values.
The determination of the stability regions and sequential transient stability analysis of integrated AC‐DC power system consisting of bipolar dc lines via the controlling unstable equilibrium points
Authors: Ayan K. , Arifoǧlu U.
Published: July 2007

In this paper, a sequential transient stability analysis of an integrated AC‐DC power system consisting of bipolar MTDC is accomplished by taking into account the variation of the amplitude of the bus voltages during transient. This variation of the bus voltages is provided by the sequential AC‐DC load flow algorithm of the AC‐DC power system with respect to the active power modified at each integration step. The algorithm has two superiority. Firstly, as DC system variables are not included in Jacobian matrix the convergence time of the algorithm and necessity of computer memory is little. Secondly, both this algorithm has been developed for bipolar system and this algorithm also can be run with current controlled, voltage controlled and power controlled terminals without making any changes in proposed method. Thus, the AC‐DC transient stability analysis is accomplished considering the true value of the ratio of the reactive power absorbed from the converters to the active power transferred through DC line by the sequential approach. In the transient analysis of the system, the DC lines loads are represented as the current injections to the buses where they are connected and its effect is reflected at the internal buses in the form of additional bus power injections. The effect of these is again reflected at the internal buses in the form of additional bus power injections. Thus, the problem in constructing a valid Lyapunov function due to the transfer conductance is eliminated.
Hepatitis B virus DNA is subject to extensive editing by the human deaminase APOBEC3C
Authors: Baumert Thomas F. , Rösler Christine , Malim Michael H. , von Weizsäcker Fritz
Published: July 2007

APOBEC3G (A3G) and APOBEC3C (A3C), 2 members of the APOBEC family, are cellular cytidine deaminases displaying broad antiretroviral activity. A3G inhibits hepatitis B virus (HBV) production by interfering with HBV replication without hypermutating the majority of HBV genomes. In contrast, A3C has little effect on HBV DNA synthesis. The aim of this study was to further dissect the mechanisms by which A3G and A3C interfere with the HBV life cycle. Immunoprecipitation experiments demonstrated that both A3G and A3C bind to the HBV core protein. A ribonuclease (RNase) treatment resulted in the nearly complete dissociation of the HBV core protein from A3G, whereas the HBV core-A3C complex was more stable. Interestingly, the majority of the newly synthesized HBV DNA genomes displayed extensive G-to-A mutations in the presence of A3C, whereas no A3C-induced HBV RNA mutations were detected. These findings support a model in which the RNA-dependent entrapment of A3G into the preassembly complex hampers subsequent steps in capsid formation. On the other hand, A3C is readily packaged into replication-competent capsids and efficiently deaminates newly synthesized HBV DNA.

Conclusion:

These findings demonstrate that HBV is highly vulnerable to the editing activity of an endogenous human deaminase and suggest that A3C could contribute to innate anti-HBV host responses. (Hepatology 2007.)
Transforming growth factor-β and substrate stiffness regulate portal fibroblast activation in culture
Authors: Li Zhaodong , Dranoff Jonathan A. , Chan Erick P. , Uemura Masayuki , Sévigny Jean , Wells Rebecca G.
Published: July 2007

Myofibroblasts derived from portal fibroblasts are important fibrogenic cells in the early stages of biliary fibrosis. In contrast to hepatic stellate cells, portal fibroblasts have not been well studied

in vitro,

and little is known about their myofibroblastic differentiation. In this article we report the isolation and characterization of rat portal fibroblasts in culture. We demonstrate that primary portal fibroblasts undergo differentiation to α-smooth muscle actin–expressing myofibroblasts over 10–14 days. Marker analysis comparing portal fibroblasts to hepatic stellate cells demonstrated that these are distinct populations and that staining with elastin and desmin can differentiate between them. Portal fibroblasts expressed elastin at all stages in culture but never expressed desmin, whereas hepatic stellate cells consistently expressed desmin but never elastin. Immunostaining of rat liver tissue confirmed these results

in vivo.

Characterization of portal fibroblast differentiation in culture demonstrated that these cells required transforming growth factor-β (TGF-β): cells remained quiescent in the presence of a TGF-β receptor kinase inhibitor, whereas exogenous TGF-β1 enhanced portal fibroblast α-smooth muscle actin expression and stress fiber formation. In contrast, platelet-derived growth factor inhibited myofibroblastic differentiation. Portal fibroblasts were also dependent on mechanical tension for myofibroblastic differentiation, and cells cultured on polyacrylamide supports of variable stiffness demonstrated an increasingly myofibroblastic phenotype as stiffness increased.

Conclusion:

Portal fibroblasts are morphologically and functionally distinct from hepatic stellate cells. Portal fibroblast myofibroblastic differentiation can be modeled in culture and requires both TGF-β and mechanical tension. (HEPATOLOGY 2007.)
Econometric models based on count data. Comparisons and applications of some estimators and tests
Authors: Cameron A. Colin , Trivedi Pravin K.
Published: July 2007

This paper deals with specification, estimation and tests of single equation reduced form type equations in which the dependent variable takes only non‐negative integer values. Beginning with Poisson and compound Poisson models, which involve strong assumptions, a variety of possible stochastic models and their implications are discussed. A number of estimators and their properties are considered in the light of uncertainty about the data generation process. The paper also considers the role of tests in sequential revision of the model specification beginr ing with the Poisson case and provides a detailed application of the estimators and tests to a model of the number of doctor consultations.
An empirical analysis of self‐employment in the U.K.
Authors: Rees Hedley , Shah Anup
Published: July 2007

The paper formulates a self‐employment/paid‐employment choice problem that draws upon Knight's notion that the individual responds to the risk‐adjusted relative earnings opportunities in each sector. Based on a development of such choice‐theoretic considerations, an econometric model is developed with the purpose of empirically examining the determinants of self‐employment. The model features simultaneous determination of employment status and earnings, which allows for self‐selectivity. The model is estimated using a sample of 4762 individuals from the General Household Survey for 1978. The estimation of the earnings equation enables a calculation of the self‐employment/paid‐employment earnings differential and the estimated probit equation for self‐employment/paid‐employment status facilitates the prediction of the probability of self‐employment. The paper finds that there is positive selection bias in the observed earnings of employees, that the probability of self‐employment depends positively on the earnings difference between the two sectors and that education and age are significant determinants of self‐employment.
Formulating wald tests of the restrictions implied by the rational expectations hypothesis
Authors: Gregory Allan W. , Veall Michael R.
Published: July 2007

In this paper, we examine the small sample properties of alternative formulations of Wald tests of non‐linear restrictions implied by the rational expectations hypothesis. A Monte Carlo analysis is presented as well as an example using Canadian aggregate time series data. The evidence indicates that Wald test results are extremely sensitive in small samples to the way in which the non‐linear restrictions in such models are parameterized, with a multiplicative form yielding tests of most accurate size. Least squares degrees of freedom adjustments also improve the sample performance of the tests.
An empirical model of a household's choice of activities
Authors: Montmarquette Claude , Monty Luc
Published: July 2007

This paper analyses how households of school‐age children choose to participate in the labour force, volunteer work, democratic assemblies, and child‐related leisure activities. Contrary to previous studies, the model presented in this paper treats all of these household's choice of activities involving time allocation decisions as simultaneously determined. The empirical section uses qualitative data estimation techniques which permit a wide range of joint and interaction effects among the variables of the model. The results confirm the importance of time‐opportunity cost variables as major determinants of household's choice of activities with the mother's education consituting a particularly important factor.
A micro‐econometric analysis of vacation behaviour
Authors: Van Soest Arthur , Kooreman Peter
Published: July 2007

We present a micro‐econometric limited dependent variable model, which simultaneously explains the decision whether to go on vacation or not, the choice of destination and the decision on the level of the vacation expenditures. The model has been estimated on the basis of a cross‐section of Dutch households; we find a striking difference in income elasticity between domestic vacations and vacations abroad, and a large impact of owning certain durables (such as a boat) on the choice of destination. To illustrate the potential of the model, some simulations are performed.
In pursuit of monopoly power: Recent quantitative work in industrial: Economics
Authors: Geroski P. A.
Published: July 2007

This paper reviews empirical work in industrial organization devoted to the question of inferring the existence of monopoly power from data on observed market outcomes like profits and prices. The discussion covers inter‐industry work long conventional in the area, newer developments in intra‐industry analysis and case studies, and concludes with some remarks on a small but growing body of work assessing market dynamics. The wide range of studies reviewed all generally reveal that market power exists in many industries.
The dynamics of exchange rate volatility: A multivariate latent factor ARCH model
Authors: Diebold Francis X. , Nerlove Marc
Published: July 2007

We study temporal volatility patterns in seven nominal dollar spot exchange rates, all of which display strong evidence of autoregressive conditional heteroskedasticity (ARCH). We first formulate and estimate univariate models, the results of which are subsequently used to guide specification of a multivariate model. The key element of our multivariate approach is exploitation of factor structure, which facilitates tractable estimation via a substantial reduction in the number of parameters to be estimated. Such a latent‐variable model is shown to provide a good description of multivariate exchange rate movements: the ARCH effects capture volatility clustering, and the factor structure captures commonality in volatility movements across exchange rates.
Production of public safety: Are socioeconomic characteristics of local communities important factors?
Authors: Gyimah‐Brempong Kwabena
Published: July 2007

This paper uses the translog cost function and cross section data from Florida to investigate the importance of socioeconomic characteristics (SEC) of local communities in the production of public safety. The approach used is noteworthy in four respects: (1) the underlying production function is specified as an AGEM production function; (2) SEC are treated as non‐purchased fixed inputs; (3) two variables are used to proxy SEC in estimation; and (4) the paper test for the existence of endogenous quality differences in safety. It is shown that exclusion of SEC from the production (cost) function of safety leads to misspecification; and hence they provide a misleading guide to policy.
Tests of international CAPM with time‐varying covariances
Authors: Engel Charles , Rodrigues Anthony P.
Published: July 2007

We perform maximum‐likelihood estimation of a model of international asset pricing based on CAPM. We test the restrictions imposed by CAPM against a more general asset pricing model. The ‘betas’ in our CAPM vary over time as the supplies of assets change and as the conditional covariances or returns on those assets change. We let the covariances change over time as a function of macroeconomic data, and an alternative model allows the covariances to follow a multivariate ARCH process. We also can identify a modified CAPM model with measurement error. We find that the estimated CAPM performs much better when variances are not constant over time. Nonetheless, CAPM is rejected in favour of the lessrestricted model of asset pricing.
Fiscal and monetary policy co‐ordination: A differential game approach
Authors: Petit Maria Luisa
Published: July 2007

In this paper the interaction between the Treasury and the central bank is examined in the case of both cooperative and non‐cooperative behaviour. Differential games are used in the framework of a continuous‐time econometric model of the Italian economy. The Nash and the Stackelberg non‐cooperative equilibrium solutions are computed, and the case for cooperation is analysed by considering the Nash and the Kalai‐Smorodinsky bargaining models. It is shown that, in the Italian case, the government has a stronger bargaining position than the central bank. A comparison is then made between the different solutions to show that the drawbacks that emerge from non‐cooperation are not simply those depending on the players' payoffs. Other features are in fact considered which constitute a further argument for policy co‐ordination.